日期时间格式化 SimpleDateFormat与DateTimeFormatter

原文：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。

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy MM dd");
LocalDate date = LocalDate.parse("2017 06 17", formatter);
System.out.println(formatter.format(date));

 自测类

import java.text.SimpleDateFormat;
import java.time.LocalDate;
import java.time.format.DateTimeFormatter;
import java.util.Locale;

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

    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];
                            
                            //线程安全
                            LocalDate date = LocalDate.parse(str1, formatter);
                            String str2 =  formatter.format(date);
                            
                            //线程不安全
//                            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();
        }
    }
}