1.前言
Handler是Android最重要的知识点，无论你是初级开发者还是大牛，Handler都是你必须要了解的。它的作用是线程间通讯，由于Android中只允许在主线程中更新UI，而且又有ANR机制，主线程中不应该处理耗时问题.那么在通过网络接口获取数据的时候就一定需要子线程中操作，然后通知主线程去更新UI。于是Android中提供了Handler机制去解决这个问题。当然这也是几乎所有Android面试中必问的问题。如何了解？网络上关于Handler的博客数不胜数，但是了解一个知识点的最好方法是看源码，Handler相关的源码并不多，主要是Handler.java,Message.java,Looper.java,以及MessageQueue.java，代码量并不多，每个类几百行代码还有很多是注释，花个一两个小时就可以深入了解这个模块。
2.知识点介绍
面试中如果问到Handler的原理你应该怎么回答？
Handler是消息处理者，它负责发送消息和处理Message。消息创建后会放入MessageQueue中，Looper是轮询器，其中有一个死循环，不停地从MessageQueue中取出Message，取出之后再交给Handler进行处理。如果这是你的答案，恭喜你答对了，通过十几秒精干的回答面试官一眼就能看出来你压根不了解Handler原理，只是背了面试题。
那么如何让自己显得不那么初级呢？首先，Handler是用来处理线程间通讯的，不提线程肯定是不合格的。
- HandlerThread，Looper，MessageQueue和线程的关系？
- 主线程和子线程之中如何通讯，子线程和子线程如何通讯？
- 为什么创建主线程的Handler不需要调用Looper.prepare()?
这些问题都是你需要了解的。
如果你更加深入了解Handler，那么你还需要明白哪些知识点呢？
- ThreadLocal是什么，为什么Looper中需要使用ThreadLocal？
- Message的回收机制，生产者消费者模式。
- 创建Handler时引发内存泄漏的问题，如何避免？
- 什么是栅栏消息（屏障消息），如何使用？
当然如果你还深入了解MessageQueue的链表结构那就更好啦~
3.源码分析
3.1 Message
Message是消息的载体，可以把它当做一个JavaBean，而且是实现了Parcelable接口的JavaBean，其主要功能是封装数据，以及Message回收机制。
首先看Message的成员变量
what是msg的标识，通过msg.what来区分不同的msg。
arg1,arg2,obj用来让msg携带简单的数据，两个int类型，或者任意一个对象
如果上面的都不够用，bundle则能让msg携带更多的数据
/**
* 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;
/**
* 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.
*/
/*package*/ Bundle data;
那么再看一些不常见的
比如这个replyTo是个啥？
/**
* 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;
没错，这是个Messenger ？Message，Messenger 傻傻分不清楚？从字面意思来看一个是信使一个是信息？
其实也就是字面上的意思，Messenger 通过Message来传递信息。既然如此那为什么使用Handler的时候没见过呢？因为这玩意压根不是线程间通讯用的，而是进程间通讯用的。what？不是说Android线程间通讯用handler进程间通讯用AIDL吗？怎么Handler中又扯到了IPC?
(⊙o⊙)…呃，这是因为Messenger 就是AIDL的封装，一种简化的使用方式。而Messenger需要使用Message来发送进程间消息，所以在这里能见到它。
言归正传，这个Messenger 名字叫replyTo，其作用是将发送者的Messenger对象放入message中，使得接收者在收到消息后可以获得这个Messenger对象来给发送者回复。有兴趣的玩家可以自己研究一下AIDL嗷！
剩下成员的都列出来
/*package*/ int flags;
/*package*/ long when;
/*package*/ Bundle data;
/*package*/ Handler target;
/*package*/ Runnable callback;
// sometimes we store linked lists of these things
/*package*/ Message next;
private 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;
使用Handler的时候都知道Message可以new出来，但是应该使用
handler.obtainMessage() 来获取msg对象。
这是因为Message有一套自己的回收复用机制，如果每用到一个msg就new一个，会产生大量垃圾，增加触发gc频率。要知道虽然现在ART虚拟机gc的时候不会stopTheWorld了，但是也会影响性能的。
那么现在就来了解一哈Message的回收机制吧
/**
* 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();
}
/**
* Recycles a Message that may be in-use.
* Used internally by the MessageQueue and Looper when disposing of queued Messages.
*/
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 = -1;
when = 0;
target = null;
callback = null;
data = null;
synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
我们在使用Handler中obtainMessage()方法时会调用Message的obtain()方法
在Looper.loop() 循环体中处理消息的时候dispatchMessage给handler后，会执行message的recycleUnchecked() 方法对使用过的msg回收利用。
说实话在看源码之前我一直以为Message的回收机制会用享元模式弄一个类似常量池的东西 = 。=
从上述代码中可以看出，每一个Message对象都会有一个next变量和一个静态sPool变量用来存储缓存的message，是一个典型的链表结构。同时有一个静态变量进行计数，最大可缓存消息为50（MAX_POOL_SIZE）个，注意这并不说明Handler如果缓存满了就不能处理消息了，只是不再回收。这不是还有一句return new Message(); 嘛。
总结一下也就是当一个Message被使用完之后，回收msg链表长度小于50，那么执行回收策略，将sPool的值赋予这个对象的next，将这个对象自己赋值给sPool，链表长度+1。当需要使用msg的时候，obtainMessage会判断sPool这个静态变量是否为空，如果不为空说明之前有被回收的msg，把这个被回收的msg取出来，将这个msg的next，也就是被回收时的sPool（上一个被回收对象）赋值给现在的sPool，同时链表长度-1。
obtain() 还有一些重载函数
/**
* 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;
if (orig.data != null) {
m.data = new Bundle(orig.data);
}
m.target = orig.target;
m.callback = orig.callback;
return m;
}
这个函数形参是一个Message对象，此时obtain会得到一个这个对象的深拷贝对象。
还有很多都是复制原msg的一些属性的obtain方法，太多就不贴出来了，可以自己看源码。
/**
* 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;
if (o.data != null) {
this.data = (Bundle) o.data.clone();
} else {
this.data = null;
}
}
还有这个方法，可以得到一个几乎一样的msg。
到这Message.java的源码主要部分就介绍完了，剩下的还有一些get/set方法和序列化反序列化方法。
3.2 Handler
Handler.java第一句定义的变量就是
/*
* Set this flag to true to detect anonymous, local or member classes
* that extend this Handler class and that are not static. These kind
* of classes can potentially create leaks.
*/
private static final boolean FIND_POTENTIAL_LEAKS = false;
可见Handler导致Android内存泄漏是个多么经典的实例啊。简直就是官方认定的教材。
解决Handler内存泄漏问题需要创建Handler静态子类，但是变成静态类之后handleMessage更改UI没有context怎么办？当然是创建一个Activity的弱引用啦，只要GC的时候会干掉它就不会导致内存泄漏了。
具体的代码网上都有我就不贴出来了。
继续看代码
/**
* Callback interface you can use when instantiating a Handler to avoid
* having to implement your own subclass of Handler.
*
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
public interface Callback {
public boolean handleMessage(Message msg);
}
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
通过接口回调获得message的处理逻辑。
/**
* Default constructor associates this handler with the {@link Looper} for the
* current thread.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*/
public Handler() {
this(null, false);
}
/**
* Constructor associates this handler with the {@link Looper} for the
* current thread and takes a callback interface in which you can handle
* messages.
*
* If this thread does not have a looper, this handler won't be able to receive messages
* so an exception is thrown.
*
* @param callback The callback interface in which to handle messages, or null.
*/
public Handler(Callback callback) {
this(callback, false);
}
/**
* Use the provided {@link Looper} instead of the default one.
*
* @param looper The looper, must not be null.
*/
public Handler(Looper looper) {
this(looper, null, false);
}
/**
* Use the provided {@link Looper} instead of the default one and take a callback
* interface in which to handle messages.
*
* @param looper The looper, must not be null.
* @param callback The callback interface in which to handle messages, or null.
*/
public Handler(Looper looper, Callback callback) {
this(looper, callback, false);
}
/**
* Use the {@link Looper} for the current thread
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with respect to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
public Handler(boolean async) {
this(null, async);
}
/**
* Use the {@link Looper} for the current thread with the specified callback interface
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with respect to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
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;
}
/**
* Use the provided {@link Looper} instead of the default one and take a callback
* interface in which to handle messages. Also set whether the handler
* should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with respect to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param looper The looper, must not be null.
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
Handler的各种构造函数
形参介绍：
looper：轮询器
callback：使用时传入的handleMessage实现
async：标识同步还是异步，这个在后面说栅栏消息的时候会用到
/**
* Causes the Runnable r to be added to the message queue.
* The runnable will be run on the thread to which this handler is
* attached.
*
* @param r The Runnable that will be executed.
*
* @return Returns true if the Runnable was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
/**
* Enqueue a message into the message queue after all pending messages
* before (current time + delayMillis). You will receive it in
* {@link #handleMessage}, in the thread attached to this handler.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
/**
* Enqueue a message into the message queue after all pending messages
* before the absolute time (in milliseconds) <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* Time spent in deep sleep will add an additional delay to execution.
* You will receive it in {@link #handleMessage}, in the thread attached
* to this handler.
*
* @param uptimeMillis The absolute time at which the message should be
* delivered, using the
* {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
if (queue == null) {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
return enqueueMessage(queue, msg, uptimeMillis);
}
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
private static Message getPostMessage(Runnable r, Object token) {
Message m = Message.obtain();
m.obj = token;
m.callback = r;
return m;
}
使用Handler传递一个Runnable对象，可以看到sendMessageDelayed() 其实调用的是sendMessageAtTime() ，最终调用到MessageQueue的enqueueMessage()。
sendMessage()和sendEmptyMessage()其实都是一样，只不过sendEmptyMessage()的时候会自动obtain一个Message。同样sendMessage调用sendEmptyMessageDelayed()，sendEmptyMessageDelayed()调用sendMessageDelayed()
/**
* Pushes a message onto the end of the message queue after all pending messages
* before the current time. It will be received in {@link #handleMessage},
* in the thread attached to this handler.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
/**
* Sends a Message containing only the what value.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean sendEmptyMessage(int what)
{
return sendEmptyMessageDelayed(what, 0);
}
/**
* Sends a Message containing only the what value, to be delivered
* after the specified amount of time elapses.
* @see #sendMessageDelayed(android.os.Message, long)
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
/**
* Sends a Message containing only the what value, to be delivered
* at a specific time.
* @see #sendMessageAtTime(android.os.Message, long)
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageAtTime(msg, uptimeMillis);
}
/**
* Enqueue a message into the message queue after all pending messages
* before (current time + delayMillis). You will receive it in
* {@link #handleMessage}, in the thread attached to this handler.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
/**
* Enqueue a message into the message queue after all pending messages
* before the absolute time (in milliseconds) <var>uptimeMillis</var>.
* <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
* Time spent in deep sleep will add an additional delay to execution.
* You will receive it in {@link #handleMessage}, in the thread attached
* to this handler.
*
* @param uptimeMillis The absolute time at which the message should be
* delivered, using the
* {@link android.os.SystemClock#uptimeMillis} time-base.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting. Note that a
* result of true does not mean the message will be processed -- if
* the looper is quit before the delivery time of the message
* occurs then the message will be dropped.
*/
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
MessageQueue queue = mQueue;
if (queue == null) {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
return false;
}
return enqueueMessage(queue, msg, uptimeMillis);
}
还有一些方法是对MessageQueue方法的封装比如enqueueMessage,removeMessages,removeCallbacksAndMessages,hasMessages等
还有两方法是有关Messenger的
final IMessenger getIMessenger() {
synchronized (mQueue) {
if (mMessenger != null) {
return mMessenger;
}
mMessenger = new MessengerImpl();
return mMessenger;
}
}
private final class MessengerImpl extends IMessenger.Stub {
public void send(Message msg) {
msg.sendingUid = Binder.getCallingUid();
Handler.this.sendMessage(msg);
}
}
还有几个是google不推荐使用的方法，源码中有，但是不放入API中，就不贴出来了
3.3 Looper
Looper的代码只有300多行，核心方法主要就是prepare()和loop()方法。
我们首先看它的构造
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
可以看到每个Looper对象会创建一个MessageQueue，并且绑定当前的线程。
Looper的构造函数是私有的，也就是说外界不能通过new的方式去初始化一个Looper对象。而是应该使用Looper.prepare()
Looper和MessageQueue可以有很多个，但是每个线程只能绑定一个Looper和MessageQueue。
/**
* Returns true if the current thread is this looper's thread.
*/
public boolean isCurrentThread() {
return Thread.currentThread() == mThread;
}
它还提供了一个isCurrentThread() 方法，也就是说可以通过Looper对象来判断这个looper是不是当前线程的looper。
接下来看Prepare的逻辑
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));
}
ThreadLocal：
可以看到Looper在prepare的时候new了个Looper对象到了ThreadLocal里。
ThreadLocal是什么？虽然它名字里有个Thread，但是它不是一个Thread。而是一个线程内部的存储类。
可以在指定线程内存储数据，数据存储以后，只有指定线程可以得到存储数据。ThreadLocal提供了线程内存储变量的能力，这些变量不同之处在于每一个线程读取的变量是对应的互相独立的。通过get和set方法就可以得到当前线程对应的值。也就是说它给每个线程存储专属的数据。
详细了解请移步
ThreadLocal
回到prepare()方法，Looper.prepare()的时候将looper对象存入了本线程的threadLocal中
再往下看，代码中专门给主线程写了prepareMainLooper()
/**
* Initialize the current thread as a looper, marking it as an
* application's main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself. See also: {@link #prepare()}
*/
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
/**
* Return the {@link MessageQueue} object associated with the current
* thread. This must be called from a thread running a Looper, or a
* NullPointerException will be thrown.
*/
public static @NonNull MessageQueue myQueue() {
return myLooper().mQueue;
}
这就是主线程中创建Handler不需要调用Looper.prepare()的原因。
那么是哪里调用它的呢？贴一段ActivityThread的main方法，一目了然。
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
SamplingProfilerIntegration.start();
// CloseGuard defaults to true and can be quite spammy. We
// disable it here, but selectively enable it later (via
// StrictMode) on debug builds, but using DropBox, not logs.
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
OK，再看重点，loop方法
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
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 slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
final long end;
try {
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (slowDispatchThresholdMs > 0) {
final long time = end - start;
if (time > slowDispatchThresholdMs) {
Slog.w(TAG, "Dispatch took " + time + "ms on "
+ Thread.currentThread().getName() + ", h=" +
msg.target + " cb=" + msg.callback + " msg=" + msg.what);
}
}
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();
}
}
可以看到loop方法中有一个死循环，一直调用MessageQueue的next() 方法从queue中取msg，只有msg为空的时候才会退出循环。拿到消息以后msg.target.dispatchMessage(msg); 将msg丢给Handler来处理。当msg处理完毕，调用msg.recycleUnchecked(); 回收这个msg。
3.4 MessageQueue
终于写到最后一个部分了，这也是最复杂的一个类，将近1000行代码，使用很多native方法。
private native static long nativeInit();
private native static void nativeDestroy(long ptr);
private native void nativePollOnce(long ptr, int timeoutMillis); /*non-static for callbacks*/
private native static void nativeWake(long ptr);
private native static boolean nativeIsPolling(long ptr);
private native static void nativeSetFileDescriptorEvents(long ptr, int fd, int events);
MessageQueue的构造方法调用的是nativeInit()方法
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}
再看MessageQueue.cpp
void MessageQueue::init(const sp<SurfaceFlinger>& flinger)
{
mFlinger = flinger;
mLooper = new Looper(true);
mHandler = new Handler(*this);
}
创建了一个C++的Looper和Handler对象。这里其实是Android在native层也有一套Message机制用于底层通讯，这套机制是c++编写的。本篇我们主要看java层的机制。
我们先看Handler发送消息时MessageQueue做了什么，handler中是调用了
enqueueMessage 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;
}
主要看这一段,解释放在注释里
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
//如果队列中没有消息，或者当前进入的消息比消息队列中的消息等待时间短，那么就放在消息队列的头部
// 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.
//判断唤醒条件，当前当前消息队列头部消息是屏障消息，且当前插入的消息为异步消息
//且当前消息队列处于无消息可处理的状态
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
//循环遍历消息队列，把当前进入的消息放入合适的位置（比较等待时间）
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;
}
//调用nativeWake，以触发nativePollOnce函数结束等待
if (needWake) {
nativeWake(mPtr);
}
再看获取消息。
在Looper.loop()时，looper不断从MessageQueue中获取msg，调用的是next() 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();
}
//执行native层消息机制层
//timeOutMillis参数为超时等待时间。如果为-1，则表示无限等待，直到有事件发生为止。
//如果值为0，则无需等待立即返回。该方法可能会阻塞
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.
//判断是否是栅栏，同时获取消息队列最近的异步消息
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
//获取消息，判断等待时间，如果还需要等待则等待相应时间后唤醒
// 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 {
// 不需要等待时间或者等待时间已经到了，那么直接返回该消息
// 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.
//判断是否已经退出了
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.
//获取空闲时处理任务的handler 用于发现线程何时阻塞等待更多消息的回调接口。
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
//如果空闲时处理任务的handler个数为0，继续让线程阻塞
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
//判断当前空闲时处理任务的handler是否是为空
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.
//只有第一次迭代的时候，才会执行下面代码
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);
}
//如果不保存空闲任务，执行完成后直接删除
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
// 重置空闲的handler个数，因为不需要重复执行
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.
//当执行完空闲的handler的时候，新的native消息可能会进入，所以唤醒Native消息机制层
nextPollTimeoutMillis = 0;
}
}
看到没，这是啥
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());
}
这就是栅栏消息的处理逻辑，msg不为空，而target也就是Handler为空的时候，这个消息是一个栅栏消息。碰到这种栅栏消息，这个do…while循环会拦住所有同步消息，跳出循环时，msg指向离表头最近的一个异步消息。为什么要重点提这个屏障消息，虽然在app开发中几乎不会碰到这玩意，但是很不巧我吃过它大亏。屏障消息app开发中几乎不会用到，但是系统会用啊。
我这个大亏来自于一次压力测试，不停地hotboot开机关机3个小时之后报ANR，这个ANR指向我的app的MainActivity中的Handler，而且每次出现都是在我的app里。当时的我直接懵逼，加个log测一次3小时呢，3小时的log你知道有多少吗？我电脑打开都死机了，测试设备还不好借，what are you弄啥哩？
花了多久就不说了，看到这里你肯定能猜到ANR的原因就是因为我的MessageQueue收到屏障消息但是一直没有解除导致我的Message爆了。谁发的呢？
Android应用框架中为了更快的响应UI刷新事件在ViewRootImpl.scheduleTraversals中使用了同步屏障
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
//设置同步障碍，确保mTraversalRunnable优先被执行
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
//内部通过Handler发送了一个异步消息
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
if (!mUnbufferedInputDispatch) {
scheduleConsumeBatchedInput();
}
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}
没错就是它！那个CR的root cause就是因为压测的时候v-sync消息送不上来了，导致这里处理mTraversalRunnable 的时候
oid doTraversal() {
if (mTraversalScheduled) {
mTraversalScheduled = false;
mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
if (mProfile) {
Debug.startMethodTracing("ViewAncestor");
}
performTraversals();
if (mProfile) {
Debug.stopMethodTracing();
mProfile = false;
}
}
}
它一直没有removeSyncBarrier ，然后我的App就一直不能处理同步消息，还是在压测中，太惨了。
好了其实这个栅栏消息应该放在后面讲的，但是我看到它就想起被压测支配的恐惧。现在我们回到next()方法。处理完了栅栏消息之后的逻辑如下
if (msg != null) {
//获取消息，判断等待时间，如果还需要等待则等待相应时间后唤醒
if (now < msg.when) {
// Next message is not ready. Set a timeout to wake up when it is ready.
//判断当前消息时间，是不是比当前时间大，计算时间差
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// 不需要等待时间或者等待时间已经到了，那么直接返回该消息
// 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;
}
剩下的一些removeMsg的逻辑自己看代码吧，Handler主要的知识点都提到了。
除此之外可以看到MessageQueue中有好几个native方法，没错Android native也有一套Handler机制用于底层媒体通讯，具体结构和java层差不多，有兴趣可以自己了解一哈~终于写完了，突然发现还差几个字到3w字，这句凑数的
