概述

ThreadLocal，即線程本地變量，它為每一個使用該變量的線程提供獨立的變量副本，每一個線程都可以獨立地改變自己的變量副本，而不會影響其它線程。

構造方法

1. ThreadLocal()

成員變量

1. 222

成員方法

1. initialValue()

    protected T initialValue() {
        return null;
    }

返回變量的初始值，默認返回為空，可在子類中復寫該方法；

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();
    }

    // 獲取線程的本地變量集合
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

返回當前線程對應的本地變量副本值；

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);
    }

    // 為線程創建本地變量集合
    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

設置當前線程對應的本地變量副本值；

4. remove()

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

移除當前線程對應的本地變量副本；

實例

public class Main {
    
    private ThreadLocal<Integer> mThreadLocal = new ThreadLocal<Integer>() {
        
        @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());
            }
        }
    }
}