什么时候使用多线程:
1. 耗时操作使用多线程, 耗时操作放在UI线程中会导致用户的操作无法得到响应.
2. 阻塞操作使用多线程, 理由同上.
3. 多核CUP的设备使用多线程, 可以有效提高CPU的利用率.
4. 并行操作使用多线程.
android中的多线程模型主要涉及的类有:Looper, Handler, MessageQueue, Message等.
一：Looper类：
1 static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
2 private static Looper sMainLooper; // guarded by Looper.class
3
4 final MessageQueue mQueue;
5 final Thread mThread;
6 public static void loop() {
7 final Looper me = myLooper();
8 if (me == null) {
9 throw new RuntimeException("No Looper; Looper.prepare() wasn‘t called on this thread.");
10 }
11 final MessageQueue queue = me.mQueue;
12
13 // Make sure the identity of this thread is that of the local process,
14 // and keep track of what that identity token actually is.
15 Binder.clearCallingIdentity();
16 final long ident = Binder.clearCallingIdentity();
17
18 for (;;) {
19 Message msg = queue.next(); // might block
20 if (msg == null) {
21 // No message indicates that the message queue is quitting.
22 return;
23 }
24
25 // This must be in a local variable, in case a UI event sets the logger
26 Printer logging = me.mLogging;
27 if (logging != null) {
28 logging.println(">>>>> Dispatching to " + msg.target + " " +
29 msg.callback + ": " + msg.what);
30 }
31
32 msg.target.dispatchMessage(msg);//超级重要
33
34 if (logging != null) {
35 logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
36 }
37
38 // Make sure that during the course of dispatching the
39 // identity of the thread wasn‘t corrupted.
40 final long newIdent = Binder.clearCallingIdentity();
41 if (ident != newIdent) {
42 Log.wtf(TAG, "Thread identity changed from 0x"
43 + Long.toHexString(ident) + " to 0x"
44 + Long.toHexString(newIdent) + " while dispatching to "
45 + msg.target.getClass().getName() + " "
46 + msg.callback + " what=" + msg.what);
47 }
48
49 msg.recycle();
50 }
51 }
52 public void quit() {
53 mQueue.quit(false);
54 }
55 public void quitSafely() {//最好用此方法
56 mQueue.quit(true);
57 }
Looper类用来创建消息队列. 每个线程最多只能有一个消息队列：Looper唯一的构造方法如下
1 private Looper(boolean quitAllowed) {
2 mQueue = new MessageQueue(quitAllowed);
3 mThread = Thread.currentThread();
4 }
android中UI线程默认具有消息队列, 但非UI线程在默认情况下是不具备消息队列的. 如果需要在非UI线程中开启消息队列, 需要调用Looper.prepare()方法, 在该方法的执行过程中会创建一个Looper对象
1 public static void prepare() {
2 prepare(true);
3 }
4
5 private static void prepare(boolean quitAllowed) {
6 if (sThreadLocal.get() != null) {
7 throw new RuntimeException("Only one Looper may be created per thread");
8 }
9 sThreadLocal.set(new Looper(quitAllowed));
10 }
而Looper的构造函数中会创建一个MessageQueue instance(Looper的构造函数是私有的, 在Looper类之外无法创建其对象).
此后再为该线程绑定一个Handler instance, 然后调用Looper.loop()方法, 就可以不断的从消息队列中取出消息和处理消息了.
1 public Handler() {
2 this(null, false);
3 }
4 public Handler(Callback callback) {
5 this(callback, false);
6 }
7 public Handler(Looper looper) {
8 this(looper, null, false);
9 }
10 public Handler(Looper looper, Callback callback) {
11 this(looper, callback, false);
12 }
13 public Handler(Callback callback, boolean async) {
14 if (FIND_POTENTIAL_LEAKS) {
15 final Class<? extends Handler> klass = getClass();
16 if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
17 (klass.getModifiers() & Modifier.STATIC) == 0) {
18 Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
19 klass.getCanonicalName());
20 }
21 }
22
23 mLooper = Looper.myLooper();
24 if (mLooper == null) {
25 throw new RuntimeException(
26 "Can‘t create handler inside thread that has not called Looper.prepare()");
27 }
28 mQueue = mLooper.mQueue;
29 mCallback = callback;
30 mAsynchronous = async;
31 }
32 public Handler(Looper looper, Callback callback, boolean async) {
33 mLooper = looper;
34 mQueue = looper.mQueue;
35 mCallback = callback;
36 mAsynchronous = async;
37 }
(创建handler的时候必须关联一个looper)
Looper.myLoop()方法可以得到线程的Looper对象, 如果为null, 说明此时该线程尚未开启消息队列.
1 public static Looper myLooper() {
2 return sThreadLocal.get();
3 }
二：Handler：
Handler类用于处理消息. 该类具有四个构造函数:(如上)
1. public Handler(). 创建好的Handler instance将绑定在代码所在的线程的消息队列上, 因此一定要确定该线程开启了消息队列, 否则程序将发生错误. 使用这个构造函数创建Handler instance, 一般来说, 我们需要重写Hanler类的handleMessage()方法, 以便在之后的消息处理时调用.
2. public Handler(Callback callback). Callback是Handler内部定义的一个接口, 因此想要使用这个构造函数创建Handler对象, 需要自定义一个类实现Callback接口, 并重写接口中定义的handleMessage()方法. 这个构造函数其实与无参的构造函数类似, 也要确保代码所在的线程开启了消息队列. 不同的是在之后处理消息时, 将优先调用callback的handleMessage()方法,返回true，则消息处理结束；返回false。则会调用Handler对象的handleMssage()方法.
1 final MessageQueue mQueue;
2 final Looper mLooper;
3 final Callback mCallback;
4 public interface Callback {
5 public boolean handleMessage(Message msg);
6 }
7 private static void handleCallback(Message message) {
8 message.callback.run();
9 }
1 /**
2 * Handle system messages here.
3 */
4 public void dispatchMessage(Message msg) {
5 if (msg.callback != null) {
6 handleCallback(msg);//如上的方法
7 } else {
8 if (mCallback != null) {
9 if (mCallback.handleMessage(msg)) {
10 return;
11 }
12 }
13 handleMessage(msg);
14 }
15 }
3. public Handler(Looper looper). 这个构造函数表示创建一个Handler instance, 并将其绑定在looper所在的线程上. 此时looper不能为null. 此时一般也需要重写Hanler类的handleMessage()方法
4. public Handler(Looper looper, Callback callback). 可以结合2和3理解.
1 public final Message obtainMessage()
2 {
3 return Message.obtain(this);
4 }
5 private static Message getPostMessage(Runnable r) {
6 Message m = Message.obtain();
7 m.callback = r;
8 return m;
9 }
10 public final boolean post(Runnable r)
11 {
12 return sendMessageDelayed(getPostMessage(r), 0);
13 }
14 public final boolean postAtTime(Runnable r, long uptimeMillis)
15 {
16 return sendMessageAtTime(getPostMessage(r), uptimeMillis);
17 }
18 public final boolean postDelayed(Runnable r, long delayMillis)
19 {
20 return sendMessageDelayed(getPostMessage(r), delayMillis);
21 }
22 public final boolean postAtFrontOfQueue(Runnable r)
23 {
24 return sendMessageAtFrontOfQueue(getPostMessage(r));
25 }
26 public final boolean sendMessage(Message msg)
27 {
28 return sendMessageDelayed(msg, 0);
29 }
30 public final boolean sendEmptyMessage(int what)
31 {
32 return sendEmptyMessageDelayed(what, 0);
33 }
34 public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
35 Message msg = Message.obtain();
36 msg.what = what;
37 return sendMessageDelayed(msg, delayMillis);
38 }
39 public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
40 Message msg = Message.obtain();
41 msg.what = what;
42 return sendMessageAtTime(msg, uptimeMillis);
43 }
44 //Handler所有post、send方法，最后调的都是此方法...
45 public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
46 MessageQueue queue = mQueue;
47 if (queue == null) {
48 RuntimeException e = new RuntimeException(
49 this + " sendMessageAtTime() called with no mQueue");
50 Log.w("Looper", e.getMessage(), e);
51 return false;
52 }
53 return enqueueMessage(queue, msg, uptimeMillis);
54 }
55 //入队，其实调的是MessageQueue的入队方法
56 private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
57 msg.target = this;
58 if (mAsynchronous) {
59 msg.setAsynchronous(true);
60 }
61 return queue.enqueueMessage(msg, uptimeMillis);
62 }
63 //Handler的hasMessage、removeCallback是MessageQueue的方法
64 public final void removeMessages(int what) {
65 mQueue.removeMessages(this, what, null);
66 }
67 public final void removeCallbacksAndMessages(Object token) {
68 mQueue.removeCallbacksAndMessages(this, token);
69 }
70 public final boolean hasMessages(int what) {
71 return mQueue.hasMessages(this, what, null);
72 }
73 public final boolean hasCallbacks(Runnable r) {
74 return mQueue.hasMessages(this, r, null);
75 }
三，MessageQueue：
MessageQueue类用于表示消息队列. 队列中的每一个Message都有一个when字段, 这个字段用来决定Message应该何时出对处理. 消息队列中的每一个Message根据when字段的大小由小到大排列, 排在最前面的消息会首先得到处理, 因此可以说消息队列并不是一个严格的先进先出的队列.
方法主要有：
Message next() {...}
void quit(boolean safe) {
if (!mQuitAllowed) {
throw new RuntimeException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuitting) {
return;
}
mQuitting = true;
if (safe) {
removeAllFutureMessagesLocked();
} else {
removeAllMessagesLocked();
}
// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}
//入队
boolean enqueueMessage(Message msg, long when) {...}
//判断消息队列中是否包含某个消息
boolean hasMessages(Handler h, int what, Object object) {...}
//将某个消息从队列中移出去，该消息被回收到消息池中了
void removeMessages(Handler h, int what, Object object) {...}
四：Message：
Message类用于表示消息. Message对象可以通过arg1, arg2, obj字段和setData()携带数据, 此外还具有很多字段. when字段决定Message应该何时出对处理, target字段用来表示将由哪个Handler对象处理这个消息, next字段表示在消息队列中排在这个Message之后的下一个Message, callback字段如果不为null表示这个Message包装了一个runnable对象, what字段表示code, 即这个消息具体是什么类型的消息. 每个what都在其handler的namespace中, 我们只需要确保将由同一个handler处理的消息的what属性不重复就可以.
1 public int what; 2 public int arg1; 3 public int arg2; 4 public Object obj; 5 public Messenger replyTo; 6 /** If set message is in use */ 7 static final int FLAG_IN_USE = 1 << 0; 8 /** If set message is asynchronous */ 9 static final int FLAG_ASYNCHRONOUS = 1 << 1; 10 /** Flags to clear in the copyFrom method */ 11 static final int FLAGS_TO_CLEAR_ON_COPY_FROM = FLAG_IN_USE; 12 int flags; 13 long when; 14 Bundle data; 15 Handler target; 16 Runnable callback; 17 // sometimes we store linked lists of these things 18 Message next; 19 20 private static final Object sPoolSync = new Object(); 21 private static Message sPool; 22 private static int sPoolSize = 0; 23 24 private static final int MAX_POOL_SIZE = 50;
1 public static Message obtain() {
2 synchronized (sPoolSync) {
3 if (sPool != null) {
4 Message m = sPool;
5 sPool = m.next;
6 m.next = null;
7 sPoolSize--;
8 return m;
9 }
10 }
11 return new Message();
12 }
13 public static Message obtain(Message orig) {
14 Message m = obtain();
15 m.what = orig.what;
16 m.arg1 = orig.arg1;
17 m.arg2 = orig.arg2;
18 m.obj = orig.obj;
19 m.replyTo = orig.replyTo;
20 if (orig.data != null) {
21 m.data = new Bundle(orig.data);
22 }
23 m.target = orig.target;
24 m.callback = orig.callback;
25
26 return m;
27 }
28 public static Message obtain(Handler h) {
29 Message m = obtain();
30 m.target = h;
31
32 return m;
33 }
34 ......
35 void clearForRecycle() {
36 flags = 0;
37 what = 0;
38 arg1 = 0;
39 arg2 = 0;
40 obj = null;
41 replyTo = null;
42 when = 0;
43 target = null;
44 callback = null;
45 data = null;
46 }
47 public void recycle() {
48 clearForRecycle();
49
50 synchronized (sPoolSync) {
51 if (sPoolSize < MAX_POOL_SIZE) {
52 next = sPool;
53 sPool = this;
54 sPoolSize++;
55 }
56 }
57 }
58 public void copyFrom(Message o)
59 public long getWhen()
60 public void setTarget(Handler target)
61 public Handler getTarget()
62 public Runnable getCallback()
63 public Bundle getData() {
64 if (data == null) {
65 data = new Bundle();
66 }
68 return data;
69 }
70 public Bundle peekData() {
71 return data;
72 }
73 public void setData(Bundle data)
74 public void sendToTarget()
将消息压入消息队列: Message对象的target字段关联了哪个线程的消息队列, 这个消息就会被压入哪个线程的消息队列中.
1. 调用Handler类中以send开头的方法可以将Message对象压入消息队列中, 调用Handler类中以post开头的方法可以将一个runnable对象包装在一个Message对象中, 然后再压入消息队列, 此时入队的Message其callback字段不为null, 值就是这个runnable对象. 调用Handler对象的这些方法入队的Message, 其target属性会被赋值为这个handler对象.
2. 调用Message对象的sendToTarget()方法可以将其本身压入与其target字段(即handler对象)所关联的消息队列中.
将未来得及处理的消息从消息队列中删除:
调用Handler对象中以remove开头的方法就可以.
从消息队列中取出消息并处理消息: 所有在消息队列中的消息, 都具有target字段. 消息是在target所关联的线程上被取出和处理的.
1 /**
2 * Handle system messages here.
3 */
4 public void dispatchMessage(Message msg) {
5 if (msg.callback != null) {
6 handleCallback(msg);
7 } else {
8 if (mCallback != null) {
9 if (mCallback.handleMessage(msg)) {
10 return;
11 }
12 }
13 handleMessage(msg);
14 }
15 }
1. 如果取出的Message对象的callback字段不为null, 那么就调用callback字段的run()方法(callback字段的类型是runnable). 注意此时并不开启一个新的线程运行run()方法, 而是直接在handler对象(即Message的target字段)所关联的线程上运行.
2. 如果取出的Message对象的callback字段为null, 且Handler对象中的callback字段也为null, 那么这个消息将由Handler对象的handleMessage(msg)方法处理. 注意Message对象的callback字段是Runnable类型的而Handler对象的callback字段是Callback类型的, Handler对象的callback字段是在创建Handler instance的时候指定的, 如果没有指定则这个字段为null, 详见Handler类的四个构造方法.
3. 如果取出的Message对象的callback字段为null, 且Handler对象中的callback字段不为null, 那么这个消息将由Handler对象中的callback字段的handleMessage方法优先处理.
线程间通信: 有了以上的叙述, 线程间的通信也就好理解了. 假如一个handler关联了A线程上的消息队列, 那么我们可以在B线程上调用handler的相关方法向A线程上的消息队列压入一个Message, 这个Message将在A线程上得到处理.
转载：android笔记--android中的多线程--Handler, Looper, MessageQueue, Message类