從JNI調用Java方法

Question

在某些C庫中存在具有簽名的函數：int someFunction（int a）;

我需要從該函數（從本機代碼）調用Java方法，以免破壞接口。 例如：

int someFunction(int a) { 
    ... 
    jbyteArray result = (jbyteArray)(*jEnv)->CallStaticObjectMethod(jEnv, clazz, methodId); 
    ... 
} 

我不能把任何額外的參數，像JNIEnv由JNI需要someFunction所以我聲明全局變量和函數初始化：

jmethodID globalMethodId; 
jclass globalClass; 
JavaVM* globalJVM; 

void initJNI(JNIEnv * env, jclass clazz) { 
    globalClass = (*env)->NewGlobalRef(env, clazz); 
    (*env)->GetJavaVM(env, &globalJVM); 

    globalMethodId = (*env)->GetStaticMethodID(env, clazz, "javaMethodName", "()[B"); 
} 

JNIEnv * GetJniEnv() { 
    JNIEnv * env; 
    if((*globalJVM)->AttachCurrentThread(globalJVM, &env, NULL) != JNI_OK) { 
     env = NULL; 
    } 
    return env; 
} 

和我的someFunction功能如下

int someFunction(int a) { 
    JNIEnv * jEnv = GetJniEnv(); 
    jbyteArray result = (jbyteArray)(*jEnv)->CallStaticObjectMethod(jEnv, globalClass, globalMethodId); 
    ... 
} 

這個新庫的用戶必須調用initJNI函數，然後調用一些函數。

它不是容易出錯的方式嗎？多線程怎麼樣？有更好的解決方案嗎？

Answer 1

我知道這是舊的，但人們仍然從搜索中找到它。

下面是完整的解決方案，如何從Java調用C代碼，並從JNI的C代碼回調Java代碼：

項目中的JNI目錄

LOCAL_PATH := $(call my-dir) 
include $(CLEAR_VARS) 
LOCAL_ALLOW_UNDEFINED_SYMBOLS=false 
LOCAL_MODULE := testjni 
LOCAL_SRC_FILES := testjni.c 
LOCAL_LDLIBS := -llog -ljnigraphics 
include $(BUILD_SHARED_LIBRARY) 

創建應用程序創建Android.mk。 MK在您的項目JNI目錄

APP_ABI := armeabi-v7a 
APP_PLATFORM := android-8 

項目中的JNI DIR生成testjni.c

#include <jni.h> 
#include <string.h> 
#include <sys/stat.h> 
#include <sys/types.h> 
#include <pthread.h> 

#include <android/log.h> 

#define LOGD(...) __android_log_print(ANDROID_LOG_DEBUG,"TAG",__VA_ARGS__) 

JavaVM *myVm; 

static void *android_gui = NULL; 

jint JNI_OnLoad(JavaVM *vm, void *reserved) 
{ 
    // Keep a reference on the Java VM. 
    myVm = vm; 

    LOGD("JNI interface loaded."); 

    return JNI_VERSION_1_2; 
} 


void jni_setDataEnv(JNIEnv *p_env, int width, int height) 
{ 
    if (android_gui == NULL) 
     return; 

    jclass cls = (*p_env)->GetObjectClass (p_env, android_gui); 

    jmethodID methodId = (*p_env)->GetMethodID (p_env, cls, "setData", "(II)V"); 

    //add or delete II's depinding your number or variable passsed 

    (*p_env)->CallVoidMethod (p_env, android_gui, methodId, width, height); 

    (*p_env)->DeleteLocalRef(p_env, cls); 
} 

void jni_setData(int width, int height) 
{ 

    JNIEnv *p_env; 

    (*myVm)->AttachCurrentThread (myVm, &p_env, NULL); 

    jni_setDataEnv(p_env, width, height); 

    int check = (*myVm)->GetEnv(myVm, (void *) &p_env, JNI_VERSION_1_2); 

    LOGD(" Detach destructor = %d", check); 

    if (check != JNI_EDETACHED) 
    { 

     int detach = (*myVm)->DetachCurrentThread (myVm); 

     LOGD("Detach result ==: %d", detach); 

    } 


} 

//change this to your package 

JNIEXPORT void JNICALL Java_com_example_test_LibTest_initM(JNIEnv *env, jobject thiz, jobject gui) 
{ 

    android_gui = (*env)->NewGlobalRef(env, gui); 

} 


//when you need , in your c code call jni_setData(100.200) and 
//MainActivity method , setData() will receive this values 

void *call_from_thread() 
{ 
    jni_setData(1,2); 
    return NULL; 
} 

JNIEXPORT void JNICALL Java_com_example_test_LibTest_test(JNIEnv *env, jobject thiz) 
{ 


    pthread_t t; 
    //do hard work in a thread 
    pthread_create(&t, NULL, call_from_thread, NULL); 


} 

在JNI文件夾運行NDK建造

編譯庫libtest.so

創建：MyInterface.java在Java的Android項目的src

package com.example.test; 

public interface MyInterface 
{ 

    void setData(int width, int height); 

} 

創建LibTest.java //用於發送數據從Java C代碼

package com.example.test; 

public class LibTest 
{ 

    static 
    { 
    System.loadLibrary("testjni"); 
    } 

    private static LibTest sInstance; 

    public static LibTest getInstance() 
    { 
     synchronized (LibTest.class) 
      { 
      if (sInstance == null) 
       { 
       /* First call */ 
       sInstance = new LibTest(); 
       } 
      } 

     return sInstance; 
    } 

    public native void initM(MyInterface myinterface); 

    public native void test(); 

} 

創建MainActivity.java

package com.example.test; 

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

public class MainActivity extends Activity implements MyInterface 
{ 
    int mHeight; 
    int mWidth; 

    LibTest libtest; 

    @Override 
    public void onCreate(Bundle savedInstanceState) 
    { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 

     libtest = LibTest.getInstance(); 

     libtest.initM(MainActivity.this); 


     //tell jni to send us some data 
     //we receive data in 'setData(int width, int height)' method 

     libtest.test(); 

    } 

@Override 
public void setData(int width, int height) 
{ 
    /// Here you receive data from C code 
    //We can store the values C send to Java so we use latter 

    mHeight = height; 
    mWidth = width; 

    Log.d("TAG" ,"GOT SOME DATA FROM C CODE" + mHeight + " " + mWidth); 


} 

}