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package testDate;import java.text.ParseException;import java.text.SimpleDateFormat;import java.util.Date;import java.util.Locale;public class DateTester { static SimpleDateFormat df = new SimpleDateFormat("dd-MMM-yyyy", Locale.US); static String testdata[] = { "01-Jan-1999", "14-Feb-2001", "31-Dec-2007" }; public static void main(String[] args) { Runnable r[] = new Runnable[testdata.length]; for (int i = 0; i < r.length; i++) { final int i2 = i; r[i] = new Runnable() { public void run() { try { for (int j = 0; j < 2000; j++) { String str = testdata[i2]; String str2 = null; /* synchronized(df) */{ Date d = df.parse(str); str2 = df.format(d); System.out.println("i: " + i2 + "\tj: " + j + "\tThreadID: " + Thread.currentThread().getId() + "\tThreadName: " + Thread.currentThread().getName() + "\t" + str + "\t" + str2); } if (!str.equals(str2)) { throw new RuntimeException("date conversion failed after " + j + " iterations. Expected " + str + " but got " + str2); } } } catch (ParseException e) { throw new RuntimeException("parse failed"); } } }; new Thread(r[i]).start(); } }}
异常:
i: 1 j: 0 ThreadID: 19 ThreadName: Thread-6 14-Feb-2001 14-Jan-1999i: 2 j: 0 ThreadID: 20 ThreadName: Thread-7 31-Dec-2007 14-Jan-1999i: 0 j: 0 ThreadID: 18 ThreadName: Thread-5 01-Jan-1999 14-Jan-1999Exception in thread "Thread-7" Exception in thread "Thread-5" Exception in thread "Thread-6" java.lang.RuntimeException: date conversion failed after 0 iterations. Expected 31-Dec-2007 but got 14-Jan-1999java.lang.RuntimeException: date conversion failed after 0 iterations. Expected 01-Jan-1999 but got 14-Jan-1999java.lang.RuntimeException: date conversion failed after 0 iterations. Expected 14-Feb-2001 but got 14-Jan-1999 at testDate.DateTester$1.run(DateTester.java:31) at testDate.DateTester$1.run(DateTester.java:31) at testDate.DateTester$1.run(DateTester.java:31) at java.lang.Thread.run(Thread.java:785) at java.lang.Thread.run(Thread.java:785) at java.lang.Thread.run(Thread.java:785)
恩,原因你是知道了,这是由于 SimpleDateFormat 的非线程安全问题引起的,
我们现在简化下问题,错误的代码应该是这样的:
public class DateUtil { SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy"); public String formatDate(Date input) { return sdf.format(input); }}
(1)使用局部变量:
import java.text.SimpleDateFormat;import java.util.Date;public class DateUtil { private static final String SIMPLE_FORMAT = "dd/MM/yyyy"; public String formatDate(Date input) { if(input == null){ return null; } SimpleDateFormat sdf = new SimpleDateFormat(SIMPLE_FORMAT);//local variable return sdf.format(input); }}
(2)使用 ThreadLocal
这里每个线程将有它自己的 SimpleDateFormat 副本。import java.text.SimpleDateFormat;import java.util.Date;public class DateUtil { // anonymous inner class. Each thread will have its own copy of the SimpleDateFormat private final static ThreadLocaltl = new ThreadLocal () { protected SimpleDateFormat initialValue() { return new SimpleDateFormat("dd/MM/yyyy"); } } public String formatDate(Date input) { if (input == null) { return null; } return tl.get().format(input); }}
PS:顺便聊聊这个 ThreadLocal:
ThreadLocal 按我的理解是一个Map容器,视作其key是当前线程,value就是我们想保证数据安全一致的某对象。从它的功能上来说,应该叫做 ThreadLocalVariable(线程局部变量)更合适些。
具体的含义与作用请参考如下两篇文摘:ThreadLocal 那点事儿
(3)同步代码块 synchronized(code)
或者使用装饰器设计模式包装下 SimpleDateFormat ,使之变得线程安全。(4)使用第三方的日期处理函数:
比如 JODA 来避免这些问题,你也可以使用 commons-lang 包中的 FastDateFormat 工具类。(5)最后的提问:
上面几种方案中,有最佳方案吗?如果不是最佳,各有什么优劣?参考 :
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