ThreadLocal的简单使用和实现原理

我们先看以下代码，不用ThreadLocal会发生什么情况

package com.qjc.thread.threadLocal;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

//ThreadLocal顾名思义表示线程的局部变量，及只有当前线程可以访问，自然是线程安全的
public class ThreadLocalTest {

    private static final SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

    public static class ParseDate implements Runnable {
        int i = 0;

        public ParseDate(int i) {
            this.i = i;
        }

        @Override
        public void run() {
            try {
                Date parse = sdf.parse("2018-04-19 15:12:" + i % 60);
                System.out.println(i + ":" + parse);
            } catch (ParseException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(10);
        for (int i = 0; i < 1000; i++) {
            executorService.execute(new ParseDate(i));
        }
        executorService.shutdown();
    }
}

控制台输出了这么一个异常

很明显是异常出现在线程上，表明这样做是线程不安全的

下面，我们用ThreadLocal，代码如下：

package com.qjc.thread.threadLocal;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

//ThreadLocal顾名思义表示线程的局部变量，及只有当前线程可以访问，自然是线程安全的
public class ThreadLocalTest {

    // private static final SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    static ThreadLocal<SimpleDateFormat> threadLocal = new ThreadLocal<>();

    public static class ParseDate implements Runnable {
        int i = 0;

        public ParseDate(int i) {
            this.i = i;
        }

        @Override
        public void run() {
            try {
                // 用ThreadLocal
                if (threadLocal.get() == null) {
                    threadLocal.set(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));
                }
                Date parse = threadLocal.get().parse("2018-04-19 15:12:" + i % 60);
                // 不用ThreadLocal
                // Date parse = sdf.parse("2018-04-19 15:12:" + i % 60);
                System.out.println(i + ":" + parse);
            } catch (ParseException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(10);
        for (int i = 0; i < 1000; i++) {
            executorService.execute(new ParseDate(i));
        }
        executorService.shutdown();
    }
}

控制台，正常输出，没有异常出现，表明线程安全。

我们分析一下ThreadLocal的实现原理：

public void set(T value) {
        Thread t = Thread.currentThread();//获取当前线程对象
        ThreadLocalMap map = getMap(t);//拿到线程的ThreadLocalMap，它是Thread内部的成员(Thread类中的)：ThreadLocal.ThreadLocalMap threadLocals = null;
        //并将值设入ThreadLocalMap
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }
    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);//从此可以看出，ThreadLocalMap的key就是ThreadLocal当前对象，value就是我们设置的值
    }

public T get() {
        Thread t = Thread.currentThread();//获取当前线程对象
        ThreadLocalMap map = getMap(t);//获取当前线程的ThreadLocalMap对象
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);//然后通过将自己作为key取得内部的实际数据
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }