anr,多线程,handler

应该了解的知识：

1.什么是进程、什么是线程

2.为什么要使用多线程

3.Handler机制

4.Handler原理

什么是进程、什么是线程

进程是一个程序的完全执行，包括了程序和程序需要的资源，是分配资源的基本单位；线程是独立运行和独立调度的基本单位，一边听歌一边打字一边玩游戏看似是同时进行实际上是CPU运行时候，快速切换资源，让用户感觉是同时进行的，我们可以将线程理解为程序中的一段可执行的代码块。

为什么要使用多线程--》可以参考别人的专门理解的理解：http://blog.youkuaiyun.com/itjavawfc/article/details/50360776

 1）耗时的操作使用线程，提高应用程序响应
2）并行操作时使用线程，如C/S架构的服务器端并发线程响应用户的请求。
3）多CPU系统中，使用线程提高CPU利用率
4）改善程序结构。一个既长又复杂的进程可以考虑分为多个线程，成为几个独立或半独立的运行部分，这样的程序会利于理解和修改。 

Handler 讲解

先看handler源码的部分解释：

/**
 * A Handler allows you to send and process {@link Message} and Runnable
 * objects associated with a thread's {@link MessageQueue}.  Each Handler
 * instance is associated with a single thread and that thread's message
 * queue.  When you create a new Handler, it is bound to the thread /
 * message queue of the thread that is creating it -- from that point on,
 * it will deliver messages and runnables to that message queue and execute
 * them as they come out of the message queue.
 * 
 * <p>There are two main uses for a Handler: (1) to schedule messages and
 * runnables to be executed as some point in the future; and (2) to enqueue
 * an action to be performed on a different thread than your own.
 * 
 * <p>Scheduling messages is accomplished with the
 * {@link #post}, {@link #postAtTime(Runnable, long)},
 * {@link #postDelayed}, {@link #sendEmptyMessage},
 * {@link #sendMessage}, {@link #sendMessageAtTime}, and
 * {@link #sendMessageDelayed} methods.  The <em>post</em> versions allow
 * you to enqueue Runnable objects to be called by the message queue when
 * they are received; the <em>sendMessage</em> versions allow you to enqueue
 * a {@link Message} object containing a bundle of data that will be
 * processed by the Handler's {@link #handleMessage} method (requiring that
 * you implement a subclass of Handler).
 * 
 * <p>When posting or sending to a Handler, you can either
 * allow the item to be processed as soon as the message queue is ready
 * to do so, or specify a delay before it gets processed or absolute time for
 * it to be processed.  The latter two allow you to implement timeouts,
 * ticks, and other timing-based behavior.
 * 
 * <p>When a
 * process is created for your application, its main thread is dedicated to
 * running a message queue that takes care of managing the top-level
 * application objects (activities, broadcast receivers, etc) and any windows
 * they create.  You can create your own threads, and communicate back with
 * the main application thread through a Handler.  This is done by calling
 * the same <em>post</em> or <em>sendMessage</em> methods as before, but from
 * your new thread.  The given Runnable or Message will then be scheduled
 * in the Handler's message queue and processed when appropriate.
 */ 

提取信息时：Handler是用来发送并处理消息和runnable线程对象的，同时2）排列一个在不同的线程上执行的操作，而不是自己的。

Handler的原理实际上就是说Android中异步线程的实现原理：

                                                           

     在线程内部有一个或多个Handler对象，外部程序通过改Handler对象向线程发送异步消息，消息经由Handler传递道MessageQueue对象中。线程内部智能包含一个MessageQueue对象，Looper对象像一个抽水机一样，不断的读取线程MessageQueue中的消息，并回掉给Handler对象中的回掉函数handlerMessage().

分析线程内部的Handler、MessageQueue/Looper类的调用过程

1）.线程局部存储Thread Local Storage

程序员通过调用Looper类的静态方法prepare（）为线程创建MessageQueue对象：看源码

  /**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))
    }

我们在看看Looper(quitAllowed)源码：

    /**
     *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;
    }

    private Looper(boolean quitAllowed) {
      mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

可以看到我们通过上面代码创建了Message对象。

     上面的sThreadLocal的类型是ThreadLocal,该类的作用是提供“线程局部存储”也叫线程本地变量.我们希望，当同时从一个线程中引用该变量时，其值总是相同的，不同线程引用该变量的时候，其值是不同的，即我们需要一个作用域为线程的变量的定义，这就是线程变量局部存储。

     ThreadLocal就是提供了这种功能的类，Looper内部的sThreadLocal变量是当该进程第一次调用Looper.prepare()时候被复制的，之后该进程中的其它线程调用prepare（）函数的时候，sThreadLocak变量就已经被赋值了。sThreadLocak对象内部会根据调用prepare()线程的id保存一个数据对象，这个数据对象就是所谓的“线程局部存储”对象，该对象是通过sThreadLocal的set()方法设置进去的，Looper类保存的这个对象是一个Looper对象。

   prepare()函数中首先调用sThreadLocal.get（）函数获取该县城对应的Looper对象，如果线程已经存在Looper对象，肯定出错。否则为该线程创建一个新的Looper对象。

   为什么一个线程中只能有一个Looper对象呢？因为异步线程需要的，每个Looper对象都会定义一个MessageQueue对象，一个异步线程只能有一个消息队列，所有只有一个Looper对象。

    讲那么多理论的东西可能都绕地球好多圈了，不太理解：其实在一个线程中的Looper对象创建MessageQueue中的过程中即执行Looper.praper（）方法的时候，使用sThreadLocal来找该设置Looper对象到线程中去，通过线程的id设置道sThreadLoad对象中去。一个线程中就只有一个Looper对象了。prepare()首先调用sThreadLocal().get()获得该线程的Looperd对象.若果存在直接报异常，不存在直接创建，这样保证每个线程只有一个Looper对象和MessageQueue对象.

Looper

      Looper作用：第一、为该类的静态函数prepare()线程创建一个消息队列；第二、提供静态函数loop()，使调用该函数线程进行无线循环，并从消息队列中读取消息。

第一点就是上面的sThreadLocal中的相关代码创建MessageQueue对象。

     当需要一个线程变为异步消息处理线程的时候，在Thread类的run方法调用Looper.prepare()为该线程创建一个MessageQueue对象，在调用Looper.loop（）函数，进行消息处理循环：

    /**
     * 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;
            }

            // This must be in a local variable, in case a UI event sets the logger
            Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            msg.target.dispatchMessage(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();
        }
    }

该方法的解释：

Run the message queue in this thread. Be sure to call

就是返回线程中的消息队列，并回掉！

获得Looper对象final Looper me = myLooper();其实通过sThreadLocal()调用获得

/**
 * 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();
}

进入无线循环while(true):

for (;;) {
    Message msg = queue.next(); // might block
    if (msg == null) {
        // No message indicates that the message queue is quitting.
        return;
    }

可以看看next()，当message为空，则线程挂起。

msg.target.dispatchMessage()，完成消息的处理。

msg.target.dispatchMessage(msg);要看看里面的源码：
 /**
     * 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);
        }
    }
处理完消息后ong
msg.recycleUnchecked();回收Message对象占用的系统资源。

MessageQueue消息队列：消息读/取不能同时进行，有枷锁机制

/**
 * Low-level class holding the list of messages to be dispatched by a
 * {@link Looper}.  Messages are not added directly to a MessageQueue,
 * but rather through {@link Handler} objects associated with the Looper.
 * 
 * <p>You can retrieve the MessageQueue for the current thread with
 * {@link Looper#myQueue() Looper.myQueue()}.
 */
public final class MessageQueue {

看源码：保存message的list【容器】,message不是直接添加道MessageQueue中取，通过looper对象来的。

Handler：

在Looper.looper（）中取出消息后，回掉msg.target对象的handleMessage()添加函数，而msg.target的类型正是handler。

Handler处理消息地方：

/**
     * 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);
        }
    }

实际项目中的运用：三点要理解

第一种方式：

先：重写创建handler对象，重写handler的handler的handlerMessage（）方法

private Handler handler = new Handler(){
		public void handleMessage(Message msg) {
			//消息接收到了
			int what = msg.what;
			switch (what) {
			case 1:
				String text = (String) msg.obj;
				ts.setText(text);
				break;
			case 2:
				//刷新其他UI
				break;
			default:
				break;
			}
		};
	};

其次创建消息：

Message msg = new Message();//初始化方式
msg_ob.obj = "你家的牛在我菜园吃菜";
msg_ob.what = 1;//企业开发用常量

handler.sendMessage(msg);

下面是否封装的一个HandlerUtils工具:

public class MsgUtils {

	static public void SendMSG(Handler handler,Object obj,int what,int arg1){
		Message msg = new Message();
		msg.obj = obj;
		msg.what = what;
		msg.arg1 = arg1;
		handler.sendMessage(msg);
	}
	
	static public void SendMSG(Handler handler,Object obj,int what){
		Message msg = new Message();
		msg.obj = obj;
		msg.what = what;
		handler.sendMessage(msg);
	}
	
	static public void SendMSG(Handler handler,int what){
		Message msg = new Message();
		msg.what = what;
		handler.sendMessage(msg);
	}
	
	static public void SendMSG(Handler handler,int what,int arg1){
		Message msg = new Message();
		msg.what = what;
		msg.arg1 = arg1;
		handler.sendMessage(msg);
	}
	
	static public void SendMSG(Handler handler,Object obj){
		Message msg = new Message();
		msg.obj = obj;
		handler.sendMessage(msg);
	}
	
	static public void SendMSG(Handler handler,int what,Object obj,int arg1){
		Message msg = new Message();
		msg.obj = obj;
		msg.arg1 = arg1;
		msg.what = what;
		handler.sendMessage(msg);
	}
}

第二种方式：获取message消息方式不一样

	Message msg_ob = handler.obtainMessage();//使用了池的概念
	msg_ob.obj = "你家的牛在我菜园吃菜";
	msg_ob.what = 1;//企业开发用常量
	msg_ob.sendToTarget();

直接通过handler的obtainMessage（）方法使用了池的概念

源码如下：

/**
     * Returns a new {@link android.os.Message Message} from the global message pool. More efficient than
     * creating and allocating new instances. The retrieved message has its handler set to this instance (Message.target == this).
     *  If you don't want that facility, just call Message.obtain() instead.
     */
    public final Message obtainMessage()
    {
        return Message.obtain(this);
    }

    /**
     * Same as {@link #obtainMessage()}, except that it also sets the what member of the returned Message.
     * 
     * @param what Value to assign to the returned Message.what field.
     * @return A Message from the global message pool.
     */
    public final Message obtainMessage(int what)
    {
        return Message.obtain(this, what);
    }
    

/**
     * 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;
    }

第三种方式，实现一个ruannable接口，然后将这个线程作为消息发送：

	private Runnable run_handler = new Runnable() {
		
		public void run() {
			i++;
			ts.setText("runnable 刷新UI"+i+"次");
			handler.postDelayed(run_handler, 100);
		}
	};

这种方式的好处是随时添加删除：

handler.post(run_handler);

handler.removeCallbacks(run_handler);//停止循环

 /**
     * 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);
    }
    
    /**
     * Causes the Runnable r to be added to the message queue, to be run
     * at a specific time given by <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.
     * The runnable will be run on the thread to which this handler is attached.
     *
     * @param r The Runnable that will be executed.
     * @param uptimeMillis The absolute time at which the callback should run,
     *         using the {@link android.os.SystemClock#uptimeMillis} time-base.
     *  
     * @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.  Note that a
     *         result of true does not mean the Runnable 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 postAtTime(Runnable r, long uptimeMillis)
    {
        return sendMessageAtTime(getPostMessage(r), uptimeMillis);
    }