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SimpleDateFormat线程不安全及解决的方法
一. 为什么SimpleDateFormat不是线程安全的?
Java源代码例如以下:
/** * Date formats are not synchronized. * It is recommended to create separate format instances for each thread. * If multiple threads access a format concurrently, it must be synchronized * externally. */ public class SimpleDateFormat extends DateFormat { public Date parse(String text, ParsePosition pos){ calendar.clear(); // Clears all the time fields // other logic ... Date parsedDate = calendar.getTime(); } } abstract class DateFormat{ // other logic ... protected Calendar calendar; public Date parse(String source) throws ParseException{ ParsePosition pos = new ParsePosition(0); Date result = parse(source, pos); if (pos.index == 0) throw new ParseException("Unparseable date: \"" + source + "\"" , pos.errorIndex); return result; } }假设我们把SimpleDateFormat定义成static成员变量,那么多个thread之间会共享这个sdf对象, 所以Calendar对象也会共享。
假定线程A和线程B都进入了parse(text, pos) 方法。 线程B运行到calendar.clear()后,线程A运行到calendar.getTime(), 那么就会有问题。
二. 问题重现:
public class DateFormatTest { 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) { 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(); } } }
创建三个进程, 使用静态成员变量SimpleDateFormat的parse和format方法。然后比較经过这两个方法折腾后的值是否相等:
程序假设出现下面错误,说明传入的日期就串掉了,即SimpleDateFormat不是线程安全的:
Exception in thread "Thread-0" java.lang.RuntimeException: parse failed at cn.test.DateFormatTest$1.run(DateFormatTest.java:27) at java.lang.Thread.run(Thread.java:662) Caused by: java.lang.RuntimeException: Thread-0, Expected 01-Jan-1999 but got 01-Jan-2000 at cn.test.DateFormatTest$1.run(DateFormatTest.java:22) ... 1 more
三. 解决方式:
1. 解决方式a:
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. 解决方式b:
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它线程才干进入
3. 解决方式c: (推荐)
使用ThreadLocal: 每一个线程都将拥有自己的SimpleDateFormat对象副本。
写一个工具类:
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); } }測试代码:
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(); } } }
SimpleDateFormat线程不安全及解决的方法
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