Android 的消息机制

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

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
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

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
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( ) 这个方法得到的却是不一样的,这就很有趣了!

源码分析

ThreadLocal 的 set( ) 方法,如下:

1
2
3
4
5
6
7
8
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:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
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:

1
2
3
4
5
6
7
8
9
10
11
12
13
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

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
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)。

1
2
3
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

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
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

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
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

1
2
3
4
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:

1
2
3
4
5
6
7
8
9
10
11
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。

1
2
3
4
5
6
7
8
9
10
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:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
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

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24

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:

1
2
3
4
5
6
7
8
public static void main(String[] args) {
// 省略N行...

Looper.prepareMainLooper();

// 省略N行...
Looper.loop();
}

prepareMainLooper() 正调用了 prepare(),如下所示:

1
2
3
4
5
6
7
8
9
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}