常见问题
- 一个线程中有几个Handler？
N个。
因为Handler可以在Activity可以new，在Service里面也可以new，而Activity全部都跑在了主线程里面，这就证明了主线程中可以有多个Handler。
- 一个线程中有几个Looper？
1个。 在 Looper 创建的时候就 mThread = Thread.currentThread();获取了当前线程; 在 loop 时进行线程比较;
- 多个 Handler 往 MessageQueue 中添加数据，内部是如何确保线程安全的 ？
synchronized(this) 同步锁
- Looper.loop() 为什么不会阻塞主线程？
loop 本事就是在运行在主线的死循环里, 通过 epoll机制 , I/O 多路复用
- Handler内存泄漏场景？
handler 持有 activity 的引用, 当发送延迟消息之后, activity 销毁, 这个是时候 handleMessage 时出现内存泄露的情况;
- IntentService源码解析？
IntentService
- Handler 生产消费模式内存情况？
- 子线程到主线程的通讯的实现方式？
通过内存共享
Handler、Looper、Message、MessageQueue的引用关系
ActivityThread -> Looper -> MessageQueue -> Message -> Handler -> Activity;
-
ActivityThread 执行 Looper.loop()
-
Looper 初始化 new MessageQueue
-
MessageQueue 持有 mMessages
-
Message 的 target = Handler、 callback = Runnable
Looper
-
Looper.loop()
-
构造方法 Thread 和 mQueue
@UnsupportedAppUsage
final MessageQueue mQueue;
final Thread mThread;
- sThreadLocal
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
Handler
-
构造里的 Looper
-
dispatchMessage
-
handleMessage
MessageQueue
-
next()
-
epoll 机制
-
postSyncBarrier
-
idleHandler
Message
- obatin message 对象池
/** @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();
}
Handler 相关总结
- enqueueMessage 时有一个时间参数；
- MessageQueue 里面的消息是无上限的；
- AMS 管理四大组件使用 handler 机制，一切的生命周期变化都是通过 message 发送, 主要是 ApplicationThread 这个 binder 来发送生命周期的变化给到 ActivityThread 来处理
- 60Hz 屏幕刷新的同步 vsync 信号量，每一次信号会发送3个 message， 一个刷线，一个屏障，一个清除屏障的消息；具体查看 view.invalidate 文章
- message 的执行的耗时时间，可以进行性能分析
- eventbus、rxandroid、retrofit、 都用到 Handler 把子线程消息传递给主线程通信；
- 主线程就是UI线程，展示UI 垂直同步信号
MessageQueue epoll机制
延迟消息使用的是：Linux 的 epoll_wait 机制
1、 消息是通过 enqueueMessage(msg) 方法插入消息链表中的，并且按照 message.when 时间排序，链表头的延迟时间小，尾部延迟时间最大。
2、 Looper.loop()通过 MessageQueue 中的next()去取消息。
3、 next()中如果当前链表头部的 message 是延迟消息，则根据延迟时间进行消息队列会阻塞，不返回给 Looper message，并设置定时唤醒，唤醒后，返回message给looper处理。
4、 如果在 looper 处于休眠中(要么链表为空，要么整个链表均为延时message)，此时若有新的消息插入到链表头部则直接唤醒线程，looper继续判断步骤3)。
5、 最终Looper将表头message交给handler中的handleMessage处理，然后继续调用MessageQueue的next()方法，如果刚刚的延迟消息还是时间未到，则计算时间继续阻塞。
epoll_create、epoll_ctl、epoll_wait
阻塞的时候使用的是 mEpollFd 文件句柄 ,
唤醒的时候使用的是 mWakeEventFd 文件句柄 ,
下面分析这两个文件句柄之间的联系 ;
Looper 构造函数 , 调用了 rebuildEpollLocked() 方法 ,
在 rebuildEpollLocked 方法 中调用 mEpollFd = epoll_create(EPOLL_SIZE_HINT) , 创建了一个句柄 ,
调用 int result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem) 注册事件监听
注册 mEpollFd 句柄 , 监听 mWakeEventFd 句柄的 eventItem 事件 ,
监听的事件是 eventItem.events = EPOLLIN 事件 ,
该事件代表 , 向 mWakeEventFd 文件句柄写入数据 , 此时就对应解除 epoll_wait 阻塞 ;
int epoll_create ( int size );//创建一个epoll监听 返回文件描述符
//对事件进行操作
// epfd：epoll描述符，由create创建的
// op：具体操作
// EPOLL_CTL_ADD：向事件表中注册fd上的事件
// EPOLL_CTL_MOD：修改fd上的注册事件
// EPOLL_CTL_DEL：删除fd上的注册事件
// fd：要注册观察的文件描述符，可以是具体的文件，socket，管道等
// event：具体要检测的事件
// EPOLLIN：有数据流入，即文件非空
// EPOLLOUT：有数据可以写入，即文件非满
int epoll_ctl ( int epfd, int op, int fd, struct epoll_event *event );
//阻塞等待事件发生
// epfd：create创建的epoll描述符
// events：存放事件的数组
// maxevents：最大可存放的事件个数，数组events的大小
// timeout：超时时间，为0时立即返回，为-1时一直阻塞，否则等待timeout时间后返回
int epoll_wait ( int epfd, struct epoll_event* events, int maxevents, int timeout );
MessageQueue 消息屏障
当 View Vsync 垂直同步信号发送之后执行
ViewRootImpl 的 scheduleTraversals 和 doTraversals 执行插入屏障消息，
之后在插入异步的 doTraversal 中执行 performTraversal 之后移除消息；
/**
* Posts a synchronization barrier to the Looper's message queue.
*
* Message processing occurs as usual until the message queue encounters the
* synchronization barrier that has been posted. When the barrier is encountered,
* later synchronous messages in the queue are stalled (prevented from being executed)
* until the barrier is released by calling { @link #removeSyncBarrier} and specifying
* the token that identifies the synchronization barrier.
*
* This method is used to immediately postpone execution of all subsequently posted
* synchronous messages until a condition is met that releases the barrier.
* Asynchronous messages (see { @link Message#isAsynchronous} are exempt from the barrier
* and continue to be processed as usual.
*
* This call must be always matched by a call to { @link #removeSyncBarrier} with
* the same token to ensure that the message queue resumes normal operation.
* Otherwise the application will probably hang!
*
* @return A token that uniquely identifies the barrier. This token must be
* passed to { @link #removeSyncBarrier} to release the barrier.
*
* @hide
*/
@UnsupportedAppUsage
@TestApi
public int postSyncBarrier() {
return postSyncBarrier(SystemClock.uptimeMillis());
}
private int postSyncBarrier(long when) {
// Enqueue a new sync barrier token.
// We don't need to wake the queue because the purpose of a barrier is to stall it.
synchronized (this) {
final int token = mNextBarrierToken++;
final Message msg = Message.obtain();
msg.markInUse();
msg.when = when;
msg.arg1 = token;
Message prev = null;
Message p = mMessages;
if (when != 0) {
while (p != null && p.when <= when) {
prev = p;
p = p.next;
}
}
if (prev != null) { // invariant: p == prev.next
msg.next = p;
prev.next = msg;
} else {
msg.next = p;
mMessages = msg;
}
return token;
}
}
/**
* Removes a synchronization barrier.
*
* @param token The synchronization barrier token that was returned by
* {@link #postSyncBarrier}.
*
* @throws IllegalStateException if the barrier was not found.
*
* @hide
*/
@UnsupportedAppUsage
@TestApi
public void removeSyncBarrier(int token) {
// Remove a sync barrier token from the queue.
// If the queue is no longer stalled by a barrier then wake it.
synchronized (this) {
Message prev = null;
Message p = mMessages;
while (p != null && (p.target != null || p.arg1 != token)) {
prev = p;
p = p.next;
}
if (p == null) {
throw new IllegalStateException("The specified message queue synchronization "
+ " barrier token has not been posted or has already been removed.");
}
final boolean needWake;
if (prev != null) {
prev.next = p.next;
needWake = false;
} else {
mMessages = p.next;
needWake = mMessages == null || mMessages.target != null;
}
p.recycleUnchecked();
// If the loop is quitting then it is already awake.
// We can assume mPtr != 0 when mQuitting is false.
if (needWake && !mQuitting) {
nativeWake(mPtr);
}
}
}
Looper
prepare() -> new Looper() -> new MessageQueue
loop() -> loopOnce -> msg.target.dispatchMessage(msg);
/** Initialize the current thread as a looper.
* This gives you a chance to create handlers that then reference
* this looper, before actually starting the loop. Be sure to call
* {@link #loop()} after calling this method, and end it by calling
* {@link #quit()}.
*/
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));
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
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;
// 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", -1);
me.mSlowDeliveryDetected = false;
for (;;) {
if (!loopOnce(me, ident, thresholdOverride)) {
return;
}
}
```
private static boolean loopOnce(final Looper me,
final long ident, final int thresholdOverride) {
Message msg = me.mQueue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return false;
}
// 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;
final boolean hasOverride = thresholdOverride >= 0;
if (hasOverride) {
slowDispatchThresholdMs = thresholdOverride;
slowDeliveryThresholdMs = thresholdOverride;
}
final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0 || hasOverride)
&& (msg.when > 0);
final boolean logSlowDispatch = (slowDispatchThresholdMs > 0 || hasOverride);
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 (me.mSlowDeliveryDetected) {
if ((dispatchStart - msg.when) <= 10) {
Slog.w(TAG, "Drained");
me.mSlowDeliveryDetected = false;
}
} else {
if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
msg)) {
// Once we write a slow delivery log, suppress until the queue drains.
me.mSlowDeliveryDetected = 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();
return true;
}
}
MessageQueue 入队出队
-
enqueueMessage() 入队
-
next() 出队
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;
}
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;
}
}
native 方法
// 销毁 MessageQueue 时调用的方法
private native static void nativeDestroy(long ptr);
// 线程阻塞方法
@UnsupportedAppUsage
private native void nativePollOnce(long ptr, int timeoutMillis);
// 线程唤醒方法
private native static void nativeWake(long ptr);
Message
- next = message 单项链表指向下一个message
- when 时间点
- target = Handler
- callback = Runnable
- data
*/
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;
/**
* The targeted delivery time of this message. The time-base is
* { @link SystemClock#uptimeMillis}.
* @hide Only for use within the tests.
*/
@UnsupportedAppUsage
@VisibleForTesting(visibility = VisibleForTesting.Visibility.PACKAGE)
public long when;
/*package*/ Bundle data;
@UnsupportedAppUsage
/*package*/ Handler target;
@UnsupportedAppUsage
/*package*/ Runnable callback;
// sometimes we store linked lists of these things
@UnsupportedAppUsage
/*package*/ Message next;
/** @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();
}
/**
* Same as { @link #obtain()}, but copies the values of an existing
* message (including its target) into the new one.
* @param orig Original message to copy.
* @return A Message object from the global pool.
*/
public static Message obtain(Message orig) {
Message m = obtain();
m.what = orig.what;
m.arg1 = orig.arg1;
m.arg2 = orig.arg2;
m.obj = orig.obj;
m.replyTo = orig.replyTo;
m.sendingUid = orig.sendingUid;
m.workSourceUid = orig.workSourceUid;
if (orig.data != null) {
m.data = new Bundle(orig.data);
}
m.target = orig.target;
m.callback = orig.callback;
return m;
}
/**
* Same as { @link #obtain()}, but sets the value for the <em>target</em> member on the Message returned.
* @param h Handler to assign to the returned Message object's <em>target</em> member.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h) {
Message m = obtain();
m.target = h;
return m;
}
/**
* Same as { @link #obtain(Handler)}, but assigns a callback Runnable on
* the Message that is returned.
* @param h Handler to assign to the returned Message object's <em>target</em> member.
* @param callback Runnable that will execute when the message is handled.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h, Runnable callback) {
Message m = obtain();
m.target = h;
m.callback = callback;
return m;
}
/**
* Same as { @link #obtain()}, but sets the values for both <em>target</em> and
* <em>what</em> members on the Message.
* @param h Value to assign to the <em>target</em> member.
* @param what Value to assign to the <em>what</em> member.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h, int what) {
Message m = obtain();
m.target = h;
m.what = what;
return m;
}
/**
* Same as { @link #obtain()}, but sets the values of the <em>target</em>, <em>what</em>, and <em>obj</em>
* members.
* @param h The <em>target</em> value to set.
* @param what The <em>what</em> value to set.
* @param obj The <em>object</em> method to set.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h, int what, Object obj) {
Message m = obtain();
m.target = h;
m.what = what;
m.obj = obj;
return m;
}
/**
* Same as { @link #obtain()}, but sets the values of the <em>target</em>, <em>what</em>,
* <em>arg1</em>, and <em>arg2</em> members.
*
* @param h The <em>target</em> value to set.
* @param what The <em>what</em> value to set.
* @param arg1 The <em>arg1</em> value to set.
* @param arg2 The <em>arg2</em> value to set.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h, int what, int arg1, int arg2) {
Message m = obtain();
m.target = h;
m.what = what;
m.arg1 = arg1;
m.arg2 = arg2;
return m;
}
/**
* Same as { @link #obtain()}, but sets the values of the <em>target</em>, <em>what</em>,
* <em>arg1</em>, <em>arg2</em>, and <em>obj</em> members.
*
* @param h The <em>target</em> value to set.
* @param what The <em>what</em> value to set.
* @param arg1 The <em>arg1</em> value to set.
* @param arg2 The <em>arg2</em> value to set.
* @param obj The <em>obj</em> value to set.
* @return A Message object from the global pool.
*/
public static Message obtain(Handler h, int what,
int arg1, int arg2, Object obj) {
Message m = obtain();
m.target = h;
m.what = what;
m.arg1 = arg1;
m.arg2 = arg2;
m.obj = obj;
return m;
}
/** @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++;
}
}
}
/**
* Make this message like o. Performs a shallow copy of the data field.
* Does not copy the linked list fields, nor the timestamp or
* target/callback of the original message.
*/
public void copyFrom(Message o) {
this.flags = o.flags & ~FLAGS_TO_CLEAR_ON_COPY_FROM;
this.what = o.what;
this.arg1 = o.arg1;
this.arg2 = o.arg2;
this.obj = o.obj;
this.replyTo = o.replyTo;
this.sendingUid = o.sendingUid;
this.workSourceUid = o.workSourceUid;
if (o.data != null) {
this.data = (Bundle) o.data.clone();
} else {
this.data = null;
}
}
/**
* Return the targeted delivery time of this message, in milliseconds.
*/
public long getWhen() {
return when;
}
public void setTarget(Handler target) {
this.target = target;
}
/**
* Retrieve the { @link android.os.Handler Handler} implementation that
* will receive this message. The object must implement
* { @link android.os.Handler#handleMessage(android.os.Message)
* Handler.handleMessage()}. Each Handler has its own name-space for
* message codes, so you do not need to
* worry about yours conflicting with other handlers.
*/
public Handler getTarget() {
return target;
}
/**
* Retrieve callback object that will execute when this message is handled.
* This object must implement Runnable. This is called by
* the <em>target</em> { @link Handler} that is receiving this Message to
* dispatch it. If
* not set, the message will be dispatched to the receiving Handler's
* { @link Handler#handleMessage(Message)}.
*/
public Runnable getCallback() {
return callback;
}
/** @hide */
@UnsupportedAppUsage
public Message setCallback(Runnable r) {
callback = r;
return this;
}
/**
* Obtains a Bundle of arbitrary data associated with this
* event, lazily creating it if necessary. Set this value by calling
* { @link #setData(Bundle)}. Note that when transferring data across
* processes via { @link Messenger}, you will need to set your ClassLoader
* on the Bundle via { @link Bundle#setClassLoader(ClassLoader)
* Bundle.setClassLoader()} so that it can instantiate your objects when
* you retrieve them.
* @see #peekData()
* @see #setData(Bundle)
*/
public Bundle getData() {
if (data == null) {
data = new Bundle();
}
return data;
}
/**
* Like getData(), but does not lazily create the Bundle. A null
* is returned if the Bundle does not already exist. See
* { @link #getData} for further information on this.
* @see #getData()
* @see #setData(Bundle)
*/
public Bundle peekData() {
return data;
}
/**
* Sets a Bundle of arbitrary data values. Use arg1 and arg2 members
* as a lower cost way to send a few simple integer values, if you can.
* @see #getData()
* @see #peekData()
*/
public void setData(Bundle data) {
this.data = data;
}
/**
* Chainable setter for { @link #what}
*
* @hide
*/
public Message setWhat(int what) {
this.what = what;
return this;
}
/**
* Sends this Message to the Handler specified by { @link #getTarget}.
* Throws a null pointer exception if this field has not been set.
*/
public void sendToTarget() {
target.sendMessage(this);
}
/**
* Returns true if the message is asynchronous, meaning that it is not
* subject to { @link Looper} synchronization barriers.
*
* @return True if the message is asynchronous.
*
* @see #setAsynchronous(boolean)
*/
public boolean isAsynchronous() {
return (flags & FLAG_ASYNCHRONOUS) != 0;
}
/**
* Sets whether the message is asynchronous, meaning that it is not
* subject to { @link Looper} synchronization barriers.
* <p>
* Certain operations, such as view invalidation, may introduce synchronization
* barriers into the { @link Looper}'s message queue to prevent subsequent messages
* from being delivered until some condition is met. In the case of view invalidation,
* messages which are posted after a call to { @link android.view.View#invalidate}
* are suspended by means of a synchronization barrier until the next frame is
* ready to be drawn. The synchronization barrier ensures that the invalidation
* request is completely handled before resuming.
* </p><p>
* Asynchronous messages are exempt from synchronization barriers. They typically
* represent interrupts, input events, and other signals that must be handled independently
* even while other work has been suspended.
* </p><p>
* Note that asynchronous messages may be delivered out of order with respect to
* synchronous messages although they are always delivered in order among themselves.
* If the relative order of these messages matters then they probably should not be
* asynchronous in the first place. Use with caution.
* </p>
*
* @param async True if the message is asynchronous.
*
* @see #isAsynchronous()
*/
public void setAsynchronous(boolean async) {
if (async) {
flags |= FLAG_ASYNCHRONOUS;
} else {
flags &= ~FLAG_ASYNCHRONOUS;
}
}
/*package*/ boolean isInUse() {
return ((flags & FLAG_IN_USE) == FLAG_IN_USE);
}
@UnsupportedAppUsage
/*package*/ void markInUse() {
flags |= FLAG_IN_USE;
}
/** Constructor (but the preferred way to get a Message is to call { @link #obtain() Message.obtain()}).
*/
public Message() {
}
@Override
public String toString() {
return toString(SystemClock.uptimeMillis());
}
@UnsupportedAppUsage
String toString(long now) {
StringBuilder b = new StringBuilder();
b.append("{ when=");
TimeUtils.formatDuration(when - now, b);
if (target != null) {
if (callback != null) {
b.append(" callback=");
b.append(callback.getClass().getName());
} else {
b.append(" what=");
b.append(what);
}
if (arg1 != 0) {
b.append(" arg1=");
b.append(arg1);
}
if (arg2 != 0) {
b.append(" arg2=");
b.append(arg2);
}
if (obj != null) {
b.append(" obj=");
b.append(obj);
}
b.append(" target=");
b.append(target.getClass().getName());
} else {
b.append(" barrier=");
b.append(arg1);
}
b.append(" }");
return b.toString();
}
void dumpDebug(ProtoOutputStream proto, long fieldId) {
final long messageToken = proto.start(fieldId);
proto.write(MessageProto.WHEN, when);
if (target != null) {
if (callback != null) {
proto.write(MessageProto.CALLBACK, callback.getClass().getName());
} else {
proto.write(MessageProto.WHAT, what);
}
if (arg1 != 0) {
proto.write(MessageProto.ARG1, arg1);
}
if (arg2 != 0) {
proto.write(MessageProto.ARG2, arg2);
}
if (obj != null) {
proto.write(MessageProto.OBJ, obj.toString());
}
proto.write(MessageProto.TARGET, target.getClass().getName());
} else {
proto.write(MessageProto.BARRIER, arg1);
}
proto.end(messageToken);
}
public static final @android.annotation.NonNull Parcelable.Creator<Message> CREATOR
= new Parcelable.Creator<Message>() {
public Message createFromParcel(Parcel source) {
Message msg = Message.obtain();
msg.readFromParcel(source);
return msg;
}
public Message[] newArray(int size) {
return new Message[size];
}
};
public int describeContents() {
return 0;
}
public void writeToParcel(Parcel dest, int flags) {
if (callback != null) {
throw new RuntimeException(
"Can't marshal callbacks across processes.");
}
dest.writeInt(what);
dest.writeInt(arg1);
dest.writeInt(arg2);
if (obj != null) {
try {
Parcelable p = (Parcelable)obj;
dest.writeInt(1);
dest.writeParcelable(p, flags);
} catch (ClassCastException e) {
throw new RuntimeException(
"Can't marshal non-Parcelable objects across processes.");
}
} else {
dest.writeInt(0);
}
dest.writeLong(when);
dest.writeBundle(data);
Messenger.writeMessengerOrNullToParcel(replyTo, dest);
dest.writeInt(sendingUid);
dest.writeInt(workSourceUid);
}
private void readFromParcel(Parcel source) {
what = source.readInt();
arg1 = source.readInt();
arg2 = source.readInt();
if (source.readInt() != 0) {
obj = source.readParcelable(getClass().getClassLoader());
}
when = source.readLong();
data = source.readBundle();
replyTo = Messenger.readMessengerOrNullFromParcel(source);
sendingUid = source.readInt();
workSourceUid = source.readInt();
}
}
内存抖动
短时间内申请内存和释放内存；
例如；Handler.sendMessage(msg) 时 new Message 短时间内申请很多内存
obtain(); 复用静态池里 sPool 对象，避免申请内存从而回收GC造成的内存抖动；
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();
}
Handler
sendMessage -> enqueueMessage
dispatchMessage -> handleMessage
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;
mIsShared = false;
}
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);
}
HandlerThread 运行在子线程的 Handler
区别 Looper.prepareMain();
/**
* A { @link Thread} that has a { @link Looper}.
* The { @link Looper} can then be used to create { @link Handler}s.
* <p>
* Note that just like with a regular { @link Thread}, { @link #start()} must still be called.
*/
public class HandlerThread extends Thread {
int mPriority;
int mTid = -1;
Looper mLooper;
private @Nullable Handler mHandler;
public HandlerThread(String name) {
super(name);
mPriority = Process.THREAD_PRIORITY_DEFAULT;
}
/**
* Constructs a HandlerThread.
* @param name
* @param priority The priority to run the thread at. The value supplied must be from
* { @link android.os.Process} and not from java.lang.Thread.
*/
public HandlerThread(String name, int priority) {
super(name);
mPriority = priority;
}
/**
* Call back method that can be explicitly overridden if needed to execute some
* setup before Looper loops.
*/
protected void onLooperPrepared() {
}
@Override
public void run() {
mTid = Process.myTid();
Looper.prepare();
synchronized (this) {
mLooper = Looper.myLooper();
notifyAll();
}
Process.setThreadPriority(mPriority);
onLooperPrepared();
Looper.loop();
mTid = -1;
}
/**
* This method returns the Looper associated with this thread. If this thread not been started
* or for any reason isAlive() returns false, this method will return null. If this thread
* has been started, this method will block until the looper has been initialized.
* @return The looper.
*/
public Looper getLooper() {
if (!isAlive()) {
return null;
}
// If the thread has been started, wait until the looper has been created.
synchronized (this) {
while (isAlive() && mLooper == null) {
try {
wait();
} catch (InterruptedException e) {
}
}
}
return mLooper;
}
/**
* @return a shared { @link Handler} associated with this thread
* @hide
*/
@NonNull
public Handler getThreadHandler() {
if (mHandler == null) {
mHandler = new Handler(getLooper());
}
return mHandler;
}
/**
* Quits the handler thread's looper.
* <p>
* Causes the handler thread's looper to terminate without processing any
* more messages in the message queue.
* </p><p>
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the { @link Handler#sendMessage(Message)} method will return false.
* </p><p class="note">
* Using this method may be unsafe because some messages may not be delivered
* before the looper terminates. Consider using { @link #quitSafely} instead to ensure
* that all pending work is completed in an orderly manner.
* </p>
*
* @return True if the looper looper has been asked to quit or false if the
* thread had not yet started running.
*
* @see #quitSafely
*/
public boolean quit() {
Looper looper = getLooper();
if (looper != null) {
looper.quit();
return true;
}
return false;
}
/**
* Quits the handler thread's looper safely.
* <p>
* Causes the handler thread's looper to terminate as soon as all remaining messages
* in the message queue that are already due to be delivered have been handled.
* Pending delayed messages with due times in the future will not be delivered.
* </p><p>
* Any attempt to post messages to the queue after the looper is asked to quit will fail.
* For example, the { @link Handler#sendMessage(Message)} method will return false.
* </p><p>
* If the thread has not been started or has finished (that is if
* { @link #getLooper} returns null), then false is returned.
* Otherwise the looper is asked to quit and true is returned.
* </p>
*
* @return True if the looper looper has been asked to quit or false if the
* thread had not yet started running.
*/
public boolean quitSafely() {
Looper looper = getLooper();
if (looper != null) {
looper.quitSafely();
return true;
}
return false;
}
/**
* Returns the identifier of this thread. See Process.myTid().
*/
public int getThreadId() {
return mTid;
}
}
IdleHandler 空闲消息
Idler
在 messageQueue.next() 方法中处理 idleHandler 消息
在 handleResumeActivity 最后一行使用了 Idler ,作用调用 AMS 通知上一个 activity 执行 onStop 方法
Looper.myQueue().addIdleHandler(new Idler());
private class Idler implements MessageQueue.IdleHandler {
@Override
public final boolean queueIdle() {
ActivityClientRecord a = mNewActivities;
boolean stopProfiling = false;
if (mBoundApplication != null && mProfiler.profileFd != null
&& mProfiler.autoStopProfiler) {
stopProfiling = true;
}
if (a != null) {
mNewActivities = null;
IActivityTaskManager am = ActivityTaskManager.getService();
ActivityClientRecord prev;
do {
if (localLOGV) Slog.v(
TAG, "Reporting idle of " + a +
" finished=" +
(a.activity != null && a.activity.mFinished));
if (a.activity != null && !a.activity.mFinished) {
try {
am.activityIdle(a.token, a.createdConfig, stopProfiling);
a.createdConfig = null;
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
}
prev = a;
a = a.nextIdle;
prev.nextIdle = null;
} while (a != null);
}
if (stopProfiling) {
mProfiler.stopProfiling();
}
applyPendingProcessState();
return false;
}
}
GcIdler
final class GcIdler implements MessageQueue.IdleHandler {
@Override
public final boolean queueIdle() {
doGcIfNeeded();
purgePendingResources();
return false;
}
}
final class PurgeIdler implements MessageQueue.IdleHandler {
@Override
public boolean queueIdle() {
purgePendingResources();
return false;
}
}
Instrument.class 使用 ActivityGoing
private void prePerformCreate(Activity activity) {
if (mWaitingActivities != null) {
synchronized (mSync) {
final int N = mWaitingActivities.size();
for (int i=0; i<N; i++) {
final ActivityWaiter aw = mWaitingActivities.get(i);
final Intent intent = aw.intent;
if (intent.filterEquals(activity.getIntent())) {
aw.activity = activity;
mMessageQueue.addIdleHandler(new ActivityGoing(aw));
}
}
}
}
}
private final class ActivityGoing implements MessageQueue.IdleHandler {
private final ActivityWaiter mWaiter;
public ActivityGoing(ActivityWaiter waiter) {
mWaiter = waiter;
}
public final boolean queueIdle() {
synchronized (mSync) {
mWaitingActivities.remove(mWaiter);
mSync.notifyAll();
}
return false;
}
}
private static final class Idler implements MessageQueue.IdleHandler {
private final Runnable mCallback;
private boolean mIdle;
public Idler(Runnable callback) {
mCallback = callback;
mIdle = false;
}
public final boolean queueIdle() {
if (mCallback != null) {
mCallback.run();
}
synchronized (this) {
mIdle = true;
notifyAll();
}
return false;
}
public void waitForIdle() {
synchronized (this) {
while (!mIdle) {
try {
wait();
} catch (InterruptedException e) {
}
}
}
}
}
