SimpleDateFormat的线程安全问题与解决方案

参考地址：https://www.cnblogs.com/zemliu/p/3290585.html

使用ThreadLocal解决sdf多线程问题的例子

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;


public class DateUtil {

    /** 锁对象 */
    private static final Object lockObj = new Object();

    /** 存放不同的日期模板格式的sdf的Map */
    private static Map<String, ThreadLocal<SimpleDateFormat>> sdfMap = new HashMap<String, ThreadLocal<SimpleDateFormat>>();

    /**
     * 返回一个ThreadLocal的sdf,每个线程只会new一次sdf
     * 
     * @param pattern
     * @return
     */
    private static SimpleDateFormat getSdf(final String pattern) {
        ThreadLocal<SimpleDateFormat> tl = sdfMap.get(pattern);

        // 此处的双重判断和同步是为了防止sdfMap这个单例被多次put重复的sdf
        if (tl == null) {
            synchronized (lockObj) {
                tl = sdfMap.get(pattern);
                if (tl == null) {
                    // 只有Map中还没有这个pattern的sdf才会生成新的sdf并放入map
                    System.out.println("put new sdf of pattern " + pattern + " to map");

                    // 这里是关键,使用ThreadLocal<SimpleDateFormat>替代原来直接new SimpleDateFormat
                    tl = new ThreadLocal<SimpleDateFormat>() {

                        @Override
                        protected SimpleDateFormat initialValue() {
                            System.out.println("thread: " + Thread.currentThread() + " init pattern: " + pattern);
                            return new SimpleDateFormat(pattern);
                        }
                    };
                    sdfMap.put(pattern, tl);
                }
            }
        }

        return tl.get();
    }

    /**
     * 是用ThreadLocal<SimpleDateFormat>来获取SimpleDateFormat,这样每个线程只会有一个SimpleDateFormat
     * 
     * @param date
     * @param pattern
     * @return
     */
    public static String format(Date date, String pattern) {
        return getSdf(pattern).format(date);
    }

    public static Date parse(String dateStr, String pattern) throws ParseException {
        return getSdf(pattern).parse(dateStr);
    }

}

测试类

import java.text.ParseException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;


public class Test {

    public static void main(String[] args) {
        final String patten1 = "yyyy-MM-dd";
        final String patten2 = "yyyy-MM";

        Thread t1 = new Thread() {

            @Override
            public void run() {
                try {
                    DateUtil.parse("1992-09-13", patten1);
                } catch (ParseException e) {
                    e.printStackTrace();
                }
            }
        };

        Thread t2 = new Thread() {

            @Override
            public void run() {
                try {
                    DateUtil.parse("2000-09", patten2);
                } catch (ParseException e) {
                    e.printStackTrace();
                }
            }
        };

        Thread t3 = new Thread() {

            @Override
            public void run() {
                try {
                    DateUtil.parse("1992-09-13", patten1);
                } catch (ParseException e) {
                    e.printStackTrace();
                }
            }
        };

        Thread t4 = new Thread() {

            @Override
            public void run() {
                try {
                    DateUtil.parse("2000-09", patten2);
                } catch (ParseException e) {
                    e.printStackTrace();
                }
            }
        };

        Thread t5 = new Thread() {

            @Override
            public void run() {
                try {
                    DateUtil.parse("2000-09-13", patten1);
                } catch (ParseException e) {
                    e.printStackTrace();
                }
            }
        };

        Thread t6 = new Thread() {

            @Override
            public void run() {
                try {
                    DateUtil.parse("2000-09", patten2);
                } catch (ParseException e) {
                    e.printStackTrace();
                }
            }
        };

        System.out.println("单线程执行: ");
        ExecutorService exec = Executors.newFixedThreadPool(1);
        exec.execute(t1);
        exec.execute(t2);
        exec.execute(t3);
        exec.execute(t4);
        exec.execute(t5);
        exec.execute(t6);
        exec.shutdown();

        sleep(1000);

        System.out.println("双线程执行: ");
        ExecutorService exec2 = Executors.newFixedThreadPool(2);
        exec2.execute(t1);
        exec2.execute(t2);
        exec2.execute(t3);
        exec2.execute(t4);
        exec2.execute(t5);
        exec2.execute(t6);
        exec2.shutdown();
    }

    private static void sleep(long millSec) {
        try {
            TimeUnit.MILLISECONDS.sleep(millSec);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

输出

单线程执行: 
put new sdf of pattern yyyy-MM-dd to map
thread: Thread[pool-1-thread-1,5,main] init pattern: yyyy-MM-dd
put new sdf of pattern yyyy-MM to map
thread: Thread[pool-1-thread-1,5,main] init pattern: yyyy-MM
双线程执行: 
thread: Thread[pool-2-thread-1,5,main] init pattern: yyyy-MM-dd
thread: Thread[pool-2-thread-2,5,main] init pattern: yyyy-MM
thread: Thread[pool-2-thread-1,5,main] init pattern: yyyy-MM
thread: Thread[pool-2-thread-2,5,main] init pattern: yyyy-MM-dd

从输出我们可以看出:

1) 1个线程执行这6个任务的时候,这个线程首次使用过的时候会new一个新的sdf,并且以后都一直用这个sdf,而不是每次处理任务都新建一个新的sdf

2) 2个线程执行6个任务的时候也是同理,但是2个线程的sdf是分开的,每个线程都有自己的"yyyy-MM-dd", "yyyy-MM"的sdf,所以他们不会有线程安全安全问题

试想,如果使用的是new的实现方法,那么不管是用1个线程去执行,还是用2个线程去执行这6个任务,都需要new 6个sdf