BAEL-4165 - Custom DLL Load - Fixing the "java.lang.UnsatisfiedLinkError" Error (#14393)

* first draft

* review 1

* review 1 - test setup

java-native/src/main/cpp/unsatisfiedlink/com_baeldung_unsatisfiedlink_JniUnsatisfiedLink.cpp

@@ -0,0 +1,13 @@
+#include "com_baeldung_unsatisfiedlink_JniUnsatisfiedLink.h"
+#include <iostream>
+
+/*
+ * Class:     com_baeldung_unsatisfiedlink_JniUnsatisfiedLink
+ * Method:    test
+ * Signature: ()Ljava/lang/String;
+ */
+JNIEXPORT jstring JNICALL Java_com_baeldung_unsatisfiedlink_JniUnsatisfiedLink_test (JNIEnv* env, jobject thisObject) {
+	std::string test = "Test OK";
+    std::cout << test << std::endl;
+    return env->NewStringUTF(test.c_str());
+}

java-native/src/main/cpp/unsatisfiedlink/com_baeldung_unsatisfiedlink_JniUnsatisfiedLink.h

@@ -0,0 +1,21 @@
+/* DO NOT EDIT THIS FILE - it is machine generated */
+#include <jni.h>
+/* Header for class com_baeldung_unsatisfiedlink_JniUnsatisfiedLink */
+
+#ifndef _Included_com_baeldung_unsatisfiedlink_JniUnsatisfiedLink
+#define _Included_com_baeldung_unsatisfiedlink_JniUnsatisfiedLink
+#ifdef __cplusplus
+extern "C" {
+#endif
+/*
+ * Class:     com_baeldung_unsatisfiedlink_JniUnsatisfiedLink
+ * Method:    test
+ * Signature: ()V
+ */
+JNIEXPORT jstring JNICALL Java_com_baeldung_unsatisfiedlink_JniUnsatisfiedLink_test
+  (JNIEnv *, jobject);
+
+#ifdef __cplusplus
+}
+#endif
+#endif

java-native/src/main/cpp/unsatisfiedlink/generateNativeLib.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+# Create the header with javac -h . ClassName.java
+# Remember to set your JAVA_HOME env var
+# Don't forget to set java.library.path to point to the folder where you have the lib you're loading.
+MYSELF="$(readlink -f "$0")"
+MYDIR="${MYSELF%/*}"
+
+cd "$MYDIR"
+class_name=com_baeldung_unsatisfiedlink_JniUnsatisfiedLink
+lib_name=test
+
+g++ -c -fPIC -I"${JAVA_HOME}/include" -I"${JAVA_HOME}/include/linux" ${class_name}.cpp -o ${class_name}.o
+g++ -shared -fPIC -o lib${lib_name}.so ${class_name}.o -lc
+
+# 32-bit version
+g++ -m32 -c -fPIC -I"${JAVA_HOME}/include" -I"${JAVA_HOME}/include/linux" ${class_name}.cpp -o ${class_name}32.o
+g++ -m32 -shared -fPIC -o lib${lib_name}32.so ${class_name}32.o -lc
+
+# dummy version
+touch ${class_name}-dummy.o
+
+ls lib*

java-native/src/main/java/com/baeldung/unsatisfiedlink/JniUnsatisfiedLink.java

@@ -0,0 +1,15 @@
+package com.baeldung.unsatisfiedlink;
+
+public class JniUnsatisfiedLink {
+
+    public static final String LIB_NAME = "test";
+
+    public static void main(String[] args) {
+        System.loadLibrary(LIB_NAME);
+        new JniUnsatisfiedLink().test();
+    }
+
+    public native String test();
+
+    public native String nonexistentDllMethod();
+}

java-native/src/main/resources/unsatisfiedlink/jni.policy

@@ -0,0 +1,3 @@
+grant {
+    permission java.lang.RuntimePermission "loadLibrary.test";
+};