Handler源码解析二------- Looper源码

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原创最新推荐文章于 2026-07-09 09:55:52 发布 336 阅读 AI 写同款· GEO 优化 ›

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Handler源码解析二------- Looper

Handler相关的线程处理，还有Looper、Message和MessageQueue，下面会一一介绍。

本篇介绍Looper的源码

分析的是android api23 的源码，先上全部源码，在一个一个方法介绍

/*
 * Copyright (C) 2006 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.os;

import android.annotation.NonNull;
import android.annotation.Nullable;
import android.util.Log;
import android.util.Printer;

/**
  * Class used to run a message loop for a thread.  Threads by default do
  * not have a message loop associated with them; to create one, call
  * {@link #prepare} in the thread that is to run the loop, and then
  * {@link #loop} to have it process messages until the loop is stopped.
  *
  * <p>Most interaction with a message loop is through the
  * {@link Handler} class.
  *
  * <p>This is a typical example of the implementation of a Looper thread,
  * using the separation of {@link #prepare} and {@link #loop} to create an
  * initial Handler to communicate with the Looper.
  *
  * <pre>
  *  class LooperThread extends Thread {
  *      public Handler mHandler;
  *
  *      public void run() {
  *          Looper.prepare();
  *
  *          mHandler = new Handler() {
  *              public void handleMessage(Message msg) {
  *                  // process incoming messages here
  *              }
  *          };
  *
  *          Looper.loop();
  *      }
  *  }</pre>
  */
public final class Looper {
    /*
     * API Implementation Note:
     *
     * This class contains the code required to set up and manage an event loop
     * based on MessageQueue.  APIs that affect the state of the queue should be
     * defined on MessageQueue or Handler rather than on Looper itself.  For example,
     * idle handlers and sync barriers are defined on the queue whereas preparing the
     * thread, looping, and quitting are defined on the looper.
     */

    private static final String TAG = "Looper";

    // sThreadLocal.get() will return null unless you've called prepare().
    static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
    private static Looper sMainLooper;  // guarded by Looper.class

    final MessageQueue mQueue;
    final Thread mThread;

    private Printer mLogging;

     /** 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));
    }

    /**
     * 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();
        }
    }

    /**
     * Returns the application's main looper, which lives in the main thread of the application.
     */
    public static Looper getMainLooper() {
        synchronized (Looper.class) {
            return sMainLooper;
        }
    }

    /**
     * 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();
        }
    }

    /**
     * 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;
    }

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

    /**
     * Returns true if the current thread is this looper's thread.
     */
    public boolean isCurrentThread() {
        return Thread.currentThread() == mThread;
    }

    /**
     * Control logging of messages as they are processed by this Looper.  If
     * enabled, a log message will be written to <var>printer</var>
     * at the beginning and ending of each message dispatch, identifying the
     * target Handler and message contents.
     *
     * @param printer A Printer object that will receive log messages, or
     * null to disable message logging.
     */
    public void setMessageLogging(@Nullable Printer printer) {
        mLogging = printer;
    }

    /**
     * Quits the looper.
     * <p>
     * Causes the {@link #loop} method 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>
     *
     * @see #quitSafely
     */
    public void quit() {
        mQueue.quit(false);
    }

    /**
     * Quits the looper safely.
     * <p>
     * Causes the {@link #loop} method to terminate as soon as all remaining messages
     * in the message queue that are already due to be delivered have been handled.
     * However pending delayed messages with due times in the future will not be
     * delivered before the loop terminates.
     * </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>
     */
    public void quitSafely() {
        mQueue.quit(true);
    }

    /**
     * Gets the Thread associated with this Looper.
     *
     * @return The looper's thread.
     */
    public @NonNull Thread getThread() {
        return mThread;
    }

    /**
     * Gets this looper's message queue.
     *
     * @return The looper's message queue.
     */
    public @NonNull MessageQueue getQueue() {
        return mQueue;
    }

    /**
     * Dumps the state of the looper for debugging purposes.
     *
     * @param pw A printer to receive the contents of the dump.
     * @param prefix A prefix to prepend to each line which is printed.
     */
    public void dump(@NonNull Printer pw, @NonNull String prefix) {
        pw.println(prefix + toString());
        mQueue.dump(pw, prefix + "  ");
    }

    @Override
    public String toString() {
        return "Looper (" + mThread.getName() + ", tid " + mThread.getId()
                + ") {" + Integer.toHexString(System.identityHashCode(this)) + "}";
    }
}

先看最上面的类注释

*
* <p>This is a typical example of the implementation of a Looper thread,
* using the separation of {@link #prepare} and {@link #loop} to create an
* initial Handler to communicate with the Looper.
*
* <pre>
*  class LooperThread extends Thread {
*      public Handler mHandler;
*
*      public void run() {
*          Looper.prepare();
*
*          mHandler = new Handler() {
*              public void handleMessage(Message msg) {
*                  // process incoming messages here
*              }
*          };
*
*          Looper.loop();
*      }
*  }</pre>

这是一个典型的子线程实现looper的例子，源码中就有的。

在继续看类的成员变量

/*
 * API Implementation Note:
 *
 * This class contains the code required to set up and manage an event loop
 * based on MessageQueue.  APIs that affect the state of the queue should be
 * defined on MessageQueue or Handler rather than on Looper itself.  For example,
 * idle handlers and sync barriers are defined on the queue whereas preparing the
 * thread, looping, and quitting are defined on the looper.
 */

private static final String TAG = "Looper";

// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper;  // guarded by Looper.class

final MessageQueue mQueue;
final Thread mThread;

private Printer mLogging;

这些中第一个是TAG，打log用的。

第二个sThreadLocal是个线程变量，这个Looper是基于线程的，每个线程有一个Looper对象，这个Looper其实是存储在线程里的线程变量中的。

第三个是 sMainLooper 主线程looper，这里没看到子线程有同样的表示，还是主线程有特权啊。

第四个mQueue 是每一个looper都对应一个消息队列，这里持有这个队列的引用

第五个mThread 是looper执行的当前线程

第六个那个是调试打日志用的，这里不具体介绍了

然后把这几个主要的成员变量使用的地方介绍一下：

 /** 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));
}

/**
 * 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();
}

上面这两处是引用sThreadLocal的地方。

第一个是静态方法，创建looper到时候调用的：Looper.prepare();这个就可以new出Looper来了

第二个是判断是否有这个sThreadLocal存在，注释中也介绍了，如果为null说明Looper没有和调用的线程关联上

下面继续介绍sMainLooper

/**
 * 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();
    }
}

/**
 * Returns the application's main looper, which lives in the main thread of the application.
 */
public static Looper getMainLooper() {
    synchronized (Looper.class) {
        return sMainLooper;
    }
}

这两个也是静态方法，其中调用prepare（）方法，这里注解介绍了，是系统调用，不需要咱们来调用这两个方法，调用的话会报错

第二个是获得这个sMainLooper对象。

下面继续介绍

final MessageQueue mQueue;

/**
 * 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();
    }
}

/**
 * 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();
}

public void quit() {
    mQueue.quit(false);
}

public void quitSafely() {
    mQueue.quit(true);
}

/**
 * Gets this looper's message queue.
 *
 * @return The looper's message queue.
 */
public @NonNull MessageQueue getQueue() {
    return mQueue;
}

/**
 * Dumps the state of the looper for debugging purposes.
 *
 * @param pw A printer to receive the contents of the dump.
 * @param prefix A prefix to prepend to each line which is printed.
 */
public void dump(@NonNull Printer pw, @NonNull String prefix) {
    pw.println(prefix + toString());
    mQueue.dump(pw, prefix + "  ");
}

一共这么几个地方，

dump（）方法就是调试日志用的。

getQueue（）方法就是获得他的引用

quitSafely() 这个是安全退出

quit() 和退出，这两个的具体实现实在MessageQueue中，之后会介绍，这两个区别在退出时如果有任务没执行完成该如何处理。

Looper（）这个是构造方法了，构造方法是privae的，以后写程序也可以考虑这种方式，用static的方法去new 这个对象。

在这个构造方法中new出这个MessageQueue，所以没有Looper都会对应一个MessageQueue和一个Thread。

之后是myQueue()方法代码是 return myLooper().mQueue;

这个myLooper（）实现是这样的

public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}

其实就是从线程中取出looper，然后从looper中取出mQueue，这个和那个getQueue（）方法貌似得到的一样，只是从两个地方取的，目前没发现区别。

然后继续到了最主要的loop（）方法

final Looper me = myLooper();
if (me == null) {
    throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}

前面是如果Looper为null就抛出异常了

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();

然后验证一下身份

之后是一串for循环

for (;;) {

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

查询queue中是否有消息，没有就返回了

// 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);
}

打印的日志

msg.target.dispatchMessage(msg);

这个msg是消息Message，从MessageQueue中获取到的，需要处理的。

这个target其实是handler，也就是调用了handler的dispatchMessage（）讲这个消息处理掉了，这里不明白的可以看下上一篇的handler解析

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);
}

又是调试日志处理

msg.recycleUnchecked();

将消息回收了。

for循环结束，方法结束

然后继续看Looper类的成员变量

final Thread mThread;

看看他的引用的地方

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

这个是私有的构造方法，前面介绍过了，讲mThread 引用为当前线程。

/**
 * Returns true if the current thread is this looper's thread.
 */
public boolean isCurrentThread() {
    return Thread.currentThread() == mThread;
}

看看是否为当前调用的线程

/**
 * Gets the Thread associated with this Looper.
 *
 * @return The looper's thread.
 */
public @NonNull Thread getThread() {
    return mThread;
}

从looper中获得线程的引用，程序代码判断处理用的。

@Override
public String toString() {
    return "Looper (" + mThread.getName() + ", tid " + mThread.getId()
            + ") {" + Integer.toHexString(System.identityHashCode(this)) + "}";
}