使用JNI在C中调用java代码

1.定义个类来定义java的native方法

public class DataProvider {

static{

System.loadLibrary("Hello");

}

public native void callCcode();

public native void callCcode2();

public native void callCcode3();

//C调用java空方法

public void helloFromJava(){

System.out.println("hello from java");

}

//C调用java中的带两个int参数的方法

public static int Add(int x,int y){

System.out.println("java "+ (x+y));

return x+y;

}

//C调用java中参数为string的方法

public void printString(String s){

System.out.println("java "+ s);

}

}

2.用javah编译那个native的类.

3.新建jni的文件夹.把刚刚编译生成的.h的头文件给拷贝进来.

4.在jni的文件夹下新建XX.c文件

5.编写c代码

#include

#include

#include "com_mccxxiv_ndk4_DemoActivity.h"

#include

#define LOG_TAG "System.out"

#define LOGD(...) __android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, __VA_ARGS__)

#define LOGI(...) __android_log_print(ANDROID_LOG_INFO, LOG_TAG, __VA_ARGS__)

JNIEXPORT void JNICALL Java_com_mccxxiv_ndk4_DataProvider_callCcode

(JNIEnv * env, jobject obj){

// new 出来java的对象 然后调用java对象里面的方法

char* classname = "com/mccxxiv/ndk4/DataProvider";

jclass clazz; //定义一个java中的类

clazz = (*env)->FindClass(env,classname); //该方法返回一个类

if(clazz==0){

LOGI("CAN'T FIND CLAZZ ");

}else{

LOGI("FIND CLASS");

}

//该方法返回指定类中的指定方法

//最后一个参数是方法的签名.

//获取方法的签名是用dos进到工程的classes目录下用javap -s 包名+类名

jmethodID java_method = (*env)->GetMethodID(env,clazz,"helloFromJava","()V");

if(java_method==0){

LOGI("CAN'T FIND java_method ");

}else{

LOGI("FIND java_method");

}

//调用java中无返回值的方法.第二个参数obj为类的对象.

//所以把native的方法放到和方法在同一个类中(DataProvider)

//如果native方法和c要调用的java方法不在同一个类中.在c代码中需调用AllocObject()方法来手动的指定obj对象

//方法的返回值不同调用的CallXXXMethod中的XXX不同.返回值是什么类型XXX就是什么.

//静态的方法也是一样的.XXX就是StaticXXX.少一个obj的参数.

(*env)->CallVoidMethod(env,obj,java_method);

}

JNIEXPORT void JNICALL Java_com_mccxxiv_ndk4_DataProvider_callCcode2

(JNIEnv * env, jobject obj){

char* classname = "com/mccxxiv/ndk4/DataProvider";

jclass clazz;

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

if(clazz==0){

LOGI("CAN'T FIND CLAZZ ");

}else{

LOGI("FIND CLASS");

}

jmethodID java_method = (*env)->GetMethodID(env,clazz,"Add","(II)I");

if(java_method==0){

LOGI("CAN'T FIND java_method ");

}else{

LOGI("FIND java_method");

}

jint result = (*env)->CallIntMethod(env,obj,java_method,5,8);

LOGI("c daima  %d",result);

}

JNIEXPORT void JNICALL Java_com_mccxxiv_ndk4_DataProvider_callCcode3

(JNIEnv * env, jobject obj){

char* classname = "com/mccxxiv/ndk4/DataProvider";

jclass clazz;

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

if(clazz==0){

LOGI("CAN'T FIND CLAZZ ");

}else{

LOGI("FIND CLASS");

}

jmethodID java_method = (*env)->GetMethodID(env,clazz,"printString","(Ljava/lang/String;)V");

if(java_method==0){

LOGI("CAN'T FIND java_method ");

}else{

LOGI("FIND java_method");

}

(*env)->CallVoidMethod(env,obj,java_method,(*env)->NewStringUTF(env,"haha from c"));

}

6.在jni文件夹下新建Android.mk文件

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS) #类似于工具初始化的操作

LOCAL_MODULE    := Hello

LOCAL_SRC_FILES := Hello.c

LOCAL_LDLIBS += -llog

include $(BUILD_SHARED_LIBRARY)

7.用ndk-build命令编译

8.在主Activity中调用native方法

public class DemoActivity extends Activity {

@Override

public void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.main);

}

public void click(View view){

DataProvider provider = new DataProvider();

provider.callCcode();

}

public void click2(View view){

DataProvider provider = new DataProvider();

provider.callCcode2();

}

public void click3(View view){

DataProvider provider = new DataProvider();

provider.callCcode3();

}

}