ThreadLocal实现局部变量的多线程同步

两个例子来说明：

1）线程不安全的示例：

public class TestMain {

@Test
public void main() {
int a = 0;
ThreadNotSafe rt = new ThreadNotSafe(a);
Thread thread1 = new Thread(rt);
thread1.start();
Thread thread2 = new Thread(rt);
thread2.start();
Thread thread3 = new Thread(rt);
thread3.start();
}

｝

public class ThreadNotSafe implements Runnable{


int a;
public ThreadNotSafe(int a){
this.a = a;
}

public void run() {
for(int i=0;i<5;i++){
System.out.println(Thread.currentThread().getName() +": " + a++);
}
}

}

运行结果：

Thread-0: 0
Thread-0: 2
Thread-0: 3
Thread-0: 4
Thread-0: 5
Thread-1: 1
Thread-1: 6
Thread-1: 8
Thread-1: 9
Thread-1: 10
Thread-2: 7
Thread-2: 11
Thread-2: 12
Thread-2: 13
Thread-2: 14

2）ThreadLocal线程安全的示例：

public class TestMain {

@Test
public void threadSafe(){
ThreadLocal<Integer> aTL = new ThreadLocal<Integer>(){
public Integer initialValue(){
return 0;
}
};
RunnableThread rt = new RunnableThread(aTL);
Thread thread1 = new Thread(rt);
thread1.start();
Thread thread2 = new Thread(rt);
thread2.start();
Thread thread3 = new Thread(rt);
thread3.start();
}

}

public class ThreadSafe implements Runnable{
ThreadLocal<Integer> a;
public ThreadSafe(ThreadLocal<Integer> a){
this.a = a;
}
public void run() {
for(int i=0;i<5;i++){
try {
Thread.sleep(10L);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
this.a.set(this.a.get() + 1);
System.out.println(Thread.currentThread().getName() +": " + this.a.get());
}
}
}

运行结果：（不知为何这里只输出到3，不过不影响本文的主题）
Thread-0: 1
Thread-2: 1
Thread-1: 1
Thread-0: 2
Thread-2: 2
Thread-1: 2
Thread-0: 3
Thread-2: 3
Thread-1: 3

以上两个例子，三条线程都共用了一个实例。

结果一：每个线程对a值的操作都会影响到其它线程；

结果二：每个线程对a值的操作对其它线程都没有影响，每个线程输出的独立的序列，因为ThreadLocal对每条线程生成了一个单独的副本。