java多线程之DateTimeFormatter和SimpleDateFormat

本文参考自：https://www.jianshu.com/p/b212afa16f1f

1.SimpleDateFormat为什么不是线程安全的？

如果我们把SimpleDateFormat定义成static成员变量，那么多个thread之间会共享这个SimpleDateFormat对象， 所以Calendar对象也会共享。

public static SimpleDateFormat formater = new SimpleDateFormat(
            "yyyy-MM-dd HH:mm:ss");

System.out.println(formater.format(new Date())+" Exception made...");

DateFormat.java

  public final String format(Date date)
    {
        return format(date, new StringBuffer(),
                      DontCareFieldPosition.INSTANCE).toString();
    }

    public abstract StringBuffer format(Date date, StringBuffer toAppendTo,
                                        FieldPosition fieldPosition);

SimpleDateFormat.java

@Override
public StringBuffer format(Date date, StringBuffer toAppendTo,
                           FieldPosition pos)
{
//  如此轻易地使用内部变量，肯定不能线程安全
//  线程都对pos进行写操作，必然会影响其他线程的读操作
    pos.beginIndex = pos.endIndex = 0;
    return format(date, toAppendTo, pos.getFieldDelegate());
}

private StringBuffer format(Date date, StringBuffer toAppendTo,
                            FieldDelegate delegate) {
    // Convert input date to time field list
   // 这里已经彻底毁坏线程的安全性
    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;
}

1.1 复现错误
代码参考

public class DateFormatTest {
    private static SimpleDateFormat sdf = new SimpleDateFormat("dd-MMM-yyyy", Locale.US);
    private static String date[] = { "01-Jan-1999", "09-Jan-2000", "08-Jan-2001" , "07-Jan-2002" , "06-Jan-2003" , "05-Jan-2004" , "04-Jan-2005" , "03-Jan-2006" , "02-Jan-2007" };

public static void main(String[] args) {
    for (int i = 0; i < date.length; i++) {
        final int temp = i;
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    while (true) {
                        String str1 = date[temp];
                        String str2 = sdf.format(sdf.parse(str1));
                        System.out.println(Thread.currentThread().getName() + ", " + str1 + "," + str2);
                        if(!str1.equals(str2)){
                            throw new RuntimeException(Thread.currentThread().getName()
                                    + ", Expected " + str1 + " but got " + str2);
                        }
                    }
                } catch (Exception e) {
                    throw new RuntimeException("parse failed", e);
                }
            }
        }).start();
    }
}

}

2.SimpleDateFormat线程不安全的解决方法

2.1 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创建一个SimpleDateFormat对象，方法结束又要作为垃圾回收。
2.2 加一把线程同步锁：synchronized(lock)

public class SyncDateFormatTest {
    private static SimpleDateFormat sdf = new SimpleDateFormat("dd-MMM-yyyy", Locale.US);
    private static String date[] = { "01-Jan-1999", "01-Jan-2000", "01-Jan-2001" };
 
    public static void main(String[] args) {
        for (int i = 0; i < date.length; i++) {
            final int temp = i;
            new Thread(new Runnable() {
                @Override
                public void run() {
                    try {
                        while (true) {
                            synchronized (sdf) {
                                String str1 = date[temp];
                                Date date = sdf.parse(str1);
                                String str2 = sdf.format(date);
                                System.out.println(Thread.currentThread().getName() + ", " + str1 + "," + str2);
                                if(!str1.equals(str2)){
                                    throw new RuntimeException(Thread.currentThread().getName() 
                                            + ", Expected " + str1 + " but got " + str2);
                                }
                            }
                        }
                    } catch (Exception e) {
                        throw new RuntimeException("parse failed", e);
                    }
                }
            }).start();
        }
    }
}

缺点：性能较差，每次都要等待锁释放后其他线程才能进入
2.3 使用ThreadLocal
每个线程都将拥有自己的SimpleDateFormat对象副本。

public class DateUtil {
    private static ThreadLocal local = new ThreadLocal();
 
    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);
    }
}

public class ThreadLocalDateFormatTest {
    private static String date[] = { "01-Jan-1999", "01-Jan-2000", "01-Jan-2001" };
 
    public static void main(String[] args) {
        for (int i = 0; i < date.length; i++) {
            final int temp = i;
            new Thread(new Runnable() {
                @Override
                public void run() {
                    try {
                        while (true) {
                            String str1 = date[temp];
                            Date date = DateUtil.parse(str1);
                            String str2 = DateUtil.format(date);
                            System.out.println(str1 + "," + str2);
                            if(!str1.equals(str2)){
                                throw new RuntimeException(Thread.currentThread().getName() 
                                        + ", Expected " + str1 + " but got " + str2);
                            }
                        }
                    } catch (Exception e) {
                        throw new RuntimeException("parse failed", e);
                    }
                }
            }).start();
        }
    }
}

3.使用DateTimeFormatter代替SimpleDateFormat
代码参考DateTimeFormatterTest.java
jdk1.8中新增了 LocalDate 与 LocalDateTime等类来解决日期处理方法，同时引入了一个新的类DateTimeFormatter来解决日期格式化问题。
LocalDateTime,DateTimeFormatter两个类都没有线程问题，只要你自己不把它们创建为共享变量就没有线程问题。
可以使用Instant代替 Date，LocalDateTime代替 Calendar，DateTimeFormatter 代替 SimpleDateFormat。

使用DateTimeFormatter完成格式化

DateTimeFormatter[] formatters = new DateTimeFormatter[]{
        // 直接使用常量创建DateTimeFormatter格式器
        DateTimeFormatter.ISO_LOCAL_DATE,
        DateTimeFormatter.ISO_LOCAL_TIME,
        DateTimeFormatter.ISO_LOCAL_DATE_TIME,
        // 使用本地化的不同风格来创建DateTimeFormatter格式器
        DateTimeFormatter.ofLocalizedDateTime(FormatStyle.FULL, FormatStyle.MEDIUM),
        DateTimeFormatter.ofLocalizedTime(FormatStyle.LONG),
        // 根据模式字符串来创建DateTimeFormatter格式器
        DateTimeFormatter.ofPattern("Gyyyy%%MMM%%dd HH:mm:ss")
};
LocalDateTime date = LocalDateTime.now();
// 依次使用不同的格式器对LocalDateTime进行格式化
for(int i = 0 ; i < formatters.length ; i++)
{
    // 下面两行代码的作用相同
    System.out.println(date.format(formatters[i]));
    System.out.println(formatters[i].format(date));
}

使用DateTimeFormatter解析字符串

// 定义一个任意格式的日期时间字符串
String str1 = "2014==04==12 01时06分09秒";
// 根据需要解析的日期、时间字符串定义解析所用的格式器
DateTimeFormatter fomatter1 = DateTimeFormatter
    .ofPattern("yyyy==MM==dd HH时mm分ss秒");
// 执行解析
LocalDateTime dt1 = LocalDateTime.parse(str1, fomatter1);
System.out.println(dt1); // 输出 2014-04-12T01:06:09
// ---下面代码再次解析另一个字符串---
String str2 = "2014$$$四月$$$13 20小时";
DateTimeFormatter fomatter2 = DateTimeFormatter
    .ofPattern("yyy$$$MMM$$$dd HH小时");
LocalDateTime dt2 = LocalDateTime.parse(str2, fomatter2);
System.out.println(dt2); // 输出 2014-04-13T20:00