Merge remote-tracking branch 'upstream/master'

core-java-modules/core-java-8-2/README.md

@@ -8,4 +8,5 @@ This module contains articles about Java 8 core features
 - [Java 8 Stream skip() vs limit()](https://www.baeldung.com/java-stream-skip-vs-limit)
 - [Guide to Java BiFunction Interface](https://www.baeldung.com/java-bifunction-interface)
 - [Interface With Default Methods vs Abstract Class](https://www.baeldung.com/java-interface-default-method-vs-abstract-class)
+- [Convert Between Byte Array and UUID in Java](https://www.baeldung.com/java-byte-array-to-uuid)
 - [[<-- Prev]](/core-java-modules/core-java-8)

core-java-modules/core-java-8-2/src/main/java/com/baeldung/game/RockPaperScissorsGame.java

@@ -1,17 +1,24 @@
 package com.baeldung.game;
 
-import java.util.HashMap;
-import java.util.Map;
-import java.util.Random;
-import java.util.Scanner;
+import java.util.*;
 
 class RockPaperScissorsGame {
 
-    private static Map<Integer, String> movesMap = new HashMap<Integer, String>() {{
-        put(0, "rock");
-        put(1, "paper");
-        put(2, "scissors");
-    }};
+    enum Move {
+        ROCK("rock"),
+        PAPER("paper"),
+        SCISSORS("scissors");
+
+        private String value;
+
+        Move(String value) {
+            this.value = value;
+        }
+
+        public String getValue() {
+            return value;
+        }
+    }
 
     public static void main(String[] args) {
         Scanner scanner = new Scanner(System.in);
@@ -31,7 +38,7 @@ class RockPaperScissorsGame {
                 break;
             }
 
-            if (!movesMap.containsValue(playerMove)) {
+            if (Arrays.stream(Move.values()).noneMatch(x -> x.getValue().equals(playerMove))) {
                 System.out.println("Your move isn't valid!");
                 continue;
             }
@@ -51,15 +58,15 @@ class RockPaperScissorsGame {
     }
 
     private static boolean isPlayerWin(String playerMove, String computerMove) {
-        return playerMove.equals("rock") && computerMove.equals("scissors")
-                || (playerMove.equals("scissors") && computerMove.equals("paper"))
-                || (playerMove.equals("paper") && computerMove.equals("rock"));
+        return playerMove.equals(Move.ROCK.value) && computerMove.equals(Move.SCISSORS.value)
+                || (playerMove.equals(Move.SCISSORS.value) && computerMove.equals(Move.PAPER.value))
+                || (playerMove.equals(Move.PAPER.value) && computerMove.equals(Move.ROCK.value));
     }
 
     private static String getComputerMove() {
         Random random = new Random();
         int randomNumber = random.nextInt(3);
-        String computerMove = movesMap.get(randomNumber);
+        String computerMove = Move.values()[randomNumber].getValue();
         System.out.println("Computer move: " + computerMove);
         return computerMove;
     }

core-java-modules/core-java-collections-4/README.md

@@ -7,3 +7,4 @@
 - [ArrayList vs. LinkedList vs. HashMap in Java](https://www.baeldung.com/java-arraylist-vs-linkedlist-vs-hashmap)
 - [Java Deque vs. Stack](https://www.baeldung.com/java-deque-vs-stack)
 - [Collection.toArray(new T[0]) or .toArray(new T[size])](https://www.baeldung.com/java-collection-toarray-methods)
+- [Create an Empty Map in Java](https://www.baeldung.com/java-create-empty-map)

core-java-modules/core-java-collections-4/src/main/java/com/baeldung/maps/initialize/EmptyMapInitializer.java

@@ -0,0 +1,56 @@
+package com.baeldung.maps.initialize;
+
+import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.Maps;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.LinkedHashMap;
+import java.util.Map;
+import java.util.NavigableMap;
+import java.util.SortedMap;
+import java.util.TreeMap;
+
+public class EmptyMapInitializer {
+
+  public static Map<String, String> articleMap;
+  static {
+    articleMap = new HashMap<>();
+  }
+
+  public static Map<String, String> createEmptyMap() {
+    return Collections.emptyMap();
+  }
+
+  public void createMapUsingConstructors() {
+    Map hashMap = new HashMap();
+    Map linkedHashMap = new LinkedHashMap();
+    Map treeMap = new TreeMap();
+  }
+
+  public Map<String, String> createEmptyMapUsingMapsObject() {
+    Map<String, String> emptyMap = Maps.newHashMap();
+    return emptyMap;
+  }
+
+  public Map createGenericEmptyMapUsingMapsObject() {
+    Map genericEmptyMap = Maps.<String, Integer>newHashMap();
+    return genericEmptyMap;
+  }
+
+  public static Map<String, String> createMapUsingGuava() {
+    Map<String, String> emptyMapUsingGuava =
+        Maps.newHashMap(ImmutableMap.of());
+    return emptyMapUsingGuava;
+  }
+
+  public SortedMap<String, String> createEmptySortedMap() {
+    SortedMap<String, String> sortedMap = Collections.emptySortedMap();
+    return sortedMap;
+  }
+
+  public NavigableMap<String, String> createEmptyNavigableMap() {
+    NavigableMap<String, String> navigableMap = Collections.emptyNavigableMap();
+    return navigableMap;
+  }
+
+}

core-java-modules/core-java-collections-4/src/test/java/com/baeldung/maps/initialize/EmptyMapInitializerUnitTest.java

@@ -0,0 +1,31 @@
+package com.baeldung.maps.initialize;
+
+import java.util.Map;
+import org.junit.Test;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class EmptyMapInitializerUnitTest {
+
+  @Test(expected=UnsupportedOperationException.class)
+  public void givenEmptyMap_whenAddingEntries_throwsException() {
+    Map<String, String> map = EmptyMapInitializer.createEmptyMap();
+    map.put("key", "value");
+  }
+
+  @Test
+  public void givenEmptyMap_whenChecked_returnsTrue() {
+    assertTrue(EmptyMapInitializer.articleMap.isEmpty());
+  }
+
+  @Test
+  public void givenEmptyMap_whenCreatedUsingGuava_returnsEmptyOrNot() {
+    Map<String, String> emptyMapUsingGuava =
+        EmptyMapInitializer.createMapUsingGuava();
+    assertTrue(emptyMapUsingGuava.isEmpty());
+    emptyMapUsingGuava.put("key", "value");
+    assertFalse(emptyMapUsingGuava.isEmpty());
+  }
+
+}

core-java-modules/core-java-security-3/README.md

@@ -10,4 +10,5 @@ This module contains articles about core Java Security
 - [HMAC in Java](https://www.baeldung.com/java-hmac)
 - [Generating a Secure AES Key in Java](https://www.baeldung.com/java-secure-aes-key)
 - [Computing an X509 Certificate’s Thumbprint in Java](https://www.baeldung.com/java-x509-certificate-thumbprint)
+- [Error: “trustAnchors parameter must be non-empty”](https://www.baeldung.com/java-trustanchors-parameter-must-be-non-empty)
 - More articles: [[<-- prev]](/core-java-modules/core-java-security-2)

docker/README.md

@@ -6,3 +6,4 @@
 - [How To Configure Java Heap Size Inside a Docker Container](https://www.baeldung.com/ops/docker-jvm-heap-size)
 - [Dockerfile Strategies for Git](https://www.baeldung.com/ops/dockerfile-git-strategies)
 - [How to Get Docker-Compose to Always Use the Latest Image](https://www.baeldung.com/ops/docker-compose-latest-image)
+- [How to Include Files Outside of Docker’s Build Context](https://www.baeldung.com/ops/docker-include-files-outside-build-context)

docker/docker-include-outside-build-context/projects/config/Dockerfile

@@ -0,0 +1,2 @@
+FROM nginx:latest
+COPY nginx.conf /etc/nginx/nginx.conf

docker/docker-include-outside-build-context/projects/config/nginx.conf

@@ -0,0 +1,8 @@
+events {}  
+  
+http {  
+ server {  
+    listen 80;
+    index index.html;
+  }  
+}

docker/docker-include-outside-build-context/projects/sample-site/docker/Dockerfile

@@ -0,0 +1,3 @@
+FROM nginx:latest
+COPY sample-site/html/* /etc/nginx/html/
+COPY config/nginx.conf /etc/nginx/nginx.conf

docker/docker-include-outside-build-context/projects/sample-site/docker/Dockerfile-from-base

@@ -0,0 +1,2 @@
+FROM sample-site-base:latest
+COPY html/* /etc/nginx/html/

docker/docker-include-outside-build-context/projects/sample-site/docker/Dockerfile-script

@@ -0,0 +1,3 @@
+FROM nginx:latest
+COPY html/* /etc/nginx/html/
+COPY config/nginx.conf /etc/nginx/nginx.conf

docker/docker-include-outside-build-context/projects/sample-site/docker/build-docker.sh

@@ -0,0 +1,6 @@
+mkdir tmp-context
+cp -R ../html tmp-context/
+cp -R ../../config tmp-context/
+cp Dockerfile-script tmp-context/Dockerfile
+docker build -t sample-site:latest tmp-context
+rm -rf tmp-context

docker/docker-include-outside-build-context/projects/sample-site/html/index.html

@@ -0,0 +1,10 @@
+<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <title>Sample Site</title>
+</head>
+<body>
+  <h2>Welcome to the first page of the site</h2>
+</body>
+</html>

gradle/gradle-source-vs-target-compatibility/README.md

@@ -0,0 +1,3 @@
+### Relevant Articles:
+
+- [Gradle: sourceCompatiblity vs targetCompatibility](https://www.baeldung.com/gradle-sourcecompatiblity-vs-targetcompatibility)

java-collections-maps-3/README.md

@@ -7,3 +7,4 @@
 - [Update the Value Associated With a Key in a HashMap](https://www.baeldung.com/java-hashmap-update-value-by-key)
 - [Java Map – keySet() vs. entrySet() vs. values() Methods](https://www.baeldung.com/java-map-entries-methods)
 - [Java IdentityHashMap Class and Its Use Cases](https://www.baeldung.com/java-identityhashmap)
+- [How to Invert a Map in Java](https://www.baeldung.com/java-invert-map)

java-collections-maps-3/src/main/java/com/baeldung/map/invert/InvertHashMapExample.java

@@ -0,0 +1,59 @@
+package com.baeldung.map.invert;
+
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.stream.Collectors;
+
+public class InvertHashMapExample {
+
+    public static void main(String[] args) {
+
+        Map<String, Integer> map = new HashMap<>();
+        map.put("first", 1);
+        map.put("second", 2);
+        System.out.println(map);
+
+        invertMapUsingForLoop(map);
+        invertMapUsingStreams(map);
+        invertMapUsingMapper(map);
+
+        map.put("two", 2);
+        invertMapUsingGroupingBy(map);
+    }
+
+    public static <V, K> Map<V, K> invertMapUsingForLoop(Map<K, V> map) {
+        Map<V, K> inversedMap = new HashMap<V, K>();
+        for (Entry<K, V> entry : map.entrySet()) {
+            inversedMap.put(entry.getValue(), entry.getKey());
+        }
+        System.out.println(inversedMap);
+        return inversedMap;
+    }
+
+    public static <V, K> Map<V, K> invertMapUsingStreams(Map<K, V> map) {
+        Map<V, K> inversedMap = map.entrySet()
+            .stream()
+            .collect(Collectors.toMap(Entry::getValue, Entry::getKey));
+        System.out.println(inversedMap);
+        return inversedMap;
+    }
+
+    public static <K, V> Map<V, K> invertMapUsingMapper(Map<K, V> sourceMap) {
+        Map<V, K> inversedMap = sourceMap.entrySet()
+            .stream()
+            .collect(Collectors.toMap(Entry::getValue, Entry::getKey, (oldValue, newValue) -> oldValue));
+        System.out.println(inversedMap);
+        return inversedMap;
+    }
+
+    public static <V, K> Map<V, List<K>> invertMapUsingGroupingBy(Map<K, V> map) {
+        Map<V, List<K>> inversedMap = map.entrySet()
+            .stream()
+            .collect(Collectors.groupingBy(Map.Entry::getValue, Collectors.mapping(Map.Entry::getKey, Collectors.toList())));
+        System.out.println(inversedMap);
+        return inversedMap;
+    }
+
+}

java-collections-maps-3/src/test/java/com/baeldung/map/invert/InvertHashMapUnitTest.java

@@ -0,0 +1,72 @@
+package com.baeldung.map.invert;
+
+import static org.junit.Assert.assertNotEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.TestInstance;
+
+@TestInstance(TestInstance.Lifecycle.PER_CLASS)
+public class InvertHashMapUnitTest {
+
+    Map<String, Integer> sourceMap;
+
+    @BeforeEach
+    void setup() {
+        sourceMap = new HashMap<>();
+        sourceMap.put("Sunday", 0);
+        sourceMap.put("Monday", 1);
+        sourceMap.put("Tuesday", 2);
+        sourceMap.put("Wednesday", 3);
+        sourceMap.put("Thursday", 4);
+        sourceMap.put("Friday", 5);
+        sourceMap.put("Saturday", 6);
+    }
+
+    @Test
+    void givenSourceMap_whenUsingForLoop_returnsInvertedMap() {
+        Map<Integer, String> inversedMap = InvertHashMapExample.invertMapUsingForLoop(sourceMap);
+        
+        assertNotNull(inversedMap);
+        assertEquals(sourceMap.size(), inversedMap.size());
+        assertEquals("Monday", inversedMap.get(1));
+    }
+
+    @Test
+    void givenSourceMap_whenUsingStreams_returnsInvertedMap() {
+        Map<Integer, String> inversedMap = InvertHashMapExample.invertMapUsingStreams(sourceMap);
+        
+        assertNotNull(inversedMap);
+        assertEquals(sourceMap.size(), inversedMap.size());
+        assertEquals("Monday", inversedMap.get(1));
+    }
+
+    @Test
+    void givenSourceMap_whenUsingMapper_returnsInvertedMap() {
+        Map<Integer, String> inversedMap = InvertHashMapExample.invertMapUsingMapper(sourceMap);
+        
+        assertNotNull(inversedMap);
+        assertEquals(sourceMap.size(), inversedMap.size());
+        assertEquals("Monday", inversedMap.get(1));
+    }
+
+    @Test
+    void givenSourceMapWithDuplicateValues_whenUsingGroupBy_returnsInvertedMap() {
+        sourceMap.put("MONDAY", 1);
+        Map<Integer, List<String>> inversedMap = InvertHashMapExample.invertMapUsingGroupingBy(sourceMap);
+        
+        assertNotNull(inversedMap);
+        assertNotEquals(sourceMap.size(), inversedMap.size()); // duplicate keys are merged now
+        assertEquals(2, inversedMap.get(1).size());
+        assertTrue(inversedMap.get(1).contains("Monday"));
+        assertTrue(inversedMap.get(1).contains("MONDAY"));
+    }
+
+}

java-native/README.md

@@ -7,3 +7,4 @@ This module contains articles about the Java Native Interface (JNI).
 - [Guide to JNI (Java Native Interface)](https://www.baeldung.com/jni)
 - [Using JNA to Access Native Dynamic Libraries](https://www.baeldung.com/java-jna-dynamic-libraries)
 - [Check if a Java Program Is Running in 64-Bit or 32-Bit JVM](https://www.baeldung.com/java-detect-jvm-64-or-32-bit)
+- [How to use JNI’s RegisterNatives() method?](https://www.baeldung.com/jni-registernatives)

javax-servlets-2/README.md

@@ -0,0 +1,5 @@
+## Servlets
+
+This module contains articles about Servlets.
+
+### Relevant Articles:

javax-servlets-2/pom.xml

@@ -0,0 +1,60 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<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.javax-servlets</groupId>
+    <artifactId>javax-servlets-2</artifactId>
+    <version>1.0-SNAPSHOT</version>
+    <name>javax-servlets-2</name>
+    <packaging>war</packaging>
+
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>parent-modules</artifactId>
+        <version>1.0.0-SNAPSHOT</version>
+    </parent>
+
+    <dependencies>
+        <!-- File Uploading -->
+        <dependency>
+            <groupId>commons-fileupload</groupId>
+            <artifactId>commons-fileupload</artifactId>
+            <version>${commons-fileupload.version}</version>
+        </dependency>
+        <!-- Servlet -->
+        <dependency>
+            <groupId>javax.servlet</groupId>
+            <artifactId>javax.servlet-api</artifactId>
+            <version>${javax.servlet-api.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>javax.servlet.jsp</groupId>
+            <artifactId>javax.servlet.jsp-api</artifactId>
+            <version>${javax.servlet.jsp-api.version}</version>
+            <scope>provided</scope>
+        </dependency>
+        <dependency>
+            <groupId>javax.servlet</groupId>
+            <artifactId>jstl</artifactId>
+            <version>${jstl.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.httpcomponents</groupId>
+            <artifactId>httpclient</artifactId>
+            <version>${org.apache.httpcomponents.version}</version>
+            <scope>test</scope>
+            <exclusions>
+                <exclusion>
+                    <artifactId>commons-logging</artifactId>
+                    <groupId>commons-logging</groupId>
+                </exclusion>
+            </exclusions>
+        </dependency>
+    </dependencies>
+
+    <properties>
+        <org.apache.httpcomponents.version>4.5.13</org.apache.httpcomponents.version>
+        <javax.servlet-api.version>4.0.1</javax.servlet-api.version>
+    </properties>
+</project>

javax-servlets-2/src/main/java/com/baeldung/user/check/User.java

@@ -0,0 +1,80 @@
+package com.baeldung.user.check;
+
+import java.io.Serializable;
+import java.util.ArrayList;
+import java.util.Date;
+import java.util.HashMap;
+import java.util.List;
+
+/**
+ * @since 6 de fev de 2022
+ * @author ulisses
+ */
+public class User implements Serializable {
+    private static final long serialVersionUID = 1L;
+
+    protected static final HashMap<String, User> DB = new HashMap<>();
+    static {
+        DB.put("admin", new User("admin", "password"));
+        DB.put("user", new User("user", "pass"));
+    }
+
+    private String name;
+    private String password;
+
+    private List<Date> logins = new ArrayList<Date>();
+
+    public User(String name, String password) {
+        this.name = name;
+        this.password = password;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public List<Date> getLogins() {
+        return logins;
+    }
+
+    public void setLogins(List<Date> logins) {
+        this.logins = logins;
+    }
+
+    public String getPassword() {
+        return password;
+    }
+
+    public void setPassword(String password) {
+        this.password = password;
+    }
+
+    @Override
+    public int hashCode() {
+        final int prime = 31;
+        int result = 1;
+        result = prime * result + ((name == null) ? 0 : name.hashCode());
+        return result;
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (this == obj)
+            return true;
+        if (obj == null)
+            return false;
+        if (getClass() != obj.getClass())
+            return false;
+        User other = (User) obj;
+        if (name == null) {
+            if (other.name != null)
+                return false;
+        } else if (!name.equals(other.name))
+            return false;
+        return true;
+    }
+}

javax-servlets-2/src/main/java/com/baeldung/user/check/UserCheckFilter.java

@@ -0,0 +1,46 @@
+package com.baeldung.user.check;
+
+import java.io.IOException;
+
+import javax.servlet.Filter;
+import javax.servlet.FilterChain;
+import javax.servlet.ServletException;
+import javax.servlet.ServletRequest;
+import javax.servlet.ServletResponse;
+import javax.servlet.annotation.WebFilter;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+
+@WebFilter("/user-check/*")
+public class UserCheckFilter implements Filter {
+    public static void forward(HttpServletRequest request, HttpServletResponse response, String page) throws ServletException, IOException {
+        request.getRequestDispatcher("/WEB-INF/user.check" + page)
+            .forward(request, response);
+    }
+
+    @Override
+    public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {
+        if (!(req instanceof HttpServletRequest)) {
+            throw new ServletException("Can only process HttpServletRequest");
+        }
+
+        if (!(res instanceof HttpServletResponse)) {
+            throw new ServletException("Can only process HttpServletResponse");
+        }
+
+        HttpServletRequest request = (HttpServletRequest) req;
+        HttpServletResponse response = (HttpServletResponse) res;
+
+        request.setAttribute("origin", request.getRequestURI());
+
+        if (!request.getRequestURI()
+            .contains("login") && request.getSession(false) == null) {
+            response.setStatus(HttpServletResponse.SC_UNAUTHORIZED);
+            forward(request, response, "/login.jsp");
+            // we return here so the original servlet is not processed
+            return;
+        }
+
+        chain.doFilter(request, response);
+    }
+}

javax-servlets-2/src/main/java/com/baeldung/user/check/UserCheckLoginServlet.java

@@ -0,0 +1,56 @@
+package com.baeldung.user.check;
+
+import java.io.IOException;
+import java.util.Date;
+
+import javax.servlet.ServletException;
+import javax.servlet.annotation.WebServlet;
+import javax.servlet.http.HttpServlet;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import javax.servlet.http.HttpSession;
+
+@WebServlet("/user-check/login")
+public class UserCheckLoginServlet extends HttpServlet {
+    private static final long serialVersionUID = 1L;
+
+    @Override
+    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
+        if (request.getSession(false) != null) {
+            response.sendRedirect(request.getContextPath() + "/user-check/home");
+            return;
+        }
+
+        String referer = (String) request.getAttribute("origin");
+        request.setAttribute("origin", referer);
+        UserCheckFilter.forward(request, response, "/login.jsp");
+    }
+
+    @Override
+    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
+        String key = request.getParameter("name");
+        String pass = request.getParameter("password");
+
+        User user = User.DB.get(key);
+        if (user == null || !user.getPassword()
+            .equals(pass)) {
+            response.setStatus(HttpServletResponse.SC_UNAUTHORIZED);
+            request.setAttribute("origin", request.getParameter("origin"));
+            request.setAttribute("error", "invalid login");
+            UserCheckFilter.forward(request, response, "/login.jsp");
+            return;
+        }
+
+        user.getLogins()
+            .add(new Date());
+
+        HttpSession session = request.getSession();
+        session.setAttribute("user", user);
+
+        String origin = request.getParameter("origin");
+        if (origin == null || origin.contains("login"))
+            origin = "./";
+
+        response.sendRedirect(origin);
+    }
+}

javax-servlets-2/src/main/java/com/baeldung/user/check/UserCheckLogoutServlet.java

@@ -0,0 +1,26 @@
+package com.baeldung.user.check;
+
+import java.io.IOException;
+
+import javax.servlet.ServletException;
+import javax.servlet.annotation.WebServlet;
+import javax.servlet.http.HttpServlet;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import javax.servlet.http.HttpSession;
+
+@WebServlet("/user-check/logout")
+public class UserCheckLogoutServlet extends HttpServlet {
+    private static final long serialVersionUID = 1L;
+
+    @Override
+    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
+        HttpSession session = request.getSession(false);
+        if (session != null) {
+            session.invalidate();
+        }
+
+        request.setAttribute("loggedOut", true);
+        response.sendRedirect("./");
+    }
+}

javax-servlets-2/src/main/java/com/baeldung/user/check/UserCheckServlet.java

@@ -0,0 +1,28 @@
+package com.baeldung.user.check;
+
+import java.io.IOException;
+
+import javax.servlet.ServletException;
+import javax.servlet.annotation.WebServlet;
+import javax.servlet.http.HttpServlet;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import javax.servlet.http.HttpSession;
+
+@WebServlet(name = "home", urlPatterns = { "/user-check/", "/user-check", "/user-check/home" })
+public class UserCheckServlet extends HttpServlet {
+    private static final long serialVersionUID = 1L;
+
+    @Override
+    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
+        HttpSession session = request.getSession(false);
+        if (session == null || session.getAttribute("user") == null) {
+            throw new IllegalStateException("user not logged in");
+        }
+
+        User user = (User) session.getAttribute("user");
+        request.setAttribute("user", user);
+
+        UserCheckFilter.forward(request, response, "/home.jsp");
+    }
+}

javax-servlets-2/src/main/resources/logback.xml

@@ -0,0 +1,13 @@
+<?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>
+
+    <root level="INFO">
+        <appender-ref ref="STDOUT" />
+    </root>
+</configuration>

javax-servlets-2/src/main/webapp/WEB-INF/user.check/home.jsp

@@ -0,0 +1,31 @@
+<%@ page contentType="text/html;charset=UTF-8" session="false"%>
+<%@ taglib uri="http://java.sun.com/jstl/core_rt" prefix="c"%>
+
+<html>
+    <head>
+        <title>login success - current session info</title>
+    </head>
+    <body>
+        <section>
+            <h2>user info</h2>
+            <div>
+                <span>name: ${user.name}</span>
+            </div>
+    
+            <div>
+                <span>logins:</span>
+                <ul>
+                    <c:forEach var="login" items="${user.logins}">
+                        <li>${login}</li>
+                    </c:forEach>
+                </ul>
+            </div>
+    
+            <div>
+                <a href="${pageContext.request.contextPath}/user-check/logout">
+                    logout
+                </a>
+            </div>
+        </section>
+    </body>
+</html>

javax-servlets-2/src/main/webapp/WEB-INF/user.check/login.jsp

@@ -0,0 +1,33 @@
+<%@ page contentType="text/html;charset=UTF-8" session="false"%>
+<%@ taglib uri="http://java.sun.com/jstl/core_rt" prefix="c"%>
+
+<html>
+    <head>
+        <title>login</title>
+    </head>
+    <body>
+        <form action="${pageContext.request.contextPath}/user-check/login"
+          method="POST">
+            <input type="hidden" name="origin" value="${origin}">
+            <c:if test="${not empty origin}">
+                <div>* redirected to login from: ${origin}</div>
+            </c:if>
+    
+            <c:if test="${not empty error}">
+                <div>* error: ${error}</div>
+            </c:if>
+    
+            <fieldset>
+                <legend>credentials</legend>
+                
+                <label for="name">name</label> 
+                <input type="text" name="name">
+    
+                <label for="password">password</label> 
+                <input type="password" name="password"> 
+                
+                <input type="submit">
+            </fieldset>
+        </form>
+    </body>
+</html>