java多线程之DateTimeFormatter和SimpleDateFormat

ゝ一纸荒年。 2024-02-19 20:39 51阅读 0赞

本文参考自：[https://www.jianshu.com/p/b212afa16f1f][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<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);
        }
    }
    
    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

[https_www.jianshu.com_p_b212afa16f1f]: https://www.jianshu.com/p/b212afa16f1f