《建造我的Android技能塔之沙粒二：Handler机制》
Handler机制
一、ThreadLocal
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THreadLocal是一个线程内部的数据存储类，通过它可以在指定的线程中获取数据，数据存储以后，只有在指定线程中可以获取到存储的数据，对于其他线程 来说，则无法获取到数据。
-
使用场景：
① 当某些数据是以线程为作用域并且不同的线程具有不同的Looper，这时候通过ThreadLocal就可以轻松实现Looper在线程中的存取。
②复杂逻辑下对象的传递，比如监听器的传递，有些时候一个线程中的任务过于复杂，这可能表现为函数调用栈比较深以及代码入口的多样性，在这种情况下，有需要监听器能够贯穿整个线程的执行过程，这个时候就可以采用ThreadLocal，采用ThreadLocal可以让监听器作为线程内的全局对象而存在，在线程内部只需要get就可以获取到监听器。
-
示例：
public static final String TAG = "StudyTest";
private ThreadLocal<Boolean> mThreadLocal = new ThreadLocal<Boolean>();
@Test
public void useAppContext() {
mThreadLocal.set(true);
Log.d(TAG,"[ThreadLocal#main] mThreadLocal = " + mThreadLocal.get() );
new Thread("Thread1"){
@Override
public void run() {
mThreadLocal.set(false);
Log.d(TAG,"[Thread#1] mThreadLocal = " + mThreadLocal.get());
}
}.start();
new Thread("Thread2"){
@Override
public void run() {
Log.d(TAG,"[Thread#2] mThreadLocal = " + mThreadLocal.get());
}
}.start();
}
D/StudyTest: [ThreadLocal#main] mThreadLocal = true
D/StudyTest: [Thread#1] mThreadLocal = false
D/StudyTest: [Thread#2] mThreadLocal = null
从以上可以看出，虽然在不同线程中访问的是同一个ThreadLocal对象，但是他们通过ThreadLocal获取的值却是不一样的。
? ThreadLocal之所以有这么神奇的效果，是因为不同线程访问同一个THreadLocal的get方法，ThreadLocal内部会从各自的线程中取出一个数组，然后再从数组中根据当前ThreadLocal的索引去查找对应的value的值。
? 显然不同的线程中的数组是不同的，这就是为什么通过ThreadLocal的索引去查找出对应的value值。
-
ThreadLocal内部实现
ThreadLocal 是一个泛型类。
ThreadLocal set方法
/**
* Sets the current thread's copy of this thread-local variable
* to the specified value. Most subclasses will have no need to
* override this method, relying solely on the {@link #initialValue}
* method to set the values of thread-locals.
*
* @param value the value to be stored in the current thread's copy of
* this thread-local.
*/
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 set方法
/**
* Set the value associated with key.
*
* @param key the thread local object
* @param value the value to be set
*/
private void set(ThreadLocal<?> key, Object value) {
// We don't use a fast path as with get() because it is at
// least as common to use set() to create new entries as
// it is to replace existing ones, in which case, a fast
// path would fail more often than not.
Entry[] tab = table;
int len = tab.length;
int i = key.threadLocalHashCode & (len-1);
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
ThreadLocal<?> k = e.get();
if (k == key) {
e.value = value;
return;
}
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
tab[i] = new Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
ThreadLocal get方法
/**
* Returns the value in the current thread's copy of this
* thread-local variable. If the variable has no value for the
* current thread, it is first initialized to the value returned
* by an invocation of the {@link #initialValue} method.
*
* @return the current thread's value of this thread-local
*/
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();
}
/**
* Variant of set() to establish initialValue. Used instead
* of set() in case user has overridden the set() method.
*
* @return the initial value
*/
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;
}
/**
* Create the map associated with a ThreadLocal. Overridden in
* InheritableThreadLocal.
*
* @param t the current thread
* @param firstValue value for the initial entry of the map
*/
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
二、 消息队列工作原理
? 消息队列指的是 MessageQueue，MessageQueue主要包含两个操作：插入和读取。 读取操作本身会伴随着删除操作，插入和读取对应的方法分别是enqueueMessage（）和next（）方法的实现。
MessageQueue实际上是一个单链表。
enqueueMessage 方法
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
synchronized (this) {
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
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
Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
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;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
// If first time idle, then get the number of idlers to run.
// Idle handles only run if the queue is empty or if the first message
// in the queue (possibly a barrier) is due to be handled in the future.
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null; // release the reference to the handler
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
pendingIdleHandlerCount = 0;
// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
nextPollTimeoutMillis = 0;
}
}
next是一个无限循环的方法，若果消息队列中没有消息，那么next方法会一直阻塞在这里。当有新的消息到来时，next方法会返回这条消息并将其从单链表中移除。
三、Looper工作原理
? Looper负责不停的从MessageQueue中查看是否有新消息，如果有新消息就会立刻处理，否则就一直阻塞在那里。下面是Looper的构造方法，在Looper创建时，他会自动创建一个消息队列，然后将当前线程的对象保存起来。
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
? 创建Looper
new Thread("Thread2"){
@Override
public void run() {
Looper.prepare();
Handler handler = new Handler();
Looper.loop();
}
}.start();
? Looper除了prepare方法外，还提供了prepareMainLooper方法，这个方法主要是给主线程也就是ActivityThread创建Looper使用的。其本质也是prepare方法。
? 由于主线程的Looper比较特殊，所以Looper提供了一个getMainLooper方法，通过它可以在任何地方获取到主线程的Looper。
? Looper退出方法：1. quit： 直接退出；2.quitSafely ：设定一个退出标记，然后把消息队列中的已有消息处理完毕后才安全的退出。
? Looper最重要的是loop方法，只有调用了loop方法后，消息系统才会真正的起作用。
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
if (me.mInLoop) {
Slog.w(TAG, "Loop again would have the queued messages be executed"
+ " before this one completed.");
}
me.mInLoop = true;
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();
// Allow overriding a threshold with a system prop. e.g.
// adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
final int thresholdOverride =
SystemProperties.getInt("log.looper."
+ Process.myUid() + "."
+ Thread.currentThread().getName()
+ ".slow", 0);
boolean slowDeliveryDetected = false;
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);
}
// Make sure the observer won't change while processing a transaction.
final Observer observer = sObserver;
final long traceTag = me.mTraceTag;
long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
if (thresholdOverride > 0) {
slowDispatchThresholdMs = thresholdOverride;
slowDeliveryThresholdMs = thresholdOverride;
}
final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);
final boolean needStartTime = logSlowDelivery || logSlowDispatch;
final boolean needEndTime = logSlowDispatch;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
final long dispatchEnd;
Object token = null;
if (observer != null) {
token = observer.messageDispatchStarting();
}
long origWorkSource = ThreadLocalWorkSource.setUid(msg.workSourceUid);
try {
msg.target.dispatchMessage(msg);
if (observer != null) {
observer.messageDispatched(token, msg);
}
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} catch (Exception exception) {
if (observer != null) {
observer.dispatchingThrewException(token, msg, exception);
}
throw exception;
} finally {
ThreadLocalWorkSource.restore(origWorkSource);
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (logSlowDelivery) {
if (slowDeliveryDetected) {
if ((dispatchStart - msg.when) <= 10) {
Slog.w(TAG, "Drained");
slowDeliveryDetected = false;
}
} else {
if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
msg)) {
// Once we write a slow delivery log, suppress until the queue drains.
slowDeliveryDetected = true;
}
}
}
if (logSlowDispatch) {
showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", msg);
}
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方法是一个死循环，唯一跳出循环的方式就是MessageQueue的next方法返回了null。当Looper的quit方法呗调用时，Looper就会调用MessageQueue的quit方法或者quitSafely方法来通知消息队列的退出，当消息队列被标记为退出状态时，next方法就会返回null.。也就是说Looper必须退出，否则loop方法无限循环下去。
? loop方法回调用MessageQueue的next方法来获取消息，而next方法是一个阻塞操作，没有消息时，会一直阻塞在那里，也导致loop方法一直阻塞在那里。
? 如果MessageQueue的next方法反悔了消息，Looper就会处理这条消息：msg.target.dispatchMessage(msg)，这里的msg.target是发送这条消息的Handler对象，这样Handler发送的消息最终又交给它的dispatchMessage方法来处理了。这里不同的是，Handler的dispatchMessage方法是在创建Handler时所使用的Looper中执行的，这样就成功的将代码逻辑切换到指定线程中去了。
四、Handler工作原理
?
public boolean sendMessageAtTime(@NonNull 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(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
msg.target = this;
msg.workSourceUid = ThreadLocalWorkSource.getUid();
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
可以发现，Handler发送消息的过程就是想消息队列中插入一条消息，，MessageQueue的next方法就会返回这条消息给Looper，Looper收到消息后就开始处理了，最终消息由Looper交给Handler处理，即Handler的dispatchMessage方法会被调用，这时Handler就进入了处理消息阶段。dispatchMessage消息如下：
/**
* Handle system messages here.
*/
public void dispatchMessage(@NonNull Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
Handler处理消息的过程如下：
首先，检查Message的callback是一个Runnable对象，实际上就是Handler的post方法所传递的Runnable参数。handleCallback逻辑如下：
private static void handleCallback(Message message) {
message.callback.run();
}
callback是一个接口：
/**
* Callback interface you can use when instantiating a Handler to avoid
* having to implement your own subclass of Handler.
*/
public interface Callback {
/**
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
boolean handleMessage(@NonNull Message msg);
}
? 通过callback可采用如下方式来创建Handler对象： Handler h = new Handler（callback）；
? CallBack 的作用：可以用来创建一个Handler的实例但并不需要派生Handler的子类。
? 最后，调用Handler的handleMessage方法来处理。Handler处理消息可以归纳为一个流程图：
Handler还有个特殊的构造方法——通过一个特定的Looper来构造Handler。
/**
* Use the provided {@link Looper} instead of the default one.
*
* @param looper The looper, must not be null.
*/
public Handler(@NonNull Looper looper) {
this(looper, null, false);
}
很明显当前线程如果没有Looper的话，就会抛出异常。
public Handler(@Nullable 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 " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
五、主线程的消息循环
? Android中的主线程就是ActivityThread，主线程的入口方法为main，在Main方法中系统会通过Looper.prepareMainLooper()来创建主线程的Looper以及MessageQueue，并通过Looper.loop()来开启主线程的 消息循环。
? 主线程的消息循环开始以后，ActivityThread还需要一个Handler也就是ActivityThread.H，它内部定义了一组消息类型，主要包含了四大组件的启动和停止的过程。
class H extends Handler {
5 public static final int BIND_APPLICATION = 110;
6 @UnsupportedAppUsage
7 public static final int EXIT_APPLICATION = 111;
8 @UnsupportedAppUsage
9 public static final int RECEIVER = 113;
10 @UnsupportedAppUsage
11 public static final int CREATE_SERVICE = 114;
12 @UnsupportedAppUsage
13 public static final int SERVICE_ARGS = 115;
14 @UnsupportedAppUsage
15 public static final int STOP_SERVICE = 116;
16
17 public static final int CONFIGURATION_CHANGED = 118;
18 ...
19 @UnsupportedAppUsage
20 public static final int BIND_SERVICE = 121;
21 @UnsupportedAppUsage
22 public static final int UNBIND_SERVICE = 122;
23 ...
24
25 public static final int EXECUTE_TRANSACTION = 159;
26
27 public static final int RELAUNCH_ACTIVITY = 160;
28 public static final int PURGE_RESOURCES = 161;
29
30 public void handleMessage(Message msg) {
31 if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
32 switch (msg.what) {
33 case BIND_APPLICATION:
34 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "bindApplication");
35 AppBindData data = (AppBindData)msg.obj;
36 handleBindApplication(data);
37 Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
38 break;
39 case EXIT_APPLICATION:
40 if (mInitialApplication != null) {
41 mInitialApplication.onTerminate();
42 }
43 Looper.myLooper().quit();
44 break;
45 case RECEIVER:
46 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "broadcastReceiveComp");
47 handleReceiver((ReceiverData)msg.obj);
48 Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
49 break;
50 case CREATE_SERVICE:
51 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, ("serviceCreate: " + String.valueOf(msg.obj)));
52 handleCreateService((CreateServiceData)msg.obj);
53 Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
54 break;
55 case BIND_SERVICE:
56 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceBind");
57 handleBindService((BindServiceData)msg.obj);
58 Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
59 break;
60 case UNBIND_SERVICE:
61 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceUnbind");
62 handleUnbindService((BindServiceData)msg.obj);
63 schedulePurgeIdler();
64 Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
65 break;
66 case SERVICE_ARGS:
67 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, ("serviceStart: " + String.valueOf(msg.obj)));
68 handleServiceArgs((ServiceArgsData)msg.obj);
69 Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
70 break;
71 case STOP_SERVICE:
72 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceStop");
73 handleStopService((IBinder)msg.obj);
74 schedulePurgeIdler();
75 Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
76 break;
77 case CONFIGURATION_CHANGED:
78 handleConfigurationChanged((Configuration) msg.obj);
79 break;
80
81 ...
82
83 case EXECUTE_TRANSACTION:
84 final ClientTransaction transaction = (ClientTransaction) msg.obj;
85 mTransactionExecutor.execute(transaction);
86 if (isSystem()) {
87 // Client transactions inside system process are recycled on the client side
88 // instead of ClientLifecycleManager to avoid being cleared before this
89 // message is handled.
90 transaction.recycle();
91 }
92 // TODO(lifecycler): Recycle locally scheduled transactions.
93 break;
94 case RELAUNCH_ACTIVITY:
95 handleRelaunchActivityLocally((IBinder) msg.obj);
96 break;
97 case PURGE_RESOURCES:
98 schedulePurgeIdler();
99 break;
100 }
101 Object obj = msg.obj;
102 if (obj instanceof SomeArgs) {
103 ((SomeArgs) obj).recycle();
104 }
105 if (DEBUG_MESSAGES) Slog.v(TAG, "<<< done: " + codeToString(msg.what));
106 }
107 }
本篇到这就结束啦，这篇内容是今天学习《Android开发艺术探索》做的笔记！ 希望不足之处大家多多补充和指正啊！