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();  } }

Thread-0    1th    1 Thread-0    2th    2 Thread-0    3th    3 Thread-3    1th    1 Thread-1    1th    1 Thread-1    2th    2 Thread-2    1th    1 Thread-1    3th    3 Thread-3    2th    2 Thread-3    3th    3 Thread-2    2th    2 Thread-2    3th    3