从 JNI 调用 Java 方法

Question

在某些 C 库中存在带有签名的函数：int someFunction(int a);

我需要从此函数（从本机代码）调用 Java 方法，以免破坏接口。 例如：

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

我不能在someFunction 中添加JNI 所需的任何额外参数，例如JNIEnv，因此我声明了用于初始化的全局变量和函数：

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;
}

而我的someFunction函数如下

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

并且这个新库的用户必须调用 initJNI 函数，然后调用 someFunction。

这不是容易出错的方式吗？多线程呢？有没有更好的解决方案。

Answer 1

我知道这是旧的，但人们仍然可以通过搜索找到它。

这里是完整的解决方案，如何从java调用C代码，以及从jni c代码回调java代码：

在你的项目 jni 目录中创建 Android.mk

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)

在你的项目 jni 目录中创建 Application.mk

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

在你的项目 jni 目录中创建 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 文件夹中运行 ndk-build 编译库 libtest.so

在您的 java android 项目 src 中创建：MyInterface.java

package com.example.test;

public interface MyInterface
{

    void setData(int width, int height);

}

创建LibTest.java //用于从java向C代码发送数据

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();

}

创建 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 );


}

}