Android多线程分析之二：Thread
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在前文《Android多线程分析之中的一个：使用Thread异步下载图像》中演示了怎样使用 Thread 处理异步事务。演示样例中这个 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。就须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 源代码路径：
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，这个类的源代码在android/libcore/luni/src/main/java/java/lang/Thread.java。它实现了 Runnable 接口。Runnable 仅仅有一个无參无返回值的 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，源代码凝视写的非常清晰。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例如以下：
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。我们能够将 Android Thread 和 dalvik Thread 的关联起来；
group：每个线程都属于一个group。当线程被创建时就会加入一个特定的group，当线程执行结束。会从这个 group 中移除；
daemon：当前线程是不是守护线程，守护线程仅仅会在没有非守护线程执行的情况下才会执行；
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，默觉得 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，假设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 并非这个 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，以下来具体分析 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();
}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);
}从上面的调用栈能够看到，每个 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，仅仅须要依据这个 key 查询获取就可以，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 关联的就是相应的 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 线程相应的 Android Thread。
接着分析上面的代码，假设没有给新线程指定 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 方法将新线程加入到 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，能够对一组 thread 进行一些线程操作。
上面分析的是 Android Thread 的构造过程，从上面的分析能够看出。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。
以下我们来分析 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 方法非常easy，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 的实如今 Thread.cpp 中。因为这个函数的内容非常长，在这里仅仅列出关键的地方：
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 执行，下来我们来看新线程的 thread entry 方法 interpThreadStart。相同仅仅列出关键的地方：
/*
* 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 以及其他一些属性，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。然后改动状态为 THREAD_RUNNING。并调用相应 Android Thread 的 run 方法。执行客户代码：
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成从 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。然后通过其相应的 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 该线程的其他线程；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。