ThreadLocal详解

最新推荐文章于 2025-09-09 22:58:36 发布

cisco273443746

最新推荐文章于 2025-09-09 22:58:36 发布

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文章标签： java 开发语言

本文链接：https://blog.youkuaiyun.com/cisco273443746/article/details/84818436

ThreadLocal 线程局部变量所以只能当前线程可以访问即线程安全

下面看简单示例

public class ParseDateDemo {
    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 date = sdf.parse("2018-12-05 09:33:" + i % 60);
                System.out.println(i + ":" + date);
            } 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));
        }
    }
}

执行结果

Exception in thread "pool-1-thread-8" java.lang.NumberFormatException: multiple points
Exception in thread "pool-1-thread-3" java.lang.NumberFormatException: For input string: ""

异常原因：因为SimpleDateFormat不是线程安全的在线程池中共享此对象必然导致错误!!

下面是给SimpleDateFormat加上ThreadLocal后代码示例

public class ParseDateDemo {
    //private static final SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
	static ThreadLocal<SimpleDateFormat> tl = new ThreadLocal<SimpleDateFormat>();

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

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

        @Override
        public void run() {
            try {
            	if(tl.get()==null) {
            		tl.set(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));
            	}
                Date date = tl.get().parse("2018-12-05 09:33:" + i % 60);
                System.out.println(i + ":" + date);
            } 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));
        }
    }
}

需要特别注意:ThreadLocal只是为每一个线程都提供了变量的副本，使得每个线程在某一时间访问到的并不是同一个对象，这样就隔离了多个线程对数据的数据共享，并且为每个线程分配一个对象并不是ThreadLocal操作而是应用层操作