看个关于日志的知识点
在jni中输出日志的时候：

#define TAG "JNI"
#define LOGI(...) __android_log_print(ANDROID_LOG_INFO, TAG, __VA_ARGS__)

然后就直接可以在jni中使用LOGI来输出日志了

提出问题：

如何在jni中开启一个新线程，并且回调到我们的MainActivity中的方法

每个线程的JNIEnv的实例是相互独立的，所以我们要在jni中开启新线程就必须为该线程创建JNIEnv实例

1.每个进程有一个JavaVM
2.每个线程有一个JNIEnv

MainActivity.java

    private native void setJNIEnv();

    private native void newThreadInJNI();

    private static void callStaticBack(int value)
    {
        Log.e("abc","from static----->"+value);

    }

    private  void callBack(int value)
    {
        Log.e("abc","from ----->"+value);
    }

1.在onCreate中调用setJNIEnv()方法
2.点击按钮调用newThreadInJNI()方法

其余两个方法是从jni新线程中回调的
native-lib.cpp

#include <jni.h>
#include <string>
#include<stdlib.h>


#include <android/log.h>
#include <assert.h>
#include <pthread.h>

#define TAG "HZP_JNI"
#define LOGI(...) __android_log_print(ANDROID_LOG_INFO, TAG, __VA_ARGS__)
// 获取数组的大小
#define NELEM(x) ((int) (sizeof(x) / sizeof((x)[0])))

JavaVM *javaVM=NULL;
jobject job=NULL;//相当于MainActivity

JNIEXPORT void JNICALL native_setJNIEnv
        (JNIEnv *env, jclass clazz) {
    env->GetJavaVM(&javaVM);

    //保存MainActivity对象
    job=env->NewGlobalRef(clazz);
}

void *start_thread(void * args)
{
    JNIEnv * env;
    if((javaVM->AttachCurrentThread(&env,NULL))!=JNI_OK)
    {
        LOGI("%s AttachCurrentThread error failed ",__FUNCTION__);
        return NULL;
    }
    jclass jcl;
    jmethodID  jsmd;
    jmethodID  jmd;
    jcl=env->GetObjectClass(job);
    if(jcl==NULL)
    {
        LOGI("findClass error....");
        goto  error;
    }


    LOGI("call static back begin");
    jsmd=env->GetStaticMethodID(jcl,"callStaticBack","(I)V");
    if(jsmd==NULL)
    {
        LOGI("GetStaticMethodID error....");
        goto  error;
    }
    env->CallStaticVoidMethod(jcl,jsmd,(int)((size_t)args));


    LOGI("call back begin");
    jmd=env->GetMethodID(jcl,"callBack","(I)V");
    if(jmd==NULL)
    {
        LOGI("GetMethodID error....");
        goto  error;
    }
    env->CallVoidMethod(job,jmd,(int)((size_t)args));



    error:
        if((javaVM->DetachCurrentThread())!=JNI_OK)
         {
           LOGI("%s DetachCurrentThread error failed ",__FUNCTION__);
         }
         pthread_exit(0);
}

JNIEXPORT void JNICALL native_newThreadInJNI
        (JNIEnv *env, jclass clazz) {

    pthread_t pt[5];

    for(int i=0;i<5;i++)
    {
        pthread_create(&pt[i],NULL,&start_thread,(void*)i);
    }

}
const JNINativeMethod gMethods[] = {
        {
                "setJNIEnv","()V",(void*)native_setJNIEnv
        },
        {
                "newThreadInJNI","()V",(void*)native_newThreadInJNI
        }
};
int registerNatives(JNIEnv* engv)
{
    LOGI("registerNatives begin");
    jclass  clazz;
    clazz = engv -> FindClass("com/example/huozhenpeng/myapplication/MainActivity");

    if (clazz == NULL) {
        LOGI("clazz is null");
        return JNI_FALSE;
    }

    if (engv ->RegisterNatives( clazz, gMethods, NELEM(gMethods)) < 0) {
        LOGI("RegisterNatives error");
        return JNI_FALSE;
    }

    return JNI_TRUE;
}


JNIEXPORT jint JNI_OnLoad(JavaVM* vm, void* reserved)
{

    LOGI("jni_OnLoad begin");

    JNIEnv* env = NULL;
    jint result = -1;

    if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {
        LOGI("ERROR: GetEnv failed\n");
        return -1;
    }
    assert(env != NULL);

    registerNatives(env);

    return JNI_VERSION_1_4;
}

我们用logcat看下日志：

hhh:~ huozhenpeng$ adb logcat -v threadtime -> ./log.txt

10-04 19:22:00.330 23213 23876 I HZP_JNI : call static back begin
10-04 19:22:00.330 23213 23879 I HZP_JNI : call static back begin
10-04 19:22:00.330 23213 23879 E abc     : from static----->4
10-04 19:22:00.330 23213 23876 E abc     : from static----->1
10-04 19:22:00.330 23213 23879 I HZP_JNI : call back begin
10-04 19:22:00.330 23213 23876 I HZP_JNI : call back begin
10-04 19:22:00.331 23213 23879 E abc     : from ----->4
10-04 19:22:00.331 23213 23876 E abc     : from ----->1
10-04 19:22:00.332 23213 23878 I HZP_JNI : call static back begin
10-04 19:22:00.332 23213 23878 E abc     : from static----->3
10-04 19:22:00.332 23213 23878 I HZP_JNI : call back begin
10-04 19:22:00.332 23213 23877 I HZP_JNI : call static back begin
10-04 19:22:00.332 23213 23878 E abc     : from ----->3
10-04 19:22:00.332 23213 23877 E abc     : from static----->2
10-04 19:22:00.332 23213 23877 I HZP_JNI : call back begin
10-04 19:22:00.333 23213 23877 E abc     : from ----->2
10-04 19:22:00.334 23213 23875 I HZP_JNI : call static back begin
10-04 19:22:00.335 23213 23875 E abc     : from static----->0
10-04 19:22:00.335 23213 23875 I HZP_JNI : call back begin
10-04 19:22:00.335 23213 23875 E abc     : from ----->0

//保存MainActivity对象
job=env->NewGlobalRef(clazz);
这块可能造成内存泄露，我们顶一个释放内存的方法：

private native void releaseGloableRefJNI();

 @Override
    protected void onDestroy() {
        super.onDestroy();
        releaseGloableRefJNI();
    }

const JNINativeMethod gMethods[] = {
      
        {
                "setJNIEnv","()V",(void*)native_setJNIEnv
        },
        {
                "newThreadInJNI","()V",(void*)native_newThreadInJNI
        }
        ,
        {
                "releaseGloableRefJNI","()V",(void*)native_releaseGloableRefJNI
        }

};

JNIEXPORT void JNICALL native_releaseGloableRefJNI
        (JNIEnv *env, jclass clazz) {

   env->DeleteGlobalRef(job);

}