併發編程-SimpleDateFormat線程不安全及解決辦法

時間 2019-12-25

標籤併發編程 simpledateformat 線程安全解決辦法欄目系統安全简体版

原文原文鏈接

一. 爲何SimpleDateFormat不是線程安全的？java

Java源碼以下：安全

一. 爲何SimpleDateFormat不是線程安全的？ide

Java源碼以下：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

若是咱們把SimpleDateFormat定義成static成員變量，那麼多個thread之間會共享這個sdf對象，因此Calendar對象也會共享。.net

假定線程A和線程B都進入了parse(text, pos) 方法，線程B執行到calendar.clear()後，線程A執行到calendar.getTime(), 那麼就會有問題。線程

二. 問題重現：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(){對象

               @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. 解決方案：

1. 解決方案a：

將SimpleDateFormat定義成局部變量：

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

String str1 = "01-Jan-2010";

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

缺點：每調用一次方法就會建立一個SimpleDateFormat對象，方法結束又要做爲垃圾回收。

2. 解決方案b:

加一把線程同步鎖：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();
                   }
               }
           });
       }
   }
}

缺點：性能較差，每次都要等待鎖釋放後其餘線程才能進入

3. 解決方案c: (推薦)

使用ThreadLocal: 每一個線程都將擁有本身的SimpleDateFormat對象副本。

寫一個工具類：

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); } }