SimpleDateFormat线程不安全及解决办法

一. 为什么SimpleDateFormat不是线程安全的？

Java源码如下：

/*** 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;    }}

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执行到calendar.clear()后，线程A执行到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() {@Overridepublic 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，然后比较经过这两个方法折腾后的值是否相等：

程序如果出现以下错误，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 failedat 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-2000at cn.test.DateFormatTest$1.run(DateFormatTest.java:22)... 1 more

三. 解决方案：

1. 解决方案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. 解决方案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() {@Overridepublic 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

3. 解决方案c: (推荐)

使用ThreadLocal: 每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);}}

测试代码：

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() {@Overridepublic 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();}}}