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 threads
		ExecutorService exec = Executors.newFixedThreadPool(5);
		List<Future<Date>> results = new ArrayList<Future<Date>>();
		// perform 10 date conversions
		for (int i = 0; i < 10; i++) {
			results.add(exec.submit(task));
		}
		exec.shutdown();
		// look at the results
		for (Future<Date> result : results) {
			System.out.println(result.get());
		}
	}

}

问题1：

Fri Oct 22 00:00:00 CST 2010
Fri Oct 22 00:00:00 CST 2010
Fri Oct 22 00:00:00 CST 2010
Mon Nov 22 00:00:00 CST 2010
Exception 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)

Fri Oct 22 00:00:00 CST 2010
Fri Oct 22 00:00:00 CST 2010
Fri Oct 22 00:00:00 CST 2010
Exception in thread "main" java.util.concurrent.ExecutionException: java.lang.ArrayIndexOutOfBoundsException: -1
	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.ArrayIndexOutOfBoundsException: -1
	at 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)

Sat Oct 01 00:00:00 CST 1
Sat Oct 01 00:00:00 CST 1
Sun Oct 30 00:00:00 CST 2016
Sun Oct 30 00:00:00 CST 2016
Exception 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

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>() {
		@Override
		protected DateFormat initialValue() {
			return new SimpleDateFormat("yyyy-MM-dd HH");
		}
	};

	public Date convert(String source) throws ParseException {
		Date d = df.get().parse(source);
		return d;
	}
}

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