实例讲解Java并发编程之ThreadLocal类

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