1.概述
Handler的作用是將一個任務切換到指定的線程去執行。
UI操作只能在主線程進行,這個限制是在ViewRootImpl#checkThread中實現的:
void checkThread() {
if (mThread != Thread.currentThread()) {
throw new CalledFromWrongThreadException(
"Only the original thread that created a view hierarchy can touch its views.");
}
}
Handler工作原理:
- Handler創建時,會採用當前線程的Looper來構建內部的消息循環,如果當前線程沒有Looper,會拋出異常。
- 通過Handler的send發送一個msg,或者直接post一個Runnable,最終都會由Looper去執行handleMessage或Runnable,而Looper是在創建Handler所在的線程中運行的,所以實現了線程切換。
2.消息機制分析
- ThreadLocal的工作原理
ThreadLocal是一個線程內部的存儲類,可以在指定的線程中存儲數據,且這份數據僅供這個線程獲取。其應用場景是,當某些數據是以線程爲作用域並且不同線程具有不得數據副本。如Handler中的Looper,每個線程都有自己的Looper且互不相同。
另外一個使用場景,是在複雜邏輯下的對象傳遞,使該對象作爲線程內的全局對象存在,在線程內部通過get方法就可以獲得該對象。
ThreadLocal是個泛型類,創建一個ThreadLocal實例:private ThreadLocal<Boolean> mBooleanThreadLocal = new ThreadLocal();
當分別從主線程,子線程去獲取該ThreadLocal的值時,他們分別會得到自己的線程中對應的ThreaLocal的值:
mBooleanThreadLocal.set(true);
Boolean value = mBooleanThreadLocal.get(); // true
new Thread(new Runnable() {
@Override
public void run() {
Boolean value = mBooleanThreadLocal.get(); // null
mBooleanThreadLocal.set(false);
value = mBooleanThreadLocal.get(); // false
}
}).start();
ThreadLocal#set,每個線程中都有一個ThreadLocalMap,用於存儲<ThreadLocal<T>, T>
,每在一個線程中調用ThreadLocal#set,就會將這個類型的ThreadLocal以及其泛型的值存儲進map:
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
ThreadLocal#get,首先會嘗試從ThreadLocalMap取與該ThreadLocal類型對應的其泛型的值,如果找不到,調用setInitialValue方法,通過initialValue方法得到初始值,然後如果ThreadLocalMap也不存在,則同時創建ThreadLocalMap,最後添加進map中:
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
private T setInitialValue() {
T value = initialValue();
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
}
initialValue方法的默認實現返回了null,可以依據具體ThreadLocal的泛型類型,重寫該方法。
2. MessageQueue工作原理
enqueueMessage方法,MessageQueue內部通過單鏈表維護msg隊列,入隊的過程與隊列類似,只是其中會根據msg的when字段,判斷合適的入隊位置,並不一定是隊尾:
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
synchronized (this) {
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
msg.recycle();
return false;
}
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// Inserted within the middle of the queue. Usually we don't have to wake
// up the event queue unless there is a barrier at the head of the queue
// and the message is the earliest asynchronous message in the queue.
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
next方法,會嘗試獲取下一個msg,如果獲取到則從隊列中刪除並返回這個msg;如果沒有msg則阻塞。新msg的到來nativePollOnce就會返回:
Message next() {
...
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
// Next message is not ready. Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
...
}
}
- Looper工作原理
Looper的構造方法,創建了MessageQueue,同時將當前的線程對象保存起來:
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
啓動Looper的例子:
class LooperThread extends Thread {
public Handler mHandler;
public void run() {
Looper.prepare();
mHandler = new Handler() {
public void handleMessage(Message msg) {
// process incoming messages here
}
};
Looper.loop();
}
}
在prepare方法中,創建了Looper對象,同時使用ThreadLocal存儲了該Looper:
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
除了prepare方法,Looper提供了prepareMainLooper方法,用於ActivityThread創建主線程Looper使用,其本質也是通過prepare來實現的。同時提供了getMainLooper方法,在其他線程中獲取主線程的Looper。
Looper完成操作之後,需要調用quit或quitSafely退出,前者會直接退出,後者設置了一個標記,待所有的msg處理完畢再退出。如果子線程不調用quit,該線程會一直阻塞,直到調用quit時直接退出。Looper調用了quit之後,Handler發出的msg將失敗,send方法會返回false。
public void quit() {
mQueue.quit(false);
}
public void quitSafely() {
mQueue.quit(true);
}
loop方法,在該方法中真正處理了MessageQueue中的msg。首先通過ThreadLocal獲得當前線程的Looper實例,然後開始取其MessageQueue中的msg,如果Looper調用了quit方法,這裏取得的msg就是null,對應的Looper就會退出loop;如果取到了msg,就會調用msg.target.dispatchMessage(msg)
執行Handler的dispatchMessage方法。MessageQueue中沒有msg時,則阻塞:
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;
...
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
...
try {
msg.target.dispatchMessage(msg);
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
...
}
- Handler的工作原理
Handler主要負責msg的發送和接收。post方法會將Runnable封裝成msg,然後調用send發出msg:
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
send的一系列方法,最終會將msg添加到MessageQueue中:
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
if (queue == null) {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
return enqueueMessage(queue, msg, uptimeMillis);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
MessageQueue#enqueueMessage方法在msg添加之後,會執行nativeWake方法,如果之前MessageQueue因爲是空的而阻塞在next方法中,這時阻塞解除,開始獲取msg。Looper得到MessageQueue的msg,調用Handler的dispatchMessage方法。
這裏分幾種情況:對於通過post方法傳入的Runnable,則直接通過handleCallback方法去執行他;對於send方法傳入的msg,首先會交由其mCallback的handleMessage方法去處理,該回調可以在創建Handler時指定,就不必重寫Handler的handleMessage方法;如果該回調沒有處理,即返回了false,則交由Handler自己的handleMessage處理:
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
private static void handleCallback(Message message) {
message.callback.run();
}
Handler的構造方法中,會判斷當前線程的Looper是否存在,不存在則拋出異常。同時會獲取Looper的MessageQueue以便後續添加msg:
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
3.主線程消息循環
Android的主線程就是ActivityThread,其main方法中,創建了主線程Looper,並啓動了消息循環:
public static void main(String[] args) {
...
Looper.prepareMainLooper();
...
ActivityThread thread = new ActivityThread();
thread.attach(false, startSeq);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
負責處理消息的Handler即ActivityThread.H,其中就包括了四大組件的啓動和停止過程:
class H extends Handler {
public static final int BIND_APPLICATION = 110;
public static final int EXIT_APPLICATION = 111;
public static final int RECEIVER = 113;
public static final int CREATE_SERVICE = 114;
public static final int SERVICE_ARGS = 115;
public static final int STOP_SERVICE = 116;
主線程消息循環模型:
- ActivityThread提供ApplicationThread與AMS進行IPC
- AMS完成了ActivityThread的RPC之後,回調ApplicationThread中的遠程方法
- ApplicationThread爲Binder對象,其方法運行在Binder線程池中
- ApplicationThread通過H發送msg給ActivityThread的MessageQueue,Looper取出msg,在主線程中處理相應的msg