ThreadLocal应用示例

ThreadLocal从字面意思来看是线程本地（变量），有什么作用呢？

存放在ThreadLocal中的变量值只在当前线程中可见及使用。那仅在当前线程可用，自然就不存在多线程并发冲突问题，是一种空间（每个线程单独存储变量）换时间的解决多线程并发问题的方式。

下面举例说明：

package threadLocalDemo;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Demo {

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

    public static void main(String[] args) throws InterruptedException {
        //创建一个仅有一个线程的线程池（模拟在一个线程中的前后操作）
        ExecutorService executorService = Executors.newFixedThreadPool(1);
        //线程提交任务：线程中设置ThreadLocal变量并打印
        executorService.submit(() -> {
            threadLocal.set("hello");
            System.out.println(Thread.currentThread().getName()+":"+threadLocal.get());
        });
        //线程延迟1s提交任务:获取ThreadLocal变量并打印
        Thread.sleep(1000);
        executorService.submit(() -> {
            System.out.println(Thread.currentThread().getName()+":"+threadLocal.get());
        });
        //线程延迟1s提交任务:获取ThreadLocal变量并打印
        Thread.sleep(1000);
        executorService.submit(() -> {
            System.out.println(Thread.currentThread().getName()+":"+threadLocal.get());
        });
        executorService.shutdown();
    }
}

打印结果如下：

pool-1-thread-1:hello
pool-1-thread-1:hello
pool-1-thread-1:hello

说明只要在当前线程中设置ThreadLocal变量后，线程的后续操作时都可以拿到此变量。

另外：在某些场景下，我们在同一线程的前后操作中是需要修改或者清空ThreadLocal变量的，当然也有相关的API，举例如下：

package threadLocalDemo;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class AnotherDemo {

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

    public static void main(String[] args) throws InterruptedException {
        //创建一个仅有一个线程的线程池（模拟在一个线程中的前后操作）
        ExecutorService executorService = Executors.newFixedThreadPool(1);
        //线程提交任务：线程中设置ThreadLocal变量并打印
        executorService.submit(() -> {
            threadLocal.set("hello");
            System.out.println(Thread.currentThread().getName()+":"+threadLocal.get());
        });
        //线程延迟1s提交任务:获取ThreadLocal变量并打印
        Thread.sleep(1000);
        executorService.submit(() -> {
            //修改变量
            threadLocal.set("world");
            System.out.println(Thread.currentThread().getName()+":"+threadLocal.get());
            //删除变量
            threadLocal.remove();
        });
        //线程延迟1s提交任务:获取ThreadLocal变量并打印
        Thread.sleep(1000);
        executorService.submit(() -> {
            System.out.println(Thread.currentThread().getName()+":"+threadLocal.get());
        });
        executorService.shutdown();
    }
}

打印结果如下：

pool-1-thread-1:hello
pool-1-thread-1:world
pool-1-thread-1:null