Looper中ThreadLocal的简介

一 ThreadLocal的简介

This class provides thread-local variables. These variables differ from their normal counterparts in that each thread that accesses one (via its {@code get} or {@code set} method) has its own, independently initialized copy of the variable. {@code ThreadLocal} instances are typically private static fields in classes that wish to associate state with a thread (e.g., a user ID or Transaction ID).

以上是摘抄于java中的ThreadLocal.java中的解释.

概意有如下几点(英文不是很好,抱歉)

1. ThreadLocal这个类提供线程本地变量
2. 每个线程可以通过get()或者set()得到或设置自己拥有的变量,且不相互映像

我们通过实例简单测试一下:

        //private ThreadLocal<String> mThreadLocal = new ThreadLocal<>();
        mThreadLocal.set(Thread.currentThread().getName()); //保存当前主线程的名字
        Log.d(TAG, "----Thread#main------mThreadLocal.get():" + mThreadLocal.get());

        new Thread("Thread#1") {
            @Override
            public void run() {
                super.run();
                //保存线程1中的名字
                mThreadLocal.set(Thread.currentThread().getName());
                Log.d(TAG, "----Thread#1------mThreadLocal.get():" + mThreadLocal.get());
            }
        }.start();

        new Thread("Thread#2") {
            @Override
            public void run() {
                super.run();
                //保存线程2中的名字
                mThreadLocal.set(Thread.currentThread().getName());
                Log.d(TAG, "----Thread#2------mThreadLocal.get():" + mThreadLocal.get());
            }
        }.start();

以下就是输出的日志

----Thread#main------mThreadLocal.get():main
----Thread#1------mThreadLocal.get():Thread#1
----Thread#2------mThreadLocal.get():Thread#2

从日志中可以看出(1)ThreadLocal中保存的都是每个线程中的局部变量,而且每个线程只能改变自己线程中的值.

ThreadLocal中部分方法的简介

1. get() 获取当前线程保存的局部变量
2. set() 设置当前线程的变量值
3. remove() //移除当前变量的值
4. setInitialValue()  私有的,设置初始化值

我们平时常用的也就get(),set()和remove()

    public T get() {
        Thread t = Thread.currentThread();
        //获取当前线程中ThreadLocalMap变量,ThreadLocalMap是ThreadLocal中定义的类
        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;
            }
        }
        //如果map为空,就初始化map
        return setInitialValue();
    }

    private T setInitialValue() {
        T value = initialValue(); // 返回 null
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
        return value;
    }


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


     public void remove() {
         ThreadLocalMap m = getMap(Thread.currentThread());
         if (m != null)
             m.remove(this);
     }

二 ThreadLocal在Looper的作用

而我们在(Android消息机制Handler,Looper,Message,MessageQueue关系之一)提到过,base\core\java\android\os\Looper.java中出现了ThreadLocal

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

     //主线程中获取Looper

    public static Looper getMainLooper() {
        synchronized (Looper.class) {
            //在prepareMainLooper中初始化的
            //因为一个应用只有一个主线,所以不用动态的获取
            return sMainLooper;
        }
    }

    public static void prepareMainLooper() {
        prepare(false);//这里初始化并保存当前线程的Looper对象
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper(); //这里获取的主线程中的Looper
        }
    }

//最终都是调用sThreadLocal.get方法获取
 public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }

ThreadLocal中存储的是Looper,而且每个线程中只能有一个Looper

所以我们可通过线程来获取当前线程中的Looper