Android多线程分析之二:Thread的实现

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android多线程分析之二:Thread

罗朝辉 (http://blog.csdn.net/kesalin) CC 许可,转载请注明出处

在前文《Android多线程分析之一:使用Thread异步下载图像》中演示了如何使用 Thread 处理异步事务。示例中这个 Java Thread 类都是位于 Framework 层的类,它自身是通过 JNI 转调 dalvik 里面的 Thread 相关方法实现的。因此要分析 Androd 中的线程,就需要分析这两层中的与线程相关的代码,这就是本文要探讨的主题。本文将把 Framework 层中的 Java Thread 称为 Android 线程/Thread,而把 dalvik 中的  Thread 成为 dalvik 线程/Thread。 

本文涉及到的 Android 源码路径:
android/libcore/luni/src/main/java/java/lang/Runnable.java
android/libcore/luni/src/main/java/java/lang/Thread.java
android/libcore/luni/src/main/java/java/lang/ThreadGroup.java
android/libcore/luni/src/main/java/java/lang/VMThread.java
android/dalvik/vm/native/java_lang_VMThread.cpp
android/dalvik/vm/Thread.cpp

首先来分析 Android Thread,这个类的源码在android/libcore/luni/src/main/java/java/lang/Thread.java,它实现了 Runnable 接口。Runnable 只有一个无参无返回值的 void run() 的接口:

/**
 * Represents a command that can be executed. Often used to run code in a
 * different @link Thread.
 */
public interface Runnable 

    /**
     * Starts executing the active part of the class' code. This method is
     * called when a thread is started that has been created with a class which
     * implements @code Runnable.
     */
    public void run();

Android Thread 存在六种状态,这些状态定义在枚举 State 中,源码注释写的很清晰,在这里就不罗嗦了:

    /**
     * A representation of a thread's state. A given thread may only be in one
     * state at a time.
     */
    public enum State 
        /**
         * The thread has been created, but has never been started.
         */
        NEW,
        /**
         * The thread may be run.
         */
        RUNNABLE,
        /**
         * The thread is blocked and waiting for a lock.
         */
        BLOCKED,
        /**
         * The thread is waiting.
         */
        WAITING,
        /**
         * The thread is waiting for a specified amount of time.
         */
        TIMED_WAITING,
        /**
         * The thread has been terminated.
         */
        TERMINATED
    

Android Thread 类中一些关键成员变量如下:

    volatile VMThread vmThread;
    volatile ThreadGroup group;
    volatile boolean daemon;    
    volatile String name;
    volatile int priority;
    volatile long stackSize;
    Runnable target;
    private static int count = 0;
    private long id;
    ThreadLocal.Values localValues;

vmThread:可视为对 dalvik thread 的简单封装,Thread 类通过 VMThread 里面的 JNI 方法来调用 dalvik 中操作线程的方法,通过它的成员变量 thread 和 vmata,我们可以将 Android Thread 和 dalvik Thread 的关联起来;
group:每一个线程都属于一个group,当线程被创建时就会加入一个特定的group,当线程运行结束,会从这个 group 中移除;
daemon:当前线程是不是守护线程,守护线程只会在没有非守护线程运行的情况下才会运行;
priority:线程优先级,Java Thread 类的线程优先级取值范围为 [1, 10],默认优先级为 5;
stackSize:线程栈大小,默认为 0,即使用默认的线程栈大小(由 dalvik 中的全局变量 gDvm.stackSize 决定);
target:一个 Runnable 对象,Thread 的 run() 方法中会转调该 target 的 run() 方法,这是线程真正处理事务的地方;
id:线程 id,通过递增 count 得到该id,如果没有显式给线程设置名字,那么就会使用 Thread+id 当作线程的名字。注意这不是真正意义上的线程 id,即在 logcat 中打印的 tid 并不是这个 id,那 tid 是指 dalvik 线程的 id;
localValues:线程本地存储(TLS)数据;


接下来,我们来看Android Thread 的构造函数,大部分构造函数都是通过转调静态函数 create 实现的,下面来详细分析 create 这个关键函数:

    private void create(ThreadGroup group, Runnable runnable, String threadName, long stackSize) 
        Thread currentThread = Thread.currentThread();
        if (group == null) 
            group = currentThread.getThreadGroup();
        

        if (group.isDestroyed()) 
            throw new IllegalThreadStateException("Group already destroyed");
        

        this.group = group;

        synchronized (Thread.class) 
            id = ++Thread.count;
        

        if (threadName == null) 
            this.name = "Thread-" + id;
         else 
            this.name = threadName;
        

        this.target = runnable;
        this.stackSize = stackSize;

        this.priority = currentThread.getPriority();

        this.contextClassLoader = currentThread.contextClassLoader;

        // Transfer over InheritableThreadLocals.
        if (currentThread.inheritableValues != null) 
            inheritableValues = new ThreadLocal.Values(currentThread.inheritableValues);
        

        // add ourselves to our ThreadGroup of choice
        this.group.addThread(this);
    

首先,通过静态函数 currentThread 获取创建线程所在的当前线程,然后将当前线程的一些属性传递给即将创建的新线程。这是通过 VMThread 转调 dalvik 中的代码实现的。

    public static Thread currentThread() 
        return VMThread.currentThread();
    


VMThread 的 currentThread 是一个 native 方法,其 JNI 实现为 android/dalvik/vm/native/java_lang_VMThread.cpp 中的 Dalvik_java_lang_VMThread_currentThread 方法:

static void Dalvik_java_lang_VMThread_currentThread(const u4* args,
    JValue* pResult)

    UNUSED_PARAMETER(args);

    RETURN_PTR(dvmThreadSelf()->threadObj);

该方法里的 dvmThreadSelf() 方法定义在 android/dalvik/vm/Thread.cpp 中:
Thread* dvmThreadSelf()

    return (Thread*) pthread_getspecific(gDvm.pthreadKeySelf);

从上面的调用栈可以看到,每一个 dalvik 线程都会将自身存放在key 为 pthreadKeySelf 的线程本地存储中,获取当前线程时,只需要根据这个 key 查询获取即可,dalvik Thread 有一个名为 threadObj 的成员变量:

    /* the java/lang/Thread that we are associated with */
    Object*     threadObj;

dalvik Thread 这个成员变量 threadObj 关联的就是对应的 Android Thread 对象,所以通过 native 方法 VMThread.currentThread() 返回的是存储在 TLS 中的当前 dalvik 线程对应的 Android Thread。

接着分析上面的代码,如果没有给新线程指定 group 那么就会指定 group 为当前线程所在的 group 中,然后给新线程设置 name,priority 等。最后通过调用 ThreadGroup 的 addThread 方法将新线程添加到 group 中:

    /**
     * Called by the Thread constructor.
     */
    final void addThread(Thread thread) throws IllegalThreadStateException 
        synchronized (threadRefs) 
            if (isDestroyed) 
                throw new IllegalThreadStateException();
            
            threadRefs.add(new WeakReference<Thread>(thread));
        
    

ThreadGroup 的代码相对简单,它有一个名为 threadRefs 的列表,持有属于同一组的 thread 引用,可以对一组 thread 进行一些线程操作。

上面分析的是 Android Thread 的构造过程,从上面的分析可以看出,Android Thread 的构造方法仅仅是设置了一些线程属性,并没有真正去创建一个新的 dalvik Thread,dalvik Thread 创建过程要等到客户代码调用 Android Thread 的 start() 方法才会进行。下面我们来分析 Java Thread 的 start() 方法:

public synchronized void start() 

        if (hasBeenStarted) 
            throw new IllegalThreadStateException("Thread already started."); // TODO Externalize?
        

        hasBeenStarted = true;

        VMThread.create(this, stackSize);
    

Android Thread 的 start 方法很简单,仅仅是转调 VMThread 的 native 方法 create,其 JNI 实现为 android/dalvik/vm/native/java_lang_VMThread.cpp 中的 Dalvik_java_lang_VMThread_create 方法:

static void Dalvik_java_lang_VMThread_create(const u4* args, JValue* pResult)

    Object* threadObj = (Object*) args[0];
    s8 stackSize = GET_ARG_LONG(args, 1);

    /* copying collector will pin threadObj for us since it was an argument */
    dvmCreateInterpThread(threadObj, (int) stackSize);
    RETURN_VOID();

dvmCreateInterpThread 的实现在 Thread.cpp 中,由于这个函数的内容很长,在这里只列出关键的地方:

bool dvmCreateInterpThread(Object* threadObj, int reqStackSize)

    Thread* self = dvmThreadSelf();
    ...
    Thread* newThread = allocThread(stackSize); 
    newThread->threadObj = threadObj;
    ...
    Object* vmThreadObj = dvmAllocObject(gDvm.classJavaLangVMThread, ALLOC_DEFAULT);
    dvmSetFieldInt(vmThreadObj, gDvm.offJavaLangVMThread_vmData, (u4)newThread);
    dvmSetFieldObject(threadObj, gDvm.offJavaLangThread_vmThread, vmThreadObj);
    ...
    pthread_t threadHandle;
    int cc = pthread_create(&threadHandle, &threadAttr, interpThreadStart, newThread);

    /*
     * Tell the new thread to start.
     *
     * We must hold the thread list lock before messing with another thread.
     * In the general case we would also need to verify that newThread was
     * still in the thread list, but in our case the thread has not started
     * executing user code and therefore has not had a chance to exit.
     *
     * We move it to VMWAIT, and it then shifts itself to RUNNING, which
     * comes with a suspend-pending check.
     */
    dvmLockThreadList(self);

    assert(newThread->status == THREAD_STARTING);
    newThread->status = THREAD_VMWAIT;
    pthread_cond_broadcast(&gDvm.threadStartCond);

    dvmUnlockThreadList();
    ...


/*
 * Alloc and initialize a Thread struct.
 *
 * Does not create any objects, just stuff on the system (malloc) heap.
 */
static Thread* allocThread(int interpStackSize)

    Thread* thread;
    thread = (Thread*) calloc(1, sizeof(Thread));
    ...
    thread->status = THREAD_INITIALIZING;

首先,通过调用 allocThread 创建一个名为 newThread 的 dalvik Thread  并设置一些属性,将设置其成员变量 threadObj 为传入的 Android Thread,这样 dalvik Thread 就与Android Thread 关联起来了;然后创建一个名为 vmThreadObj 的 VMThread 对象,设置其成员变量 vmData 为 newThread,设置 Android Thread threadObj 的成员变量 vmThread 为这个 vmThreadObj,这样 Android Thread 通过 VMThread 的成员变量 vmData 就和 dalvik Thread 关联起来了。

然后,通过 pthread_create 创建 pthread 线程,并让这个线程 start,这样就会进入该线程的 thread entry 运行,下来我们来看新线程的 thread entry 方法 interpThreadStart,同样只列出关键的地方:

/*
 * pthread entry function for threads started from interpreted code.
 */
static void* interpThreadStart(void* arg)

    Thread* self = (Thread*) arg;

    std::string threadName(dvmGetThreadName(self));
    setThreadName(threadName.c_str());

    /*
     * Finish initializing the Thread struct.
     */
    dvmLockThreadList(self);
    prepareThread(self);

    while (self->status != THREAD_VMWAIT)
        pthread_cond_wait(&gDvm.threadStartCond, &gDvm.threadListLock);

    dvmUnlockThreadList();

    /*
     * Add a JNI context.
     */
    self->jniEnv = dvmCreateJNIEnv(self);

    /*
     * Change our state so the GC will wait for us from now on.  If a GC is
     * in progress this call will suspend us.
     */
    dvmChangeStatus(self, THREAD_RUNNING);

    /*
     * Execute the "run" method.
     *
     * At this point our stack is empty, so somebody who comes looking for
     * stack traces right now won't have much to look at.  This is normal.
     */
    Method* run = self->threadObj->clazz->vtable[gDvm.voffJavaLangThread_run];
    JValue unused;

    ALOGV("threadid=%d: calling run()", self->threadId);
    assert(strcmp(run->name, "run") == 0);
    dvmCallMethod(self, run, self->threadObj, &unused);
    ALOGV("threadid=%d: exiting", self->threadId);

    /*
     * Remove the thread from various lists, report its death, and free
     * its resources.
     */
    dvmDetachCurrentThread();

    return NULL;


/*
 * Finish initialization of a Thread struct.
 *
 * This must be called while executing in the new thread, but before the
 * thread is added to the thread list.
 *
 * NOTE: The threadListLock must be held by the caller (needed for
 * assignThreadId()).
 */
static bool prepareThread(Thread* thread)

    assignThreadId(thread);
    thread->handle = pthread_self();
    thread->systemTid = dvmGetSysThreadId();

    setThreadSelf(thread);
    ...

    return true;


/*
 * Explore our sense of self.  Stuffs the thread pointer into TLS.
 */
static void setThreadSelf(Thread* thread)

    int cc;

    cc = pthread_setspecific(gDvm.pthreadKeySelf, thread);
    ...

在新线程的 thread entry 方法 interpThreadStart 中,首先设置线程的名字,然后通过调用 prepareThread 设置线程 id 以及其它一些属性,并调用 setThreadSelf 将新 dalvik Thread 自身保存在 TLS 中,这样之后就能通过  dvmThreadSelf 方法从 TLS 中获取它。然后修改状态为 THREAD_RUNNING,并调用对应 Android Thread 的 run 方法,运行客户代码:

    public void run() 
        if (target != null) 
            target.run();
        
    


对于继承自 Android Thread 带有 Looper 的 Android HandlerThread 来说,会调用它覆写 run 方法():(关于 Looper 的话题下一篇会讲到,这里暂且略过)

    public void run() 
        mTid = Process.myTid();
        Looper.prepare();
        synchronized (this) 
            mLooper = Looper.myLooper();
            notifyAll();
        
        Process.setThreadPriority(mPriority);
        onLooperPrepared();
        Looper.loop();
        mTid = -1;
    

target 在前面已经做了介绍,它是线程真正处理逻辑事务的地方。 一旦逻辑事务处理完毕从 run 中返回,线程就会回到 interpThreadStart 方法中继续执行 dvmDetachCurrentThread 方法:

/*
 * Detach the thread from the various data structures, notify other threads
 * that are waiting to "join" it, and free up all heap-allocated storage.
 * /
void dvmDetachCurrentThread()

    Thread* self = dvmThreadSelf();
    Object* vmThread;
    Object* group;
    ...
    group = dvmGetFieldObject(self->threadObj, gDvm.offJavaLangThread_group);

    /*
     * Remove the thread from the thread group.
     */
    if (group != NULL) 
        Method* removeThread =
            group->clazz->vtable[gDvm.voffJavaLangThreadGroup_removeThread];
        JValue unused;
        dvmCallMethod(self, removeThread, group, &unused, self->threadObj);
    

    /*
     * Clear the vmThread reference in the Thread object.  Interpreted code
     * will now see that this Thread is not running.  As this may be the
     * only reference to the VMThread object that the VM knows about, we
     * have to create an internal reference to it first.
     */
    vmThread = dvmGetFieldObject(self->threadObj,
                    gDvm.offJavaLangThread_vmThread);
    dvmAddTrackedAlloc(vmThread, self);
    dvmSetFieldObject(self->threadObj, gDvm.offJavaLangThread_vmThread, NULL);

    /* clear out our struct Thread pointer, since it's going away */
    dvmSetFieldObject(vmThread, gDvm.offJavaLangVMThread_vmData, NULL);

    ...

    /*
     * Thread.join() is implemented as an Object.wait() on the VMThread
     * object.  Signal anyone who is waiting.
     */
    dvmLockObject(self, vmThread);
    dvmObjectNotifyAll(self, vmThread);
    dvmUnlockObject(self, vmThread);

    dvmReleaseTrackedAlloc(vmThread, self);
    vmThread = NULL;

    ...

    dvmLockThreadList(self);

    /*
     * Lose the JNI context.
     */
    dvmDestroyJNIEnv(self->jniEnv);
    self->jniEnv = NULL;

    self->status = THREAD_ZOMBIE;

    /*
     * Remove ourselves from the internal thread list.
     */
    unlinkThread(self);

    ...

    releaseThreadId(self);
    dvmUnlockThreadList();

    setThreadSelf(NULL);

    freeThread(self);


/*
 * Free a Thread struct, and all the stuff allocated within.
 */
static void freeThread(Thread* thread)

    ...
    free(thread);

在 dvmDetachCurrentThread 函数里,首先获取当前线程 self,这里获得的就是当前执行 thread entry 的新线程,然后通过其对应的 Android Thread 对象 threadObj 获取该对象所在 group,然后将 threadObj 这个 Android Thread 对象从 group 中移除;接着清除 Android 与 dalvik 线程之间的关联关系,并通知 join 该线程的其它线程;最后,设置线程状态为 THREAD_ZOMBIE,清除 TLS 中存储的线程值,并通过调用 freeThread 释放内存,至此线程就终结了。


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