JNI系列——C文件中的方法调用Java中方法

2022-02-01 07:35:37

1.创建xxx.jni包并在该包下实现一些Java的方法，和要调用的本地方法

2.实现MainActivity中的按钮点击事件—即点击按钮调用本地的方法

3.在C文件中的方法中回调Java的方法

　　3.1：通过env获取字节码对象—>jclass (*FindClass)(JNIEnv*, const char*)

　　　　第二个参数为要回调Java方法所在类的全路径如：jclass claz = (*env)->FindClass(env, "com/rocky/jniccallbackdemo/jni/JNI");

　　3.2：通过字节码对象获取方法ID—>jmethodID (*GetMethodID)(JNIEnv*, jclass, const char*, const char*);

　　　　第二个参数是由3.1获取的字节码对象，第三个对象是要回调的Java方法名，第四个参数该Java方法签名，因为Java中有重载，需要此参数确定具体是哪个方法如：jmethodID methodId = (*env)->GetMethodID(env, claz, "helloFromJava", "()V")

　　　　注：对应方法签名，需要到工程目录下的bin/classes目录下执行：javap -s com.rocky.jniccallbackdemo.jni.JNI（该方法所在类的全类名）

　　3.3：通过字节码对象创建Java对象—>jobject (*AllocObject)(JNIEnv*, jclass);
　　　　如：jobject obj = (*env)->AllocObject(env, claz);

注：如果本地方法和要回调的Java方法在同一个类中，则第二参数就是本地方法传过来的第二个参数，即第三步可以可以省略。

　　3.4：通过Java对象回调Java方法

　　调用Java的无返回值方法：void (*CallVoidMethod)(JNIEnv*, jobject, jmethodID, ...)，如： (*env)->CallVoidMethod(env, clazz, methodId);

DEMO:

JNI.java

public class JNI

{

    static {

        System.loadLibrary("callback");

    }

    private Context mContext = null;

    public JNI(Context context)

    {

        this.mContext = context;

    }

    public void helloFromJava()

    {

        System.out.println("Hello From Java");

    }

    public void printString(String str)

    {

        System.out.println(str);

    }

    public void showToast(String str)

    {

        Toast.makeText(mContext, str, Toast.LENGTH_SHORT).show();

    }

    public native void callbackVoidMethod();

    public native void callbackStringMethod();

    public native void callbackIntMethod();

    public native void callbackShowToastMethod();

}

MainActivity.java

public class MainActivity extends Activity implements OnClickListener

{

    Context mContext = null;

    Button bt_callback_void = null;

    Button bt_callback_string = null;

    Button bt_callback_int = null;

    Button bt_callback_toast = null;

    JNI jni = null;

    @Override

    protected void onCreate(Bundle savedInstanceState)

    {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        mContext = this;

        jni = new JNI(mContext);

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

        bt_callback_void.setOnClickListener(this);

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

        bt_callback_int.setOnClickListener(this);

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

        bt_callback_string.setOnClickListener(this);

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

        bt_callback_toast.setOnClickListener(this);

    }

    public int add(int x, int y)

    {

        return x + y;

    }

    @Override

    public void onClick(View v)

    {

        switch (v.getId())

        {

        case R.id.bt_callback_void:

            jni.callbackVoidMethod();

            break;

        case R.id.bt_callback_int:

            jni.callbackIntMethod();

            break;

        case R.id.bt_callback_string:

            jni.callbackStringMethod();

            break;

        case R.id.bt_callback_toast:

            jni.callbackShowToastMethod();

            break;

        default:

            break;

        }

    }

}

callback.c

#include <jni.h>

#include <android/log.h>

#define LOG_TAG "rocky"

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

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

#define LOGW(...) __android_log_print(ANDROID_LOG_WARN, LOG_TAG, __VA_ARGS__)

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

#define LOGV(...) __android_log_print(ANDROID_LOG_VERBOSE, LOG_TAG, __VA_ARGS__)

JNIEXPORT void JNICALL Java_com_rocky_jniccallbackdemo_jni_JNI_callbackVoidMethod

  (JNIEnv *env, jobject clazz)

{

    //1.获取字节码对象  jclass      (*FindClass)(JNIEnv*, const char*);

    jclass claz = (*env)->FindClass(env, "com/rocky/jniccallbackdemo/jni/JNI");

    //2.通过字节码对象获取方法ID    jmethodID   (*GetMethodID)(JNIEnv*, jclass, const char*, const char*);

    jmethodID methodId = (*env)->GetMethodID(env, claz, "helloFromJava", "()V");

    //3.通过字节码对象创建Java对象(此处因为本地方法和回调方法在同一个类中，可以省略)

    //4.通过Java对象回调Java方法  void        (*CallVoidMethod)(JNIEnv*, jobject, jmethodID, ...);

    (*env)->CallVoidMethod(env, clazz, methodId);

}

/*

 * Class:     com_rocky_jniccallbackdemo_jni_JNI

 * Method:    callbackIntMethod

 * Signature: ()V

 */

JNIEXPORT void JNICALL Java_com_rocky_jniccallbackdemo_jni_JNI_callbackIntMethod

  (JNIEnv *env, jobject clazz)

{

    jclass claz = (*env)->FindClass(env, "com/rocky/jniccallbackdemo/MainActivity");

    jmethodID methodId = (*env)->GetMethodID(env, claz, "add", "(II)I");

    //3.通过字节码对象创建Java对象(此处为MainActivity对象）  jobject     (*AllocObject)(JNIEnv*, jclass);

    //    jobject obj = (*env)->AllocObject(env, claz);

        //4.通过Java对象回调Java方法  void        (*CallVoidMethod)(JNIEnv*, jobject, jmethodID, ...);

    jobject obj = (*env)->AllocObject(env, claz);

    jint res = (*env)->CallIntMethod(env, obj, methodId, , );

    LOGI("res = %d\n", res);

}

JNIEXPORT void JNICALL Java_com_rocky_jniccallbackdemo_jni_JNI_callbackStringMethod

  (JNIEnv *env, jobject clazz)

{

    jclass claz = (*env)->FindClass(env, "com/rocky/jniccallbackdemo/jni/JNI");

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

    jstring result = (*env)->NewStringUTF(env, "cccccccccccccc");

    (*env)->CallVoidMethod(env, clazz, methodId, result);

}

JNIEXPORT void JNICALL Java_com_rocky_jniccallbackdemo_jni_JNI_callbackShowToastMethod

  (JNIEnv *env, jobject clazz)

{

    jclass claz = (*env)->FindClass(env, "com/rocky/jniccallbackdemo/jni/JNI");

    jmethodID methodId = (*env)->GetMethodID(env, claz, "showToast", "(Ljava/lang/String;)V");

    jstring result = (*env)->NewStringUTF(env, "rocky--------rocky");

    (*env)->CallVoidMethod(env, clazz, methodId, result);

}

Android.mk

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE    := callback

LOCAL_SRC_FILES := callback.c

LOCAL_LDLIBS += -llog

include $(BUILD_SHARED_LIBRARY)

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