Android多線程分析之二：Thread的實現

Android多線程分析之二：Thread

羅朝輝 (http://blog.csdn.net/kesalin)CC 許可，轉載請注明出處

在前文《Android多線程分析之一：使用Thread異步下載圖像》中演示了如何使用 Thread 處理異步事務。示例中這個 Java Thread 類都是位於 Framework 層的類，它自身是通過 JNI 轉調 dalvik 裡面的 Thread 相關方法實現的。因此要分析 Androd 中的線程，就需要分析這兩層中的與線程相關的代碼，這就是本文要探討的主題。本文將把 Framework 層中的 Java Thread 稱為 Android 線程/Thread，而把 dalvik 中的 Thread 成為 dalvik 線程/Thread。

本文涉及到的 Android 源碼路徑：
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

首先來分析 Android Thread，這個類的源碼在android/libcore/luni/src/main/java/java/lang/Thread.java，它實現了 Runnable 接口。Runnable 只有一個無參無返回值的 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 存在六種狀態，這些狀態定義在枚舉 State 中，源碼注釋寫的很清晰，在這裡就不羅嗦了：

    /**
     * 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 類中一些關鍵成員變量如下：

    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：可視為對 dalvik thread 的簡單封裝，Thread 類通過 VMThread 裡面的 JNI 方法來調用 dalvik 中操作線程的方法，通過它的成員變量 thread 和 vmata，我們可以將 Android Thread 和 dalvik Thread 的關聯起來；
group：每一個線程都屬於一個group，當線程被創建時就會加入一個特定的group，當線程運行結束，會從這個 group 中移除；
daemon：當前線程是不是守護線程，守護線程只會在沒有非守護線程運行的情況下才會運行；
priority：線程優先級，Java Thread 類的線程優先級取值范圍為 [1, 10]，默認優先級為 5；
stackSize：線程棧大小，默認為 0，即使用默認的線程棧大小（由 dalvik 中的全局變量 gDvm.stackSize 決定）；
target：一個 Runnable 對象，Thread 的 run() 方法中會轉掉該 target 的 run() 方法，這是線程真正處理事務的地方；
id：線程 id，通過遞增 count 得到該id，如果沒有顯示給線程設置名字，那麼就會使用 Thread+id 當作線程的名字；
localValues：線程本地存儲（TLS）數據；

接下來，我們來看Android Thread 的構造函數，大部分構造函數都是通過轉調靜態函數 create 實現的，下面來詳細分析 create 這個關鍵函數：

    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，通過靜態函數 currentThread 獲取創建線程所在的當前線程，然後將當前線程的一些屬性傳遞給即將創建的新線程。這是通過 VMThread 轉調 dalvik 中的代碼實現的。

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

VMThread 的 currentThread 是一個 native 方法，其 JNI 實現為 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);
}

該方法裡的 dvmThreadSelf() 方法定義在 android/dalvik/vm/Thread.cpp 中：

Thread* dvmThreadSelf()
{
    return (Thread*) pthread_getspecific(gDvm.pthreadKeySelf);
}

從上面的調用棧可以看到，每一個 dalvik 線程都會將自身存放在key 為 pthreadKeySelf 的線程本地存儲中，獲取當前線程時，只需要根據這個 key 查詢獲取即可，dalvik Thread 有一個名為 threadObj 的成員變量：

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

dalvik Thread 這個成員變量 threadObj 關聯的就是對應的 Android Thread 對象，所以通過 native 方法 VMThread.currentThread() 返回的是存儲在 TLS 中的當前 dalvik 線程對應的 Android Thread。

接著分析上面的代碼，如果沒有給新線程指定 group 那麼就會指定 group 為當前線程所在的 group 中，然後給新線程設置 name，priority 等。最後通過調用 ThreadGroup 的 addThread 方法將新線程添加到 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));
        }
    }

ThreadGroup 的代碼相對簡單，它有一個名為 threadRefs 的列表，持有屬於同一組的 thread 引用，可以對一組 thread 進行一些線程操作。

上面分析的是 Android Thread 的構造過程，從上面的分析可以看出，Android Thread 的構造方法僅僅是設置了一些線程屬性，並沒有真正去創建一個新的 dalvik Thread，dalvik Thread 創建過程要等到客戶代碼調用 Android Thread 的 start() 方法才會進行。下面我們來分析 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 方法很簡單，僅僅是轉調 VMThread 的 native 方法 create，其 JNI 實現為 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 的實現在 Thread.cpp 中，由於這個函數的內容很長，在這裡只列出關鍵的地方：

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，通過調用 allocThread 創建一個名為 newThread 的 dalvik Thread 並設置一些屬性，將設置其成員變量 threadObj 為傳入的 Android Thread，這樣 dalvik Thread 就與Android Thread 關聯起來了；然後創建一個名為 vmThreadObj 的 VMThread 對象，設置其成員變量 vmData 為 newThread，設置 Android Thread threadObj 的成員變量 vmThread 為這個 vmThreadObj，這樣 Android Thread 通過 VMThread 的成員變量 vmData 就和 dalvik Thread 關聯起來了。

然後，通過 pthread_create 創建 pthread 線程，並讓這個線程 start，這樣就會進入該線程的 thread entry 運行，下來我們來看新線程的 thread entry 方法 interpThreadStart，同樣只列出關鍵的地方：

/*
 * 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);
    ...
}

在新線程的 thread entry 方法 interpThreadStart 中，首先設置線程的名字，然後通過調用 prepareThread 設置線程 id 以及其它一些屬性，並調用 setThreadSelf 將新 dalvik Thread 自身保存在 TLS 中，這樣之後就能通過 dvmThreadSelf 方法從 TLS 中獲取它。然後修改狀態為 THREAD_RUNNING，並調用對應 Android Thread 的 run 方法，運行客戶代碼：

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

target 在前面已經做了介紹，它是線程真正處理邏輯事務的地方。一旦邏輯事務處理完畢從 run 中返回，線程就會回到 interpThreadStart 方法中，繼續執行 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 函數裡，首先獲取當前線程 self，這裡獲得的就是當前執行 thread entry 的新線程，然後通過其對應的 Android Thread 對象 threadObj 獲取該對象所在 group，然後將 threadObj 這個 Android Thread 對象從 group 中移除；接著清除 Android 與 dalvik 線程之間的關聯關系，並通知 join 該線程的其它線程；最後，設置線程狀態為 THREAD_ZOMBIE，清除 TLS 中存儲的線程值，並通過調用 freeThread 釋放內存，至此線程就終結了。