Android多线程分析之二：Thread的实现

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() 方法中会转掉该 target 的 run() 方法，这是线程真正处理事务的地方；

id：Android 线程 id，通过递增 count 得到该 id，如果没有显示给线程设置名字，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：

1 2	`/* 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。