10.2.1 ThreadLocal的工作原理
ThreadLocal是一個線程內部的數據存儲類，通過它可以在指定的線程中存儲數據缺谴，數據存儲以后，只有在指定線程中可以獲取到存儲的數據耳鸯，對于其他線程來說則無法獲取到數據湿蛔。
源碼分析
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
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;
}
protected T initialValue() {
return null;
}
set(value)方法會給當前的Thread創(chuàng)建一個ThreadLocalMap，讓后以thread為key县爬，將key-value存入該map中阳啥。
get()方法會從當前Thread中獲取ThreadLocalMap，然后以Thread為key從map中取value财喳。
10.2.2 消息隊列的工作原理
MessageQueue持有Message對象察迟，Message是一個單向鏈表。
messageQueue有2個核心方法：enqueueMessage()和next()
next()方法是一個無限循環(huán)方法耳高，如果沒有消息next就被阻塞扎瓶，如果有新消息，next方法會返回這條消息并將其從單鏈表中移除泌枪。
10.2.3 Looper的工作原理
Looper會不停的從messageQueue中查看是否有新消息概荷，如果有新消息就會立刻處理，否則一直就阻塞在哪里碌燕。
構造方法
構建方法創(chuàng)造了一個MessageQueue
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
創(chuàng)建Looper
prepare()方法為當前線程創(chuàng)建一個Looper
public static void prepare() {
prepare(true);
}
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));
}
開啟循環(huán) loop()方法開啟消息循環(huán)
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
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;
// Make sure the identity of this thread is that of the local process,
// and keep track of what that identity token actually is.
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
// This must be in a local variable, in case a UI event sets the logger
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
final long end;
try {
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (slowDispatchThresholdMs > 0) {
final long time = end - start;
if (time > slowDispatchThresholdMs) {
Slog.w(TAG, "Dispatch took " + time + "ms on "
+ Thread.currentThread().getName() + ", h=" +
msg.target + " cb=" + msg.callback + " msg=" + msg.what);
}
}
if (logging != null) {
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
}
// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
final long newIdent = Binder.clearCallingIdentity();
if (ident != newIdent) {
Log.wtf(TAG, "Thread identity changed from 0x"
+ Long.toHexString(ident) + " to 0x"
+ Long.toHexString(newIdent) + " while dispatching to "
+ msg.target.getClass().getName() + " "
+ msg.callback + " what=" + msg.what);
}
msg.recycleUnchecked();
}
}
loop()解析
loop是一個死循環(huán)误证，唯一得跳出條件時MessageQueue.next()返回null。
當Looper的quite或quitSafely被調用的時候修壕，MessageQueue的quit被調用愈捅，之后消息隊列被標記位退出裝填，next會返回空慈鸠。
// Looper 的quit方法
public void quit() {
mQueue.quit(false);
}
public void quitSafely() {
mQueue.quit(true);
}
拿到message后蓝谨，會執(zhí)行
msg.target.dispatchMessage(msg)，這里的msg.targe是發(fā)送這條消息的Handler對象青团。
終止循環(huán)
Looper提供了quit和quitSafely兩個方法退出一個Looper像棘。
quit直接退出，quitSafely只做標記位壶冒，消息隊列中的消息處理完畢后才會安全的退出缕题。
10.2.4Handler的工作原理
無Looper構造方法
不傳入Looper的構造方法，handler中的Looper默認是當前線程的Looper胖腾，如果當前線程沒有Looper會拋出異常烟零。
/**
* Default constructor associates this handler with the {@link Looper} for the
* current thread.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*/
public Handler() {
this(null, false);
}
/**
* Use the {@link Looper} for the current thread with the specified callback interface
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with respect to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
final Class<? extends Handler> klass = getClass();
if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
(klass.getModifiers() & Modifier.STATIC) == 0) {
Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
klass.getCanonicalName());
}
}
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
關鍵代碼mLooper = Looper.myLooper();
再來看看Looper
Looper有個ThreadLocal<Looper>的靜態(tài)成員變量sThreadLocal瘪松，在調用prepare()時會 new 一個Looper對象出來，并將它置入sThreadLocal中锨阿。
Looper的構造方法新建了一個MessageQueue宵睦，將currentThread賦值到成員變量之中。
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
/** Initialize the current thread as a looper.
* This gives you a chance to create handlers that then reference
* this looper, before actually starting the loop. Be sure to call
* {@link #loop()} after calling this method, and end it by calling
* {@link #quit()}.
*/
public static void prepare() {
prepare(true);
}
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));
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
有Looper構造方法
有Looper的構造方法相對簡單墅诡，直接將方法入參的Looper傳入Handler之中壳嚎。
/**
* Use the provided {@link Looper} instead of the default one.
*
* @param looper The looper, must not be null.
*/
public Handler(Looper looper) {
this(looper, null, false);
}
/**
* Use the provided {@link Looper} instead of the default one and take a callback
* interface in which to handle messages.
*
* @param looper The looper, must not be null.
* @param callback The callback interface in which to handle messages, or null.
*/
public Handler(Looper looper, Callback callback) {
this(looper, callback, false);
}
/**
* Use the provided {@link Looper} instead of the default one and take a callback
* interface in which to handle messages. Also set whether the handler
* should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with respect to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param looper The looper, must not be null.
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
發(fā)送消息
無論是post(Runnable)還是sendMessage(Message)最終都是將數據轉化為Message，存入MessageQueue之中末早。
Handler.png
