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;
    }
}

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() {
				@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 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));

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() {
				@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();
		}
	}
}

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);
	}
}

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非线程安全及解决办法