说说 Message 与 MessageQueue

在计算机科学中,消息队列(英语:Message queue)是一种进程间通信或同一进程的不同线程间的通信方式.

而在 Android 中, 它是持有那些被 Looper 分发的消息的列表的底层类,消息不是直接添加到消息队列的,而是通过与 Looper 相关联的 Handler添加的.

既然是是队列,我们来看看这个队是怎排起来的.我们来看看 MessageQueue 的 enqueueMessage 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
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;
}

看到这里,没有看到我们熟悉的容器Queue,MessageQueue不是由Java 的Queue容器实现的.
那 messageQueue是怎么实现的呢,我们看下这么一句代码

1
msg.next = p;

msg 的 next 是 p, 而它们都是 Message 对象.我们找到 next 位于 Message的位置:

1
2
3
// sometimes we store linked lists of these things
@UnsupportedAppUsage
/*package*/ Message next;

从以上注解可知,Message 对象们是以链表的形式存储的. 而 enqueueMessage 方法中的 if/ else 则是要在链表头部插入还是链表中间插入的一个判断.

对应的以下便是 MessageQueue 的出队列操作:

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
106
@UnsupportedAppUsage
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;
}
}

看过了 MessageQueue,我们来看看 Message:

定义了一个消息,这个消息包含了描述和任意的数据对象,这个消息能够被发送到 Handler .这个对象包含了两个额外的两个int域以及一个额外的对象域,使你在很多情况下不必去分配内存了.

虽然Message的构造器是public的,但是获取Message对象的最好方式是通过调用 Message.obtain()或是 Handler.obtainMessage()方法获取,它们会从回收的对象池中获取出来.

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
/**
* User-defined message code so that the recipient can identify
* what this message is about. Each {@link Handler} has its own name-space
* for message codes, so you do not need to worry about yours conflicting
* with other handlers.
*/
public int what;

/**
* arg1 and arg2 are lower-cost alternatives to using
* {@link #setData(Bundle) setData()} if you only need to store a
* few integer values.
*/
public int arg1;

/**
* arg1 and arg2 are lower-cost alternatives to using
* {@link #setData(Bundle) setData()} if you only need to store a
* few integer values.
*/
public int arg2;

/**
* An arbitrary object to send to the recipient. When using
* {@link Messenger} to send the message across processes this can only
* be non-null if it contains a Parcelable of a framework class (not one
* implemented by the application). For other data transfer use
* {@link #setData}.
*
* <p>Note that Parcelable objects here are not supported prior to
* the {@link android.os.Build.VERSION_CODES#FROYO} release.
*/
public Object obj;
1
2
3
4
5
6
/**
* Optional Messenger where replies to this message can be sent. The
* semantics of exactly how this is used are up to the sender and
* receiver.
*/
public Messenger replyTo;

对消息的回复.

一些标志信息:

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
/**
* Indicates that the uid is not set;
*
* @hide Only for use within the system server.
*/
public static final int UID_NONE = -1;

/**
* Optional field indicating the uid that sent the message. This is
* only valid for messages posted by a {@link Messenger}; otherwise,
* it will be -1.
*/
public int sendingUid = UID_NONE;

/**
* Optional field indicating the uid that caused this message to be enqueued.
*
* @hide Only for use within the system server.
*/
public int workSourceUid = UID_NONE;

/** If set message is in use.
* This flag is set when the message is enqueued and remains set while it
* is delivered and afterwards when it is recycled. The flag is only cleared
* when a new message is created or obtained since that is the only time that
* applications are allowed to modify the contents of the message.
*
* It is an error to attempt to enqueue or recycle a message that is already in use.
*/
/*package*/ static final int FLAG_IN_USE = 1 << 0;

/** If set message is asynchronous */
/*package*/ static final int FLAG_ASYNCHRONOUS = 1 << 1;

/** Flags to clear in the copyFrom method */
/*package*/ static final int FLAGS_TO_CLEAR_ON_COPY_FROM = FLAG_IN_USE;

@UnsupportedAppUsage
/*package*/ int flags;

从消息池中获取Message对象

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
/** @hide */
public static final Object sPoolSync = new Object();
private static Message sPool;
private static int sPoolSize = 0;

private static final int MAX_POOL_SIZE = 50;

private static boolean gCheckRecycle = true;

/**
* Return a new Message instance from the global pool. Allows us to
* avoid allocating new objects in many cases.
*/
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}

回收的操作

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

/** @hide */
public static void updateCheckRecycle(int targetSdkVersion) {
if (targetSdkVersion < Build.VERSION_CODES.LOLLIPOP) {
gCheckRecycle = false;
}
}

/**
* Return a Message instance to the global pool.
* <p>
* You MUST NOT touch the Message after calling this function because it has
* effectively been freed. It is an error to recycle a message that is currently
* enqueued or that is in the process of being delivered to a Handler.
* </p>
*/
public void recycle() {
if (isInUse()) {
if (gCheckRecycle) {
throw new IllegalStateException("This message cannot be recycled because it "
+ "is still in use.");
}
return;
}
recycleUnchecked();
}

/**
* Recycles a Message that may be in-use.
* Used internally by the MessageQueue and Looper when disposing of queued Messages.
*/
@UnsupportedAppUsage
void recycleUnchecked() {
// Mark the message as in use while it remains in the recycled object pool.
// Clear out all other details.
flags = FLAG_IN_USE;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
sendingUid = UID_NONE;
workSourceUid = UID_NONE;
when = 0;
target = null;
callback = null;
data = null;

synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}