C++ 全局调用Java方法

版权声明：本文为卫伟学习总结文章，转载请注明出处！
一、C++主线程调用Java方法

• 1. 根据jobject获取jclass（静态方法就不用这一步了）如: jclass clz = env->GetObjectClass(jobj);
• 2.获取jmethodid 如:jmethodid jmid = env->GetMethodID(clz,"onError","(ILjava/lang/String;)V");
• 3.调用方法如: jenv->CallVoidMethod(jobj,jmid,code,jmsg)

签名方法参数类型对应表

Java类型	类型签名
boolean	Z
byte	B
char	C
short	S
int	I
long	L
float	F
double	D
类	L全限定名；，比如String,其签名为Ljava/lang/util/String;
数组	[类型签名，比如[B

通过命令获取native方法签名
在native所在类的class文件目录下，执行命令:
javap -s ***.class

二、C++子线程调用Java方法
由于JniEnv是线程相关的,所以子线程中不能使用创建线程的JniEnv；而JVM是进程相关的，所以可以通过JVM来获取当前线程的JniEnv，然后就可以调用Java的方法了。

• 1.获取JVM对象: JNI_OnLoad(JavaVM* vm, void *reserved)
• 2. 通过JVM获取JniEnv:JNIEnv *env = jvm->AttachCurrentThread(&env,0);
     jvm->DetachCurrentThread();

C++全局调用Java方法Demo

//
// Created by weiwei on 2019/11/9.
//
#include "jni.h"

#ifndef MYMUSIC_JAVALISTENER_H
#define MYMUSIC_JAVALISTENER_H


class JavaListener {

public:
JavaVM *jvm;
_JNIEnv *jenv;
jobject jobj;
jmethodID  jmid;

public:
JavaListener(JavaVM *vm, _JNIEnv *env, jobject obj);
~JavaListener();

/**
 * 1：主线程
 * 2：子线程
 * @param type
 * @param code
 * @param msg
 */
void onError(int type, int code, const char *msg);

};


#endif //MYMUSIC_JAVALISTENER_H

JavaListener.cpp文件

//
// Created by weiwei on 2019/11/9.
//
#include "JavaListener.h"

JavaListener::JavaListener(JavaVM *vm, _JNIEnv *env, jobject obj) {

   jvm = vm;
   jobj = obj;
   jenv = env;
   jclass cls = env->GetObjectClass(jobj);

   if(!cls) {
      return;
   }

   jmid = env->GetMethodID(cls,"onError","(ILjava/lang/String;)V");

   if(!jmid)
     return;

}

void JavaListener::onError(int type, int code, const char *msg) {

   if(type == 0)
   {
  JNIEnv *env;
  jvm->AttachCurrentThread(&env, 0);
  jstring jmsg = env->NewStringUTF(msg);
  env->CallVoidMethod(jobj, jmid, code, jmsg);
  env->DeleteLocalRef(jmsg);

  jvm->DetachCurrentThread();
  }
  else if(type == 1)
 {
  jstring jmsg = jenv->NewStringUTF(msg);
  jenv->CallVoidMethod(jobj, jmid, code, jmsg);
  jenv->DeleteLocalRef(jmsg);
 }
} 

JNI实现代码

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

extern "C"
{
    #include <libavformat/avformat.h>
}

#include "queue"
#include "unistd.h"

pthread_t produc;
pthread_t custom;
pthread_mutex_t mutex;
pthread_cond_t cond;

#include "JavaListener.h"

JavaVM *jvm;
JavaListener *javaListener;

pthread_t childThread;

void *childCallback(void *data) {
   JavaListener *javaListener1 = (JavaListener *) data;

   javaListener1->onError(0,101,"c++ call java meid from child thread! haha");
   pthread_exit(&childThread);
}

extern "C"
JNIEXPORT void JNICALL
Java_com_day1_jado_mymusic_ThreadDemo_callbackFromC(JNIEnv *env, jobject instance) {
   javaListener = new JavaListener(jvm,env,env->NewGlobalRef(instance)); //得到全局的 instance
    //javaListener->onError(1,100,"c++ call java meid from main thread!");
   pthread_create(&childThread, NULL, childCallback, javaListener);
}

JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *vm, void* reserved)
{
   JNIEnv *env;
   jvm = vm;
   if(vm->GetEnv((void **) &env, JNI_VERSION_1_6) != JNI_OK)
   {
    return -1;
   }
   return JNI_VERSION_1_6;
}