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package com.example.hellojni;
import android.app.Activity;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.os.Bundle;
import android.view.View.OnClickListener;

public class HelloJni extends Activity
{    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState)
    {        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        Button btn = (Button)findViewById(R.id.btn);
        final TextView txtv = (TextView)findViewById(R.id.txtv);
        btn.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {                txtv.setText(stringFromJNI());//调用native函数

            }
        });
    }

    /* A native method that is implemented by the
     * 'hello-jni' native library, which is packaged
     * with this application.
     * 声明含有native关键词的函数，就可以在类中使用了。
     */
    public native String  stringFromJNI();

    /*
     * 该函数并没有在共享库中实现，但是仍然可以声明。
     * 没有实现的native函数也可以在类中声明，native方法仅在首次调用时才开始搜索。
     * 若没有找到该方法，会抛出java.lang.UnsatisfiedLinkError异常
     */
    public native String  unimplementedStringFromJNI();

    /* this is used to load the 'hello-jni' library on application
     * startup. The library has already been unpacked into
     * /data/data/com.example.HelloJni/lib/libhello-jni.so at
     * installation time by the package manager.
     * 使用静态方式再创建类时就载入共享库，该共享库（后面会介绍）在程序安装后
     * 位于/data/data/com.example.HelloJni/lib/libhello-jni.so
     */
    static {        System.loadLibrary("hello-jni");
    }
}

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/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_example_hellojni_HelloJni */

#ifndef _Included_com_example_hellojni_HelloJni
#define _Included_com_example_hellojni_HelloJni
#ifdef __cplusplus
extern "C" {#endif
/*
 * Class:     com_example_hellojni_HelloJni
 * Method:    stringFromJNI
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_example_hellojni_HelloJni_stringFromJNI
  (JNIEnv *, jobject);

/*
 * Class:     com_example_hellojni_HelloJni
 * Method:    unimplementedStringFromJNI
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_example_hellojni_HelloJni_unimplementedStringFromJNI
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

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#include <string.h>
#include <jni.h>
#include <stdio.h>

/* This is a trivial JNI example where we use a native method
 * to return a new VM String. See the corresponding Java source
 * file located at:
 *
 *   apps/samples/hello-jni/project/src/com/example/HelloJni/HelloJni.java
 */
jstring
Java_com_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env,
                                                  jobject thiz )
{    char msg[100];
    sprintf(msg,"Hello from JNI.");
    return (*env)->NewStringUTF(env, msg);
}

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LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE    := hello-jni
LOCAL_SRC_FILES := hello-jni2.c

include $(BUILD_SHARED_LIBRARY)

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Gdbserver      : [arm-linux-androideabi-4.4.3] libs/armeabi/gdbserver
Gdbsetup       : libs/armeabi/gdb.setup
Compile thumb  : hello-jni <= hello-jni.c
SharedLibrary  : libhello-jni.so
Install        : libhello-jni.so => libs/armeabi/libhello-jni.so

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#include <string.h>
#include <jni.h>
#include <stdio.h>

jstring getHelloString();

static JNINativeMethod gMethods[] = {    {    "stringFromJNI",
    "()Ljava/lang/String;",
    (void *)getHelloString
    }
};

static int nMethods = 1;
static JNIEnv *env = NULL;

jstring getHelloString()
{    char msg[100];
    sprintf(msg,"Hello from JNI.");
    return (*env)->NewStringUTF(env, msg);
}

jint JNI_OnLoad(JavaVM *vm,void *reserved){
    jint result = -1;
    jclass clz = NULL;
    if ((*vm)->GetEnv(vm,(void**) &env, JNI_VERSION_1_4) != JNI_OK){        return -1;
    }
    clz = (*env)->FindClass(env,"com/example/hellojni/HelloJni");
    if((*env)->RegisterNatives(env,clz,gMethods,nMethods) < 0) {        return -1;
    }
    return JNI_VERSION_1_4;//根据JNI规范，JNI_OnLoad必须返回版本号常量否则出错。
}

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`RegisterNatives` 用来注册一组native函数，其中使用到了 `JNINativeMethod` 结构，具体定义如下3：

    typedef struct {
        char *name; //Java代码中声明的native函数的名称

        char *signature; //对应Java代码层native函数的签名，下面会介绍

        void *fnPtr; //共享库中函数指针

    } JNINativeMethod;

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typedef const struct JNINativeInterface *JNIEnv;
const struct JNINativeInterface ... = {
    NULL,
    NULL,
    NULL,
    NULL,
    GetVersion,

    DefineClass,
    FindClass,

    FromReflectedMethod,
    FromReflectedField,
    ToReflectedMethod,

    GetSuperclass,
    IsAssignableFrom,

    ToReflectedField,
    ....//还有很多，这里略去
};

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//调用对象方法的函数原型
NativeType Call<type>Method(JNIEnv *env, jobject obj,jmethodID methodID, ...);
NativeType Call<type>MethodA(JNIEnv *env, jobject obj,jmethodID methodID, jvalue *args);
NativeType Call<type>MethodV(JNIEnv *env, jobject obj,jmethodID methodID, va_list args);
//对对象成员操作的函数原型
NativeType Get<type>Field(JNIEnv *env, jobject obj,jfieldID fieldID);
void Set<type>Field(JNIEnv *env, jobject obj, jfieldID fieldID,NativeType value);
//取得methodID,fieldId的函数原型
jmethodID GetMethodID(JNIEnv *env, jclass clazz,const char *name, const char *sig);
jfieldID GetFieldID(JNIEnv *env, jclass clazz,const char *name, const char *sig);

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jstring NewString(JNIEnv *env, const jchar *unicodeChars,jsize len);
void ReleaseStringChars(JNIEnv *env, jstring string,const jchar *chars);
const jchar * GetStringChars(JNIEnv *env, jstring string,jboolean *isCopy);
jsize GetStringLength(JNIEnv *env, jstring string);

jstring NewStringUTF(JNIEnv *env, const char *bytes);
void ReleaseStringUTFChars(JNIEnv *env, jstring string,const char *utf);
const char * GetStringUTFChars(JNIEnv *env, jstring string,jboolean *isCopy);
jsize GetStringUTFLength(JNIEnv *env, jstring string);

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int[] iarr; //基本类型数组
float[] farr;//基本类型数组
Object[] oarr;//引用类型数组，数组元素是Object
int[][] arr2;//引用类型数组，数组元素是 int[]

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jobjectArray get2DArray(jint size){     jobjectArray result;
     int i;
     jclass intArrCls = (*env)->FindClass(env, "[I");
     if (intArrCls == NULL) {         return NULL; /* exception thrown */
     }
     result = (*env)->NewObjectArray(env, size, intArrCls,
                                     NULL);
     if (result == NULL) {         return NULL; /* out of memory error thrown 可能遇到空间不足*/
     }
     for (i = 0; i < size; i++) {         jint tmp[256];  /* make sure it is large enough! */
         int j;
         jintArray iarr = (*env)->NewIntArray(env, size);
         if (iarr == NULL) {             return NULL; /* out of memory error thrown */
         }
         for (j = 0; j < size; j++) {             tmp[j] = i + j;
         }
         (*env)->SetIntArrayRegion(env, iarr, 0, size, tmp);
         (*env)->SetObjectArrayElement(env, result, i, iarr);
         (*env)->DeleteLocalRef(env, iarr);
     }
     return result;
}

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//GlobalReference
jobject NewGlobalRef(JNIEnv *env, jobject obj);
void DeleteGlobalRef(JNIEnv *env, jobject globalRef);
//LocalReference
void DeleteLocalRef(JNIEnv *env, jobject localRef);
jobject NewLocalRef(JNIEnv *env, jobject ref);
//WeakLocalReference
jweak NewWeakGlobalRef(JNIEnv *env, jobject obj);
void DeleteWeakGlobalRef(JNIEnv *env, jweak obj);
//通用的引用操作
jobject AllocObject(JNIEnv *env, jclass clazz);
jobject NewObject(JNIEnv *env, jclass clazz,jmethodID methodID, ...);
jclass GetObjectClass(JNIEnv *env, jobject obj);
jobjectRefType GetObjectRefType(JNIEnv* env, jobject obj);
jboolean IsSameObject(JNIEnv *env, jobject ref1,jobject ref2);