Android 的消息机制
Android 的消息机制
HelloVass
发布于 2018-09-12 10:29:51
发布于 2018-09-12 10:29:51
先来谈谈 ThreadLocal
简介
ThreadLocal 是一个线程内部的数据存储类，通过他可以在指定的线程中存储数据，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
某些数据以线程为作用域并且不同线程具有不同的数据副本
举个栗子
public final class Looper {
/*
* API Implementation Note:
*
* This class contains the code required to set up and manage an event loop
* based on MessageQueue. APIs that affect the state of the queue should be
* defined on MessageQueue or Handler rather than on Looper itself. For example,
* idle handlers and sync barriers are defined on the queue whereas preparing the
* thread, looping, and quitting are defined on the looper.
*/
private static final String TAG = "Looper";
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();Handler 需要获取当前线程的 Looper，这时候 Looper 的作用域就是线程并且不同线程具有不同的 Looper。
通过一个小 Demo 测试下
public class Main {
private static ThreadLocal<String> mThreadLocal = new ThreadLocal<String>();
public static void main(String[] args) {
printInMainThread();
printInThreadOne();
printInThreadTwo();
}
private static void printInMainThread() {
mThreadLocal.set("main thread");
System.out.println("[Thread#main] mThreadLocal = " + mThreadLocal.get());
}
private static void printInThreadTwo() {
new Thread("Thread#1") {
@Override
public void run() {
mThreadLocal.set("thread 1");
System.out.println("[Thread#1] mThreadLocal = " + mThreadLocal.get());
}
}.start();
}
private static void printInThreadOne() {
new Thread("Thread#2") {
@Override
public void run() {
mThreadLocal.set("thread 2");
System.out.println("[Thread#2] mThreadLocal = " + mThreadLocal.get());
}
}.start();
}
}从控制台输出可以看到，虽然在不同的线程中访问的是同一个 ThreadLocal，但是通过 ThreadLocal.get( ) 这个方法得到的值却是不一样的，这就很有趣了！
源码分析
ThreadLocal 的 set( ) 方法，如下：
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}根据当前线程调用 getMap(Thread t) 方法找到对应的 ThreadLocalMap，如果 map 不等于 null，则调用 ThreadLocalMap 的 set(ThreadLocal<?> key, Object value) 方法。
NOTE: ThreadLocalMap 是 ThreadLocal 的一个静态内部类
看下这个 set(ThreadLocal<?> key, Object value) 方法：
private void set(ThreadLocal<?> key, Object value) {
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();
}Entry 是 ThreadLocalMap 的一个静态内部类，如下：
static class ThreadLocalMap {
// ...省略N行注释
static class Entry extends WeakReference<ThreadLocal<?>> {
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}NOTE：这里 ThreadMap 在选择 key 的时候并没有直接选择 ThreadLocal 实例，而是 ThreadLocal 实例的弱引用
再看看 ThreadLocal 的 get() 方法和 ThreadLocalMap 的 getEntry(ThreadLocal<?> key) 方法
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 Entry getEntry(ThreadLocal<?> key) {
int i = key.threadLocalHashCode & (table.length - 1);
Entry e = table[i];
if (e != null && e.get() == key)
return e;
else
return getEntryAfterMiss(key, i, e);
}根据当前线程找到对应的 ThreadLocalMap，如果 map 不等于空，接着调用 ThreadLocalMap 的 getEntry(ThreadLocal<?> key) 方法得到对应的 ThreadLocalMap.Entry 。如果 map 等于空，则调用 setInitialValue() 提供的值（默认是 null）。
protected T initialValue() {
return null;
}该方法可以由开发者来重写，提供一个初始值。
搞基三剑客
开发者日常接触最多的可能是 Handler，而支撑 Handler 运行机制的实际上还有 MessageQueue 和 Looper 这两个好基友。
MessageQueue
中文名称消息队列，实际上的数据结构并不是队列，而是一个链表，主要支持两个操作——消息入队和消息出队。
入队操作对应的方法
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;
}出队操作对应的方法
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;
}
}Looper
字面意思，循环者，在 Android 的消息机制中扮演的是消息循环的角色。具体来说，是它负责从 MessageQueue 中查看是否有新的消息投递进来，如果有则立即处理；如果没有，就会阻塞在哪里。
构造方法
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}在构造方法中创建了一个 MessageQueue。
prepare( ) 方法
初学 Android 的时候我们经常会写这样的一段代码，如下：
new Thread() {
@Override public void run() {
Looper.prepare();
Handler handler = new Handler() {
@Override public void handleMessage(Message msg) {
// do some hard work
}
};
Looper.loop();
}
}.start();如果没有调用 Looper.prepare( ) 这个方法，应用就会 Crash。
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));
}很明显，每一个线程只允许有一个 Looper，否则就会抛出 RuntimeException。
接下来，Looper 中最重要的一个方法 loop()，如下所示：
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 traceTag = me.mTraceTag;
if (traceTag != 0) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
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() 方法内是一个死循环，唯一能跳出的条件是 MessageQueue.next() 返回了 null，否则 loop 将会无限循环下去。注意，loop 方法调用的 MessageQueue.next() 是一个阻塞操作，没有消息时，会阻塞在那里，这也是loop 会阻塞的原因。
PS：关于 Looper 的阻塞，很多人还会产生这么一个疑问。
注意，这里的 msg.target 就是发送消息的 Handler 对象，所以，最后 Handler 发送的消息又交给了它的 dispatchMessage() 方法处理！记得第一次看这个逻辑的时候我也是懵逼的，为毛绕这么大一个圈消息又交给自己处理，MDZZ！实际上，仔细看的话会发现，这时候 msg.target.dispatchMessage() 这个方法是在创建 Handler 的子线程中执行的！简单来说，代码巧妙地切换到指定的这个新线程中去执行了。
Handler
构造方法
public Handler() {
this(null, false);
}
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;
}Handler 默认的构造方法会检查 Looper 是否为空，如果为空则会报错 Can’t create handler inside thread that has not called Looper.prepare()。
至于为毛在 Activity 我们创建 Handler 实例的时候没有报错呢？因为在 ActivityThread 中已经调用了 Looper.prepareMainLooper()，如下所示：
public static void main(String[] args) {
// 省略N行...
Looper.prepareMainLooper();
// 省略N行...
Looper.loop();
}而 prepareMainLooper() 正调用了 prepare()，如下所示：
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}本文参与 腾讯云自媒体分享计划，分享自作者个人站点/博客。
原始发表：2016-10-05，如有侵权请联系 cloudcommunity@tencent.com 删除
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