在真机上实践JNI

第一步，在Eclipse中新建一个Android工程，创建HelloWorldActivity

 

package com.spt3rd;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.view.View.OnClickListener;

public class HelloWorldActivity extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        mButton = (Button)findViewById(R.id.button1);
        mButton.setOnClickListener(new OnClickListener(){
                public void onClick(View v){ 
                        String s = printJNI("hello jni, I'm from Activity");
                }    
        }); 
    }   
    static
    {   
        //加载库文件
        System.loadLibrary("HelloWorldActivityJni");
    }   
    //声明原生函数 参数为String类型 返回类型为String
    private native String printJNI(String inputStr);
    private Button mButton;
}

 

第二步：生成共享库头文件

进入到代码根目录HelloWorld/

进入到bin中，执行：javah com.spt3rd.HelloWorldActivity

生成文件com_spt3rd_HelloWorldActivity.h

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_spt3rd_HelloWorldActivity */

#ifndef _Included_com_spt3rd_HelloWorldActivity
#define _Included_com_spt3rd_HelloWorldActivity
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_spt3rd_HelloWorldActivity
 * Method:    printJNI
 * Signature: (Ljava/lang/String;)Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_spt3rd_HelloWorldActivity_printJNI
  (JNIEnv *, jobject, jstring);

#ifdef __cplusplus
}
#endif
#endif

第三步：实现JNI函数

创建com_spt3rd_HelloWorldActivity.c文件

#include <jni.h>
#define LOG_TAG "HelloWorld"
#include <utils/Log.h>
/* Native interface, it will be call in java code */
JNIEXPORT jstring JNICALL Java_com_spt3rd_HelloWorldActivity_printJNI(JNIEnv *env, jobject obj,jstring inputStr)
{
　　LOGI("dufresne Hello World From libhelloworld.so!");
　　// 从 instring 字符串取得指向字符串 UTF 编码的指针
　　const char *str =
　　(const char *)(*env)->GetStringUTFChars( env,inputStr, JNI_FALSE );
　　LOGI("dufresne--->%s",(const char *)str);
　　// 通知虚拟机本地代码不再需要通过 str 访问 Java 字符串。
　　(*env)->ReleaseStringUTFChars(env, inputStr, (const char *)str );
　　return (*env)->NewStringUTF(env, "Hello World! I am Native interface");
}

/* This function will be call when the library first be load.
* You can do some init in the libray. return which version jni it support.
*/
jint JNI_OnLoad(JavaVM* vm, void* reserved)
{
　　void *venv;
　　LOGI("dufresne----->JNI_OnLoad!");
　　if ((*vm)->GetEnv(vm, (void**)&venv, JNI_VERSION_1_4) != JNI_OK) {
　　　　LOGE("dufresne--->ERROR: GetEnv failed");
　　　　return -1; 
　　} 
　　return JNI_VERSION_1_4;
}

第四步：创建jni的Android.mk

在根目录HelloWorld下创建jni文件夹，将com_spt3rd_HelloWorldActivity.h和com_spt3rd_HelloWorldActivity.c都复制到里面。

新建Android.mk

LOCAL_PATH:= $(call my-dir)
include $(CLEAR_VARS)
LOCAL_SRC_FILES:=com_spt3rd_HelloWorldActivity.c
LOCAL_C_INCLUDES := $(JNI_H_INCLUDE)
LOCAL_MODULE := libHelloWorldActivityJni
LOCAL_SHARED_LIBRARIES := libutils
LOCAL_PRELINK_MODULE := false
LOCAL_MODULE_TAGS := optional
include $(BUILD_SHARED_LIBRARY)

第五步：创建HelloWorld的Android.mk

# Copyright 2007-2008 The Android Open Source Project
LOCAL_PATH:= $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE_TAGS := optional
LOCAL_SRC_FILES := $(call all-java-files-under, src)
LOCAL_PACKAGE_NAME := HelloWorld
include $(BUILD_PACKAGE)
# This finds and builds the test apk as well, so a single make does both.
include $(call all-makefiles-under,$(LOCAL_PATH))

第六步，将整体移到Android源代码的packages/apps/下面

编译这个APK，会生成

HelloWorld.apk  和 libHelloWorldActivityJni.so

adb install HelloWorld.apk

adb push libHelloWorldActivityJni.so system/lib