Android多線程分析之二：Thread的實現
羅朝輝 (http://www.shnenglu.com/kesalin/)
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 類都是位于 Framework 層的類，它自身是通過 JNI 轉調 dalvik 里面的 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，在這里就不羅嗦了： /**
* 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 里面的 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：每一個線程都屬于一個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?。ㄓ?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：Android 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，那么就會使用 Thread+id 當作線程的名字。注意這不是真正意義上的線程 id，即在 logcat 中打印的 tid 并不是這個 id，那 tid 是指 dalvik 線程的 id；
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，然后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);
}
該方法里的 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?？梢钥吹?，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 那么就會指定 group 為當前線程所在的 group 中，然后給新線程設置 name，priority 等。最后通過調用 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>(thread));
}
}
ThreadGroup 的代碼相對簡單，它有一個名為 threadRefs 的列表，持有屬于同一組的 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，并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，由于這個函數的內容很長，在這里只列出關鍵的地方：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 并設置一些屬性，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；然后創建一個名為 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。然后，通過 pthread_create 創建 pthread 線程，并讓這個線程 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，然后通過調用 prepareThread 設置線程 id 以及其它一些屬性，并調用 setThreadSelf 將新 dalvik Thread 自身保存在 TLS 中，這樣之后就能通過 dvmThreadSelf 方法從 TLS 中獲取它。然后修改狀態為 THREAD_RUNNING，并調用對應 Android Thread 的 run() 方法，運行客戶代碼： public void run() {
if (target != null) {
target.run();
}
}
對于繼承自 Android Thread 帶有 Looper 的 Android HandlerThread 來說，會調用它覆寫 run 方法()：（關于 Looper 的話題下一篇會講到，這里暫且略過）
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 函數里，首先獲取當前線程 self，這里獲得的就是當前執行 thread entry 的新線程，然后通過其對應的 Android Thread 對象 threadObj 獲取該對象所在 group，然后將 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，并通知 join 該線程的其它線程；最后，設置線程狀態為 THREAD_ZOMBIE，清除 TLS 中存儲的線程值，并通過調用 freeThread 釋放內存，至此線程就終結了。