Handler运行机制 Looper MessageQueue ThreadLocal

曾经终败给现在 2022-06-02 09:22 182阅读 0赞

> 你只有努力奔跑，才能留在原地。

### 一. Handler ###

post or send message post 最终还是走send一样的逻辑代码：

public final boolean post(Runnable r)
        {
           return  sendMessageDelayed(getPostMessage(r), 0);
        }
    
    public final boolean sendMessage(Message msg)
        {
            return sendMessageDelayed(msg, 0);
        }

发送消息到messageQueue, messageQueue通过enqueueMessage method已单链表数据结构保存消息

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
            msg.target = this;
            if (mAsynchronous) {
                msg.setAsynchronous(true);
            }
            return queue.enqueueMessage(msg, uptimeMillis);
        }

处理Looper.loop 循环取出的消息

/** * Handle system messages here. */
        public void dispatchMessage(Message msg) {
            if (msg.callback != null) {    // msg.callback 是post method 传入的回调
                handleCallback(msg);      // 该方法执行message.callback.run(); 
            } else {
                if (mCallback != null) {     //public Handler(Callback callback, boolean async) 构造Handler的时候 mCallback = callback; 
                    if (mCallback.handleMessage(msg)) {
                        return;
                    }
                }
                handleMessage(msg);
            }
        }

### 二. Looper ###

**get message**  
通过loop() 方法进而MessageQueue.next()方法，循环取出MessageQueue里的Message。此处可能会死循环！

--------------------

/** * 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;
                }
                ...
        }

> Message msg = queue.next(); 当msg == null，才退出循环；但是messageQueue.next(), 可能会一直死循环(block)!

**MessageQueue.java**

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 (;;) {
                ...
            }
        }

get message 之后进而给 handler 处理相应的消息

msg.target.dispatchMessage(msg);    //msg.target 就是对应的handler, handler send message 的时候这是handler send消息的时候保存在Messasg对象

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
            msg.target = this;    //这里传入的 handler类似接口回调
            if (mAsynchronous) {
                msg.setAsynchronous(true);
            }
            return queue.enqueueMessage(msg, uptimeMillis);
        }```

### 三. MessageQueue ###

由Looper构造方法创建，当调用Looper.prepare(); 系统将会创建一个对应的MessageQueue；

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();
    }

Looper对象持有MessageQueue, Handler 通过拿到当前线程的Looper对象，进而拿到Looper的MessageQueue，再通过上面的post或者send Message到MessageQueue; Looper.loop(), 一直循环取出Message, 进而给对应的Handler处理(dispatchMessage method 处理)

### 四. ThreadLocal ###

ThreadLocal一般用来保存每个Thread特有的对象或者属性;  
Looper对象就是保存在ThreadLocal里面的，通过ThreadLocal.set() save Looper对象，ThreadLocal.get() get Looper对象; 线程中只能有不超过一个Looper对象，并且Thread 跟Looper对象一对一的关系；这样用ThreadLocal来存取Looper, 就不用一个类型LooperManager类来管理所有Thread的Looper对象了。

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

验证一下ThreadLocal.get(), ThreadLocal.set() 方法，以及ThreadLocal在多个Thread中是怎么保持自我的。

private static void testThreadLocal() {
            final ThreadLocal<Integer> threadLocal = new ThreadLocal<>();
    
            new Thread() {
                @Override
                public void run() {
                    super.run();
                    threadLocal.set(100);
                    System.out.println("Thread name: " + Thread.currentThread() + " threadLocal value: " + threadLocal.get());
                }
            }.start();
    
            new Thread() {
                @Override
                public void run() {
                    super.run();
                    threadLocal.set(200);
                    System.out.println("Thread name: " + Thread.currentThread() + " threadLocal value: " + threadLocal.get());
                }
            }.start();
    
            System.out.println("Thread name: " + Thread.currentThread() + " threadLocal value: " + threadLocal.get());
        }

**log:**

Thread name: Thread[Thread-0,5,main] threadLocal value: 100
    Thread name: Thread[Thread-1,5,main] threadLocal value: 200
    Thread name: Thread[main,5,main] threadLocal value: null

### 五. 使用中可能会遇到的问题 ###