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Android多线程分析之二：Thread的实现
Android多线程分析之二：Thread的实现
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;
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。