仿写一个Java Thread 底层实现

Java里的线程和操作系统的线程是一一对应的，为了方便理解Thread 类的底层实现，我们可以自己来仿造一个MyThread, 同样接受一个Runable接口，调用start实现run方法。

这是要实现的类

public class MyThread {
    Runnable mRunnable;
    public MyThread(Runnable runnable) {
        mRunnable = runnable;
    }
    native public void start();
}

那么我们先去实现native方法

#define TAG    "native-lib" // 这个是自定义的LOG的标识
#define LOGD(...)  __android_log_print(ANDROID_LOG_DEBUG,TAG,__VA_ARGS__) // 定义LOGD类型

JavaVM *jvm = 0;
jclass RunnableClass = NULL;
extern "C"
JNIEXPORT void JNICALL
Java_com_test_MyThread_start(JNIEnv *env, jobject thiz) {
    LOGD("java invoke start()---------------start");
    jclass MyThreadClass = env->FindClass("com/test/MyThread");
    RunnableClass = (jclass) env->NewGlobalRef(env->FindClass("java/lang/Runnable"));
    jfieldID runnableID = env->GetFieldID(MyThreadClass, "mRunnable", "Ljava/lang/Runnable;");
    jobject mRunnable = env->NewGlobalRef(env->GetObjectField(thiz, runnableID));

    pthread_t tid;
    int ret_t = -1;
    ret_t = pthread_create(&tid, NULL, new_thread, mRunnable);
    pthread_detach(tid);
    if (ret_t != 0) {
        printf("pthread_create error \n");
        return;
    }
    LOGD("java invoke start()---------------end");
    return;
}

java 调用了start方法进入 c 层面，接下来 主要是通过pthread创建新线程，在新线程里面反调java里面的run方法。

新建的线程 void *new_thread(void *arg)

void *new_thread(void *arg) {
    LOGD("new_thread-----------------");
    JNIEnv *env;
    jobject mRunnable = static_cast<jobject>(arg);
    LOGD("new_thread-----------------AttachCurrentThread()");
    //将当前线程注册到虚拟机中
    jvm->AttachCurrentThread(&env, NULL);
    jmethodID run = NULL;
    LOGD("new_thread-----------------GetMethodID");
    // 拿到run方法
    run = env->GetMethodID(RunnableClass, "run", "()V");
    LOGD("new_thread-----------------CallVoidMethod");
    env->CallVoidMethod(mRunnable, run);
    env->DeleteGlobalRef(mRunnable);
    LOGD("new_thread-----------------end");
    //返回值一定要
    return nullptr;
}

加载库的时候做一下初始化

/*加载库的时候，初始化JavaVM对象，全局复用*/
jint JNI_OnLoad(JavaVM *vm, void *reserved) {
    JNIEnv *env;
    if (JNI_OK != vm->GetEnv((void **) &env, JNI_VERSION_1_6)) {
        printf("JNI_OnLoad error \n");
        return -1;
    }
    jvm = vm;
    LOGD("JNI_OnLoad-----------------------------------");
    return JNI_VERSION_1_6;
}

最后在java 里面加载so文件

public class MyThread {
    Runnable mRunnable;

    static {
        System.loadLibrary("native-lib");
        System.out.println("load native-lib");
    }

    public MyThread(Runnable runnable) {
        mRunnable = runnable;
    }

    native public void start();
}

测试

new MyThread(new Runnable() {
            @Override
            public void run() {
                System.out.println("Hello World!");
            }
        }).start();

结果

D/native-lib: JNI_OnLoad-----------------------------------
I/System.out: load native-lib
D/native-lib: java invoke start()---------------start
    java invoke start()---------------end
D/native-lib: new_thread-----------------
    new_thread-----------------AttachCurrentThread()
    new_thread-----------------GetMethodID
    new_thread-----------------CallVoidMethod
I/System.out: Hello World!
D/native-lib: new_thread-----------------end