什么是ThreadLocal

它是一个线程本地变量，访问这个变量的每个线程都会有这个变量的一个本地拷贝（副本），多个线程操作的时候，实际是操作自己本地内存里面的变量，从而起到线程隔离的作用，避免线程安全问题。

如下面的演示程序：

Main.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
public class Main {
    static ThreadLocal<Integer> threadLocal = new ThreadLocal<>();
    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread(() -> {
            System.out.println(threadLocal.get());
            threadLocal.set(0);
            System.out.println(threadLocal.get());
        }, "1");

        Thread thread2 = new Thread(() -> {
            System.out.println(threadLocal.get());
            threadLocal.set(2);
            System.out.println(threadLocal.get());
        }, "2");
        thread.start();
        thread.join();
        thread2.start();
        thread2.join();
    }

}

它的输出是 null 0 null 2

实现的原理

通过一个Key-Value形式，将当前线程的值存入ThreadLocal中，也就是ThreadLocalMap，每个Thread对象都有一个ThreadLocalMap，Key为ThreadLocal对象，Valueh为需要缓存的值。

ThreadLocal.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25

public void set(T value) {
    Thread t = Thread.currentThread();//获取当前线程
    ThreadLocalMap map = getMap(t);//拿到当前线程的ThreadLocalMap
    if (map != null) {
        map.set(this, value);//存在这个Map就直接修改
    } else {
        createMap(t, value);//不存在就创建
    }
}

public T get() {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    //不为空则从Map中取出来
    if (map != null) {
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    return setInitialValue();
}

拿到Map

ThreadLocal.java
1
2
3
ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;
}
ThreadLocal.java
1
2
3
void createMap(Thread t, T firstValue) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);
}
ThreadLoca.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
private T setInitialValue() {
    T value = initialValue();
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        map.set(this, value);
    } else {
        createMap(t, value);
    }
    if (this instanceof TerminatingThreadLocal) {
        TerminatingThreadLocal.register((TerminatingThreadLocal<?>) this);
    }
    return value;
}