BAEL-1637: Guide to JNI(Java Native Interface) (#4066)

* BAEL-1546: Java 8 Math additions

* Applied feedback to Unit Tests

* BAEL-1546 Added missing test annotations

* Added code for BAEL-1637

* Added script for Windows C++ code compile

* Added compilation script for MacOS

* Added some Unit tests

jni/pom.xml

@@ -0,0 +1,15 @@
+<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>jni</artifactId>
+  <version>0.0.1-SNAPSHOT</version>
+  
+  <dependencies>
+    <dependency>
+        <groupId>junit</groupId>
+        <artifactId>junit</artifactId>
+        <version>4.8.1</version>
+        <scope>test</scope>
+    </dependency>
+  </dependencies>
+</project>

jni/src/main/cpp/com_baeldung_jni_ExampleObjectsJNI.cpp

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+#include "com_baeldung_jni_ExampleObjectsJNI.h"
+#include <iostream>
+
+/*
+ * Class:     com_baeldung_jni_ExampleObjectsJNI
+ * Method:    createUser
+ * Signature: (Ljava/lang/String;D)Lcom/baeldung/jni/UserData;
+ */
+JNIEXPORT jobject JNICALL Java_com_baeldung_jni_ExampleObjectsJNI_createUser
+  (JNIEnv *env, jobject thisObject, jstring name, jdouble balance){
+  
+    // Create the object of the class UserData
+    jclass userDataClass = env->FindClass("com/baeldung/jni/UserData");
+    jobject newUserData = env->AllocObject(userDataClass);
+	
+    // Get UserData fields to set
+    jfieldID nameField = env->GetFieldID(userDataClass , "name", "Ljava/lang/String;");
+    jfieldID balanceField = env->GetFieldID(userDataClass , "balance", "D");
+	
+    // Set the values of the new object
+    env->SetObjectField(newUserData, nameField, name);
+    env->SetDoubleField(newUserData, balanceField, balance);
+    
+    // Return the created object
+    return newUserData;
+  }
+
+/*
+ * Class:     com_baeldung_jni_ExampleObjectsJNI
+ * Method:    printUserData
+ * Signature: (Lcom/baeldung/jni/UserData;)Ljava/lang/String;
+ */
+JNIEXPORT jstring JNICALL Java_com_baeldung_jni_ExampleObjectsJNI_printUserData
+  (JNIEnv *env, jobject thisObject, jobject userData){
+  	
+  	// Find the class method id
+  	jclass userDataClass = env->GetObjectClass(userData);
+  	jmethodID methodId = env->GetMethodID(userDataClass, "getUserInfo", "()Ljava/lang/String;");
+
+    // Call the object method and get the result
+    jstring result = (jstring)env->CallObjectMethod(userData, methodId);
+	
+    // Print the result
+    std::cout << "C++: User data is: " << env->GetStringUTFChars(result, NULL) << std::endl;
+    
+    return result;
+  }
+

jni/src/main/cpp/com_baeldung_jni_ExampleObjectsJNI.h

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+/* DO NOT EDIT THIS FILE - it is machine generated */
+#include <jni.h>
+/* Header for class com_baeldung_jni_ExampleObjectsJNI */
+
+#ifndef _Included_com_baeldung_jni_ExampleObjectsJNI
+#define _Included_com_baeldung_jni_ExampleObjectsJNI
+#ifdef __cplusplus
+extern "C" {
+#endif
+/*
+ * Class:     com_baeldung_jni_ExampleObjectsJNI
+ * Method:    createUser
+ * Signature: (Ljava/lang/String;D)Lcom/baeldung/jni/UserData;
+ */
+JNIEXPORT jobject JNICALL Java_com_baeldung_jni_ExampleObjectsJNI_createUser
+  (JNIEnv *, jobject, jstring, jdouble);
+
+/*
+ * Class:     com_baeldung_jni_ExampleObjectsJNI
+ * Method:    printUserData
+ * Signature: (Lcom/baeldung/jni/UserData;)V
+ */
+JNIEXPORT jstring JNICALL Java_com_baeldung_jni_ExampleObjectsJNI_printUserData
+  (JNIEnv *, jobject, jobject);
+
+#ifdef __cplusplus
+}
+#endif
+#endif

jni/src/main/cpp/com_baeldung_jni_ExampleParametersJNI.cpp

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+#include "com_baeldung_jni_ExampleParametersJNI.h"
+#include <iostream>
+#include <string>
+
+/*
+ * Class:     com_baeldung_jni_ExampleParametersJNI
+ * Method:    sumIntegers
+ * Signature: (II)J
+ */
+JNIEXPORT jlong JNICALL Java_com_baeldung_jni_ExampleParametersJNI_sumIntegers (JNIEnv* env, jobject thisObject, jint first, jint second){
+    std::cout << "C++: The numbers received are : " << first << " and " << second << std::endl;
+    return (long)first + (long)second;
+}
+
+
+/*
+ * Class:     com_baeldung_jni_ExampleParametersJNI
+ * Method:    sayHelloToMe
+ * Signature: (Ljava/lang/String;Z)Ljava/lang/String;
+ */
+JNIEXPORT jstring JNICALL Java_com_baeldung_jni_ExampleParametersJNI_sayHelloToMe (JNIEnv* env, jobject thisObject, jstring name, jboolean isFemale){
+    const char* nameCharPointer = env->GetStringUTFChars(name, NULL);
+    std::cout << "C++: The string received is: " << nameCharPointer << std::endl;
+    std::string title;
+    if(isFemale){
+        title = "Ms. ";
+    }
+    else{
+        title = "Mr. ";
+    }
+
+    std::string fullName = title + nameCharPointer;
+    return env->NewStringUTF(fullName.c_str());
+}

jni/src/main/cpp/com_baeldung_jni_ExampleParametersJNI.h

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+/* DO NOT EDIT THIS FILE - it is machine generated */
+#include <jni.h>
+/* Header for class com_baeldung_jni_ExampleParametersJNI */
+
+#ifndef _Included_com_baeldung_jni_ExampleParametersJNI
+#define _Included_com_baeldung_jni_ExampleParametersJNI
+#ifdef __cplusplus
+extern "C" {
+#endif
+/*
+ * Class:     com_baeldung_jni_ExampleParametersJNI
+ * Method:    sumIntegers
+ * Signature: (II)J
+ */
+JNIEXPORT jlong JNICALL Java_com_baeldung_jni_ExampleParametersJNI_sumIntegers
+  (JNIEnv*, jobject, jint, jint);
+
+/*
+ * Class:     com_baeldung_jni_ExampleParametersJNI
+ * Method:    sayHelloToMe
+ * Signature: (Ljava/lang/String;Z)Ljava/lang/String;
+ */
+JNIEXPORT jstring JNICALL Java_com_baeldung_jni_ExampleParametersJNI_sayHelloToMe
+  (JNIEnv*, jobject, jstring, jboolean);
+
+#ifdef __cplusplus
+}
+#endif
+#endif

jni/src/main/cpp/com_baeldung_jni_HelloWorldJNI.cpp

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+#include "com_baeldung_jni_HelloWorldJNI.h"
+#include <iostream>
+
+/*
+ * Class:     com_baeldung_jni_HelloWorldJNI
+ * Method:    sayHello
+ * Signature: ()Ljava/lang/String;
+ */
+JNIEXPORT jstring JNICALL Java_com_baeldung_jni_HelloWorldJNI_sayHello (JNIEnv* env, jobject thisObject) {
+	std::string hello = "Hello from C++ !!";
+    std::cout << hello << std::endl;
+    return env->NewStringUTF(hello.c_str());
+}

jni/src/main/cpp/com_baeldung_jni_HelloWorldJNI.h

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+/* DO NOT EDIT THIS FILE - it is machine generated */
+#include <jni.h>
+/* Header for class com_baeldung_jni_HelloWorldJNI */
+
+#ifndef _Included_com_baeldung_jni_HelloWorldJNI
+#define _Included_com_baeldung_jni_HelloWorldJNI
+#ifdef __cplusplus
+extern "C" {
+#endif
+/*
+ * Class:     com_baeldung_jni_HelloWorldJNI
+ * Method:    sayHello
+ * Signature: ()V
+ */
+JNIEXPORT jstring JNICALL Java_com_baeldung_jni_HelloWorldJNI_sayHello
+  (JNIEnv *, jobject);
+
+#ifdef __cplusplus
+}
+#endif
+#endif

jni/src/main/cpp/generateNativeLib.bat

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+REM Create the header with javac -h . ClassName.java
+REM Remember to set your JAVA_HOME env var
+g++ -c -I%JAVA_HOME%\include -I%JAVA_HOME%\include\win32 com_baeldung_jni_HelloWorldJNI.cpp -o com_baeldung_jni_HelloWorldJNI.o
+g++ -c -I%JAVA_HOME%\include -I%JAVA_HOME%\include\win32 com_baeldung_jni_ExampleParametersJNI.cpp -o com_baeldung_jni_ExampleParametersJNI.o
+g++ -c -I%JAVA_HOME%\include -I%JAVA_HOME%\include\win32 com_baeldung_jni_ExampleObjectsJNI.cpp -o com_baeldung_jni_ExampleObjectsJNI.o
+g++ -shared -o ..\..\..\native\win32\native.dll com_baeldung_jni_HelloWorldJNI.o com_baeldung_jni_ExampleParametersJNI.o com_baeldung_jni_ExampleObjectsJNI.o -Wl,--add-stdcall-alias

jni/src/main/cpp/generateNativeLib.sh

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+# Create the header with javac -h . ClassName.java
+# Remember to set your JAVA_HOME env var
+g++ -c -fPIC -I${JAVA_HOME}/include -I${JAVA_HOME}/include/linux com_baeldung_jni_HelloWorldJNI.cpp -o com_baeldung_jni_HelloWorldJNI.o
+g++ -c -fPIC -I${JAVA_HOME}/include -I${JAVA_HOME}/include/linux com_baeldung_jni_ExampleParametersJNI.cpp -o com_baeldung_jni_ExampleParametersJNI.o
+g++ -c -fPIC -I${JAVA_HOME}/include -I${JAVA_HOME}/include/linux com_baeldung_jni_ExampleObjectsJNI.cpp -o com_baeldung_jni_ExampleObjectsJNI.o
+g++ -shared -fPIC -o ../../../native/linux_x86_64/libnative.so com_baeldung_jni_HelloWorldJNI.o com_baeldung_jni_ExampleParametersJNI.o com_baeldung_jni_ExampleObjectsJNI.o -lc
+# Don't forget to set java.library.path to point to the folder where you have the libnative you're loading.

jni/src/main/cpp/generateNativeLibMac.sh

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+# Create the header with javac -h . ClassName.java
+# Remember to set your JAVA_HOME env var
+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

jni/src/main/java/com/baeldung/jni/ExampleObjectsJNI.java

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+package com.baeldung.jni;
+
+public class ExampleObjectsJNI {
+
+    static {
+        System.loadLibrary("native");
+    }
+    
+    public static void main(String[] args) {
+        ExampleObjectsJNI instance = new ExampleObjectsJNI();
+        UserData newUser = instance.createUser("John Doe", 450.67);
+        instance.printUserData(newUser);
+    }
+    
+    public native UserData createUser(String name, double balance);
+    
+    public native String printUserData(UserData user);
+
+}

jni/src/main/java/com/baeldung/jni/ExampleParametersJNI.java

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+package com.baeldung.jni;
+
+public class ExampleParametersJNI {
+
+    static {
+        System.loadLibrary("native");
+    }
+    
+    public static void main(String[] args) {  
+        System.out.println("Java: My full name: " + new ExampleParametersJNI().sayHelloToMe("Martin", false));
+        long sumFromNative = new ExampleParametersJNI().sumIntegers(456, 44);
+        System.out.println("Java: The sum coming from native code is: " + sumFromNative);
+    }
+    
+    // Declare another method sumIntegers that receives two integers and return a long with the sum
+    public native long sumIntegers(int first, int second);
+    
+    // Declare another method sayHelloToMe that receives the name and gender and returns the proper salutation
+    public native String sayHelloToMe(String name, boolean isFemale);
+}

jni/src/main/java/com/baeldung/jni/HelloWorldJNI.java

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+package com.baeldung.jni;
+
+public class HelloWorldJNI {
+
+    static {
+        System.loadLibrary("native");
+    }
+    
+    public static void main(String[] args) {
+        new HelloWorldJNI().sayHello();
+    }
+
+    // Declare a native method sayHello() that receives no arguments and returns void
+    public native String sayHello();
+}

jni/src/main/java/com/baeldung/jni/UserData.java

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+package com.baeldung.jni;
+
+public class UserData {
+    
+    public String name;
+    public double balance;
+    
+    public String getUserInfo() {
+        return "[name]=" + name + ", [balance]=" + balance;
+    }
+}

jni/src/test/java/com/baeldung/jni/JNINativeTests.java

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+package com.baeldung.jni;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+
+import org.junit.Before;
+import org.junit.Test;
+
+public class JNINativeTests {
+    
+    @Before
+    public void setup() {
+        System.loadLibrary("native");
+    }
+    
+    @Test
+    public void whenNativeHelloWorld_thenOutputIsAsExpected() {
+        HelloWorldJNI helloWorld = new HelloWorldJNI();
+        String helloFromNative = helloWorld.sayHello();
+        assertTrue(!helloFromNative.isEmpty() && helloFromNative.equals("Hello from C++ !!"));
+    }
+    
+    @Test
+    public void whenSumNative_thenResultIsArithmeticallyCorrect() {
+        ExampleParametersJNI parametersNativeMethods = new ExampleParametersJNI();
+        assertTrue(parametersNativeMethods.sumIntegers(200, 400) == 600L);
+    }
+    
+    @Test
+    public void whenSayingNativeHelloToMe_thenResultIsAsExpected() {
+        ExampleParametersJNI parametersNativeMethods = new ExampleParametersJNI();
+        assertEquals(parametersNativeMethods.sayHelloToMe("Orange", true), "Ms. Orange");
+    }
+
+    @Test
+    public void whenCreatingNativeObject_thenObjectIsNotNullAndHasCorrectData() {
+        String name = "Iker Casillas";
+        double balance = 2378.78;
+        ExampleObjectsJNI objectsNativeMethods = new ExampleObjectsJNI();
+        UserData userFromNative = objectsNativeMethods.createUser(name, balance);
+        assertNotNull(userFromNative);
+        assertEquals(userFromNative.name, name);
+        assertTrue(userFromNative.balance == balance);
+    }
+    
+    @Test
+    public void whenNativeCallingObjectMethod_thenResultIsAsExpected() {
+        String name = "Sergio Ramos";
+        double balance = 666.77;
+        ExampleObjectsJNI objectsNativeMethods = new ExampleObjectsJNI();
+        UserData userData = new UserData();
+        userData.name = name;
+        userData.balance = balance;
+        assertEquals(objectsNativeMethods.printUserData(userData), "[name]=" + name + ", [balance]=" + balance);
+    }
+    
+}