線程封閉之ThreadLocal

多線程訪問共享可變數據時，涉及到線程間數據同步問題。

數據都被封閉在各自的線程之中，就不需要同步，這種通過將數據封閉在線程中，而避免使同步的技術，稱爲線程封閉。

ThreadLocal

• 線程級別變量
• 每個線程都有一個ThreadLocal
• 每個線程都擁有了自己獨立的一個變量
• 競爭條件被徹底消除了
• 在併發模式下是絕對安全的變量

1.示例

import java.text.SimpleDateFormat;
import java.util.Random;
import java.util.concurrent.TimeUnit;

public class ThreadLocalDemo implements Runnable {

    ThreadLocal<SimpleDateFormat> simpleDateFormatThreadLocal =
            ThreadLocal.withInitial(() -> new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));

    public static void main(String[] args) throws InterruptedException {
        ThreadLocalDemo threadLocalDemo = new ThreadLocalDemo();
        for (int i = 0; i < 10; i++) {
            Thread thread = new Thread(threadLocalDemo, String.valueOf(i));
            TimeUnit.MILLISECONDS.sleep(new Random().nextInt(1000));
            thread.start();
        }
    }

    @Override
    public void run() {
        System.out.println("Thread name = " + Thread.currentThread().getName()
                + ", default formatter = " + simpleDateFormatThreadLocal.get().toPattern());
        try {
            TimeUnit.MILLISECONDS.sleep(new Random().nextInt(1000));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        simpleDateFormatThreadLocal.set(new SimpleDateFormat());
        System.out.println("Thread name = " + Thread.currentThread().getName()
                + ", formatter = " + simpleDateFormatThreadLocal.get().toPattern());
    }
}

輸出

Thread name = 0, default formatter = yyyy-MM-dd HH:mm:ss
Thread name = 0, formatter = yy-M-d ah:mm
Thread name = 1, default formatter = yyyy-MM-dd HH:mm:ss
Thread name = 2, default formatter = yyyy-MM-dd HH:mm:ss
Thread name = 1, formatter = yy-M-d ah:mm
Thread name = 3, default formatter = yyyy-MM-dd HH:mm:ss
Thread name = 2, formatter = yy-M-d ah:mm
Thread name = 3, formatter = yy-M-d ah:mm
Thread name = 4, default formatter = yyyy-MM-dd HH:mm:ss
Thread name = 5, default formatter = yyyy-MM-dd HH:mm:ss
Thread name = 4, formatter = yy-M-d ah:mm
Thread name = 5, formatter = yy-M-d ah:mm
Thread name = 6, default formatter = yyyy-MM-dd HH:mm:ss
Thread name = 6, formatter = yy-M-d ah:mm
Thread name = 7, default formatter = yyyy-MM-dd HH:mm:ss
Thread name = 8, default formatter = yyyy-MM-dd HH:mm:ss
Thread name = 8, formatter = yy-M-d ah:mm
Thread name = 9, default formatter = yyyy-MM-dd HH:mm:ss
Thread name = 7, formatter = yy-M-d ah:mm
Thread name = 9, formatter = yy-M-d ah:mm

雖然Thread-0已經改變了formatter的值，顯然其他線程的默認formatter格式沒有受到影響。

2.原理分析

在Thread類中有如下屬性

/* ThreadLocal values pertaining to this thread. This map is maintained
 * by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;

默認情況下，threadLocals屬性值爲null。只有當調用ThreadLocal的set()、get()方法時，如果爲null則調用createMap()賦初值。

/**
 * Sets the current thread's copy of this thread-local variable
 * to the specified value.  Most subclasses will have no need to
 * override this method, relying solely on the {@link #initialValue}
 * method to set the values of thread-locals.
 *
 * @param value the value to be stored in the current thread's copy of
 *        this thread-local.
 */
public void set(T value) {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
}

/**
 * Get the map associated with a ThreadLocal. Overridden in
 * InheritableThreadLocal.
 *
 * @param  t the current thread
 * @return the map
 */
ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;
}

/**
 * Create the map associated with a ThreadLocal. Overridden in
 * InheritableThreadLocal.
 *
 * @param t the current thread
 * @param firstValue value for the initial entry of the map
 */
void createMap(Thread t, T firstValue) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);
}

 最終，變量存儲在ThreadLocalMap中，key爲ThreadLocal對象。