我们常说子线程使用Handler之前，需要先进行Looper.prepare();而主线程不需要额外添加代码，为什么主线程不需要手动操作Looper呢？它又是在哪里做prepare操作的呢？

我们来看一下。

（一）相关源码

Activity启动的源头是来自ActivityThread.java里面的main方法。main方法里会调用Looper.prepareMainLooper();

下面是通过Looper.prepareMainLooper();跟进去的源码。

    /**
     * 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();
        }
    }




Looper.java

    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));
    }



ThreadLocal.java
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }


ThreadLocal.java
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }
ThreadLocal.java
    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }



Looper.java

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




ThreadLocal.java
    /**
     * Returns the value in the current thread's copy of this
     * thread-local variable.  If the variable has no value for the
     * current thread, it is first initialized to the value returned
     * by an invocation of the {@link #initialValue} method.
     *
     * @return the current thread's value of this thread-local
     */
    public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }



ThreadLocal.java
    /**
     * Variant of set() to establish initialValue. Used instead
     * of set() in case user has overridden the set() method.
     *
     * @return the initial value
     */
    private T setInitialValue() {
        T value = initialValue();
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
        return value;
    }



ThreadLocal.java
    protected T initialValue() {
        return null;
    }

（二）总结

Looper.prepareMainLooper();与Looper.prepare();有什么区别？

区别有两点：

1. prepareMainLooper()里面对初始化Looper进行了同步，而且只允许初始化一次，多次初始化会抛出异常；
2. Looper.prepareMainLooper()里面prepare(false);传入的参数，与Looper.prepare()里面prepare(true);传入的参数值不同。这个参数是quitAllowed，这个参数最终会传入到MessageQueue里面作为它的属性，参数的含义为是否允许退出，如果为true，允许MessageQueue在清除掉所有消息之后退出。否则就会抛出异常。

下面是相关源码片段：

Looper.java
    public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }



Looper.java
    public static void prepare() {
        prepare(true);
    }



Looper.java
    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));
    }





MessageQueue.java
    void quit(boolean safe) {
        if (!mQuitAllowed) {
            throw new IllegalStateException("Main thread not allowed to quit.");
        }

        synchronized (this) {
            if (mQuitting) {
                return;
            }
            mQuitting = true;

            if (safe) {
                removeAllFutureMessagesLocked();
            } else {
                removeAllMessagesLocked();
            }

            // We can assume mPtr != 0 because mQuitting was previously false.
            nativeWake(mPtr);
        }
    }

下一次讲Looper与Handler源码分析，敬请期待！