ThreadLocal源码解析

概述

ThreadLocal，即线程本地变量，它为每一个使用该变量的线程提供独立的变量副本，每一个线程都可以独立地改变自己的变量副本，而不会影响其它线程。

构造方法

1. ThreadLocal()

成员变量

1. 222

成员方法

1. initialValue()

    protected T initialValue() {
        return null;
    }

返回变量的初始值，默认返回为空，可在子类中复写该方法；

2. get()

    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();
    }

    // 获取线程的本地变量集合
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

返回当前线程对应的本地变量副本值；

3. set(T value)

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

    // 为线程创建本地变量集合
    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

设置当前线程对应的本地变量副本值；

4. remove()

    public void remove() {
         ThreadLocalMap m = getMap(Thread.currentThread());
         if (m != null)
             m.remove(this);
     }

移除当前线程对应的本地变量副本；

实例

public class Main {
    
    private ThreadLocal mThreadLocal = new ThreadLocal() {
        
        @Override
        protected Integer initialValue() {
            return 0;
        }
    };
    
    private int getNext() {
        mThreadLocal.set(mThreadLocal.get() + 1);
        return mThreadLocal.get();
    }

    public static void main(String[] args) {
        Main main = new Main();
        Task task1 = new Task(main);
        Task task2 = new Task(main);
        Task task3 = new Task(main);
        Task task4 = new Task(main);
        task1.start();
        task2.start();
        task3.start();
        task4.start();
    }
    
    static class Task extends Thread {
        private Main main;
        
        private Task(Main main) {
            this.main = main;
        }
        
        @Override
        public void run() {
            for (int i = 0; i < 3; i++) {
                System.out.println(Thread.currentThread().getName() + " -----> " + main.getNext());
            }
        }
    }
}