并发编程-SimpleDateFormat线程不安全及解决办法

时间 2019-12-25

标签并发编程 simpledateformat 线程安全解决办法栏目系统安全繁體版

原文原文链接

一. 为何SimpleDateFormat不是线程安全的？java

Java源码以下：安全

一. 为何SimpleDateFormat不是线程安全的？ide

Java源码以下：工具

/**
* Date formats are not synchronized.
* It is recommended to create separate format instances for each thread.
* If multiple threads access a format concurrently, it must be synchronized
* externally.
*/
public class SimpleDateFormat extends DateFormat {

   public Date parse(String text, ParsePosition pos){
       calendar.clear(); // Clears all the time fields
       // other logic ...
       Date parsedDate = calendar.getTime();
   }
}
abstract class DateFormat{
   // other logic ...
   protected Calendar calendar;
   public Date parse(String source) throws ParseException{
ParsePosition pos = new ParsePosition(0);
Date result = parse(source, pos);
if (pos.index == 0)
throw new ParseException("Unparseable date: \"" + source + "\"" ,
pos.errorIndex);
return result;
}
}性能

若是咱们把SimpleDateFormat定义成static成员变量，那么多个thread之间会共享这个sdf对象，因此Calendar对象也会共享。.net

假定线程A和线程B都进入了parse(text, pos) 方法，线程B执行到calendar.clear()后，线程A执行到calendar.getTime(), 那么就会有问题。线程

二. 问题重现：orm

public class SdfThreadTest2{
   public static void main(String[] args) throws InterruptedException {
       final SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
       final String[] date = new String[]{"2017-01-01","2017-02-02","2017-03-03","2017-04-04","2017-05-05",
               "2017-06-06","2017-07-07","2017-08-08","2017-09-09","2017-10-10","2017-11-11","2017-12-12"};
       final ExecutorService es2 = Executors.newFixedThreadPool(12);
       for(int i=0;i<12;i++){
           final int j = i;
           es2.execute(new Runnable(){对象

               @Override
               public void run() {
                   try {
                       System.out.println(j+ "->" + sdf.parse((date[j])));
                   } catch (ParseException e) {
                       // TODO Auto-generated catch block
                       e.printStackTrace();
                   }
               }
           });
       }
   }
}ip

三. 解决方案：

1. 解决方案a：

将SimpleDateFormat定义成局部变量：

SimpleDateFormat sdf = new SimpleDateFormat("dd-MMM-yyyy", Locale.US);

String str1 = "01-Jan-2010";

String str2 = sdf.format(sdf.parse(str1));

缺点：每调用一次方法就会建立一个SimpleDateFormat对象，方法结束又要做为垃圾回收。

2. 解决方案b:

加一把线程同步锁：synchronized(lock)

               @Override
               public void run() {
                   try {
                       synchronized(sdf){
                           System.out.println(j+ "->" + sdf.parse((date[j])));
                       }
                   } catch (ParseException e) {
                       // TODO Auto-generated catch block
                       e.printStackTrace();
                   }
               }
           });
       }
   }
}

缺点：性能较差，每次都要等待锁释放后其余线程才能进入

3. 解决方案c: (推荐)

使用ThreadLocal: 每一个线程都将拥有本身的SimpleDateFormat对象副本。

写一个工具类：

public class DateUtil {
private static ThreadLocal<SimpleDateFormat> local = new ThreadLocal<SimpleDateFormat>();

public static Date parse(String str) throws Exception { SimpleDateFormat sdf = local.get(); if (sdf == null) { sdf = new SimpleDateFormat("dd-MMM-yyyy", Locale.US); local.set(sdf); } return sdf.parse(str); } public static String format(Date date) throws Exception { SimpleDateFormat sdf = local.get(); if (sdf == null) { sdf = new SimpleDateFormat("dd-MMM-yyyy", Locale.US); local.set(sdf); } return sdf.format(date); } }