SimpleDateFormat非线程安全

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问题的引出，这是一个多线程访问SimpleDateFormat的测试类

public class DateUtilTest {

	public static class TestSimpleDateFormatThreadSafe extends Thread {

		@Override
		public void run() {
			while ( true ) {
				try {
					this.join(2000);
				} catch ( InterruptedException e1 ) {
					e1.printStackTrace();
				}
				try {
					System.out.println(this.getName() + ":" + DateUtil.parse("2013-05-24 06:02:20"));
				} catch ( ParseException e ) {
					e.printStackTrace();
				}
			}
		}
	}


	public static void main( String[] args ) {
		for ( int i = 0 ; i < 10 ; i++ ) {
			new TestSimpleDateFormatThreadSafe().start();
		}

	}
}

  运行时会出现如下Exception

java.lang.NumberFormatException: For input string: ""
java.lang.NumberFormatException: multiple points

 原因是DateFormate类中有个Calendar变量是全局，这个变量在子类SimpleDateFormat类在两个方法中被调用

private StringBuffer format(Date date, StringBuffer toAppendTo,FieldDelegate delegate)
private void subFormat(int patternCharIndex, int count,FieldDelegate delegate, StringBuffer buffer,boolean useDateFormatSymbols)

这时多线程访问就是非线程安全的

public abstract class DateFormat extends Format {
    protected Calendar calendar;
}

 

解决办法：

方法一：每次创建都new一个新的

public class DateUtil {
    
    public static  String formatDate(Date date)throws ParseException{
         SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        return sdf.format(date);
    }
    
    public static Date parse(String strDate) throws ParseException{
         SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        return sdf.parse(strDate);
    }
}

 方法二：使用同步

public class DateSyncUtil {

    private static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
      
    public static String formatDate(Date date)throws ParseException{
        synchronized(sdf){
            return sdf.format(date);
        }  
    }
    
    public static Date parse(String strDate) throws ParseException{
        synchronized(sdf){
            return sdf.parse(strDate);
        }
    } 
}

方法三：使用ThreadLocal

public class ConcurrentDateUtil {

	private static ThreadLocal threadLocal = new ThreadLocal() {

		@Override
		protected DateFormat initialValue() {
			return new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
		}
	};


	public static Date parse( String dateStr ) throws ParseException {
		return threadLocal.get().parse(dateStr);
	}


	public static String format( Date date ) {
		return threadLocal.get().format(date);
	}
}

 另一中ThreadLocal方法

public class ThreadLocalDateUtil {
    private static final String date_format = "yyyy-MM-dd HH:mm:ss";
    private static ThreadLocal threadLocal = new ThreadLocal(); 
 
    public static DateFormat getDateFormat()   
    {  
        DateFormat df = threadLocal.get();  
        if(df==null){  
            df = new SimpleDateFormat(date_format);  
            threadLocal.set(df);  
        }  
        return df;  
    }  

    public static String formatDate(Date date) throws ParseException {
        return getDateFormat().format(date);
    }

    public static Date parse(String strDate) throws ParseException {
        return getDateFormat().parse(strDate);
    }   
}

  

 commons-lang.jar中提供了FastDateFormt，是一个thread-safe，性能上也能还是可以的

FastDateFormat fdf = FastDateFormat.getInstance("yyyy-MM-dd", TimeZone.getDefault(), Locale.getDefault());
DateFormatUtils.ISO_DATE_FORMAT.format(new Date())

 DateFormatUtils是一个FastDateFormat工具类，提供了一些format常量

ISO_DATE_FORMAT
yyyy-MM-dd"2004-01-02"
ISO_DATE_TIME_ZONE_FORMAT
yyyy-MM-ddZZ"2004-01-02-07:00"
ISO_DATETIME_FORMAT
yyyy-MM-dd'T'HH:mm:ss"2004-01-02T23:22:12"
ISO_DATETIME_TIME_ZONE_FORMAT
yyyy-MM-dd'T'HH:mm:ssZZ"2004-01-02T21:13:45-07:00"
ISO_TIME_FORMAT
'T'HH:mm:ss"T04:23:22"
ISO_TIME_NO_T_FORMAT
HH:mm:ss"05:12:34"
ISO_TIME_NO_T_TIME_ZONE_FORMAT
HH:mm:ssZZ"12:32:22-07:00"
ISO_TIME_TIME_ZONE_FORMAT
'T'HH:mm:ssZZ"T18:23:22-07:00"
SMTP_DATETIME_FORMAT
EEE, dd MMM yyyy HH:mm:ss Z"Wed, 01 Feb 2004 20:03:01 CST"