Merge branch 'master' of https://github.com/JonCook/tutorials

core-java-modules/core-java-concurrency-advanced/README.md

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+## Core Java Concurrency Advanced Examples
+
+This module contains articles about advanced topics about multithreading with core Java.
+
+### Relevant Articles: 
+- [Introduction to Thread Pools in Java](https://www.baeldung.com/thread-pool-java-and-guava)
+- [Guide to CountDownLatch in Java](https://www.baeldung.com/java-countdown-latch)
+- [Guide to java.util.concurrent.Locks](https://www.baeldung.com/java-concurrent-locks)
+- [An Introduction to ThreadLocal in Java](https://www.baeldung.com/java-threadlocal)
+- [LongAdder and LongAccumulator in Java](https://www.baeldung.com/java-longadder-and-longaccumulator)
+- [The Dining Philosophers Problem in Java](https://www.baeldung.com/java-dining-philoshophers)
+- [Guide to the Java Phaser](https://www.baeldung.com/java-phaser)
+- [An Introduction to Atomic Variables in Java](https://www.baeldung.com/java-atomic-variables)
+- [CyclicBarrier in Java](https://www.baeldung.com/java-cyclic-barrier)
+- [Guide to the Volatile Keyword in Java](https://www.baeldung.com/java-volatile)
+- More Articles: [[next -->]](/core-java-modules/core-java-concurrency-advanced-2)

core-java-modules/core-java-concurrency-advanced/pom.xml

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+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <groupId>com.baeldung</groupId>
+    <artifactId>core-java-concurrency-advanced</artifactId>
+    <version>0.1.0-SNAPSHOT</version>
+    <name>core-java-concurrency-advanced</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>parent-java</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+        <relativePath>../../parent-java</relativePath>
+    </parent>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-collections4</artifactId>
+            <version>${commons-collections4.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>commons-io</groupId>
+            <artifactId>commons-io</artifactId>
+            <version>${commons-io.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-lang3</artifactId>
+            <version>${commons-lang3.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-math3</artifactId>
+            <version>${commons-math3.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <version>${assertj.version}</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>com.jayway.awaitility</groupId>
+            <artifactId>awaitility</artifactId>
+            <version>${avaitility.version}</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <finalName>core-java-concurrency-advanced</finalName>
+        <resources>
+            <resource>
+                <directory>src/main/resources</directory>
+                <filtering>true</filtering>
+            </resource>
+        </resources>
+    </build>
+
+    <properties>
+        <!-- util -->
+        <guava.version>21.0</guava.version>
+        <commons-math3.version>3.6.1</commons-math3.version>
+        <commons-collections4.version>4.1</commons-collections4.version>
+        <collections-generic.version>4.01</collections-generic.version>
+        <!-- testing -->
+        <assertj.version>3.6.1</assertj.version>
+        <avaitility.version>1.7.0</avaitility.version>
+    </properties>
+
+</project>

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/atomic/SafeCounterWithLock.java

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+package com.baeldung.concurrent.atomic;
+
+public class SafeCounterWithLock {
+    private volatile int counter;
+    
+    int getValue() {
+        return counter;
+    }
+    
+    synchronized void increment() {
+        counter++;
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/atomic/SafeCounterWithoutLock.java

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+package com.baeldung.concurrent.atomic;
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+public class SafeCounterWithoutLock {
+    private final AtomicInteger counter = new AtomicInteger(0);
+    
+    int getValue() {
+        return counter.get();
+    }
+    
+    void increment() {
+        while(true) {
+            int existingValue = getValue();
+            int newValue = existingValue + 1;
+            if(counter.compareAndSet(existingValue, newValue)) {
+                return;
+            }
+        }
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/atomic/UnsafeCounter.java

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+package com.baeldung.concurrent.atomic;
+
+public class UnsafeCounter {
+    private int counter;
+    
+    int getValue() {
+        return counter;
+    }
+    
+    void increment() {
+        counter++;
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/countdownlatch/BrokenWorker.java

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+package com.baeldung.concurrent.countdownlatch;
+
+import java.util.List;
+import java.util.concurrent.CountDownLatch;
+
+public class BrokenWorker implements Runnable {
+    private final List<String> outputScraper;
+    private final CountDownLatch countDownLatch;
+
+    BrokenWorker(final List<String> outputScraper, final CountDownLatch countDownLatch) {
+        this.outputScraper = outputScraper;
+        this.countDownLatch = countDownLatch;
+    }
+
+    @Override
+    public void run() {
+        if (true) {
+            throw new RuntimeException("Oh dear");
+        }
+        countDownLatch.countDown();
+        outputScraper.add("Counted down");
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/countdownlatch/WaitingWorker.java

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+package com.baeldung.concurrent.countdownlatch;
+
+import java.util.List;
+import java.util.concurrent.CountDownLatch;
+
+public class WaitingWorker implements Runnable {
+
+    private final List<String> outputScraper;
+    private final CountDownLatch readyThreadCounter;
+    private final CountDownLatch callingThreadBlocker;
+    private final CountDownLatch completedThreadCounter;
+
+    WaitingWorker(final List<String> outputScraper, final CountDownLatch readyThreadCounter, final CountDownLatch callingThreadBlocker, CountDownLatch completedThreadCounter) {
+
+        this.outputScraper = outputScraper;
+        this.readyThreadCounter = readyThreadCounter;
+        this.callingThreadBlocker = callingThreadBlocker;
+        this.completedThreadCounter = completedThreadCounter;
+    }
+
+    @Override
+    public void run() {
+        // Mark this thread as read / started
+        readyThreadCounter.countDown();
+        try {
+            callingThreadBlocker.await();
+            outputScraper.add("Counted down");
+        } catch (InterruptedException e) {
+            e.printStackTrace();
+        } finally {
+            completedThreadCounter.countDown();
+        }
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/countdownlatch/Worker.java

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+package com.baeldung.concurrent.countdownlatch;
+
+import java.util.List;
+import java.util.concurrent.CountDownLatch;
+
+public class Worker implements Runnable {
+    private final List<String> outputScraper;
+    private final CountDownLatch countDownLatch;
+
+    Worker(final List<String> outputScraper, final CountDownLatch countDownLatch) {
+        this.outputScraper = outputScraper;
+        this.countDownLatch = countDownLatch;
+    }
+
+    @Override
+    public void run() {
+        // Do some work
+        System.out.println("Doing some logic");
+        outputScraper.add("Counted down");
+        countDownLatch.countDown();
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/cyclicbarrier/CyclicBarrierCompletionMethodExample.java

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+package com.baeldung.concurrent.cyclicbarrier;
+
+import java.util.concurrent.BrokenBarrierException;
+import java.util.concurrent.CyclicBarrier;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
+
+public class CyclicBarrierCompletionMethodExample {
+
+    private int count;
+    private int threadCount;
+    private final AtomicInteger updateCount;
+    
+    CyclicBarrierCompletionMethodExample(int count, int threadCount) {
+        updateCount = new AtomicInteger(0);
+        this.count = count;
+        this.threadCount = threadCount;
+    }
+
+    public int countTrips() {
+
+        CyclicBarrier cyclicBarrier = new CyclicBarrier(count, () -> {
+            updateCount.incrementAndGet();
+        });
+
+        ExecutorService es = Executors.newFixedThreadPool(threadCount);
+        for (int i = 0; i < threadCount; i++) {
+            es.execute(() -> {
+                try {
+                    cyclicBarrier.await();
+                } catch (InterruptedException | BrokenBarrierException e) {
+                    e.printStackTrace();
+                }
+            });
+        }
+        es.shutdown();
+        try {
+            es.awaitTermination(1, TimeUnit.SECONDS);
+        } catch (InterruptedException e) {
+            e.printStackTrace();
+        }
+        return updateCount.get();
+    }
+
+    public static void main(String[] args) {
+        CyclicBarrierCompletionMethodExample ex = new CyclicBarrierCompletionMethodExample(5, 20);
+        System.out.println("Count : " + ex.countTrips());
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/cyclicbarrier/CyclicBarrierDemo.java

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+package com.baeldung.concurrent.cyclicbarrier;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.Random;
+import java.util.concurrent.BrokenBarrierException;
+import java.util.concurrent.CyclicBarrier;
+
+public class CyclicBarrierDemo {
+
+    private CyclicBarrier cyclicBarrier;
+    private List<List<Integer>> partialResults = Collections.synchronizedList(new ArrayList<>());
+    private Random random = new Random();
+    private int NUM_PARTIAL_RESULTS;
+    private int NUM_WORKERS;
+
+    private void runSimulation(int numWorkers, int numberOfPartialResults) {
+        NUM_PARTIAL_RESULTS = numberOfPartialResults;
+        NUM_WORKERS = numWorkers;
+
+        cyclicBarrier = new CyclicBarrier(NUM_WORKERS, new AggregatorThread());
+        System.out.println("Spawning " + NUM_WORKERS + " worker threads to compute " + NUM_PARTIAL_RESULTS + " partial results each");
+        for (int i = 0; i < NUM_WORKERS; i++) {
+            Thread worker = new Thread(new NumberCruncherThread());
+            worker.setName("Thread " + i);
+            worker.start();
+        }
+    }
+
+    class NumberCruncherThread implements Runnable {
+
+        @Override
+        public void run() {
+            String thisThreadName = Thread.currentThread().getName();
+            List<Integer> partialResult = new ArrayList<>();
+            for (int i = 0; i < NUM_PARTIAL_RESULTS; i++) {
+                Integer num = random.nextInt(10);
+                System.out.println(thisThreadName + ": Crunching some numbers! Final result - " + num);
+                partialResult.add(num);
+            }
+            partialResults.add(partialResult);
+            try {
+                System.out.println(thisThreadName + " waiting for others to reach barrier.");
+                cyclicBarrier.await();
+            } catch (InterruptedException | BrokenBarrierException e) {
+                e.printStackTrace();
+            }
+        }
+    }
+
+    class AggregatorThread implements Runnable {
+
+        @Override
+        public void run() {
+            String thisThreadName = Thread.currentThread().getName();
+            System.out.println(thisThreadName + ": Computing final sum of " + NUM_WORKERS + " workers, having " + NUM_PARTIAL_RESULTS + " results each.");
+            int sum = 0;
+            for (List<Integer> threadResult : partialResults) {
+                System.out.print("Adding ");
+                for (Integer partialResult : threadResult) {
+                    System.out.print(partialResult + " ");
+                    sum += partialResult;
+                }
+                System.out.println();
+            }
+            System.out.println(Thread.currentThread().getName() + ": Final result = " + sum);
+        }
+
+    }
+
+    public static void main(String[] args) {
+        CyclicBarrierDemo play = new CyclicBarrierDemo();
+        play.runSimulation(5, 3);
+    }
+
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/daemon/MultipleThreadsExample.java

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+package com.baeldung.concurrent.daemon;
+
+public class MultipleThreadsExample {
+    public static void main(String[] args) {
+        NewThread t1 = new NewThread();
+        t1.setName("MyThread-1");
+        NewThread t2 = new NewThread();
+        t2.setName("MyThread-2");
+        t1.start();
+        t2.start();
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/daemon/NewThread.java

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+package com.baeldung.concurrent.daemon;
+
+public class NewThread extends Thread {
+    public void run() {
+    	long startTime = System.currentTimeMillis();
+        while (true) {
+            for (int i = 0; i < 10; i++) {
+                System.out.println(this.getName() + ": New Thread is running..." + i);
+                try {
+                    //Wait for one sec so it doesn't print too fast
+                    Thread.sleep(1000);
+                } catch (InterruptedException e) {
+                    e.printStackTrace();
+                }
+            }
+            // prevent the Thread to run forever. It will finish it's execution after 2 seconds
+            if (System.currentTimeMillis() - startTime > 2000) {
+            	Thread.currentThread().interrupt();
+            	break;
+            }
+        }
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/daemon/SingleThreadExample.java

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+package com.baeldung.concurrent.daemon;
+
+public class SingleThreadExample {
+    public static void main(String[] args) {
+        NewThread t = new NewThread();
+        t.start();
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/diningphilosophers/DiningPhilosophers.java

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+package com.baeldung.concurrent.diningphilosophers;
+
+public class DiningPhilosophers {
+
+    public static void main(String[] args) throws Exception {
+
+        Philosopher[] philosophers = new Philosopher[5];
+        Object[] forks = new Object[philosophers.length];
+
+        for (int i = 0; i < forks.length; i++) {
+            forks[i] = new Object();
+        }
+
+        for (int i = 0; i < philosophers.length; i++) {
+
+            Object leftFork = forks[i];
+            Object rightFork = forks[(i + 1) % forks.length];
+
+            if (i == philosophers.length - 1) {
+                philosophers[i] = new Philosopher(rightFork, leftFork); // The last philosopher picks up the right fork first
+            } else {
+                philosophers[i] = new Philosopher(leftFork, rightFork);
+            }
+
+            Thread t = new Thread(philosophers[i], "Philosopher " + (i + 1));
+            t.start();
+        }
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/diningphilosophers/Philosopher.java

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+package com.baeldung.concurrent.diningphilosophers;
+
+public class Philosopher implements Runnable {
+
+    private final Object leftFork;
+    private final Object rightFork;
+
+    Philosopher(Object left, Object right) {
+        this.leftFork = left;
+        this.rightFork = right;
+    }
+
+    private void doAction(String action) throws InterruptedException {
+        System.out.println(Thread.currentThread().getName() + " " + action);
+        Thread.sleep(((int) (Math.random() * 100)));
+    }
+
+    @Override
+    public void run() {
+        try {
+            while (true) {
+                doAction(System.nanoTime() + ": Thinking"); // thinking
+                synchronized (leftFork) {
+                    doAction(System.nanoTime() + ": Picked up left fork");
+                    synchronized (rightFork) {
+                        doAction(System.nanoTime() + ": Picked up right fork - eating"); // eating
+                        doAction(System.nanoTime() + ": Put down right fork");
+                    }
+                    doAction(System.nanoTime() + ": Put down left fork. Returning to thinking");
+                }
+            }
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+        }
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/locks/ReentrantLockWithCondition.java

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+package com.baeldung.concurrent.locks;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.Stack;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.locks.Condition;
+import java.util.concurrent.locks.ReentrantLock;
+
+import static java.lang.Thread.sleep;
+
+public class ReentrantLockWithCondition {
+
+    private static Logger LOG = LoggerFactory.getLogger(ReentrantLockWithCondition.class);
+
+    private Stack<String> stack = new Stack<>();
+    private static final int CAPACITY = 5;
+
+    private ReentrantLock lock = new ReentrantLock();
+    private Condition stackEmptyCondition = lock.newCondition();
+    private Condition stackFullCondition = lock.newCondition();
+
+    private void pushToStack(String item) throws InterruptedException {
+        try {
+            lock.lock();
+            if (stack.size() == CAPACITY) {
+                LOG.info(Thread.currentThread().getName() + " wait on stack full");
+                stackFullCondition.await();
+            }
+            LOG.info("Pushing the item " + item);
+            stack.push(item);
+            stackEmptyCondition.signalAll();
+        } finally {
+            lock.unlock();
+        }
+
+    }
+
+    private String popFromStack() throws InterruptedException {
+        try {
+            lock.lock();
+            if (stack.size() == 0) {
+                LOG.info(Thread.currentThread().getName() + " wait on stack empty");
+                stackEmptyCondition.await();
+            }
+            return stack.pop();
+        } finally {
+            stackFullCondition.signalAll();
+            lock.unlock();
+        }
+    }
+
+    public static void main(String[] args) {
+        final int threadCount = 2;
+        ReentrantLockWithCondition object = new ReentrantLockWithCondition();
+        final ExecutorService service = Executors.newFixedThreadPool(threadCount);
+        service.execute(() -> {
+            for (int i = 0; i < 10; i++) {
+                try {
+                    object.pushToStack("Item " + i);
+                } catch (InterruptedException e) {
+                    e.printStackTrace();
+                }
+            }
+
+        });
+
+        service.execute(() -> {
+            for (int i = 0; i < 10; i++) {
+                try {
+                    LOG.info("Item popped " + object.popFromStack());
+                } catch (InterruptedException e) {
+                    e.printStackTrace();
+                }
+            }
+
+        });
+
+        service.shutdown();
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/locks/SharedObjectWithLock.java

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+package com.baeldung.concurrent.locks;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.locks.ReentrantLock;
+
+import static java.lang.Thread.sleep;
+
+public class SharedObjectWithLock {
+
+    private static final Logger LOG = LoggerFactory.getLogger(SharedObjectWithLock.class);
+
+    private ReentrantLock lock = new ReentrantLock(true);
+
+    private int counter = 0;
+
+    void perform() {
+
+        lock.lock();
+        LOG.info("Thread - " + Thread.currentThread().getName() + " acquired the lock");
+        try {
+            LOG.info("Thread - " + Thread.currentThread().getName() + " processing");
+            counter++;
+        } catch (Exception exception) {
+            LOG.error(" Interrupted Exception ", exception);
+        } finally {
+            lock.unlock();
+            LOG.info("Thread - " + Thread.currentThread().getName() + " released the lock");
+        }
+    }
+
+    private void performTryLock() {
+
+        LOG.info("Thread - " + Thread.currentThread().getName() + " attempting to acquire the lock");
+        try {
+            boolean isLockAcquired = lock.tryLock(2, TimeUnit.SECONDS);
+            if (isLockAcquired) {
+                try {
+                    LOG.info("Thread - " + Thread.currentThread().getName() + " acquired the lock");
+
+                    LOG.info("Thread - " + Thread.currentThread().getName() + " processing");
+                    sleep(1000);
+                } finally {
+                    lock.unlock();
+                    LOG.info("Thread - " + Thread.currentThread().getName() + " released the lock");
+
+                }
+            }
+        } catch (InterruptedException exception) {
+            LOG.error(" Interrupted Exception ", exception);
+        }
+        LOG.info("Thread - " + Thread.currentThread().getName() + " could not acquire the lock");
+    }
+
+    public ReentrantLock getLock() {
+        return lock;
+    }
+
+    boolean isLocked() {
+        return lock.isLocked();
+    }
+
+    boolean hasQueuedThreads() {
+        return lock.hasQueuedThreads();
+    }
+
+    int getCounter() {
+        return counter;
+    }
+
+    public static void main(String[] args) {
+
+        final int threadCount = 2;
+        final ExecutorService service = Executors.newFixedThreadPool(threadCount);
+        final SharedObjectWithLock object = new SharedObjectWithLock();
+
+        service.execute(object::perform);
+        service.execute(object::performTryLock);
+
+        service.shutdown();
+
+    }
+
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/locks/StampedLockDemo.java

@@ -0,0 +1,104 @@
+package com.baeldung.concurrent.locks;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.locks.StampedLock;
+
+import static java.lang.Thread.sleep;
+
+public class StampedLockDemo {
+    private Map<String, String> map = new HashMap<>();
+    private Logger logger = LoggerFactory.getLogger(StampedLockDemo.class);
+
+    private final StampedLock lock = new StampedLock();
+
+    public void put(String key, String value) throws InterruptedException {
+        long stamp = lock.writeLock();
+
+        try {
+            logger.info(Thread.currentThread().getName() + " acquired the write lock with stamp " + stamp);
+            map.put(key, value);
+        } finally {
+            lock.unlockWrite(stamp);
+            logger.info(Thread.currentThread().getName() + " unlocked the write lock with stamp " + stamp);
+        }
+    }
+
+    public String get(String key) throws InterruptedException {
+        long stamp = lock.readLock();
+        logger.info(Thread.currentThread().getName() + " acquired the read lock with stamp " + stamp);
+        try {
+            sleep(5000);
+            return map.get(key);
+
+        } finally {
+            lock.unlockRead(stamp);
+            logger.info(Thread.currentThread().getName() + " unlocked the read lock with stamp " + stamp);
+
+        }
+
+    }
+
+    private String readWithOptimisticLock(String key) throws InterruptedException {
+        long stamp = lock.tryOptimisticRead();
+        String value = map.get(key);
+
+        if (!lock.validate(stamp)) {
+            stamp = lock.readLock();
+            try {
+                sleep(5000);
+                return map.get(key);
+
+            } finally {
+                lock.unlock(stamp);
+                logger.info(Thread.currentThread().getName() + " unlocked the read lock with stamp " + stamp);
+
+            }
+        }
+        return value;
+    }
+
+    public static void main(String[] args) {
+        final int threadCount = 4;
+        final ExecutorService service = Executors.newFixedThreadPool(threadCount);
+        StampedLockDemo object = new StampedLockDemo();
+
+        Runnable writeTask = () -> {
+
+            try {
+                object.put("key1", "value1");
+            } catch (InterruptedException e) {
+                e.printStackTrace();
+            }
+        };
+        Runnable readTask = () -> {
+
+            try {
+                object.get("key1");
+            } catch (InterruptedException e) {
+                e.printStackTrace();
+            }
+        };
+        Runnable readOptimisticTask = () -> {
+
+            try {
+                object.readWithOptimisticLock("key1");
+            } catch (InterruptedException e) {
+                e.printStackTrace();
+            }
+        };
+        service.submit(writeTask);
+        service.submit(writeTask);
+        service.submit(readTask);
+        service.submit(readOptimisticTask);
+
+        service.shutdown();
+
+    }
+
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/locks/SynchronizedHashMapWithRWLock.java

@@ -0,0 +1,120 @@
+package com.baeldung.concurrent.locks;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReadWriteLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+
+import static java.lang.Thread.sleep;
+
+public class SynchronizedHashMapWithRWLock {
+
+    private static Map<String, String> syncHashMap = new HashMap<>();
+    private Logger logger = LoggerFactory.getLogger(SynchronizedHashMapWithRWLock.class);
+
+    private final ReadWriteLock lock = new ReentrantReadWriteLock();
+    private final Lock readLock = lock.readLock();
+    private final Lock writeLock = lock.writeLock();
+
+    public void put(String key, String value) throws InterruptedException {
+
+        try {
+            writeLock.lock();
+            logger.info(Thread.currentThread().getName() + " writing");
+            syncHashMap.put(key, value);
+            sleep(1000);
+        } finally {
+            writeLock.unlock();
+        }
+
+    }
+
+    public String get(String key) {
+        try {
+            readLock.lock();
+            logger.info(Thread.currentThread().getName() + " reading");
+            return syncHashMap.get(key);
+        } finally {
+            readLock.unlock();
+        }
+    }
+
+    public String remove(String key) {
+        try {
+            writeLock.lock();
+            return syncHashMap.remove(key);
+        } finally {
+            writeLock.unlock();
+        }
+    }
+
+    public boolean containsKey(String key) {
+        try {
+            readLock.lock();
+            return syncHashMap.containsKey(key);
+        } finally {
+            readLock.unlock();
+        }
+    }
+
+    boolean isReadLockAvailable() {
+        return readLock.tryLock();
+    }
+
+    public static void main(String[] args) throws InterruptedException {
+
+        final int threadCount = 3;
+        final ExecutorService service = Executors.newFixedThreadPool(threadCount);
+        SynchronizedHashMapWithRWLock object = new SynchronizedHashMapWithRWLock();
+
+        service.execute(new Thread(new Writer(object), "Writer"));
+        service.execute(new Thread(new Reader(object), "Reader1"));
+        service.execute(new Thread(new Reader(object), "Reader2"));
+
+        service.shutdown();
+    }
+
+    private static class Reader implements Runnable {
+
+        SynchronizedHashMapWithRWLock object;
+
+        Reader(SynchronizedHashMapWithRWLock object) {
+            this.object = object;
+        }
+
+        @Override
+        public void run() {
+            for (int i = 0; i < 10; i++) {
+                object.get("key" + i);
+            }
+        }
+    }
+
+    private static class Writer implements Runnable {
+
+        SynchronizedHashMapWithRWLock object;
+
+        public Writer(SynchronizedHashMapWithRWLock object) {
+            this.object = object;
+        }
+
+        @Override
+        public void run() {
+            for (int i = 0; i < 10; i++) {
+                try {
+                    object.put("key" + i, "value" + i);
+                    sleep(1000);
+                } catch (InterruptedException e) {
+                    e.printStackTrace();
+                }
+            }
+        }
+    }
+
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/phaser/LongRunningAction.java

@@ -0,0 +1,27 @@
+package com.baeldung.concurrent.phaser;
+
+import java.util.concurrent.Phaser;
+
+class LongRunningAction implements Runnable {
+    private String threadName;
+    private Phaser ph;
+
+    LongRunningAction(String threadName, Phaser ph) {
+        this.threadName = threadName;
+        this.ph = ph;
+        ph.register();
+    }
+
+    @Override
+    public void run() {
+        System.out.println("This is phase " + ph.getPhase());
+        System.out.println("Thread " + threadName + " before long running action");
+        ph.arriveAndAwaitAdvance();
+        try {
+            Thread.sleep(20);
+        } catch (InterruptedException e) {
+            e.printStackTrace();
+        }
+        ph.arriveAndDeregister();
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/concurrent/volatilekeyword/SharedObject.java

@@ -0,0 +1,13 @@
+package com.baeldung.concurrent.volatilekeyword;
+
+
+public class SharedObject {
+    private volatile int count=0;
+
+    void increamentCount(){
+        count++;
+    }
+    public int  getCount(){
+        return count;
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/threadlocal/Context.java

@@ -0,0 +1,17 @@
+package com.baeldung.threadlocal;
+
+
+public class Context {
+    private final String userName;
+
+    Context(String userName) {
+        this.userName = userName;
+    }
+
+    @Override
+    public String toString() {
+        return "Context{" +
+          "userNameSecret='" + userName + '\'' +
+          '}';
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/threadlocal/SharedMapWithUserContext.java

@@ -0,0 +1,21 @@
+package com.baeldung.threadlocal;
+
+
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+public class SharedMapWithUserContext implements Runnable {
+    final static Map<Integer, Context> userContextPerUserId = new ConcurrentHashMap<>();
+    private final Integer userId;
+    private UserRepository userRepository = new UserRepository();
+
+    SharedMapWithUserContext(Integer userId) {
+        this.userId = userId;
+    }
+
+    @Override
+    public void run() {
+        String userName = userRepository.getUserNameForUserId(userId);
+        userContextPerUserId.put(userId, new Context(userName));
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/threadlocal/ThreadLocalWithUserContext.java

@@ -0,0 +1,24 @@
+package com.baeldung.threadlocal;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class ThreadLocalWithUserContext implements Runnable {
+    private static final Logger LOG = LoggerFactory.getLogger(ThreadLocalWithUserContext.class);
+    
+    private static final ThreadLocal<Context> userContext = new ThreadLocal<>();
+    private final Integer userId;
+    private UserRepository userRepository = new UserRepository();
+
+    ThreadLocalWithUserContext(Integer userId) {
+        this.userId = userId;
+    }
+
+
+    @Override
+    public void run() {
+        String userName = userRepository.getUserNameForUserId(userId);
+        userContext.set(new Context(userName));
+        LOG.debug("thread context for given userId: " + userId + " is: " + userContext.get());
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/threadlocal/UserRepository.java

@@ -0,0 +1,10 @@
+package com.baeldung.threadlocal;
+
+import java.util.UUID;
+
+
+public class UserRepository {
+    String getUserNameForUserId(Integer userId) {
+        return UUID.randomUUID().toString();
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/threadpool/CountingTask.java

@@ -0,0 +1,22 @@
+package com.baeldung.threadpool;
+
+import java.util.concurrent.ForkJoinTask;
+import java.util.concurrent.RecursiveTask;
+
+public class CountingTask extends RecursiveTask<Integer> {
+
+    private final TreeNode node;
+
+    CountingTask(TreeNode node) {
+        this.node = node;
+    }
+
+    @Override
+    protected Integer compute() {
+        return node.getValue() + node.getChildren().stream()
+          .map(childNode -> new CountingTask(childNode).fork())
+          .mapToInt(ForkJoinTask::join)
+          .sum();
+    }
+
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/threadpool/ExitingExecutorServiceExample.java

@@ -0,0 +1,29 @@
+package com.baeldung.threadpool;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ThreadPoolExecutor;
+import java.util.concurrent.TimeUnit;
+
+import com.google.common.util.concurrent.MoreExecutors;
+
+/**
+ * This class demonstrates the usage of Guava's exiting executor services that keep the VM from hanging.
+ * Without the exiting executor service, the task would hang indefinitely.
+ * This behaviour cannot be demonstrated in JUnit tests, as JUnit kills the VM after the tests.
+ */
+public class ExitingExecutorServiceExample {
+
+    public static void main(String... args) {
+
+        final ThreadPoolExecutor executor = (ThreadPoolExecutor) Executors.newFixedThreadPool(5);
+        final ExecutorService executorService = MoreExecutors.getExitingExecutorService(executor, 100, TimeUnit.MILLISECONDS);
+
+        executorService.submit((Runnable) () -> {
+            while (true) {
+            }
+        });
+
+    }
+
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/com/baeldung/threadpool/TreeNode.java

@@ -0,0 +1,25 @@
+package com.baeldung.threadpool;
+
+import com.google.common.collect.Sets;
+
+import java.util.Set;
+
+public class TreeNode {
+
+    private int value;
+
+    private Set<TreeNode> children;
+
+    TreeNode(int value, TreeNode... children) {
+        this.value = value;
+        this.children = Sets.newHashSet(children);
+    }
+
+    public int getValue() {
+        return value;
+    }
+
+    public Set<TreeNode> getChildren() {
+        return children;
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/main/java/log4j.properties

@@ -0,0 +1,9 @@
+# Set root logger level to DEBUG and its only appender to A1.
+log4j.rootLogger=DEBUG, A1
+
+# A1 is set to be a ConsoleAppender.
+log4j.appender.A1=org.apache.log4j.ConsoleAppender
+
+# A1 uses PatternLayout.
+log4j.appender.A1.layout=org.apache.log4j.PatternLayout
+log4j.appender.A1.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n

core-java-modules/core-java-concurrency-advanced/src/main/resources/logback.xml

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<configuration>
+    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
+        <encoder>
+            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n
+            </pattern>
+        </encoder>
+    </appender>
+
+    <logger name="org.springframework" level="WARN" />
+    <logger name="org.springframework.transaction" level="WARN" />
+
+    <!-- in order to debug some marshalling issues, this needs to be TRACE -->
+    <logger name="org.springframework.web.servlet.mvc" level="WARN" />
+
+    <root level="INFO">
+        <appender-ref ref="STDOUT" />
+    </root>
+</configuration>

core-java-modules/core-java-concurrency-advanced/src/test/java/com/baeldung/concurrent/accumulator/LongAccumulatorUnitTest.java

@@ -0,0 +1,38 @@
+package com.baeldung.concurrent.accumulator;
+
+import org.junit.Test;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.LongAccumulator;
+import java.util.function.LongBinaryOperator;
+import java.util.stream.IntStream;
+
+import static junit.framework.TestCase.assertEquals;
+
+public class LongAccumulatorUnitTest {
+
+    @Test
+    public void givenLongAccumulator_whenApplyActionOnItFromMultipleThrads_thenShouldProduceProperResult() throws InterruptedException {
+        // given
+        ExecutorService executorService = Executors.newFixedThreadPool(8);
+        LongBinaryOperator sum = Long::sum;
+        LongAccumulator accumulator = new LongAccumulator(sum, 0L);
+        int numberOfThreads = 4;
+        int numberOfIncrements = 100;
+
+        // when
+        Runnable accumulateAction = () -> IntStream.rangeClosed(0, numberOfIncrements).forEach(accumulator::accumulate);
+
+        for (int i = 0; i < numberOfThreads; i++) {
+            executorService.execute(accumulateAction);
+        }
+
+        // then
+        executorService.awaitTermination(500, TimeUnit.MILLISECONDS);
+        executorService.shutdown();
+        assertEquals(accumulator.get(), 20200);
+
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/test/java/com/baeldung/concurrent/adder/LongAdderUnitTest.java

@@ -0,0 +1,67 @@
+package com.baeldung.concurrent.adder;
+
+import org.junit.Test;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.LongAdder;
+import java.util.stream.IntStream;
+
+import static com.jayway.awaitility.Awaitility.await;
+import static junit.framework.TestCase.assertEquals;
+
+public class LongAdderUnitTest {
+    @Test
+    public void givenMultipleThread_whenTheyWriteToSharedLongAdder_thenShouldCalculateSumForThem() throws InterruptedException {
+        //given
+        LongAdder counter = new LongAdder();
+        ExecutorService executorService = Executors.newFixedThreadPool(8);
+
+        int numberOfThreads = 4;
+        int numberOfIncrements = 100;
+
+        //when
+        Runnable incrementAction = () -> IntStream
+          .range(0, numberOfIncrements)
+          .forEach((i) -> counter.increment());
+
+        for (int i = 0; i < numberOfThreads; i++) {
+            executorService.execute(incrementAction);
+        }
+
+        //then
+        executorService.awaitTermination(500, TimeUnit.MILLISECONDS);
+        executorService.shutdown();
+
+        assertEquals(counter.sum(), numberOfIncrements * numberOfThreads);
+        assertEquals(counter.sum(), numberOfIncrements * numberOfThreads);
+    }
+
+    @Test
+    public void givenMultipleThread_whenTheyWriteToSharedLongAdder_thenShouldCalculateSumForThemAndResetAdderAfterward() throws InterruptedException {
+        //given
+        LongAdder counter = new LongAdder();
+        ExecutorService executorService = Executors.newFixedThreadPool(8);
+
+        int numberOfThreads = 4;
+        int numberOfIncrements = 100;
+
+        //when
+        Runnable incrementAction = () -> IntStream
+          .range(0, numberOfIncrements)
+          .forEach((i) -> counter.increment());
+
+        for (int i = 0; i < numberOfThreads; i++) {
+            executorService.execute(incrementAction);
+        }
+
+        //then
+        executorService.awaitTermination(500, TimeUnit.MILLISECONDS);
+        executorService.shutdown();
+
+        assertEquals(counter.sumThenReset(), numberOfIncrements * numberOfThreads);
+
+        await().until(() -> assertEquals(counter.sum(), 0));
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/test/java/com/baeldung/concurrent/atomic/ThreadSafeCounterIntegrationTest.java

@@ -0,0 +1,38 @@
+package com.baeldung.concurrent.atomic;
+
+import static org.junit.Assert.assertEquals;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+import java.util.stream.IntStream;
+
+import org.junit.Test;
+
+public class ThreadSafeCounterIntegrationTest {
+
+    @Test
+    public void givenMultiThread_whenSafeCounterWithLockIncrement() throws InterruptedException {
+        ExecutorService service = Executors.newFixedThreadPool(3);
+        SafeCounterWithLock safeCounter = new SafeCounterWithLock();
+
+        IntStream.range(0, 1000)
+          .forEach(count -> service.submit(safeCounter::increment));
+        service.awaitTermination(100, TimeUnit.MILLISECONDS);
+
+        assertEquals(1000, safeCounter.getValue());
+    }
+    
+    @Test
+    public void givenMultiThread_whenSafeCounterWithoutLockIncrement() throws InterruptedException {
+        ExecutorService service = Executors.newFixedThreadPool(3);
+        SafeCounterWithoutLock safeCounter = new SafeCounterWithoutLock();
+
+        IntStream.range(0, 1000)
+          .forEach(count -> service.submit(safeCounter::increment));
+        service.awaitTermination(100, TimeUnit.MILLISECONDS);
+
+        assertEquals(1000, safeCounter.getValue());
+    }
+    
+}

core-java-modules/core-java-concurrency-advanced/src/test/java/com/baeldung/concurrent/atomic/ThreadUnsafeCounterManualTest.java

@@ -0,0 +1,33 @@
+package com.baeldung.concurrent.atomic;
+
+import static org.junit.Assert.assertEquals;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+import java.util.stream.IntStream;
+
+import org.junit.Test;
+
+/**
+ * This test shows the behaviour of a thread-unsafe class in a multithreaded scenario. We are calling
+ * the increment methods 1000 times from a pool of 3 threads. In most of the cases, the counter will 
+ * less than 1000, because of lost updates, however, occasionally it may reach 1000, when no threads
+ * called the method simultaneously. This may cause the build to fail occasionally. Hence excluding this
+ * test from build by adding this in manual test
+ */
+public class ThreadUnsafeCounterManualTest {
+
+    @Test
+    public void givenMultiThread_whenUnsafeCounterIncrement() throws InterruptedException {
+        ExecutorService service = Executors.newFixedThreadPool(3);
+        UnsafeCounter unsafeCounter = new UnsafeCounter();
+
+        IntStream.range(0, 1000)
+          .forEach(count -> service.submit(unsafeCounter::increment));
+        service.awaitTermination(100, TimeUnit.MILLISECONDS);
+
+        assertEquals(1000, unsafeCounter.getValue());
+    }
+    
+}

core-java-modules/core-java-concurrency-advanced/src/test/java/com/baeldung/concurrent/countdownlatch/CountdownLatchExampleIntegrationTest.java

@@ -0,0 +1,72 @@
+package com.baeldung.concurrent.countdownlatch;
+
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+import java.util.stream.Stream;
+
+import static java.util.stream.Collectors.toList;
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class CountdownLatchExampleIntegrationTest {
+    @Test
+    public void whenParallelProcessing_thenMainThreadWillBlockUntilCompletion() throws InterruptedException {
+        // Given
+        List<String> outputScraper = Collections.synchronizedList(new ArrayList<>());
+        CountDownLatch countDownLatch = new CountDownLatch(5);
+        List<Thread> workers = Stream.generate(() -> new Thread(new Worker(outputScraper, countDownLatch)))
+          .limit(5)
+          .collect(toList());
+
+        // When
+        workers.forEach(Thread::start);
+        countDownLatch.await(); // Block until workers finish
+        outputScraper.add("Latch released");
+
+        // Then
+        assertThat(outputScraper).containsExactly("Counted down", "Counted down", "Counted down", "Counted down", "Counted down", "Latch released");
+    }
+
+    @Test
+    public void whenFailingToParallelProcess_thenMainThreadShouldTimeout() throws InterruptedException {
+        // Given
+        List<String> outputScraper = Collections.synchronizedList(new ArrayList<>());
+        CountDownLatch countDownLatch = new CountDownLatch(5);
+        List<Thread> workers = Stream.generate(() -> new Thread(new BrokenWorker(outputScraper, countDownLatch)))
+          .limit(5)
+          .collect(toList());
+
+        // When
+        workers.forEach(Thread::start);
+        final boolean result = countDownLatch.await(3L, TimeUnit.SECONDS);
+
+        // Then
+        assertThat(result).isFalse();
+    }
+
+    @Test
+    public void whenDoingLotsOfThreadsInParallel_thenStartThemAtTheSameTime() throws InterruptedException {
+        // Given
+        List<String> outputScraper = Collections.synchronizedList(new ArrayList<>());
+        CountDownLatch readyThreadCounter = new CountDownLatch(5);
+        CountDownLatch callingThreadBlocker = new CountDownLatch(1);
+        CountDownLatch completedThreadCounter = new CountDownLatch(5);
+        List<Thread> workers = Stream.generate(() -> new Thread(new WaitingWorker(outputScraper, readyThreadCounter, callingThreadBlocker, completedThreadCounter))).limit(5).collect(toList());
+
+        // When
+        workers.forEach(Thread::start);
+        readyThreadCounter.await(); // Block until workers start
+        outputScraper.add("Workers ready");
+        callingThreadBlocker.countDown(); // Start workers
+        completedThreadCounter.await(); // Block until workers finish
+        outputScraper.add("Workers complete");
+
+        // Then
+        assertThat(outputScraper).containsExactly("Workers ready", "Counted down", "Counted down", "Counted down", "Counted down", "Counted down", "Workers complete");
+    }
+
+}

core-java-modules/core-java-concurrency-advanced/src/test/java/com/baeldung/concurrent/cyclicbarrier/CyclicBarrierCompletionMethodExampleUnitTest.java

@@ -0,0 +1,15 @@
+package com.baeldung.concurrent.cyclicbarrier;
+
+import static org.junit.Assert.assertEquals;
+
+import org.junit.Test;
+
+public class CyclicBarrierCompletionMethodExampleUnitTest {
+    
+    @Test
+    public void whenCyclicBarrier_countTrips() {
+        CyclicBarrierCompletionMethodExample ex = new CyclicBarrierCompletionMethodExample(7,20);
+        int lineCount = ex.countTrips();
+        assertEquals(2, lineCount);
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/test/java/com/baeldung/concurrent/locks/SharedObjectWithLockManualTest.java

@@ -0,0 +1,73 @@
+package com.baeldung.concurrent.locks;
+
+import org.junit.Test;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import static junit.framework.TestCase.assertEquals;
+
+public class SharedObjectWithLockManualTest {
+
+    @Test
+    public void whenLockAcquired_ThenLockedIsTrue() {
+        final SharedObjectWithLock object = new SharedObjectWithLock();
+
+        final int threadCount = 2;
+        final ExecutorService service = Executors.newFixedThreadPool(threadCount);
+
+        executeThreads(object, threadCount, service);
+
+        assertEquals(true, object.isLocked());
+
+        service.shutdown();
+    }
+
+    @Test
+    public void whenLocked_ThenQueuedThread() {
+        final int threadCount = 4;
+        final ExecutorService service = Executors.newFixedThreadPool(threadCount);
+        final SharedObjectWithLock object = new SharedObjectWithLock();
+
+        executeThreads(object, threadCount, service);
+
+        assertEquals(object.hasQueuedThreads(), true);
+
+        service.shutdown();
+
+    }
+
+    public void whenTryLock_ThenQueuedThread() {
+        final SharedObjectWithLock object = new SharedObjectWithLock();
+
+        final int threadCount = 2;
+        final ExecutorService service = Executors.newFixedThreadPool(threadCount);
+
+        executeThreads(object, threadCount, service);
+
+        assertEquals(true, object.isLocked());
+
+        service.shutdown();
+    }
+
+    @Test
+    public void whenGetCount_ThenCorrectCount() throws InterruptedException {
+        final int threadCount = 4;
+        final ExecutorService service = Executors.newFixedThreadPool(threadCount);
+        final SharedObjectWithLock object = new SharedObjectWithLock();
+
+        executeThreads(object, threadCount, service);
+        Thread.sleep(1000);
+        assertEquals(object.getCounter(), 4);
+
+        service.shutdown();
+
+    }
+
+    private void executeThreads(SharedObjectWithLock object, int threadCount, ExecutorService service) {
+        for (int i = 0; i < threadCount; i++) {
+            service.execute(object::perform);
+        }
+    }
+
+}

core-java-modules/core-java-concurrency-advanced/src/test/java/com/baeldung/concurrent/locks/SynchronizedHashMapWithRWLockManualTest.java

@@ -0,0 +1,55 @@
+package com.baeldung.concurrent.locks;
+
+import org.junit.Test;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import static junit.framework.TestCase.assertEquals;
+
+public class SynchronizedHashMapWithRWLockManualTest {
+
+    @Test
+    public void whenWriting_ThenNoReading() {
+        SynchronizedHashMapWithRWLock object = new SynchronizedHashMapWithRWLock();
+        final int threadCount = 3;
+        final ExecutorService service = Executors.newFixedThreadPool(threadCount);
+
+        executeWriterThreads(object, threadCount, service);
+
+        assertEquals(object.isReadLockAvailable(), false);
+
+        service.shutdown();
+    }
+
+    @Test
+    public void whenReading_ThenMultipleReadingAllowed() {
+        SynchronizedHashMapWithRWLock object = new SynchronizedHashMapWithRWLock();
+        final int threadCount = 5;
+        final ExecutorService service = Executors.newFixedThreadPool(threadCount);
+
+        executeReaderThreads(object, threadCount, service);
+
+        assertEquals(object.isReadLockAvailable(), true);
+
+        service.shutdown();
+    }
+
+    private void executeWriterThreads(SynchronizedHashMapWithRWLock object, int threadCount, ExecutorService service) {
+        for (int i = 0; i < threadCount; i++) {
+            service.execute(() -> {
+                try {
+                    object.put("key" + threadCount, "value" + threadCount);
+                } catch (InterruptedException e) {
+                    e.printStackTrace();
+                }
+            });
+        }
+    }
+
+    private void executeReaderThreads(SynchronizedHashMapWithRWLock object, int threadCount, ExecutorService service) {
+        for (int i = 0; i < threadCount; i++)
+            service.execute(() -> object.get("key" + threadCount));
+    }
+
+}

core-java-modules/core-java-concurrency-advanced/src/test/java/com/baeldung/concurrent/phaser/PhaserUnitTest.java

@@ -0,0 +1,41 @@
+package com.baeldung.concurrent.phaser;
+
+import org.junit.FixMethodOrder;
+import org.junit.Test;
+import org.junit.runners.MethodSorters;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Phaser;
+
+import static junit.framework.TestCase.assertEquals;
+
+@FixMethodOrder(MethodSorters.NAME_ASCENDING)
+public class PhaserUnitTest {
+
+    @Test
+    public void givenPhaser_whenCoordinateWorksBetweenThreads_thenShouldCoordinateBetweenMultiplePhases() {
+        //given
+        ExecutorService executorService = Executors.newCachedThreadPool();
+        Phaser ph = new Phaser(1);
+        assertEquals(0, ph.getPhase());
+
+        //when
+        executorService.submit(new LongRunningAction("thread-1", ph));
+        executorService.submit(new LongRunningAction("thread-2", ph));
+        executorService.submit(new LongRunningAction("thread-3", ph));
+
+        //then
+        ph.arriveAndAwaitAdvance();
+        assertEquals(1, ph.getPhase());
+
+        //and
+        executorService.submit(new LongRunningAction("thread-4", ph));
+        executorService.submit(new LongRunningAction("thread-5", ph));
+        ph.arriveAndAwaitAdvance();
+        assertEquals(2, ph.getPhase());
+
+
+        ph.arriveAndDeregister();
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/test/java/com/baeldung/concurrent/volatilekeyword/SharedObjectManualTest.java

@@ -0,0 +1,55 @@
+package com.baeldung.concurrent.volatilekeyword;
+
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+public class SharedObjectManualTest {
+
+    @Test
+    public void whenOneThreadWrites_thenVolatileReadsFromMainMemory() throws InterruptedException {
+        SharedObject sharedObject = new SharedObject();
+
+        Thread writer = new Thread(() -> sharedObject.increamentCount());
+        writer.start();
+        Thread.sleep(100);
+
+        Thread readerOne = new Thread(() -> {
+            int valueReadByThread2 = sharedObject.getCount();
+            assertEquals(1, valueReadByThread2);
+        });
+        readerOne.start();
+
+        Thread readerTwo = new Thread(() -> {
+            int valueReadByThread3 = sharedObject.getCount();
+            assertEquals(1, valueReadByThread3);
+        });
+        readerTwo.start();
+
+    }
+
+    @Test
+    public void whenTwoThreadWrites_thenVolatileReadsFromMainMemory() throws InterruptedException {
+        SharedObject sharedObject = new SharedObject();
+        Thread writerOne = new Thread(() -> sharedObject.increamentCount());
+        writerOne.start();
+        Thread.sleep(100);
+
+        Thread writerTwo = new Thread(() -> sharedObject.increamentCount());
+        writerTwo.start();
+        Thread.sleep(100);
+
+        Thread readerOne = new Thread(() -> {
+            int valueReadByThread2 = sharedObject.getCount();
+            assertEquals(2, valueReadByThread2);
+        });
+        readerOne.start();
+
+        Thread readerTwo = new Thread(() -> {
+            int valueReadByThread3 = sharedObject.getCount();
+            assertEquals(2, valueReadByThread3);
+        });
+        readerTwo.start();
+
+    }
+}

core-java-modules/core-java-concurrency-advanced/src/test/java/com/baeldung/threadlocal/ThreadLocalIntegrationTest.java

@@ -0,0 +1,35 @@
+package com.baeldung.threadlocal;
+
+
+import org.junit.Test;
+
+import java.util.concurrent.ExecutionException;
+
+import static org.junit.Assert.assertEquals;
+
+public class ThreadLocalIntegrationTest {
+    @Test
+    public void givenThreadThatStoresContextInAMap_whenStartThread_thenShouldSetContextForBothUsers() throws ExecutionException, InterruptedException {
+        //when
+        SharedMapWithUserContext firstUser = new SharedMapWithUserContext(1);
+        SharedMapWithUserContext secondUser = new SharedMapWithUserContext(2);
+        new Thread(firstUser).start();
+        new Thread(secondUser).start();
+
+        Thread.sleep(3000);
+        //then
+        assertEquals(SharedMapWithUserContext.userContextPerUserId.size(), 2);
+    }
+
+    @Test
+    public void givenThreadThatStoresContextInThreadLocal_whenStartThread_thenShouldStoreContextInThreadLocal() throws ExecutionException, InterruptedException {
+        //when
+        ThreadLocalWithUserContext firstUser = new ThreadLocalWithUserContext(1);
+        ThreadLocalWithUserContext secondUser = new ThreadLocalWithUserContext(2);
+        new Thread(firstUser).start();
+        new Thread(secondUser).start();
+
+        Thread.sleep(3000);
+    }
+
+}

core-java-modules/core-java-concurrency-advanced/src/test/java/com/baeldung/threadpool/CoreThreadPoolIntegrationTest.java

@@ -0,0 +1,147 @@
+package com.baeldung.threadpool;
+
+import org.junit.Test;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.concurrent.*;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import static org.junit.Assert.assertEquals;
+
+public class CoreThreadPoolIntegrationTest {
+
+    private static final Logger LOG = LoggerFactory.getLogger(CoreThreadPoolIntegrationTest.class);
+
+    @Test(timeout = 1000)
+    public void whenCallingExecuteWithRunnable_thenRunnableIsExecuted() throws InterruptedException {
+
+        CountDownLatch lock = new CountDownLatch(1);
+
+        Executor executor = Executors.newSingleThreadExecutor();
+        executor.execute(() -> {
+            LOG.debug("Hello World");
+            lock.countDown();
+        });
+
+        lock.await(1000, TimeUnit.MILLISECONDS);
+    }
+
+    @Test
+    public void whenUsingExecutorServiceAndFuture_thenCanWaitOnFutureResult() throws InterruptedException, ExecutionException {
+
+        ExecutorService executorService = Executors.newFixedThreadPool(10);
+        Future<String> future = executorService.submit(() -> "Hello World");
+        String result = future.get();
+
+        assertEquals("Hello World", result);
+
+    }
+
+    @Test
+    public void whenUsingFixedThreadPool_thenCoreAndMaximumThreadSizeAreTheSame() {
+
+        ThreadPoolExecutor executor = (ThreadPoolExecutor) Executors.newFixedThreadPool(2);
+        executor.submit(() -> {
+            Thread.sleep(1000);
+            return null;
+        });
+        executor.submit(() -> {
+            Thread.sleep(1000);
+            return null;
+        });
+        executor.submit(() -> {
+            Thread.sleep(1000);
+            return null;
+        });
+
+        assertEquals(2, executor.getPoolSize());
+        assertEquals(1, executor.getQueue().size());
+
+    }
+
+    @Test
+    public void whenUsingCachedThreadPool_thenPoolSizeGrowsUnbounded() {
+        ThreadPoolExecutor executor = (ThreadPoolExecutor) Executors.newCachedThreadPool();
+        executor.submit(() -> {
+            Thread.sleep(1000);
+            return null;
+        });
+        executor.submit(() -> {
+            Thread.sleep(1000);
+            return null;
+        });
+        executor.submit(() -> {
+            Thread.sleep(1000);
+            return null;
+        });
+
+        assertEquals(3, executor.getPoolSize());
+        assertEquals(0, executor.getQueue().size());
+
+    }
+
+    @Test(timeout = 1000)
+    public void whenUsingSingleThreadPool_thenTasksExecuteSequentially() throws InterruptedException {
+
+        CountDownLatch lock = new CountDownLatch(2);
+        AtomicInteger counter = new AtomicInteger();
+
+        ExecutorService executor = Executors.newSingleThreadExecutor();
+        executor.submit(() -> {
+            counter.set(1);
+            lock.countDown();
+        });
+        executor.submit(() -> {
+            counter.compareAndSet(1, 2);
+            lock.countDown();
+        });
+
+        lock.await(1000, TimeUnit.MILLISECONDS);
+        assertEquals(2, counter.get());
+
+    }
+
+    @Test(timeout = 1000)
+    public void whenSchedulingTask_thenTaskExecutesWithinGivenPeriod() throws InterruptedException {
+
+        CountDownLatch lock = new CountDownLatch(1);
+
+        ScheduledExecutorService executor = Executors.newScheduledThreadPool(5);
+        executor.schedule(() -> {
+            LOG.debug("Hello World");
+            lock.countDown();
+        }, 500, TimeUnit.MILLISECONDS);
+
+        lock.await(1000, TimeUnit.MILLISECONDS);
+
+    }
+
+    @Test(timeout = 1000)
+    public void whenSchedulingTaskWithFixedPeriod_thenTaskExecutesMultipleTimes() throws InterruptedException {
+
+        CountDownLatch lock = new CountDownLatch(3);
+
+        ScheduledExecutorService executor = Executors.newScheduledThreadPool(5);
+        ScheduledFuture<?> future = executor.scheduleAtFixedRate(() -> {
+            LOG.debug("Hello World");
+            lock.countDown();
+        }, 500, 100, TimeUnit.MILLISECONDS);
+
+        lock.await();
+        future.cancel(true);
+
+    }
+
+    @Test
+    public void whenUsingForkJoinPool_thenSumOfTreeElementsIsCalculatedCorrectly() {
+
+        TreeNode tree = new TreeNode(5, new TreeNode(3), new TreeNode(2, new TreeNode(2), new TreeNode(8)));
+
+        ForkJoinPool forkJoinPool = ForkJoinPool.commonPool();
+        int sum = forkJoinPool.invoke(new CountingTask(tree));
+
+        assertEquals(20, sum);
+    }
+
+}

core-java-modules/core-java-concurrency-advanced/src/test/java/com/baeldung/threadpool/GuavaThreadPoolIntegrationTest.java

@@ -0,0 +1,54 @@
+package com.baeldung.threadpool;
+
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.Executor;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.stream.Collectors;
+
+import com.google.common.util.concurrent.Futures;
+import com.google.common.util.concurrent.ListenableFuture;
+import com.google.common.util.concurrent.ListeningExecutorService;
+import com.google.common.util.concurrent.MoreExecutors;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+public class GuavaThreadPoolIntegrationTest {
+
+    @Test
+    public void whenExecutingTaskWithDirectExecutor_thenTheTaskIsExecutedInTheCurrentThread() {
+
+        Executor executor = MoreExecutors.directExecutor();
+
+        AtomicBoolean executed = new AtomicBoolean();
+
+        executor.execute(() -> {
+            try {
+                Thread.sleep(500);
+            } catch (InterruptedException e) {
+                e.printStackTrace();
+            }
+            executed.set(true);
+        });
+
+        assertTrue(executed.get());
+    }
+
+    @Test
+    public void whenJoiningFuturesWithAllAsList_thenCombinedFutureCompletesAfterAllFuturesComplete() throws ExecutionException, InterruptedException {
+
+        ExecutorService executorService = Executors.newCachedThreadPool();
+        ListeningExecutorService listeningExecutorService = MoreExecutors.listeningDecorator(executorService);
+
+        ListenableFuture<String> future1 = listeningExecutorService.submit(() -> "Hello");
+        ListenableFuture<String> future2 = listeningExecutorService.submit(() -> "World");
+
+        String greeting = Futures.allAsList(future1, future2).get().stream().collect(Collectors.joining(" "));
+        assertEquals("Hello World", greeting);
+
+    }
+
+}

core-java-modules/core-java-concurrency-advanced/src/test/resources/.gitignore

@@ -0,0 +1,13 @@
+*.class
+
+#folders#
+/target
+/neoDb*
+/data
+/src/main/webapp/WEB-INF/classes
+*/META-INF/*
+
+# Packaged files #
+*.jar
+*.war
+*.ear