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
     
       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);	}}
      
     

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();		}	}}