Java SimpleDateFormat 线程不安全问题及解决方法

Java SimpleDateFormat 是线程不安全的，当在多线程环境下使用一个DateFormat的时候是有问题的，如下面的例子：

package com.heaven.threadpool;import java.text.SimpleDateFormat;import java.util.ArrayList;import java.util.Date;import java.util.List;import java.util.concurrent.Callable;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Future;public class DateFormatThreadSafe {public static void main(String[] args) throws Exception {final SimpleDateFormat format = new SimpleDateFormat("yyyyMMdd");Callable<Date> task = new Callable<Date>() {public Date call() throws Exception {return format.parse("20101022");}};// pool with 5 threadsExecutorService exec = Executors.newFixedThreadPool(5);List<Future<Date>> results = new ArrayList<Future<Date>>();// perform 10 date conversionsfor (int i = 0; i < 10; i++) {results.add(exec.submit(task));}exec.shutdown();// look at the resultsfor (Future<Date> result : results) {System.out.println(result.get());}}}

上面代码可能会引起如下问题：

问题1：

Fri Oct 22 00:00:00 CST 2010Fri Oct 22 00:00:00 CST 2010Fri Oct 22 00:00:00 CST 2010Mon Nov 22 00:00:00 CST 2010Exception in thread "main" java.util.concurrent.ExecutionException: java.lang.NumberFormatException: For input string: ""at java.util.concurrent.FutureTask$Sync.innerGet(FutureTask.java:222)at java.util.concurrent.FutureTask.get(FutureTask.java:83)at com.heaven.threadpool.DateFormatThreadSafe.main(DateFormatThreadSafe.java:49)Caused by: java.lang.NumberFormatException: For input string: ""at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)at java.lang.Long.parseLong(Long.java:424)at java.lang.Long.parseLong(Long.java:461)at java.text.DigitList.getLong(DigitList.java:177)at java.text.DecimalFormat.parse(DecimalFormat.java:1298)at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1589)at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1312)at java.text.DateFormat.parse(DateFormat.java:335)at com.heaven.threadpool.DateFormatThreadSafe$1.call(DateFormatThreadSafe.java:31)at com.heaven.threadpool.DateFormatThreadSafe$1.call(DateFormatThreadSafe.java:1)at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)at java.util.concurrent.FutureTask.run(FutureTask.java:138)at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)at java.lang.Thread.run(Thread.java:619)

问题二：

Fri Oct 22 00:00:00 CST 2010Fri Oct 22 00:00:00 CST 2010Fri Oct 22 00:00:00 CST 2010Exception in thread "main" java.util.concurrent.ExecutionException: java.lang.ArrayIndexOutOfBoundsException: -1at java.util.concurrent.FutureTask$Sync.innerGet(FutureTask.java:222)at java.util.concurrent.FutureTask.get(FutureTask.java:83)at com.heaven.threadpool.DateFormatThreadSafe.main(DateFormatThreadSafe.java:32)Caused by: java.lang.ArrayIndexOutOfBoundsException: -1at java.text.DigitList.fitsIntoLong(DigitList.java:212)at java.text.DecimalFormat.parse(DecimalFormat.java:1296)at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1589)at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1312)at java.text.DateFormat.parse(DateFormat.java:335)at com.heaven.threadpool.DateFormatThreadSafe$1.call(DateFormatThreadSafe.java:19)at com.heaven.threadpool.DateFormatThreadSafe$1.call(DateFormatThreadSafe.java:1)at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)at java.util.concurrent.FutureTask.run(FutureTask.java:138)at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)

问题三：

Sat Oct 01 00:00:00 CST 1Sat Oct 01 00:00:00 CST 1Sun Oct 30 00:00:00 CST 2016Sun Oct 30 00:00:00 CST 2016Exception in thread "main" java.util.concurrent.ExecutionException: java.lang.NumberFormatException: For input string: "E.122E22"at java.util.concurrent.FutureTask$Sync.innerGet(FutureTask.java:222)at java.util.concurrent.FutureTask.get(FutureTask.java:83)at com.heaven.threadpool.DateFormatThreadSafe.main(DateFormatThreadSafe.java:32)Caused by: java.lang.NumberFormatException: For input string: "E.122E22"at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1224)at java.lang.Double.parseDouble(Double.java:510)at java.text.DigitList.getDouble(DigitList.java:151)at java.text.DecimalFormat.parse(DecimalFormat.java:1303)at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1936)at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1312)at java.text.DateFormat.parse(DateFormat.java:335)at com.heaven.threadpool.DateFormatThreadSafe$1.call(DateFormatThreadSafe.java:19)at com.heaven.threadpool.DateFormatThreadSafe$1.call(DateFormatThreadSafe.java:1)at java.u

解决方法一：把DateFormat 放到ThreadLocal中

package com.heaven.threadpool;import java.text.DateFormat;import java.text.ParseException;import java.text.SimpleDateFormat;import java.util.Date;public class DateFormatTest {private static final ThreadLocal<DateFormat> df = new ThreadLocal<DateFormat>() {@Overrideprotected DateFormat initialValue() {return new SimpleDateFormat("yyyy-MM-dd HH");}};public Date convert(String source) throws ParseException {Date d = df.get().parse(source);return d;}}

解决方法二：使用线程安全的DateFormat，在Github上已经有人写了一个线程安全的，我们可以直接拿来用，请点击下面链接查看

点击打开链接