Android中HandlerThread类的解析
Android中HandlerThread类的解析
Android应用中的消息循环由Looper和Handler配合完成,Looper类用于封装消息循环,类中有个MessageQueue消息队列;Handler类封装了消息投递和消息处理等功能。
系统默认情况下只有主线程(即UI线程)绑定Looper对象,因此在主线程中可以直接创建Handler的实例,但是在子线程中就不能直接new出Handler的实例了,因为子线程默认并没有Looper对象,此时会抛出RuntimeException异常:
浏览下Handler的默认构造函数就一目了然了:
- public Handler() {
- 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 = null;
- }
如果需要在子线程中使用Handler类,首先需要创建Looper类实例,这时可以通过Looper.prepare()和Looper.loop()函数来实现的。阅读Framework层源码发现,Android为我们提供了一个HandlerThread类,该类继承Thread类,并使用上面两个函数创建Looper对象,而且使用wait/notifyAll解决了多线程中子线程1获取子线程2的Looper对象为空的问题。HandlerThread类完整代码如下:
- /**
- * Handy class for starting a new thread that has a looper. The looper can then
- * be used to create handler classes. Note that start() must still be called.
- */
- public class HandlerThread extends Thread {
- private int mPriority; // 线程优先级
- private int mTid = -1; // 线程ID
- private Looper mLooper; // 我们需要的Looper对象
- 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;
- }
- /**
- * 在Looper.loop()之前执行动作的回调函数
- */
- protected void onLooperPrepared() {
- }
- public void run() {
- mTid = Process.myTid();
- Looper.prepare(); // 创建本线程的Looper对象
- synchronized (this) {
- mLooper = Looper.myLooper();
- notifyAll(); //通知所有等待该线程Looper对象的其他子线程,本线程的Looper对象已就绪
- }
- Process.setThreadPriority(mPriority);
- onLooperPrepared(); //回调函数
- Looper.loop(); //开始消息队列循环
- mTid = -1;
- }
- /**
- * This method returns the Looper associated with this thread. If this
- * thread not been started or for any reason is 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(); //Looper对象未创建好,等待
- } catch (InterruptedException e) {
- }
- }
- }
- return mLooper;
- }
- /**
- * Ask the currently running looper to quit. 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.
- */
- public boolean quit() {
- Looper looper = getLooper();
- if (looper != null) {
- looper.quit();
- return true;
- }
- return false;
- }
- /**
- * Returns the identifier of this thread. See Process.myTid().
- */
- public int getThreadId() {
- return mTid;
- }
- }
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