JNI-获取Java对象的成员变量-GeInttField()

例：

在Java中定义一个属性，然后用C语言将其设置成另外的值，并输出出来。

Java代码：

Person.java

package com.yuneec.demo;

public class Person {

    static {

        System.loadLibrary("td");

    }

    public int age = 20;

    public native void readAge();

    public int getAge() {

        return age;

    }

}

MainActivity.java

package com.yongdaimi.android.framework;

import com.yuneec.demo.Person;

import android.app.Activity;

import android.os.Bundle;

import android.util.Log;

import android.view.View;

import android.view.View.OnClickListener;

import android.widget.Button;

public class MainActivity extends Activity implements OnClickListener{

    private Button bt_test;

    private Person person;

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        bt_test = (Button) findViewById(R.id.bt_test);

        bt_test.setOnClickListener(this);

        person = new Person();

    }

    @Override

    public void onClick(View v) {

        switch (v.getId()) {

        case R.id.bt_test:

            // called by native

            person.readAge();

            // called by java

            int java_age = person.getAge();

            Log.i("xp.chen", "java_age is: "+java_age);

            break;

        default:

            break;

        }

    }

}

C代码;

test_jni.c

#ifdef __ANDROID__

#include <jni.h>

#include <android/log.h>

#include <stdio.h>

#include "jnihelp.h"

#define LOG_TAG "xp.chen"

#ifdef LOG_TAG

    #define LOGE(...)  __android_log_print(ANDROID_LOG_ERROR,LOG_TAG,__VA_ARGS__)

#else

    #define LOGE(...)

#endif

JNIEXPORT void com_yuneec_demo_Person_readAge(JNIEnv *env, jobject obj)

{

    //获致obj中对象的class

    jclass clazz = (*env)->GetObjectClass(env, obj);

    // 获取java中age字段的ID(最后一个参数是age的签名)

    jfieldID id_age = (*env)->GetFieldID(env, clazz, "age", "I");

    // 获取age字段对应的值

    jint age = (*env)->GetIntField(env, obj, id_age);

    // 输出

    // printf("current age is: %d\n", number);

    LOGE("native-age is: %d\n", age);

    // 重新修改age字段的值为100。

    (*env)->SetIntField(env, obj, id_age, );

}

static JNINativeMethod methods[] = {

     {"readAge","()V",(void*)com_yuneec_demo_Person_readAge},

};

static const char *classPathName = "com/yuneec/demo/Person";

static int registerNativeMethods(JNIEnv* env, const char* className,JNINativeMethod* gMethods, int numMethods)

{

    jclass clazz;

    clazz = (*env)->FindClass(env, className);

    if (clazz == NULL){

        return JNI_FALSE;

    }

   if ((*env)->RegisterNatives(env, clazz, gMethods, numMethods) < )

    {

        return JNI_FALSE;

    }

    return JNI_TRUE;

}

void JNI_OnUnload(JavaVM* vm, void* reserved)

{

    return;

}

jint JNI_OnLoad(JavaVM* vm, void* reserved)

{

    JNIEnv *env;

    if ((*vm)->GetEnv(vm, (void**) &env, JNI_VERSION_1_4) != JNI_OK) {

        return -;

    }

    if (JNI_TRUE != registerNativeMethods(env, classPathName,methods, sizeof(methods) / sizeof(methods[])))

    {

        return -;

    }

    return JNI_VERSION_1_4;

}

#endif

编译成功后，在eclipse中的运行结果为：

第一个是在C代码中读取到的Java中成员变量age的值。

第二个是在Java代码中读取到成员变量的值(在Native层作了修改)。