import java.util.ArrayList;
import java.util.List;
import java.util.Random;public class ThreadLocalTest {
    public static void main(String[] args) {        Judge.prepare();
        new Player(1).start();
        new Player(2).start();
        new Player(3).start();
    }

}class Judge {
    public static int MAX_VALUE = 10;
    private static int targetValue;

    public static void prepare() {        Random random = new Random();
        targetValue = random.nextInt(MAX_VALUE) + 1;
    }

    public static boolean judge(int value) {        return value == targetValue;
    }

}class Player extends Thread {
    private int playerId;

    public Player(int playerId) {        this.playerId = playerId;
    }

    @Override
    public void run() {        boolean success = false;
        while(!success) {            int value = Attempt.guess(Judge.MAX_VALUE);
            success = Judge.judge(value);
            System.out.println(String.format("Plyaer %s Attempts %s and %s", playerId, value, success ? " Success" : "Failed"));
        }
        Attempt.review(String.format("[IFNO] Plyaer %s Completed by ", playerId));
    }

}class Attempt {
    private static ThreadLocal<Record> history = new ThreadLocal<Record>();

    public static int guess(int maxValue) {        Record record = getRecord();
        Random random = new Random();
        int value = 0;
        do {            value = random.nextInt(maxValue) + 1;
        } while (record.contains(value));
        record.save(value);
        return value;
    }

    public static void review(String info) {        System.out.println(info + getRecord());
    }

    private static Record getRecord() {        Record record = history.get();
        if(record == null) {            record = new Record();
            history.set(record);
        }
        return record;
    }

}class Record {
    private List<Integer> attemptList = new ArrayList<Integer>();;

    public void save(int value) {        attemptList.add(value);
    }

    public boolean contains(int value) {        return attemptList.contains(value);
    }

    @Override
    public String toString() {        StringBuffer buffer = new StringBuffer();
        buffer.append(attemptList.size() + " Times: ");
        int count = 1;
        for(Integer attempt : attemptList) {            buffer.append(attempt);
            if(count < attemptList.size()) {                buffer.append(", ");
                count++;
            }
        }
        return buffer.toString();
    }

}

运行结果

Plyaer 2 Attempts 8 and Failed
Plyaer 3 Attempts 6 and Failed
Plyaer 1 Attempts 5 and Failed
Plyaer 2 Attempts 7 and  Success
Plyaer 3 Attempts 9 and Failed
Plyaer 1 Attempts 9 and Failed
Plyaer 3 Attempts 2 and Failed
Plyaer 1 Attempts 2 and Failed
[IFNO] Plyaer 2 Completed by 2 Times: 8, 7
Plyaer 3 Attempts 4 and Failed
Plyaer 1 Attempts 1 and Failed
Plyaer 3 Attempts 5 and Failed
Plyaer 1 Attempts 3 and Failed
Plyaer 3 Attempts 1 and Failed
Plyaer 1 Attempts 10 and Failed
Plyaer 3 Attempts 8 and Failed
Plyaer 1 Attempts 6 and Failed
Plyaer 3 Attempts 7 and  Success
Plyaer 1 Attempts 4 and Failed
[IFNO] Plyaer 3 Completed by 8 Times: 6, 9, 2, 4, 5, 1, 8, 7
Plyaer 1 Attempts 7 and  Success
[IFNO] Plyaer 1 Completed by 9 Times: 5, 9, 2, 1, 3, 10, 6, 4, 7

关于ThreadLocal的原理，可以从其get()方法的实现来看

public class ThreadLocal<T> {    ...    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();
    }

    ThreadLocalMap getMap(Thread t) {        return t.threadLocals;
    }    ...}

执行get()时首先获取当前的Thread，再获取Thread中的ThreadLocalMap - t.threadLocals，并以自身为key取出实际的value。于是可以看出，ThreadLocal的变量实际还是保存在Thread中的，容器是一个Map，Thread用到多少ThreadLocal变量，就会有多少以其为key的Entry。

转自：http://gaofeihang.blog.163.com/blog/static/84508285201293071940706/