ndk之C组件调用java方法和属性

     在ndk中,使用JNI_OnLoad方法进行java本地方法与C语言组件方法进行一一映射,然后使用C组件方法调用java的静态方法与非静态方法,静态属性与非静态属性。

1.在eclipse新建androidNdkC的android工程,修改MainActivity.java代码如下

 

package com.undergrowth.androidndkc;

import com.undergrowth.android.R;

import android.os.Bundle;
import android.app.Activity;
import android.util.Log;

public class MainActivity extends Activity {

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		 Manager manager=new Manager(MainActivity.this, 10, 20);
	        manager.managerDisplay();
	        Log.d("msg", MainActivity.class.getName());
	}


}

含有本地方法的类 Manager.java

package com.undergrowth.androidndkc;

import android.content.Context;
import android.util.Log;
/*
 * 用于提供静态方法和非静态方法
 * 用于提供静态成员与非静态成员
 * 提供本地方法
 */
public class Manager {
	private Context context;//用于接受创建对象的上下文
	public int numNot;
	public static int num;
	
	public Manager(Context context,int num,int numNot)
	{
		this.context=context;
		this.num=num;
		this.numNot=numNot;
		Log.d("msg", Manager.class.getName()+" is created!");
		managerInit();
	}
	
	//提供静态方法与非静态方法
	public static void managerStaticMethod(int value)
	{
		Log.d("msg", "managerStaticMethod is called,value from c,value is "+value);
	}
	
	public  void managerNotStaticMethod(int value)
	{
		Log.d("msg", "managerNotStaticMethod is called,value from c,value is "+value);
	}
	
	public native void managerInit(); //本地方法用于初始化
    public native void managerDisplay(); //本地方法 用于显示 即在c语言中调用java代码
    
    //加载共享库 
    static{
    	try {
    		System.loadLibrary("manager");
		} catch (Exception e) {
			e.printStackTrace();
		}
    }
}

2.然后在项目androidNdkC中新建jni文件夹 新建android.mk文件

LOCAL_PATH    :=$(call my-dir)
include $(CLEAR_VARS)
LOCAL_LDLIBS    :=-llog
LOCAL_MODULE    :=manager
LOCAL_SRC_FILES    :=manager.c
include $(BUILD_SHARED_LIBRARY) 

这里说一下LOCAL_LDLIBS   :=-llog  表示需要在链接时用到调试的liblog.so文件

  在jni中新建manager.c文件  内容如下

#include <jni.h>
#include <android/log.h>
#include <stdio.h>
jclass cla;
jmethodID aMethodId1,aMethodId2;
jfieldID aFieldId1,aFieldId2;


static void init(JNIEnv *env,jobject thiz)
{
    jclass clazz=(*env)->GetObjectClass(env,thiz); //get class byte code: object.getClass()
    cla=(jclass)(*env)->NewGlobalRef(env,clazz);
    aMethodId1=(*env)->GetStaticMethodID(env,clazz,"managerStaticMethod","(I)V");//get static method
    aMethodId2=(*env)->GetMethodID(env,clazz,"managerNotStaticMethod","(I)V");//get non-static method
    aFieldId1=(*env)->GetStaticFieldID(env,clazz,"num","I"); //get static field
        aFieldId2=(*env)->GetFieldID(env,clazz,"numNot","I");  //get non-static field
}

static void display(JNIEnv *env,jobject thiz)
{
    int num=(int)(*env)->GetStaticObjectField(env,cla,aFieldId1);
    int numNot=(int)(*env)->GetObjectField(env,thiz,aFieldId2);
    (*env)->CallStaticVoidMethod(env,cla,aMethodId1,(num+numNot));
    (*env)->CallVoidMethod(env,thiz,aMethodId2,(num-numNot));
}



static JNINativeMethod methods[]=
{
    {"managerInit","()V",(void *)init },
    {"managerDisplay","()V",(void *)display},    
};


jint register_native_method(JNIEnv *env)
{
    static char *className="com/undergrowth/androidndkc/Manager";
    jclass clazz;
    clazz=(*env)->FindClass(env,className);//similar to java reflect: Class.forName()
    if(clazz==NULL)
    {
        __android_log_print(ANDROID_LOG_DEBUG,"msg","can't find %s class.",className);
        return -1;
    }
    if((*env)->RegisterNatives(env,clazz,methods,sizeof(methods)/sizeof(methods[0]))!=JNI_OK)
    {
        __android_log_print(ANDROID_LOG_DEBUG,"msg","can not register native.");
        return -1;
    }
    return 0;

}


//in order to use JNI_VERSION_1_4 ,you must JNIEXPORT JNI_OnLoad and return JNI_VERSION_1_4
 jint JNI_OnLoad(JavaVM *vm,void *reserved) //when the native library load,the function willbe called by java vm
{
    int result=-1; //-1 presentative unknown error
    JNIEnv *env=NULL;  //current thread jni function interface
    if((*vm)->GetEnv(vm,(void **)&env,JNI_VERSION_1_4)!=JNI_OK)
    {
        __android_log_print(ANDROID_LOG_DEBUG,"msg","JNI_OnLoad init error!!");
        return result;
    }
    if(register_native_method(env)!=0)
    {
        __android_log_print(ANDROID_LOG_DEBUG,"msg","register native failed!!");
        return result;
    }
    result=JNI_VERSION_1_4;
    return result;
}

然后在androidNdkC的目录中使用ndk-build工具 编译androidNdkC  显示信息如下:

 

u1@u1:~/java/workspace/androidNdkC$ ndk-build
Compile thumb  : manager <= manager.c
SharedLibrary  : libmanager.so
Install        : libmanager.so => libs/armeabi/libmanager.so

  
3.运行 在LogCat中显示如下信息

 

4.上面即是成功的效果 嗯 在整个过程中出了一些错误 如下

08-28 15:12:37.916: W/dalvikvm(6885): JNI WARNING: jclass arg has wrong type
 (expected Ljava/lang/Class;, got Lcom/undergrowth/androidndkc/Manager;)
08-28 15:12:37.944: W/dalvikvm(6885):   
           in Lcom/undergrowth/androidndkc/Manager;.managerDisplay:()V (GetStaticObjectField)

嗯 出现如上错误 是因为 (*env)->GetStaticObjectField(env,cla,aFieldId1); 写成了 (*env)->GetStaticObjectField(env,thiz,aFieldId1);

   在获取静态成员或者静态方法的时候需要使用类的字节码才能获取

08-28 15:21:18.015: W/dalvikvm(7235): JNI WARNING: 
GetStaticObjectField for field 'num' of expected type L, got I
08-28 15:21:18.025: W/dalvikvm(7235):          
    in Lcom/undergrowth/androidndkc/Manager;.managerDisplay:()V (GetStaticObjectField)

出现这个错误 是因为版本问题 我试过 相同的代码 用2.2 2.3.3 4.0.3只有在2.2上是没有问题的 同时我也把数据类型换为其他的,也是相同的问题,所以想要上面的代码成功执行,必须要使用android2.2版本

JNI的函数查询: http://game.ceeger.com/Script/AndroidJNI/AndroidJNI.GetStaticFieldID.html