另一鲜为人知的单例写法-ThreadLocal

源代码范例

当我阅读FocusFinder和Choreographer的时候，我发现这两类的单例实现和我们寻经常使用双重检查锁非常不一样。而是用来一个ThreadLocal。这个也能够实现单例啊，那这个与双重检查锁实现的单例有什么差别呢？

1.FocusFinder

/**
 * The algorithm used for finding the next focusable view in a given direction
 * from a view that currently has focus.
 */
public class FocusFinder {

    private static final ThreadLocal<FocusFinder> tlFocusFinder =
            new ThreadLocal<FocusFinder>() {
                @Override
                protected FocusFinder initialValue() {
                    return new FocusFinder();
                }
            };

    /**
     * Get the focus finder for this thread.
     */
    public static FocusFinder getInstance() {
        return tlFocusFinder.get();
    }

    // enforce thread local access
    private FocusFinder() {}
}

2.Choreographer

public final class Choreographer {
    // Thread local storage for the choreographer.
    private static final ThreadLocal<Choreographer> sThreadInstance =
            new ThreadLocal<Choreographer>() {
        @Override
        protected Choreographer initialValue() {
            Looper looper = Looper.myLooper();
            if (looper == null) {
                throw new IllegalStateException("The current thread must have a looper!");
            }
            return new Choreographer(looper);
        }
    };

    private Choreographer(Looper looper) {
        mLooper = looper;
        mHandler = new FrameHandler(looper);
        mDisplayEventReceiver = USE_VSYNC ?

new FrameDisplayEventReceiver(looper) : null;
        mLastFrameTimeNanos = Long.MIN_VALUE;

        mFrameIntervalNanos = (long)(1000000000 / getRefreshRate());

        mCallbackQueues = new CallbackQueue[CALLBACK_LAST + 1];
        for (int i = 0; i <= CALLBACK_LAST; i++) {
            mCallbackQueues[i] = new CallbackQueue();
        }
    }

    /**
     * Gets the choreographer for the calling thread.  Must be called from
     * a thread that already has a {@link android.os.Looper} associated with it.
     *
     * @return The choreographer for this thread.
     * @throws IllegalStateException if the thread does not have a looper.
     */
    public static Choreographer getInstance() {
        return sThreadInstance.get();
    }

}

理论分析

ThreadLocal会为每个线程提供一个独立的变量副本，从而隔离了多个线程对数据的訪问冲突。

对于多线程资源共享的问题，同步机制採用了“以时间换空间”的方式，而ThreadLocal採用了“以空间换时间”的方式。前者仅提供一份变量，让不同的线程排队訪问，而后者为每个线程都提供了一份变量。因此能够同一时候訪问而互不影响。

public class ThreadLocal{

    /**
     * Provides the initial value of this variable for the current thread.
     * The default implementation returns {@code null}.
     *
     * @return the initial value of the variable.
     */
    protected T initialValue() {
        return null;
    }

    /**
     * Returns the value of this variable for the current thread. If an entry
     * doesn‘t yet exist for this variable on this thread, this method will
     * create an entry, populating the value with the result of
     * {@link #initialValue()}.
     *
     * @return the current value of the variable for the calling thread.
     */
    @SuppressWarnings("unchecked")
    public T get() {
        // Optimized for the fast path.
        Thread currentThread = Thread.currentThread();
        Values values = values(currentThread);
        if (values != null) {
            Object[] table = values.table;
            int index = hash & values.mask;
            if (this.reference == table[index]) {
                return (T) table[index + 1];
            }
        } else {
            values = initializeValues(currentThread);
        }

        return (T) values.getAfterMiss(this);
    }

    /**
     * Gets Values instance for this thread and variable type.
     */
    Values values(Thread current) {
        return current.localValues;
    }

    /**
     * Sets the value of this variable for the current thread. If set to
     * {@code null}, the value will be set to null and the underlying entry will
     * still be present.
     *
     * @param value the new value of the variable for the caller thread.
     */
    public void set(T value) {
        Thread currentThread = Thread.currentThread();
        Values values = values(currentThread);
        if (values == null) {
            values = initializeValues(currentThread);
        }
        values.put(this, value);
    }

}

实现步骤

//1.initialValue，创建ThreadLocal对象
//2.get()，获取当前线程里的values
//3.假设不存在则初始化一个空的values
//4.假设存在，则复用values

另一处经典应用

在Looper中使用ThreadLocal,使之每个Thread都有一个Looper与之相应.

public class Looper{
    // sThreadLocal.get() will return null unless you‘ve called prepare().
    static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
    /** 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()}.
     */
    public static void prepare() {
       prepare(true);
    }

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

自己也写

public class Manager {

    private static final ThreadLocal<Manager> sManager = new ThreadLocal<Manager>() {
        @Override
        protected Manager initialValue() {
            return new Manager();
        }
    };

    private Manager() {

    }

    public static Manager getInstance() {
        return sManager.get();
    }
}

參考

• 彻底理解ThreadLocal(http://blog.csdn.net/lufeng20/article/details/24314381)