JavaThread-ThreadLocal笔记

1.ThreadLocal简介
早在JDK 1.2的版本中就提供java.lang.ThreadLocal，ThreadLocal为解决多线程程序的并发问题提供了一种新的思路。使用这个工具类可以很简洁地编写出优美的多线程程序。
当使用ThreadLocal维护变量时，ThreadLocal为每个使用该变量的线程提供独立的变量副本，所以每一个线程都可以独立地改变自己的副本，而不会影响其它线程所对应的副本.
最常见的ThreadLocal使用场景为 用来解决 数据库连接、Session管理等.

2.部分源码
ThreadLocal中存在ThreadLocalMap 以不同的线程key创建不同的副本。如下ThreadLocal set() /get()方法

    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }
    public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null)
                return (T)e.value;
        }
        return setInitialValue();
    }

3.代码实例

/**
 * Created by wuhao on 15-12-23.
 */
public class ThreadLocalDemo {

    public static void main(String[] args) {

        ThreadStringSeq tss = new ThreadStringSeq();
        tss.setState("first");
        //切换state
        ThreadStringClient ts1 = new ThreadStringClient(tss);
        ThreadStringClient ts2 = new ThreadStringClient(tss);
        new Thread(ts1).start();
        new Thread(ts2).start();

        ThreadLocalSeq tls = new ThreadLocalSeq();
        System.out.println("Main:" + Thread.currentThread().getName() + " sn:" + tls.getNextState());
        //切换state
        ThreadLocalClient tl1 = new ThreadLocalClient(tls);
        ThreadLocalClient tl2 = new ThreadLocalClient(tls);
        new Thread(tl1).start();
        new Thread(tl2).start();

    }
}
class ThreadLocalSeq {
    //通过匿名内部类覆盖ThreadLocal的initialValue()方法，指定初始值
    private static ThreadLocal<String> state = new ThreadLocal<String>() {
        public String initialValue() {
            return "";
        }
    };
    //获取下一个序列值
    public String getNextState() {
        state.set(state.get() + "-->|");
        return state.get();
    }
    public void setState(String st){
        state.set(st);
    }

}

class ThreadStringSeq {
    //通过匿名内部类覆盖ThreadLocal的initialValue()方法，指定初始值
    private static String state = "";
    //获取下一个序列值
    public String getNextState() {
        state = state + "-->|";
        return state;
    }
    public void setState(String st){
        state = st;
    }

}


class ThreadLocalClient implements Runnable {
    private ThreadLocalSeq sn;
    public ThreadLocalClient(ThreadLocalSeq sn) {
        super();
        this.sn = sn;
    }
    public void run() {
        for (int i = 0; i < 3; i++) {
            //获取线程状态
            System.out.println("ThreadLocalClient:" + Thread.currentThread().getName() + " sn:" + sn.getNextState());
        }
    }
}

class ThreadStringClient implements Runnable {
    private ThreadStringSeq sn;
    public ThreadStringClient(ThreadStringSeq sn) {
        super();
        this.sn = sn;
    }
    public void run() {
        for (int i = 0; i < 3; i++) {
            //获取线程状态
            System.out.println("ThreadStringClient:" + Thread.currentThread().getName() + " sn:" + sn.getNextState());
        }
    }
}