handler机制是Android重要的多线程数据传输机制，所以想从源码来解析这个机制。
一般使用
在Activity中
public class MainActivity extends AppCompatActivity {
Handler handler = new Handler(new Handler.Callback() {
@Override
public boolean handleMessage(Message msg) {
switch (msg.what){
case 1:
showToast((String) msg.obj);
break;
}
return false;
}
});
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button button = findViewById(R.id.btn_start);
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
new Thread(new Runnable() {
@Override
public void run() {
Message message = Message.obtain();
message.what = 1;
message.obj = "Handler机制";
handler.sendMessage(message);
}
}).start();
}
});
}
private void showToast(String content){
Toast.makeText(this,content,Toast.LENGTH_SHORT).show();
}
}
Handler机制
1 最基本的Message
Message两种创建
- 直接构造
Message message = new Message();
- 通过obtain()方法
/**
* 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();
}
注释很清楚，从全局池返回一个Message实例，避免在很多情况下分配新对象
很重要的变量
Handler target;
Message next;
这两个之后会用到
2 handler.sendMessage方法
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
这些相关的sendMessage方法最终会调用一个enqueueMessage方法
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
这个方法中将Message的target赋值为handler自己，并调用了MessageQueue.enqueueMessage方法
3 MessageQueue.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;
}
核心大概这些代码，其实就是一个单向链表
boolean enqueueMessage(Message msg, long when) {
//保证线程安全性
synchronized (this) {
msg.when = when;
Message p = mMessages;
//这个消息是第一个或这个消息无需延迟 或 这个消息延迟时间比第一个消息短
if (p == null || when == 0 || when < p.when) {
//把这个消息放到链表的头
msg.next = p;
mMessages = msg;
} else {
//循环整个链表，直到结束或这个消息延迟时间比某一个消息短
// 1. 直到结束情况：把这个消息放到链表的末尾
// 2.这个消息延迟时间比某一个消息短： prev -> p 改为 prev -> msg -> p
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
}
return true;
}
本质上完成对于把你传递的消息跟之前的消息排序
4. Looper的loop方法（循环消息队列）
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();
}
}
其实核心就干了一件事msg.target.dispatchMessage(msg);
public static void loop() {
final Looper me = myLooper();
final MessageQueue queue = me.mQueue;
for (;;) {
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
try {
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
}
}
诡异的是这个和我们最开始Handler的实现的回调方法名并不是一样的
5.Handler的handleMessage方法
原来最后是由dispatchMessage方法调用的，其实也是因为两种方式来实现handleMessage，一种通过匿名接口回调实现，另外一种直接重写handleMessage
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
一些疑问
Handler与Looper的关联
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;
}
所以的构造都会调用这个，在这个里面调用获取了mLooper和mQueue
Looper的创建
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
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));
}
通过ThreadLocal的特性，保证每个线程只存在一个Looper对象。
ThreadLocal是一个关于创建线程局部变量的类。
通常情况下，我们创建的变量是可以被任何一个线程访问并修改的。而使用ThreadLocal创建的变量只能被当前线程访问，其他线程则无法访问和修改。
而这个实在ActivityThread（并没有继承线程）中被调用prepareMainLooper方法
public static void main(String[] args) {
...
Looper.prepareMainLooper();
...
Handler 为什么可以 post Runnable
其实就是Handler自己帮你把Runnable转换成Message
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
子线程展示Toast报错的原因
很多人都以为子线程展示Toast是因为不在主线程，其实并不是。
private static class TN extends ITransientNotification.Stub {
...
TN(String packageName, @Nullable Looper looper) {
...
if (looper == null) {
// Use Looper.myLooper() if looper is not specified.
looper = Looper.myLooper();
if (looper == null) {
throw new RuntimeException(
"Can't toast on a thread that has not called Looper.prepare()");
}
}
mHandler = new Handler(looper, null) {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case SHOW: {
IBinder token = (IBinder) msg.obj;
handleShow(token);
break;
}
case HIDE: {
handleHide();
// Don't do this in handleHide() because it is also invoked by
// handleShow()
mNextView = null;
break;
}
case CANCEL: {
handleHide();
// Don't do this in handleHide() because it is also invoked by
// handleShow()
mNextView = null;
try {
getService().cancelToast(mPackageName, TN.this);
} catch (RemoteException e) {
}
break;
}
}
}
};
}
}
因为Toast需要初始化一个Handler，并且最后的show仍然是在Handler的回调执行的
同理，子线程展示Dialog报错也是因为这个
如果非要在子线程展示Toast,可以这样
class LooperThread extends Thread {
public Handler mHandler;
public void run() {
Looper.prepare();
Toast.makeText(getApplicationContext(),"子线程展示Toast",Toast.LENGTH_SHORT).show();
Looper.loop();
}
}
但是不要这么做，因为Looper会阻塞这个线程
主线程不会因为Looper.loop()里的死循环卡死
这里涉及线程，先说说说进程/线程，进程：每个app运行时前首先创建一个进程，该进程是由Zygote fork出来的，用于承载App上运行的各种Activity/Service等组件。进程对于上层应用来说是完全透明的，这也是google有意为之，让App程序都是运行在Android Runtime。大多数情况一个App就运行在一个进程中，除非在AndroidManifest.xml中配置Android:process属性，或通过native代码fork进程。
线程：线程对应用来说非常常见，比如每次new Thread().start都会创建一个新的线程。该线程与App所在进程之间资源共享，从Linux角度来说进程与线程除了是否共享资源外，并没有本质的区别，都是一个task_struct结构体，在CPU看来进程或线程无非就是一段可执行的代码，CPU采用CFS调度算法，保证每个task都尽可能公平的享有CPU时间片。
有了这么准备，再说说死循环问题：
对于线程既然是一段可执行的代码，当可执行代码执行完成后，线程生命周期便该终止了，线程退出。而对于主线程，我们是绝不希望会被运行一段时间，自己就退出，那么如何保证能一直存活呢？简单做法就是可执行代码是能一直执行下去的，死循环便能保证不会被退出，例如，binder线程也是采用死循环的方法，通过循环方式不同与Binder驱动进行读写操作，当然并非简单地死循环，无消息时会休眠。但这里可能又引发了另一个问题，既然是死循环又如何去处理其他事务呢？通过创建新线程的方式。
真正会卡死主线程的操作是在回调方法onCreate/onStart/onResume等操作时间过长，会导致掉帧，甚至发生ANR，looper.loop本身不会导致应用卡死。
转载Gityuan的知乎回答
也就是说looper本身不会，它只是分发消息。ANR只是出现在处理消息的时候（也就是我们触摸屏幕等）
总结handler机制流程图
最后整个机制可以用这个流程图表示
handler机制流程图.png
