ThreadLocal使用学习

小知识，大挑战！本文正在参与“程序员必备小知识”创作活动 ThreadLocal字面上的意思是局部线程变量，每个线程通过ThreadLocal的get和set方法来访问和修改线程自己独有的变量。简单地说，ThreadLocal的作用就是为每一个线程提供了一个独立的变量副本，每一个线程都可以独立地改变自己的副本，而不会影响其它线程所对应的副本。

ThreadLocal的基本使用

ThreadLocal是一个泛型类，在创建的时候需要指定变量的类型：

private static ThreadLocal<String> threadLocal = new ThreadLocal<>();

ThreadLocal提供了set方法来设置变量的值，get方法获取变量的值，remove方法移除变量：

public class Test {
    private static ThreadLocal<String> threadLocal = new ThreadLocal<>();

    public static void main(String[] args) throws InterruptedException {
	threadLocal.set("ycf");
	System.out.println(threadLocal.get());
	threadLocal.remove();
	System.out.println(threadLocal.get());
    }
}

程序输出如下：

我们也可以给ThreadLocal设置初始值，设置初始值有两种方式：

1、重写initialValue方法：

public class ThreadLocalTest {
    private static ThreadLocal<String> threadLocal = new ThreadLocal<String>(){
        @Override
        protected String initialValue() {
            return "初始值";
        }
    };
    
    public static void main(String[] args) throws InterruptedException {
        System.out.println(threadLocal.get()); // 初始值
    }
}

2、使用ThreadLocal的withInitial方法：

public class ThreadLocalTest {
    private static ThreadLocal<String> threadLocal = ThreadLocal.withInitial(() -> "初始值");
    public static void main(String[] args) throws InterruptedException {
        System.out.println(threadLocal.get()); // 初始值
    }
}

值得注意的是remove无法移除初始值：

public class ThreadLocalTest {
    private static ThreadLocal<String> threadLocal = ThreadLocal.withInitial(() -> "初始值");
    public static void main(String[] args) throws InterruptedException {
        threadLocal.remove();
        System.out.println(threadLocal.get()); // 初始值
    }
}

演示多线程间独立

在多个线程中使用ThreadLocal：

import java.util.Random;
import java.util.concurrent.TimeUnit;

public class Test {
    private static ThreadLocal<String> threadLocal = new ThreadLocal<>();
    private static Random random = new Random(System.currentTimeMillis());

    public static void main(String[] args) throws InterruptedException {
	Thread thread1 = new Thread(() -> {
	    threadLocal.set("thread t1");
	    try {
	    TimeUnit.MICROSECONDS.sleep(random.nextInt(1000));
            System.out.println(Thread.currentThread().getName() + " " + threadLocal.get());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
	}, "thread1");

	Thread thread2 = new Thread(() -> {
            threadLocal.set("thread t2");
            try {
		TimeUnit.MICROSECONDS.sleep(random.nextInt(1000));
		System.out.println(Thread.currentThread().getName() + " " + threadLocal.get());
            } catch (InterruptedException e) {
		e.printStackTrace();
            }
	}, "thread2");

	thread1.start();
        thread2.start();
	thread1.join();
	thread2.join();

	System.out.println(Thread.currentThread().getName() + " " + threadLocal.get());
    }
}

程序输出如下：

结果证明了ThreadLocal在每个线程间是相互独立的，threadLocal在thread1、thread2和main线程间都有一份独立拷贝。

ThreadLocal基本原理

在ThreadLocal类中有一个静态内部类ThreadLocalMap(概念上类似于Map)，用键值对的形式存储每一个线程的变量副本，ThreadLocalMap中元素的key为当前ThreadLocal对象，而value对应线程的变量副本。

我们使用Map来代替ThreadLocalMap，创建一个简易的类ThreadLocal实现：

public class MyThreadLocal<T> {
    private final Map<Thread, T> threadLocalMap = new HashMap<>();

    public void set(T t) {
        synchronized (this) {
            Thread key = Thread.currentThread();
            threadLocalMap.put(key, t);
        }
    }

    public T get() {
        synchronized (this) {
            Thread key = Thread.currentThread();
            T t = threadLocalMap.get(key);
            if (t == null) {
                return initalValue();
            } else {
                return t;
            }
        }
    }

    public T initalValue() {
        return null;
    }
}

使用方式和之前的例子一致：

public class Test {
    private static MyThreadLocal<String> threadLocal = new MyThreadLocal<String>() {
	@Override
	public String initalValue() {
            return "initalValue";
	}
    };
    private static Random random = new Random(System.currentTimeMillis());

    public static void main(String[] args) throws InterruptedException {
	Thread thread1 = new Thread(() -> {
            threadLocal.set("thread t1");
            try {
		TimeUnit.MICROSECONDS.sleep(random.nextInt(1000));
		System.out.println(Thread.currentThread().getName() + " " + threadLocal.get());
            } catch (InterruptedException e) {
		e.printStackTrace();
            }
	}, "thread1");

	Thread thread2 = new Thread(() -> {
            threadLocal.set("thread t2");
            try {
		TimeUnit.MICROSECONDS.sleep(random.nextInt(1000));
		System.out.println(Thread.currentThread().getName() + " " + threadLocal.get());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
	}, "thread2");

	thread1.start();
	thread2.start();
	thread1.join();
	thread2.join();

	System.out.println(Thread.currentThread().getName() + " " + threadLocal.get());
    }
}

程序输出如下：

使用建议

1. 将ThreadLocal变量指定为private static；
2. 使用完毕后显式地调用remove方法移除。