为何 handlerThread.getLooper不会返回null

最新推荐文章于 2024-01-12 13:48:33 发布

原创最新推荐文章于 2024-01-12 13:48:33 发布 · 1.2k 阅读

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本文深入探讨了Android中HandlerThread的使用方法及其内部Looper机制。解析了getLooper方法为何不会返回空指针，详细说明了从HandlerThread启动到Looper创建的全过程。

HandlerThread的使用方法如下：

HandlerThread handlerThread = new HandlerThread("myTest");
handlerThread.start();

Looper looper = handlerThread.getLooper();
Handler handler = new Handler(looper);

为何getLooper方法不会返回空指针呢？

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;
}

/**
 * Tests if this thread is alive. A thread is alive if it has
 * been started and has not yet died.
 *
 * @return  <code>true</code> if this thread is alive;
 *          <code>false</code> otherwise.
 */
// Android-changed: Provide pure Java implementation of isAlive().
public final boolean isAlive() {
    return nativePeer != 0;
}

通过源码发现，只要start方法执行了，nativePeer（native层指针）就不会为空。

public synchronized void start() {
    /**
     * This method is not invoked for the main method thread or "system"
     * group threads created/set up by the VM. Any new functionality added
     * to this method in the future may have to also be added to the VM.
     *
     * A zero status value corresponds to state "NEW".
     */
    // Android-changed: Replace unused threadStatus field with started field.
    // The threadStatus field is unused on Android.
    if (started)
        throw new IllegalThreadStateException();

    /* Notify the group that this thread is about to be started
     * so that it can be added to the group's list of threads
     * and the group's unstarted count can be decremented. */
    group.add(this);

    // Android-changed: Use field instead of local variable.
    // It is necessary to remember the state of this across calls to this method so that it
    // can throw an IllegalThreadStateException if this method is called on an already
    // started thread.
    started = false;
    try {
        // Android-changed: Use Android specific nativeCreate() method to create/start thread.
        // start0();
        nativeCreate(this, stackSize, daemon);
        started = true;
    } finally {
        try {
            if (!started) {
                group.threadStartFailed(this);
            }
        } catch (Throwable ignore) {
            /* do nothing. If start0 threw a Throwable then
              it will be passed up the call stack */
        }
    }
}

最终HandlerThread初始化Looper后会notifyAll()。

@Override
public void run() {
    mTid = Process.myTid();
    Looper.prepare();
    synchronized (this) {
        mLooper = Looper.myLooper();
        notifyAll();
    }
    Process.setThreadPriority(mPriority);
    onLooperPrepared();
    Looper.loop();
    mTid = -1;
}