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 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。

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();
        }
    }
}

 

 

 

转载于:https://www.cnblogs.com/shihaiming/p/11082154.html