BAEL-4234: example of JNI registerNatives() method (#11745)

* BAEL-4234: example of JNI registerNatives() method

* fixed formatting in the cpp file

* removed camelcase in test package name

java-native/src/main/cpp/com_baeldung_jni_RegisterNativesHelloWorldJNI.cpp

@@ -0,0 +1,21 @@
+#include "com_baeldung_jni_RegisterNativesHelloWorldJNI.h"
+#include <iostream>
+
+
+JNIEXPORT jstring JNICALL hello (JNIEnv* env, jobject thisObject) {
+    std::string hello = "Hello from registered native C++ !!";
+    std::cout << hello << std::endl;
+    return env->NewStringUTF(hello.c_str());
+}
+
+static JNINativeMethod methods[] = {
+  {"sayHello", "()Ljava/lang/String;", (void*) &hello },
+};
+
+
+JNIEXPORT void JNICALL Java_com_baeldung_jni_RegisterNativesHelloWorldJNI_register (JNIEnv* env, jobject thsObject) {
+    jclass clazz = env->FindClass("com/baeldung/jni/RegisterNativesHelloWorldJNI");
+
+    (env)->RegisterNatives(clazz, methods, sizeof(methods)/sizeof(methods[0]));
+}
+

java-native/src/main/cpp/com_baeldung_jni_RegisterNativesHelloWorldJNI.h

@@ -0,0 +1,29 @@
+/* DO NOT EDIT THIS FILE - it is machine generated */
+#include <jni.h>
+/* Header for class com_baeldung_jni_RegisterNativesHelloWorldJNI */
+
+#ifndef _Included_com_baeldung_jni_RegisterNativesHelloWorldJNI
+#define _Included_com_baeldung_jni_RegisterNativesHelloWorldJNI
+#ifdef __cplusplus
+extern "C" {
+#endif
+/*
+ * Class:     com_baeldung_jni_RegisterNativesHelloWorldJNI
+ * Method:    sayHello
+ * Signature: ()Ljava/lang/String;
+ */
+JNIEXPORT jstring JNICALL Java_com_baeldung_jni_RegisterNativesHelloWorldJNI_sayHello
+  (JNIEnv *, jobject);
+
+/*
+ * Class:     com_baeldung_jni_RegisterNativesHelloWorldJNI
+ * Method:    register
+ * Signature: ()V
+ */
+JNIEXPORT void JNICALL Java_com_baeldung_jni_RegisterNativesHelloWorldJNI_register
+  (JNIEnv *, jobject);
+
+#ifdef __cplusplus
+}
+#endif
+#endif

java-native/src/main/cpp/generateNativeLibMac.sh

@@ -3,4 +3,5 @@
 g++ -c -fPIC -I${JAVA_HOME}/include -I${JAVA_HOME}/include/darwin com_baeldung_jni_HelloWorldJNI.cpp -o com_baeldung_jni_HelloWorldJNI.o
 g++ -c -fPIC -I${JAVA_HOME}/include -I${JAVA_HOME}/include/darwin com_baeldung_jni_ExampleParametersJNI.cpp -o com_baeldung_jni_ExampleParametersJNI.o
 g++ -c -fPIC -I${JAVA_HOME}/include -I${JAVA_HOME}/include/darwin com_baeldung_jni_ExampleObjectsJNI.cpp -o com_baeldung_jni_ExampleObjectsJNI.o
-g++ -dynamiclib -o ../../../native/macos/libnative.dylib com_baeldung_jni_HelloWorldJNI.o com_baeldung_jni_ExampleParametersJNI.o com_baeldung_jni_ExampleObjectsJNI.o -lc
+g++ -c -fPIC -I${JAVA_HOME}/include -I${JAVA_HOME}/include/darwin com_baeldung_jni_RegisterNativesHelloWorldJNI.cpp -o com_baeldung_jni_RegisterNativesHelloWorldJNI.o
+g++ -dynamiclib -o ../../../native/macos/libnative.dylib com_baeldung_jni_HelloWorldJNI.o com_baeldung_jni_ExampleParametersJNI.o com_baeldung_jni_ExampleObjectsJNI.o com_baeldung_jni_RegisterNativesHelloWorldJNI.o -lc

java-native/src/main/java/com/baeldung/jni/RegisterNativesHelloWorldJNI.java

@@ -0,0 +1,18 @@
+package com.baeldung.jni;
+
+public class RegisterNativesHelloWorldJNI {
+
+    static {
+        System.loadLibrary("native");
+    }
+
+    public static void main(String[] args) {
+        RegisterNativesHelloWorldJNI helloWorldJNI = new RegisterNativesHelloWorldJNI();
+        helloWorldJNI.register();
+        helloWorldJNI.sayHello();
+    }
+
+    public native String sayHello();
+
+    public native void register();
+}

java-native/src/test/java/com/baeldung/jni/registernatives/JNIRegisterNativesManualTest.java

@@ -0,0 +1,26 @@
+package com.baeldung.jni.registernatives;
+
+import com.baeldung.jni.RegisterNativesHelloWorldJNI;
+import org.junit.Before;
+import org.junit.Test;
+
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+
+public class JNIRegisterNativesManualTest {
+    @Before
+    public void setup() {
+        System.loadLibrary("native");
+    }
+
+    @Test
+    public void whenRegisteredNativeHelloWorld_thenOutputIsAsExpected() {
+        RegisterNativesHelloWorldJNI helloWorld = new RegisterNativesHelloWorldJNI();
+        helloWorld.register();
+
+        String helloFromNative = helloWorld.sayHello();
+
+        assertNotNull(helloFromNative);
+        assertTrue(helloFromNative.equals("Hello from registered native C++ !!"));
+    }
+}