今天說的程式碼是從java層呼叫c程式碼，然後再反調java程式碼的。這裡在java用的是靜態註冊，也就是jni方法名是根據java的檔案路徑生成的，不是動態註冊。

一、java呼叫jni，靜態註冊

先看：java註冊jni，以及呼叫jni函式：

package com.duicky;

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

/**
 * C 呼叫 Java 例子
 * 
 * @author luxiaofeng
 *
 */
public class MainActivity extends Activity {
	//也就是你mk配置檔案中的  LOCAL_MODULE    := NDK_03
	private static final String libSoName = "NDK_04";
	private static final String tag = "MainActivity";
	
	public static Context mContext = null;
	private Button btnClickStatic = null;
	private Button btnClick = null;
	
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        
        mContext = this;
        //初始化控制元件
        initViews();
    }
    
    /**
     * 初始化控制元件
     */
    private void initViews() {
        btnClick = (Button) this.findViewById(R.id.btn_click);
        btnClick.setOnClickListener(new OnClickListener() {
			
			public void onClick(View v) {
				sayHello();
			}
		});
        
        
        btnClickStatic = (Button) this.findViewById(R.id.btn_click_static);
        btnClickStatic.setOnClickListener(new OnClickListener() {
			
			public void onClick(View v) {
				getTime();
			}
		});
	}

    public native void getTime() ;
    
    public native void sayHello() ;
    
    /**
     * 載入JNI生成的so庫檔案
     */
    static {
        System.loadLibrary(libSoName);
    }
    
}
 

二、JNI的Android.mk

然後我們再看JNI的mk函式：

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_C_INCLUDES := $(LOCAL_PATH)/include
LOCAL_LDLIBS += -L$(SYSROOT)/usr/lib -llog


LOCAL_MODULE    := NDK_04
LOCAL_SRC_FILES := \
CToJava.c \
Provider.c

include $(BUILD_SHARED_LIBRARY)

三、jni函式

jni函式，當java傳下來，就帶有了JNIEnv參量

#include <string.h>
#include <android/log.h>
#include <jni.h>
#include "Provider.h"

JNIEnv* jniEnv;

/**
 *  Java 中 宣告的native getTime 方法的實現
 */
void Java_com_duicky_MainActivity_getTime(JNIEnv* env, jobject thiz)
{

	if(jniEnv == NULL) {
		jniEnv = env;
	}

	GetTime();
}

/**
 *  Java 中 宣告的native sayHello 方法的實現
 */
void Java_com_duicky_MainActivity_sayHello(JNIEnv* env, jobject thiz)
{
	if (jniEnv == NULL) {
		jniEnv = env;
	}

	SayHello();
}
 

下面這個類是c中的方法，以及獲取java類，和方法的初始化

#include "Provider.h"
#include <android/log.h>

extern JNIEnv* jniEnv;

jclass TestProvider;
jobject mTestProvider;
jmethodID getTime;
jmethodID sayHello;

int GetProviderInstance(jclass obj_class);

/**
 * 初始化 類、物件、方法
 */
int InitProvider() {

	__android_log_print(ANDROID_LOG_INFO, "JNIMsg", "InitProvider Begin  1" );

	if(jniEnv == NULL) {
		return 0;
	}

	if(TestProvider == NULL) {
		TestProvider = (*jniEnv)->FindClass(jniEnv,"com/duicky/TestProvider");
		if(TestProvider == NULL){
			return -1;
		}
		__android_log_print(ANDROID_LOG_INFO, "JNIMsg", "InitProvider Begin  2 ok" );
	}

	if (mTestProvider == NULL) {
		if (GetProviderInstance(TestProvider) != 1) {
			(*jniEnv)->DeleteLocalRef(jniEnv, TestProvider);
			return -1;
		}
		__android_log_print(ANDROID_LOG_INFO, "JNIMsg", "InitProvider Begin  3 ok" );
	}

	if (getTime == NULL) {
		getTime = (*jniEnv)->GetStaticMethodID(jniEnv, TestProvider, "getTime","()Ljava/lang/String;");
		if (getTime == NULL) {
			(*jniEnv)->DeleteLocalRef(jniEnv, TestProvider);
			(*jniEnv)->DeleteLocalRef(jniEnv, mTestProvider);
			return -2;
		}
		__android_log_print(ANDROID_LOG_INFO, "JNIMsg", "InitProvider Begin  4 ok" );
	}

	if (sayHello == NULL) {
		sayHello = (*jniEnv)->GetMethodID(jniEnv, TestProvider, "sayHello","(Ljava/lang/String;)V");
		if (sayHello == NULL) {
			(*jniEnv)->DeleteLocalRef(jniEnv, TestProvider);
			(*jniEnv)->DeleteLocalRef(jniEnv, mTestProvider);
			(*jniEnv)->DeleteLocalRef(jniEnv, getTime);
			return -3;
		}
		__android_log_print(ANDROID_LOG_INFO, "JNIMsg", "InitProvider Begin  5 ok" );
	}

	__android_log_print(ANDROID_LOG_INFO, "JNIMsg", "InitProvider Begin  6" );
	return 1;

}

int GetProviderInstance(jclass obj_class) {

	if(obj_class == NULL) {
		return 0;
	}

	jmethodID construction_id = (*jniEnv)->GetMethodID(jniEnv, obj_class,
			"<init>", "()V");

	if (construction_id == 0) {
		return -1;
	}

	mTestProvider = (*jniEnv)->NewObject(jniEnv, obj_class,
			construction_id);

	if (mTestProvider == NULL) {
		return -2;
	}

	return 1;
}

/**
 * 獲取時間 ---- 呼叫 Java 方法
 */
void GetTime() {
	if(TestProvider == NULL || getTime == NULL) {
		int result = InitProvider();
		if (result != 1) {
			return;
		}
	}

	jstring jstr = NULL;
	char* cstr = NULL;
	__android_log_print(ANDROID_LOG_INFO, "JNIMsg", "GetTime Begin" );
	jstr = (*jniEnv)->CallStaticObjectMethod(jniEnv, TestProvider, getTime);
	cstr = (char*) (*jniEnv)->GetStringUTFChars(jniEnv,jstr, 0);
	__android_log_print(ANDROID_LOG_INFO, "JNIMsg", "Success Get Time from Java , Value = %s",cstr );
	__android_log_print(ANDROID_LOG_INFO, "JNIMsg", "GetTime End" );

	(*jniEnv)->ReleaseStringUTFChars(jniEnv, jstr, cstr);
	(*jniEnv)->DeleteLocalRef(jniEnv, jstr);
}

/**
 * SayHello ---- 呼叫 Java 方法
 */
void SayHello() {
	if(TestProvider == NULL || mTestProvider == NULL || sayHello == NULL) {
		int result = InitProvider() ;
		if(result != 1) {
			return;
		}
	}

	jstring jstrMSG = NULL;
	jstrMSG =(*jniEnv)->NewStringUTF(jniEnv, "Hi,I'm From C");
	__android_log_print(ANDROID_LOG_INFO, "JNIMsg", "SayHello Begin" );
	(*jniEnv)->CallVoidMethod(jniEnv, mTestProvider, sayHello,jstrMSG);
	__android_log_print(ANDROID_LOG_INFO, "JNIMsg", "SayHello End" );

	(*jniEnv)->DeleteLocalRef(jniEnv, jstrMSG);
}

四、java函式

最後再反呼叫java的方法

package com.duicky;

public class TestProvider {

	public static String getTime() {
		LogUtils.printWithSystemOut( "Call From C Java Static Method"   );
		LogUtils.toastMessage(MainActivity.mContext, "Call From C Java Static Method"   );
		return String.valueOf(System.currentTimeMillis());
	}

	public void sayHello(String msg) {
		LogUtils.printWithSystemOut("Call From C Java Not Static Method ：" + msg);
		LogUtils.toastMessage(MainActivity.mContext, "Call From C Java Not Static Method ：" + msg);
	}

}