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SimpleDateFormat线程不安全示例及其解决方法

由 落爺英雄遲暮 提交于 2019-11-30 12:11:06

我们可以用java.text.SimpleDateFormat类完成日期的转换和格式化操作，如：

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

/**
 * @author wangmengjun
 *
 */
public class SimpleDateFormatExample {

	private static final DateFormat SIMPLE_DATE_FORMAT = new SimpleDateFormat(
			"yyyyMMdd");

	public Date parseDate(String value) throws ParseException {
		return SIMPLE_DATE_FORMAT.parse(value);
	}

}

import java.text.ParseException;
import java.util.Date;

/**
 * 
 * @author wangmengjun
 *
 */
public class Main {

	public static void main(String[] args) throws ParseException {
		SimpleDateFormatExample example = new SimpleDateFormatExample();
		Date date = example.parseDate("20161118");
		//Fri Nov 18 00:00:00 CST 2016
		System.out.println(date);
	}
}

但是，同时，我们也能从java.text.SimpleDateFormat类的javadoc中看到如下一句话。

 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.

Date formats没有同步。

建议为每一个线程创建独立的format对象。

如果多个线程并发访问一个format，那么，一定要在外部实现同步(synchronized)。

也就是说，

在多线程下我们需要做些额外的保护措施，去保证其正确处理，否则是不安全的。

让我们一起来看一下，多线程下会出现什么问题：

线程不安全示例

package my.format;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

/**
 * 
 * @author wangmengjun
 *
 */
public class SimpleDateFormatExample {

	private static final DateFormat SIMPLE_DATE_FORMAT = new SimpleDateFormat(
			"yyyyMMdd");

	public Date parseDate(String value) throws ParseException {
		return SIMPLE_DATE_FORMAT.parse(value);
	}

}

多线程测试示例：

package my.format;

import java.text.ParseException;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class Main {

	public static void main(String[] args) throws InterruptedException,
			ExecutionException, ParseException {

		int availableProcessors = Runtime.getRuntime().availableProcessors();
		ExecutorService exec = Executors
				.newFixedThreadPool(availableProcessors);

		List<Future<Date>> results = new ArrayList<>();

		final SimpleDateFormatExample sdf = new SimpleDateFormatExample();
		Callable<Date> parseDateTask = new Callable<Date>() {
			public Date call() throws Exception {
				return sdf.parseDate("20161118");
			}
		};

		for (int i = 0; i < 10; i++) {
			results.add(exec.submit(parseDateTask));
		}
		
		exec.shutdown();

		/**
		 * 查看结果
		 */
		for (Future<Date> result : results) {
			System.out.println(result.get());
		}
	}
}

运行结果主要包含如下几个错误：

  • 无异常，日期解析出现错误
Tue Nov 18 00:00:00 CST 2200
Tue Nov 18 00:00:00 CST 2200
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Thu Nov 18 00:00:00 CST 1
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
  • 有异常，java.lang.NumberFormatException

如：

Fri Nov 18 00:00:00 CST 2016
Exception in thread "main" java.util.concurrent.ExecutionException: java.lang.NumberFormatException: For input string: ""
	at java.util.concurrent.FutureTask.report(Unknown Source)
	at java.util.concurrent.FutureTask.get(Unknown Source)
	at my.format.Main.main(Main.java:40)
Caused by: java.lang.NumberFormatException: For input string: ""
	at java.lang.NumberFormatException.forInputString(Unknown Source)
	at java.lang.Long.parseLong(Unknown Source)
	at java.lang.Long.parseLong(Unknown Source)
	at java.text.DigitList.getLong(Unknown Source)
	at java.text.DecimalFormat.parse(Unknown Source)
	at java.text.SimpleDateFormat.subParse(Unknown Source)
	at java.text.SimpleDateFormat.parse(Unknown Source)
	at java.text.DateFormat.parse(Unknown Source)
	at my.format.SimpleDateFormatExample.parseDate(SimpleDateFormatExample.java:14)
	at my.format.Main$1.call(Main.java:27)
	at my.format.Main$1.call(Main.java:1)
	at java.util.concurrent.FutureTask.run(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
	at java.lang.Thread.run(Unknown Source)

再如：

Exception in thread "main" java.util.concurrent.ExecutionException: java.lang.NumberFormatException: For input string: "1111.E1111E22"
	at java.util.concurrent.FutureTask.report(Unknown Source)
	at java.util.concurrent.FutureTask.get(Unknown Source)
	at my.format.Main.main(Main.java:40)
Caused by: java.lang.NumberFormatException: For input string: "1111.E1111E22"
	at sun.misc.FloatingDecimal.readJavaFormatString(Unknown Source)
	at java.lang.Double.parseDouble(Unknown Source)
	at java.text.DigitList.getDouble(Unknown Source)
	at java.text.DecimalFormat.parse(Unknown Source)
	at java.text.SimpleDateFormat.subParse(Unknown Source)
	at java.text.SimpleDateFormat.parse(Unknown Source)
	at java.text.DateFormat.parse(Unknown Source)
	at my.format.SimpleDateFormatExample.parseDate(SimpleDateFormatExample.java:14)
	at my.format.Main$1.call(Main.java:27)
	at my.format.Main$1.call(Main.java:1)
	at java.util.concurrent.FutureTask.run(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
	at java.lang.Thread.run(Unknown Source)

 

那么问题来了，如何保证运行正常呢？

解决方法

其实，从SimpleDateFormat的javadoc中已经看到有处理的方法了。

 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.

接下来，先从这个描述信息给出相关的解决方法。

每次都新建SimpleDateFormat对象

改造SimpleDateFormatExample类，如：

package my.format;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

/**
 * 
 * @author wangmengjun
 *
 */
public class SimpleDateFormatExample {

	public Date parseDate(String value) throws ParseException {
		return new SimpleDateFormat(
				"yyyyMMdd").parse(value);
	}

}

执行上述Main.java类，得到正确结果：

Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016

访问format时，添加synchronized

改造SimpleDateFormatExample类，如：

package my.format;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

/**
 * 
 * @author wangmengjun
 *
 */
public class SimpleDateFormatExample {

	private static final DateFormat SIMPLE_DATE_FORMAT = new SimpleDateFormat(
			"yyyyMMdd");

	public Date parseDate(String value) throws ParseException {
		synchronized (SIMPLE_DATE_FORMAT) {
			return SIMPLE_DATE_FORMAT.parse(value);
		}
	}

}

或者在使用format对象的方法前添加synchronized修饰，如：

package my.format;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

/**
 * 
 * @author wangmengjun
 *
 */
public class SimpleDateFormatExample {

	private static final DateFormat SIMPLE_DATE_FORMAT = new SimpleDateFormat(
			"yyyyMMdd");

	public synchronized Date parseDate(String value) throws ParseException {
		return SIMPLE_DATE_FORMAT.parse(value);
	}

}

同样，执行上述Main.java类，可以得到正确结果：

Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016

使用TheadLocal

改造SimpleDateFormatExample类，如：

package my.format;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

/**
 * 
 * @author wangmengjun
 *
 */
public class SimpleDateFormatExample {

	private static final ThreadLocal<DateFormat> THREAD_LOCAL_DATE_FORMAT = new ThreadLocal<DateFormat>() {
		protected DateFormat initialValue() {
			return new SimpleDateFormat("yyyyMMdd");
		}
	};

	public Date parseDate(String value) throws ParseException {
		return THREAD_LOCAL_DATE_FORMAT.get().parse(value);
	}

}

同样，执行上述Main.java类，可以得到正确结果：

Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016

使用FastDateFormat

FastDateFormat类在Apache Common Langs包下面， 该类是线程安全的。

如果是Maven工程，其添加依赖包如下：

		<!-- https://mvnrepository.com/artifact/org.apache.commons/commons-lang3 -->
		<dependency>
			<groupId>org.apache.commons</groupId>
			<artifactId>commons-lang3</artifactId>
			<version>3.5</version>
		</dependency>

改造SimpleDateFormatExample类，如：

	private static final FastDateFormat SIMPLE_DATE_FORMAT = FastDateFormat
			.getInstance("yyyyMMdd");

完成的类为：

package my.format;

import java.text.ParseException;
import java.util.Date;

import org.apache.commons.lang3.time.FastDateFormat;

/**
 * 
 * @author wangmengjun
 *
 */
public class SimpleDateFormatExample {

	private static final FastDateFormat SIMPLE_DATE_FORMAT = FastDateFormat
			.getInstance("yyyyMMdd");

	public Date parseDate(String value) throws ParseException {
		return SIMPLE_DATE_FORMAT.parse(value);
	}

}

同样，执行上述Main.java类，可以得到正确结果：

Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016

使用Joda Time

DateTimeFormatter 类Joda-Time包下面， 该类是线程安全的。

如果是Maven工程，其添加依赖包如下：

		<!-- https://mvnrepository.com/artifact/joda-time/joda-time -->
		<dependency>
			<groupId>joda-time</groupId>
			<artifactId>joda-time</artifactId>
			<version>2.9.6</version>
		</dependency>

改造SimpleDateFormatExample类，如：

package my.format;

import java.text.ParseException;
import java.util.Date;

import org.joda.time.format.DateTimeFormat;
import org.joda.time.format.DateTimeFormatter;

/**
 * 
 * @author wangmengjun
 *
 */
public class SimpleDateFormatExample {

	private static final DateTimeFormatter SIMPLE_DATE_FORMAT = DateTimeFormat
			.forPattern("yyyyMMdd");

	public Date parseDate(String value) throws ParseException {
		return SIMPLE_DATE_FORMAT.parseDateTime(value).toDate();
	}

}

同样，执行上述Main.java类，可以得到正确结果：

Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016
Fri Nov 18 00:00:00 CST 2016

小结

本文首先给出了一个使用SimpleDateFormat的简单示例；接着，给出了一个在多线程下使用SimpleDateFormat出现问题的例子；紧接着又给出了几个解决SimpleDateFormat线程不安全使用的例子。

现在，Java 8中，已经对时间进行着改造，也已经逐渐向不可变和线程安全方向靠拢，有兴趣的读者可以尝试一下。

标签
date
Joda-Time
Apache Commmon
Thread-safe
threadlocal
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