［转载］—— Android JNI知识点

     public class MediaPlayer{   
         static {
                   System.loadLibrary("media_jni");
              }
     }

//#define LOG_NDEBUG 0

#define LOG_TAG "MediaPlayer-JNI"

jint JNI_OnLoad(JavaVM* vm, void* reserved)

{
    JNIEnv* env = NULL;
    jint result = -1;
    if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {
        LOGE("ERROR: GetEnv failed\n");
        goto bail;
    }
assert(env != NULL);
    if (register_android_media_MediaPlayer(env) < 0) {
        LOGE("ERROR: MediaPlayer native registration failed\n");
        goto bail;
    }
    if (register_android_media_MediaRecorder(env) < 0) {
        LOGE("ERROR: MediaRecorder native registration failed\n");
        goto bail;
    }
    if (register_android_media_MediaScanner(env) < 0) {
        LOGE("ERROR: MediaScanner native registration failed\n");
        goto bail;
}
    if (register_android_media_MediaMetadataRetriever(env) < 0) {
        LOGE("ERROR: MediaMetadataRetriever native registration failed\n");
        goto bail;
    }
    /* success -- return valid version number */
    result = JNI_VERSION_1_4;
bail:
    return result;
}

   if (register_android_media_MediaPlayer(env) < 0) {
        LOGE("ERROR: MediaPlayer native registration failed\n");
        goto bail;
    }

jint JNI_OnLoad(JavaVM* vm, void* reserved){
     JNIEnv* env = NULL;
     jint result = -1;
    if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {
          LOGE("ERROR: GetEnv failed\n");
          goto bail;
    }
}

jint JNI_OnLoad(JavaVM* vm, void* reserved){
　　JNIEnv* env = NULL;
    if (register_android_media_MediaPlayer(env) < 0) {
   }
}

//#define LOG_NDEBUG 0
#define LOG_TAG "MediaPlayer-JNI"
static JNINativeMethod gMethods[] = {
    {"setDataSource",   "(Ljava/lang/String;)V",
    (void *)android_media_MediaPlayer_setDataSource},
    {"setDataSource",    "(Ljava/io/FileDescriptor;JJ)V",
　　 (void *)android_media_MediaPlayer_setDataSourceFD},
    {"prepare",   "()V",     (void *)android_media_MediaPlayer_prepare},
    {"prepareAsync",   "()V",   (void *)android_media_MediaPlayer_prepareAsync},
    {"_start",   "()V",   (void *)android_media_MediaPlayer_start},
    {"_stop",   "()V",    (void *)android_media_MediaPlayer_stop},
    {"getVideoWidth",   "()I",      (void *)android_media_MediaPlayer_getVideoWidth},
    {"getVideoHeight",   "()I",      (void *)android_media_MediaPlayer_getVideoHeight},
    {"seekTo",          "(I)V",      (void *)android_media_MediaPlayer_seekTo},
    {"_pause",          "()V",       (void *)android_media_MediaPlayer_pause},
    {"isPlaying",        "()Z",       (void *)android_media_MediaPlayer_isPlaying},
    {"getCurrentPosition", "()I",  (void *)android_media_MediaPlayer_getCurrentPosition},
    {"getDuration",      "()I",     (void *)android_media_MediaPlayer_getDuration},
    {"_release",         "()V",      (void *)android_media_MediaPlayer_release},
    {"_reset",           "()V",        (void *)android_media_MediaPlayer_reset},
    {"setAudioStreamType","(I)V", (void *)android_media_MediaPlayer_setAudioStreamType},
    {"setLooping",      "(Z)V",      (void *)android_media_MediaPlayer_setLooping},
    {"setVolume",      "(FF)V",     (void *)android_media_MediaPlayer_setVolume},
    {"getFrameAt",     "(I)Landroid/graphics/Bitmap;",
    (void *)android_media_MediaPlayer_getFrameAt},
    {"native_setup",    "(Ljava/lang/Object;)V",
    (void *)android_media_MediaPlayer_native_setup},
    {"native_finalize",     "()V",    (void *)android_media_MediaPlayer_native_finalize},
};
static int register_android_media_MediaPlayer(JNIEnv *env){
    return AndroidRuntime::registerNativeMethods(env,
                "android/media/MediaPlayer", gMethods, NELEM(gMethods));
}
jint JNI_OnLoad(JavaVM* vm, void* reserved){
    if (register_android_media_MediaPlayer(env) < 0) {
        LOGE("ERROR: MediaPlayer native registration failed\n");
        goto bail;
    }
}

typedef struct {
const char* name;            /*Java中函数的名字*/         
const char* signature;      /*描述了函数的参数和返回值*/
void* fnPtr;               /*函数指针，指向C函数*/
} JNINativeMethod;

package com.hello.jnitest;
import android.app.Activity;
import android.os.Bundle;
public class jnitest extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        Nadd cal = new Nadd();
        setTitle("The Native Add Result is " + String.valueOf(cal.nadd(10, 19)));
    }
}

Nadd.java文件：
package com.hello.jnitest;
public class Nadd {
    static {
        System.loadLibrary("Nadd");
    }
    public native int nadd(int a, int b);
}

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_hello_jnitest_Nadd */
#ifndef _Included_com_hello_jnitest_Nadd
#define _Included_com_hello_jnitest_Nadd
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_hello_jnitest_Nadd
 * Method:    nadd
 * Signature: (II)I
 */
JNIEXPORT jint JNICALL Java_com_hello_jnitest_Nadd_nadd
(JNIEnv *, jobject, jint, jint);
#ifdef __cplusplus
}
#endif
#endif

#include <stdlib.h>
#include "com_hello_jnitest_Nadd.h"
JNIEXPORT jint JNICALL Java_com_hello_jnitest_Nadd_nadd(JNIEnv * env, jobject c, jint a, jint b){
   return (a+b);
}

# Copyright (C) 2009 The Android Open Source Project
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#      http://www.apache.org/licenses/LICENSE-2.0
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE    := myjni
LOCAL_SRC_FILES := myjni.c
include $(BUILD_SHARED_LIBRARY)

#include <string.h>
#include <jni.h>
jstring
Java_com_hello_NdkTest_NdkTest_stringFromJNI( JNIEnv* jobject thiz ){
    return (*env)->NewStringUTF(env, "Hello from My-JNI !");
}

package com.hello.NdkTest;
import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;
public class NdkTest extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        TextView  tv = new TextView(this);
        tv.setText( stringFromJNI() );
        setContentView(tv);
    }
    public native String  stringFromJNI();
    static {
        System.loadLibrary("myjni");
    }
}