1）Handler机制和底层实现
2）Handler、Thread和HandlerThread的差别
3）handler发消息给子线程，looper怎么启动？（手动调用）
4）关于Handler，在任何地方new Handler 都是什么线程下? （调用位置的线程）
5）ThreadLocal原理，实现及如何保证Local属性？
6）请解释下在单线程模型中Message、Handler、Message Queue、Looper之间的关系
一.Handler机制
1. 作用
在多线程的应用场景中，将工作线程中需更新UI的操作信息 传递到 UI主线程，从而实现 工作线程对UI的更新处理，最终实现异步消息的处理 。
Handler作用
2. 为什么要用 Handler消息传递机制？
多个线程并发更新UI的同时 保证线程安全。
3. 重要概念
几个关键词
4. 工作原理
线程（Thread）、循环器（Looper）、处理者（Handler）之间的对应关系如下：
- 1个线程（Thread）只能绑定 1个循环器（Looper），但可以有多个处理者（Handler）
- 1个循环器（Looper） 可绑定多个处理者（Handler）
- 1个处理者（Handler） 只能绑定1个1个循环器（Looper）
二. Handler、Thread和HandlerThread的区别
HandlerThread的源码
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;
}
}
五. ThreadLocal原理，实现及如何保证Local属性？
当需要使用多线程时，有个变量恰巧不需要共享，此时就不必使用synchronized这么麻烦的关键字来锁住，每个线程都相当于在堆内存中开辟一个空间，线程中带有对共享变量的缓冲区，通过缓冲区将堆内存中的共享变量进行读取和操作，ThreadLocal相当于线程内的内存，一个局部变量。每次可以对线程自身的数据读取和操作，并不需要通过缓冲区与 主内存中的变量进行交互。
并不会像synchronized那样修改主内存的数据，再将主内存的数据复制到线程内的工作内存。ThreadLocal可以让线程独占资源，存储于线程内部，避免线程堵塞造成CPU吞吐下降。
在每个Thread中包含一个ThreadLocalMap，ThreadLocalMap的key是ThreadLocal的对象，value是独享数据。
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