SimpleDateFormat线程不安全原因及解决方案

Posted 2022-01-06 yangyongjie

线程不安全验证：

/**
 * SimpleDateFormat线程安全测试
 * 〈功能详细描述〉
 *
 * @author 17090889
 * @see [相关类/方法]（可选）
 * @since [产品/模块版本] （可选）
 */
public class SimpleDateFormatTest 
    private SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    ThreadPoolExecutor poolExecutor = new ThreadPoolExecutor(10, 100, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<>(1000), new MyThreadFactory("SimpleDateFormatTest"));

    public void test() 
        while (true) 
            poolExecutor.execute(new Runnable() 
                @Override
                public void run() 
                    String dateString = simpleDateFormat.format(new Date());
                    try 
                        Date parseDate = simpleDateFormat.parse(dateString);
                        String dateString2 = simpleDateFormat.format(parseDate);
                        System.out.println(dateString.equals(dateString2));
                     catch (ParseException e) 
                        e.printStackTrace();
                    
                
            );
        
    

输出：

true
false
true
true
false

出现了false，说明线程不安全

1、format方法

public StringBuffer format(Date date, StringBuffer toAppendTo,
                               FieldPosition pos)
    
        pos.beginIndex = pos.endIndex = 0;
        return format(date, toAppendTo, pos.getFieldDelegate());
    

    // Called from Format after creating a FieldDelegate
    private StringBuffer format(Date date, StringBuffer toAppendTo,
                                FieldDelegate delegate) 
        // Convert input date to time field list
        calendar.setTime(date);

        boolean useDateFormatSymbols = useDateFormatSymbols();

        for (int i = 0; i < compiledPattern.length; ) 
            int tag = compiledPattern[i] >>> 8;
            int count = compiledPattern[i++] & 0xff;
            if (count == 255) 
                count = compiledPattern[i++] << 16;
                count |= compiledPattern[i++];
            

            switch (tag) 
            case TAG_QUOTE_ASCII_CHAR:
                toAppendTo.append((char)count);
                break;

            case TAG_QUOTE_CHARS:
                toAppendTo.append(compiledPattern, i, count);
                i += count;
                break;

            default:
                subFormat(tag, count, delegate, toAppendTo, useDateFormatSymbols);
                break;
            
        
        return toAppendTo;
    

 protected Calendar calendar;

可以看到，多个线程之间共享变量calendar，并修改calendar，因此在多线程环境下，线程是不安全的。

解决方案：

1、将SimpleDateFormat定义成局部变量

2、 加一把线程同步锁：synchronized(lock)

3、使用ThreadLocal，每个线程都拥有自己的SimpleDateFormat对象副本

　　如：

/**
 * SimpleDateFormat线程安全测试
 * 〈功能详细描述〉
 *
 * @author 17090889
 * @see [相关类/方法]（可选）
 * @since [产品/模块版本] （可选）
 */
public class SimpleDateFormatTest 
    ThreadLocal<SimpleDateFormat> local = new ThreadLocal<>();
    //    private SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    ThreadPoolExecutor poolExecutor = new ThreadPoolExecutor(10, 100, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<>(1000), new MyThreadFactory("SimpleDateFormatTest"));

    public void test() 
        while (true) 
            poolExecutor.execute(new Runnable() 
                @Override
                public void run() 
                    SimpleDateFormat simpleDateFormat = local.get();
                    if (simpleDateFormat == null) 
                        simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
                    
                    String dateString = simpleDateFormat.format(new Date());
                    try 
                        Date parseDate = simpleDateFormat.parse(dateString);
                        String dateString2 = simpleDateFormat.format(parseDate);
                        System.out.println(dateString.equals(dateString2));
                     catch (ParseException e) 
                        e.printStackTrace();
                    
                
            );
        
    

 

4、使用DateTimeFormatter代替SimpleDateFormat