详解SimpleDateFormat的线程安全问题与解决方案

Date parse() {
 calendar.clear(); // 清理calendar
 ... // 执行一些操作, 设置 calendar 的日期什么的
 calendar.getTime(); // 获取calendar的时间
}

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
/**
 * @author zhenwei.liu created on 2013 13-8-29 下午5:35
 * @version $Id$
 */
public class DateFormatTest extends Thread {
 private static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
 private String name;
 private String dateStr;
 private boolean sleep;
 public DateFormatTest(String name, String dateStr, boolean sleep) {
  this.name = name;
  this.dateStr = dateStr;
  this.sleep = sleep;
 }
 @Override
 public void run() {
  Date date = null;
  if (sleep) {
   try {
    TimeUnit.MILLISECONDS.sleep(2000);
   } catch (InterruptedException e) {
    e.printStackTrace();
   }
  }
  try {
   date = sdf.parse(dateStr);
  } catch (ParseException e) {
   e.printStackTrace();
  }
  System.out.println(name + " : date: " + date);
 }
 public static void main(String[] args) throws InterruptedException {
  ExecutorService executor = Executors.newCachedThreadPool();
  // A 会sleep 2s 后开始执行sdf.parse()
  executor.execute(new DateFormatTest("A", "1991-09-13", true));
  // B 打了断点,会卡在方法中间
  executor.execute(new DateFormatTest("B", "2013-09-13", false));
  executor.shutdown();
 }
}

parse() {
 calendar.clear()
 // 这里打一个断点
 calendar.getTime()
}

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;
/**
 * @author zhenwei.liu created on 2013 13-8-29 下午5:35
 * @version $Id$
 */
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> sdfMap = new HashMap>();
 /**
  * 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 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替代原来直接new SimpleDateFormat
     tl = new ThreadLocal() {
      @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);
 }
}

import java.text.ParseException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
/**
 * @author zhenyu.nie created on 2013 13-8-26 下午2:13
 * @version 1.0.0
 */
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();
  }
 }
}

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

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