转自：

1、定义native方法并加载动态链接库：

public class HelloJni extends Activity{    @Override    public void onCreate(Bundle savedInstanceState)    {        super.onCreate(savedInstanceState);        TextView  tv = new TextView(this);        tv.setText( stringFromJNI() );        setContentView(tv);    }    public native String  stringFromJNI();    public native String  unimplementedStringFromJNI();    static {        System.loadLibrary("hello-jni");    }}

2、实现native方法：

#include 
    
     #include 
     
      jstringJava_com_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env,jobject thiz ){return (*env)->NewStringUTF(env, "Hello from JNI !"); //in creturn env->NewStringUTF("Hello from JNI !");         //in c++}
     
    

若要定义静态方法：

JNIEXPORT jstring JNICALL Java_com_example_hellojni_HelloJni_stringFromJNI(JNIEnv * env, jclass clazz);

头文件可以用javah工具生成：

进入命令行，切换到项目的bin目录下的classes下面，执行 javah -classpath . -jni 包.类名。

或者：cd 到src目录中,执行 javah 包.类名

3、数据

原始数据类型：

引用数据类型：

4、字符窜的操作：

1）新建字符串：

jstring javaString;javaString = (*env)->NewStringUTF(env, "Hello World!");

2）把java string转换成c string

const jbyte* str;jboolean isCopy;str = (*env)->GetStringUTFChars(env, javaString, &isCopy);if (0 != str) {printf("Java string: %s", str);if (JNI_TRUE == isCopy) {printf("C string is a copy of the Java string.");} else {printf("C string points to actual string.");}(*env)->ReleaseStringUTFChars(env, javaString, str);

5、数组的操作：

1)新建java数组：

jintArray javaArray;javaArray = (*env)->NewIntArray(env, 10);if (0 != javaArray) {/* You can now use the array. */}

2）获取数组的值：

jint nativeArray[10];//将java array 复制到 c array(*env)->GetIntArrayRegion(env, javaArray, 0, 10, nativeArray);//将 c array 还原城 java array(*env)->SetIntArrayRegion(env, javaArray, 0, 10, nativeArray);

获取数组指针：

jint* nativeDirectArray;jboolean isCopy;nativeDirectArray = (*env)->GetIntArrayElements(env, javaArray, &isCopy);//...(*env)->ReleaseIntArrayElements(env, javaArray, nativeDirectArray, 0);

6、C\C++获取java成员变量

public class JavaClass {/** Instance field */private String instanceField = "Instance Field";/** Static field */private static String staticField = "Static Field";...}

1）获取Field ID

jclass clazz;clazz = (*env)->GetObjectClass(env, instance); jfieldID instanceFieldId;instanceFieldId = (*env)->GetFieldID(env, clazz,"instanceField", "Ljava/lang/String;"); jfieldID staticFieldId;staticFieldId = (*env)->GetStaticFieldID(env, clazz,"staticField", "Ljava/lang/String;");

2）获取Static Field

jstring staticField;staticField = (*env)->GetStaticObjectField(env, clazz, staticFieldId);

7、C\C++调用java函数

public class JavaClass {/*** Instance method.*/private String instanceMethod() {return "Instance Method";}/*** Static method.*/private static String staticMethod() {return "Static Method";}...}

1）获取Method ID：

成员函数：

jmethodID instanceMethodId;instanceMethodId = (*env)->GetMethodID(env, clazz,"instanceMethod", "()Ljava/lang/String;");

静态函数：

jmethodID staticMethodId;staticMethodId = (*env)->GetStaticMethodID(env, clazz,"staticMethod", "()Ljava/lang/String;");

2）调用：

jstring instanceMethodResult;instanceMethodResult = (*env)->CallStringMethod(env,instance, instanceMethodId);

变量和函数的描述符：

javap工具可以提取从编译过的class files 中提取这些描述符：

javap –classpath bin/classes –p –s com.example.hellojni.HelloJni

8、处理异常：

public class JavaClass {/*** Throwing method.*/private void throwingMethod() throws NullPointerException {　　 throw new NullPointerException("Null pointer");}/*** Access methods native method.*/private native void accessMethods();}

jthrowable ex;...(*env)->CallVoidMethod(env, instance, throwingMethodId);ex = (*env)->ExceptionOccurred(env);if (0 != ex) {(*env)->ExceptionClear(env);/* Exception handler. */}

从c\c++抛异常：

jclass clazz;...clazz = (*env)->FindClass(env, "java/lang/NullPointerException");if (0 ! = clazz) {(*env)->ThrowNew(env, clazz, "Exception message.");}

9、本地和全局变量：

1）本地变量在函数结束时自动释放内存：

jclass clazz;clazz = (*env)->FindClass(env, "java/lang/String");

2）新建全局变量：

jclass localClazz;jclass globalClazz;...localClazz = (*env)->FindClass(env, "java/lang/String");globalClazz = (*env)->NewGlobalRef(env, localClazz);...(*env)->DeleteLocalRef(env, localClazz);

3）释放全局变量：

(*env)->DeleteGlobalRef(env, globalClazz);

4）弱全局引用：

弱全局引用可以被内存自动回收：

jclass weakGlobalClazz;weakGlobalClazz = (*env)->NewWeakGlobalRef(env, localClazz);if (JNI_FALSE == (*env)->IsSameObject(env, weakGlobalClazz, NULL)) {/* Object is still live and can be used. */} else {/* Object is garbage collected and cannot be used. */}(*env)->DeleteWeakGlobalRef(env, weakGlobalClazz);

10、线程：

synchronized(obj) {/* Synchronized thread-safe code block. */}

if (JNI_OK == (*env)->MonitorEnter(env, obj)) {/* Error handling. */}/* Synchronized thread-safe code block. */if (JNI_OK == (*env)->MonitorExit(env, obj)) {/* Error handling. */}

native线程不能和java直接通信，必须先绑定到javaVM

JavaVM* cachedJvm;...JNIEnv* env;.../* Attach the current thread to virtual machine. */(*cachedJvm)->AttachCurrentThread(cachedJvm, &env, NULL);/* Thread can communicate with the Java applicationusing the JNIEnv interface. *//* Detach the current thread from virtual machine. */(*cachedJvm)->DetachCurrentThread(cachedJvm);