一个线程中有几个Looper？怎么保证？

首先看看Looper源码

怎么获取当前线程Looper

/**
 * 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();
}

一个线程中只能有一个Looper

在prepare的时候会获取ThreadLocal，如果已经存在Looper则抛出异常，否则创建Looper对象

  /** 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()}.
      */
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));
}

怎么保证？

通过ThreadLocal来保证

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

其实ThreadLocal只是个傀儡，真正发挥作用的，是线程内的ThreadLocalMap。

ThreadLocal只是作为一个key，并且提供方法，将线程的Looper保存进(取出)ThreadLocalMap，而已，以此来实现每个线程中只需创建一个Looper。

这就是为什么使用ThreadLocal的原因。