Java并发编程之ThreadLocal类详解

概括起来说，对于多线程资源共享的问题，同步机制采用了“以时间换空间”的方式，而ThreadLocal采用了“以空间换时间”的方式。前者仅提供一份变量，让不同的线程排队访问，而后者为每一个线程都提供了一份变量，因此可以同时访问而互不影响。

模拟ThreadLocal

     import java.util.Collections;      import java.util.HashMap;      import java.util.Map;             public class SimpleThreadLocal<T> {      private Map<Thread, T> valueMap = Collections      .synchronizedMap(new HashMap<Thread, T>());             public void set(T newValue) {      valueMap.put(Thread.currentThread(), newValue); // ①键为线程对象，值为本线程的变量副本      }             public T get() {      Thread currentThread = Thread.currentThread();      T o = valueMap.get(currentThread); // ②返回本线程对应的变量      if (o == null && !valueMap.containsKey(currentThread)) { // ③如果在Map中不存在，放到Map中保存起来。      o = initialValue();      valueMap.put(currentThread, o);      }      return o;      }             public void remove() {      valueMap.remove(Thread.currentThread());      }             protected T initialValue() {      return null;      }      }

     class Count {      private SimpleThreadLocal<Integer> count = new SimpleThreadLocal<Integer>() {      @Override      protected Integer initialValue() {      return 0;      }      };             public Integer increase() {      count.set(count.get() + 1);      return count.get();      }             }             class TestThread implements Runnable {      private Count count;             public TestThread(Count count) {      this.count = count;      }             @Override      public void run() {      // TODO Auto-generated method stub      for (int i = 1; i <= 3; i++) {      System.out.println(Thread.currentThread().getName() + "\t" + i      + "th\t" + count.increase());      }      }      }             public class TestThreadLocal {      public static void main(String[] args) {      Count count = new Count();      Thread t1 = new Thread(new TestThread(count));      Thread t2 = new Thread(new TestThread(count));      Thread t3 = new Thread(new TestThread(count));      Thread t4 = new Thread(new TestThread(count));      t1.start();      t2.start();      t3.start();      t4.start();      }      }