理解Android线程创建流程

Posted by Gityuan on September 24, 2016

基于Android 6.0源码剖析,分析Android线程的创建过程

/android/libcore/libart/src/main/java/java/lang/Thread.java
/art/runtime/native/java_lang_Thread.cc
/art/runtime/native/java_lang_Object.cc
/art/runtime/thread.cc

/system/core/libutils/Threads.cpp
/system/core/include/utils/AndroidThreads.h
/frameworks/base/core/jni/AndroidRuntime.cpp

一.概述

Android线程,一般地就是指Android虚拟机线程,而虚拟机线程由是通过系统调用而创建的Linux线程。纯粹的Linux线程与虚拟机线程的区别在于虚拟机线程具有运行Java代码的runtime. 除了虚拟机线程,还有Native线程,对于Native线程有分为是否具有访问Java代码的两类线程。接下来,本文分析介绍这3类线程的创建过程。

二. Java线程

2.1 Thread.start

[-> Thread.java]

public synchronized void start() {
     checkNotStarted(); //保证线程只有启动一次
     hasBeenStarted = true;
     //[见流程2.2]
     nativeCreate(this, stackSize, daemon);
}

nativeCreate()这是一个native方法,那么其所对应的JNI方法在哪呢?在java_lang_Thread.cc中通过gMethods是一个JNINativeMethod数组,其中一项为:

NATIVE_METHOD(Thread, nativeCreate, "(Ljava/lang/Thread;JZ)V"),

这里的NATIVE_METHOD定义在java_lang_Object.cc文件,如下:

#define NATIVE_METHOD(className, functionName, signature) \
    { #functionName, signature, reinterpret_cast<void*>(className ## _ ## functionName) }

将宏定义展开并代入,可得所对应的方法名为Thread_nativeCreate,那么接下来进入该方法。

2.2 Thread_nativeCreate

[-> java_lang_Thread.cc]

static void Thread_nativeCreate(JNIEnv* env, jclass, jobject java_thread, 
                                  jlong stack_size, jboolean daemon) {
  //【见小节2.3】
  Thread::CreateNativeThread(env, java_thread, stack_size, daemon == JNI_TRUE);
}

2.3 CreateNativeThread

[-> thread.cc]

void Thread::CreateNativeThread(JNIEnv* env, jobject java_peer, size_t stack_size, bool is_daemon) {
  Thread* self = static_cast<JNIEnvExt*>(env)->self;
  Runtime* runtime = Runtime::Current();

  ...
  Thread* child_thread = new Thread(is_daemon);
  child_thread->tlsPtr_.jpeer = env->NewGlobalRef(java_peer);
  stack_size = FixStackSize(stack_size);

  env->SetLongField(java_peer, WellKnownClasses::java_lang_Thread_nativePeer,
                    reinterpret_cast<jlong>(child_thread));

  std::unique_ptr<JNIEnvExt> child_jni_env_ext(
      JNIEnvExt::Create(child_thread, Runtime::Current()->GetJavaVM()));

  int pthread_create_result = 0;
  if (child_jni_env_ext.get() != nullptr) {
    pthread_t new_pthread;
    pthread_attr_t attr;
    child_thread->tlsPtr_.tmp_jni_env = child_jni_env_ext.get();
    //创建线程【见小节2.4】
    pthread_create_result = pthread_create(&new_pthread,
                         &attr, Thread::CreateCallback, child_thread);

    if (pthread_create_result == 0) {
      child_jni_env_ext.release();
      return;
    }
  }
  
  ...
}

2.4 pthread_create

pthread_create是pthread库中的函数,通过syscall再调用到clone来请求内核创建线程。

  • 原型:int pthread_create((pthread_t thread, pthread_attr_t *attr, void *(start_routine)(void *), void *arg)
  • 头文件:#include
  • 输入参数:thread:线程标识符; attr:线程属性设置; start_routine:线程函数的起始地址; arg:传递给start_routine的参数;
  • 返回值:成功则返回0;出错则返回-1。
  • 功能:创建线程,并调用线程起始地址所指向的函数start_routine。

关于pthread_create的分析,在后续Linux系列文章会再进一步深入分析。

三. Native线程(C/C++)

3.1 Thread.run

[-> Threads.cpp]

status_t Thread::run(const char* name, int32_t priority, size_t stack)
{
    Mutex::Autolock _l(mLock);
    //保证只会启动一次
    if (mRunning) {
        return INVALID_OPERATION;
    }
    ...
    mRunning = true;

    bool res;
    
    if (mCanCallJava) {
        //还能调用Java代码的Native线程【见小节4.1】
        res = createThreadEtc(_threadLoop,
                this, name, priority, stack, &mThread);
    } else {
        //只能调用C/C++代码的Native线程【见小节3.2】
        res = androidCreateRawThreadEtc(_threadLoop,
                this, name, priority, stack, &mThread);
    }

    if (res == false) {
        ...//清理
        return UNKNOWN_ERROR;
    }
    return NO_ERROR;
}

mCanCallJava在Thread对象创建时,在构造函数中默认设置mCanCallJava=true.

  • 当mCanCallJava=true,则代表创建的是不仅能调用C/C++代码,还能能调用Java代码的Native线程
  • 当mCanCallJava=false,则代表创建的是只能调用C/C++代码的Native线程。

3.2 androidCreateRawThreadEtc

[-> Threads.cpp]

int androidCreateRawThreadEtc(android_thread_func_t entryFunction,
                               void *userData,
                               const char* threadName __android_unused,
                               int32_t threadPriority,
                               size_t threadStackSize,
                               android_thread_id_t *threadId)
{
    pthread_attr_t attr;
    pthread_attr_init(&attr);
    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);

    if (threadPriority != PRIORITY_DEFAULT || threadName != NULL) {
        thread_data_t* t = new thread_data_t;
        t->priority = threadPriority;
        t->threadName = threadName ? strdup(threadName) : NULL;
        t->entryFunction = entryFunction;
        t->userData = userData;
        entryFunction = (android_thread_func_t)&thread_data_t::trampoline;
        userData = t;
    }

    if (threadStackSize) {
        pthread_attr_setstacksize(&attr, threadStackSize);
    }

    errno = 0;
    pthread_t thread;
    //通过pthread_create创建线程
    int result = pthread_create(&thread, &attr,
                    (android_pthread_entry)entryFunction, userData);
    pthread_attr_destroy(&attr);
    if (result != 0) {
        ... //创建失败,则返回
        return 0;
    }

    if (threadId != NULL) {
        *threadId = (android_thread_id_t)thread; 
    }
    return 1;
}

此处entryFunction所指向的是由[小节3.1]传递进来的,其值为_threadLoop。

3.3 _threadLoop

[-> Threads.cpp]

int Thread::_threadLoop(void* user)
{
    //user是指Thread对象
    Thread* const self = static_cast<Thread*>(user);

    sp<Thread> strong(self->mHoldSelf);
    wp<Thread> weak(strong);
    self->mHoldSelf.clear();

    //该参数对于gdb调试很有作用
    self->mTid = gettid();

    bool first = true;
    do {
        bool result;
        if (first) {
            first = false;
            //首次运行时会调用readyToRun()做一些初始化准备工作
            self->mStatus = self->readyToRun();
            result = (self->mStatus == NO_ERROR);
            if (result && !self->exitPending()) {
                //
                result = self->threadLoop();
            }
        } else {
            result = self->threadLoop();
        }

        {
          Mutex::Autolock _l(self->mLock);
          if (result == false || self->mExitPending) {
              self->mExitPending = true;
              self->mRunning = false;
              self->mThread = thread_id_t(-1);
              self->mThreadExitedCondition.broadcast();
              break;
          }
        }

        strong.clear(); //释放强引用
        strong = weak.promote(); //重新请求强引用,用于下一次的循环
    } while(strong != 0);

    return 0;
}

不断循环地调用成员方法threadLoop()。当满足以下任一条件,则该线程将退出循环:

  1. 当前线程状态存在错误,即mStatus != NO_ERROR;
  2. 当前线程即将退出, 即mExitPending = true; 调用Thread::requestExit()可触发该过程。
  3. 当前线程的强引用释放后,无法将弱引用提升成强引用的情况。

对于Native线程的实现方法,往往是通过继承Thread对象,通过覆写父类的readyToRun()和threadLoop()完成自定义线程的功能。

四. Native线程(Java)

4.1 createThreadEtc

[-> AndroidThreads.h]

inline bool createThreadEtc(thread_func_t entryFunction,
                            void *userData,
                            const char* threadName = "android:unnamed_thread",
                            int32_t threadPriority = PRIORITY_DEFAULT,
                            size_t threadStackSize = 0,
                            thread_id_t *threadId = 0)
{
    //【见小节4.2】
    return androidCreateThreadEtc(entryFunction, userData, threadName,
        threadPriority, threadStackSize, threadId) ? true : false;
}

4.2 androidCreateThreadEtc

[-> Threads.cpp]

int androidCreateThreadEtc(android_thread_func_t entryFunction,
                            void *userData,
                            const char* threadName,
                            int32_t threadPriority,
                            size_t threadStackSize,
                            android_thread_id_t *threadId)
{
    //【见小节4.3】
    return gCreateThreadFn(entryFunction, userData, threadName,
        threadPriority, threadStackSize, threadId);
}

此处gCreateThreadFn默认指向androidCreateRawThreadEtc函数。 文章Android系统启动-zygote篇的小节[3.3.1]已介绍 通过androidSetCreateThreadFunc()方法,gCreateThreadFn指向javaCreateThreadEtc函数。

4.3 javaCreateThreadEtc

[-> AndroidRuntime.cpp]

int AndroidRuntime::javaCreateThreadEtc(
                               android_thread_func_t entryFunction,
                               void* userData,
                               const char* threadName,
                               int32_t threadPriority,
                               size_t threadStackSize,
                               android_thread_id_t* threadId)
{
   void** args = (void**) malloc(3 * sizeof(void*)); 
   int result;

   if (!threadName)
       threadName = "unnamed thread";

   args[0] = (void*) entryFunction;
   args[1] = userData;
   args[2] = (void*) strdup(threadName); 
   
   //【见小节4.4】
   result = androidCreateRawThreadEtc(AndroidRuntime::javaThreadShell, args,
       threadName, threadPriority, threadStackSize, threadId);
   return result;
}

4.4 androidCreateRawThreadEtc

[-> Threads.cpp]

int androidCreateRawThreadEtc(android_thread_func_t entryFunction,
                               void *userData,
                               const char* threadName __android_unused,
                               int32_t threadPriority,
                               size_t threadStackSize,
                               android_thread_id_t *threadId)
{
    ...
    if (threadStackSize) {
        pthread_attr_setstacksize(&attr, threadStackSize);
    }
    
    ...
    //通过pthread_create创建线程
    int result = pthread_create(&thread, &attr,
                    (android_pthread_entry)entryFunction, userData);
    pthread_attr_destroy(&attr);
    
    ...
    return 1;
}

此处entryFunction所指向的是由[小节4.3]传递进来的AndroidRuntime::javaThreadShell,接下来,进入该方法。

4.5 javaThreadShell

[-> AndroidRuntime.cpp]

int AndroidRuntime::javaThreadShell(void* args) {
    void* start = ((void**)args)[0]; //指向_threadLoop
    void* userData = ((void **)args)[1]; //线程对象
    char* name = (char*) ((void **)args)[2]; //线程名    
    free(args);
    JNIEnv* env;
    int result;

    //hook虚拟机【见小节4.5.1】
    if (javaAttachThread(name, &env) != JNI_OK)
        return -1;

    // 调用_threadLoop()方法见小节4.5.2】
    result = (*(android_thread_func_t)start)(userData);

    //unhook虚拟机见小节4.5.3】
    javaDetachThread();
    free(name);

    return result;
}

该方法主要功能:

  1. 调用javaAttachThread():将当前线程hook到当前进程所在的虚拟机,从而既能执行C/C++代码,也能执行Java代码。
  2. 调用_threadLoop():执行当前线程的核心逻辑代码;
  3. 调用javaDetachThread():到此说明线程_threadLoop方法执行完成,则从当前进程的虚拟机中移除该线程。

4.5.1 javaAttachThread

[-> AndroidRuntime.cpp]

static int javaAttachThread(const char* threadName, JNIEnv** pEnv)
{
    JavaVMAttachArgs args;
    JavaVM* vm;
    jint result;

    vm = AndroidRuntime::getJavaVM();

    args.version = JNI_VERSION_1_4;
    args.name = (char*) threadName;
    args.group = NULL;
    // 将当前线程hook到当前进程所在的虚拟机
    result = vm->AttachCurrentThread(pEnv, (void*) &args);

    return result;
}

4.5.2 _threadLoop

[-> Threads.cpp]

int Thread::_threadLoop(void* user)
{
    ...
    do {
        if (first) {
            ...
            self->mStatus = self->readyToRun();
            result = (self->mStatus == NO_ERROR);
            if (result && !self->exitPending()) {
                result = self->threadLoop();
            }
        } else {
            result = self->threadLoop();
        }
        
        Mutex::Autolock _l(self->mLock);
        //当result=false则退出该线程
        if (result == false || self->mExitPending) {
            self->mExitPending = true;
            self->mRunning = false;
            self->mThread = thread_id_t(-1);
            self->mThreadExitedCondition.broadcast();
            break;
        }
        }

        //释放强引用,让线程有机会退出
        strong.clear();
        //再次获取强引用,用于下一轮循环
        strong = weak.promote();
    } while(strong != 0);
    return 0;
}

该过程与【小节3.3】完全一致,见上文。

4.5.3 javaDetachThread

[-> AndroidRuntime.cpp]

static int javaDetachThread(void)
{
    JavaVM* vm;
    jint result;

    vm = AndroidRuntime::getJavaVM();
    //当前进程的虚拟机中移除该线程
    result = vm->DetachCurrentThread();
    return result;
}

在创建Native进程的整个过程,涉及到JavaVM的AttachCurrentThread和DetachCurrentThread方法,都已深入虚拟机内部原理,本文就先讲到这里,不再深入,后续有精力再深入研究虚拟机,准备写一系列相关文章。

五. 总结

本文介绍了3类线程的创建过程,它们都有一个共同的特点,那就是真正的线程创建过程都是通过调用pthread_create方法(见小节[2.3],[3.2],[4.4]),该方法经过层层调用,最终都会进入clone系统调用,这是linux创建线程或进程的通用接口。

Native线程中是否可以执行Java代码的区别,在于通过javaThreadShell()方法从而实现在_threadLoop()执行前后增加分别将当前线程增加hook到虚拟机和从虚拟机移除的功能。调用过程:

android_thread_create

  1. Native线程(Java版):该过程相对比较复杂,见如上流程图:
  2. Native线程: 相对简单,只有上图中的紫色部分:thread.run -> androidCreateRawThreadEtc -> _threadLoop
  3. Java线程: Thread.start -> Thread_nativeCreate -> CreateNativeThread

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