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简介

        本文用示例介绍Java中的SingleDateFormat的线程安全的操作方法，也介绍SingleDateFormat为什么不是线程安全的。这两个问题也是面试中常见的问题。

线程不安全实例

package org.example.a;import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.concurrent.*;public class Demo{public static void main(String[] args) {final DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");Callable<Date> task = new Callable<Date>() {public Date call() throws Exception {return dateFormat.parse("2016-12-18 15:00:34");}};// 创建5个线程的线程池ExecutorService exec = Executors.newFixedThreadPool(5);List<Future<Date>> results = new ArrayList<Future<Date>>();for (int i = 0; i < 10; i++) {results.add(exec.submit(task));}exec.shutdown();// 输出结果for (Future<Date> result : results) {try {System.out.println(result.get());} catch (InterruptedException e) {e.printStackTrace();} catch (ExecutionException e) {e.printStackTrace();}}}
}

执行结果（出现异常）

Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
java.util.concurrent.ExecutionException: java.lang.NumberFormatException: For input string: "E34"at java.util.concurrent.FutureTask.report(FutureTask.java:122)at java.util.concurrent.FutureTask.get(FutureTask.java:192)at org.example.a.Demo.main(Demo.java:29)
Caused by: java.lang.NumberFormatException: For input string: "E34"at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)at java.lang.Long.parseLong(Long.java:589)at java.lang.Long.parseLong(Long.java:631)at java.text.DigitList.getLong(DigitList.java:195)at java.text.DecimalFormat.parse(DecimalFormat.java:2051)at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:2162)at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)at java.text.DateFormat.parse(DateFormat.java:364)at org.example.a.Demo$1.call(Demo.java:15)at org.example.a.Demo$1.call(Demo.java:13)at java.util.concurrent.FutureTask.run(FutureTask.java:266)at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)at java.lang.Thread.run(Thread.java:745)
java.util.concurrent.ExecutionException: java.lang.NumberFormatException: For input string: "E342"at java.util.concurrent.FutureTask.report(FutureTask.java:122)at java.util.concurrent.FutureTask.get(FutureTask.java:192)
Sun Dec 18 15:00:34 CST 2016    at org.example.a.Demo.main(Demo.java:29)
Caused by: java.lang.NumberFormatException: For input string: "E342"at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:2043)at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110)at java.lang.Double.parseDouble(Double.java:538)at java.text.DigitList.getDouble(DigitList.java:169)at java.text.DecimalFormat.parse(DecimalFormat.java:2056)at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:2162)at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)at java.text.DateFormat.parse(DateFormat.java:364)at org.example.a.Demo$1.call(Demo.java:15)at org.example.a.Demo$1.call(Demo.java:13)at java.util.concurrent.FutureTask.run(FutureTask.java:266)at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)at java.lang.Thread.run(Thread.java:745)java.util.concurrent.ExecutionException: java.lang.NumberFormatException: For input string: ""at java.util.concurrent.FutureTask.report(FutureTask.java:122)at java.util.concurrent.FutureTask.get(FutureTask.java:192)at org.example.a.Demo.main(Demo.java:29)
Caused by: java.lang.NumberFormatException: For input string: ""at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)at java.lang.Long.parseLong(Long.java:601)at java.lang.Long.parseLong(Long.java:631)at java.text.DigitList.getLong(DigitList.java:195)at java.text.DecimalFormat.parse(DecimalFormat.java:2051)at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:2162)at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)at java.text.DateFormat.parse(DateFormat.java:364)at org.example.a.Demo$1.call(Demo.java:15)at org.example.a.Demo$1.call(Demo.java:13)at java.util.concurrent.FutureTask.run(FutureTask.java:266)at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)at java.lang.Thread.run(Thread.java:745)
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016 

线程不安全原因

SimpleDateFormat线程不安全及解决办法_timmyroy的博客-CSDN博客

        SimpleDateFormat类内部有一个Calendar对象引用,它用来储存和这个SimpleDateFormat相关的日期信息,例如sdf.parse(dateStr),sdf.format(date) 诸如此类的方法参数传入的日期相关String,Date等等, 都是交由Calendar引用来储存的.这样就会导致一个问题：如果你的SimpleDateFormat是多个thread 之间共享的, 那么也共享这个Calendar引用。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。

线程安全的方案

方案1：新建局部变量（开销大）

缺点：每调用一次方法就会创建一个SimpleDateFormat对象，方法结束又要作为垃圾回收。

package org.example.a;import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.concurrent.*;public class Demo{public static void main(String[] args) {Callable<Date> task = new Callable<Date>() {public Date call() throws Exception {DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");return dateFormat.parse("2016-12-18 15:00:34");}};// 创建5个线程的线程池ExecutorService exec = Executors.newFixedThreadPool(5);List<Future<Date>> results = new ArrayList<Future<Date>>();for (int i = 0; i < 10; i++) {results.add(exec.submit(task));}exec.shutdown();// 输出结果for (Future<Date> result : results) {try {System.out.println(result.get());} catch (InterruptedException e) {e.printStackTrace();} catch (ExecutionException e) {e.printStackTrace();}}}
}

执行结果（正常）

Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016 

方案2：方法加synchronized（性能差）

package org.example.a;import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.concurrent.*;public class Demo{public static void main(String[] args) {final DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");Callable<Date> task = new Callable<Date>() {public Date call() throws Exception {synchronized (dateFormat) {return dateFormat.parse("2016-12-18 15:00:34");}}};// 创建5个线程的线程池ExecutorService exec = Executors.newFixedThreadPool(5);List<Future<Date>> results = new ArrayList<Future<Date>>();for (int i = 0; i < 10; i++) {results.add(exec.submit(task));}exec.shutdown();// 输出结果for (Future<Date> result : results) {try {System.out.println(result.get());} catch (InterruptedException e) {e.printStackTrace();} catch (ExecutionException e) {e.printStackTrace();}}}
}

执行结果（正常）

Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016

方案3：使用ThreadLocal（推荐） 

package org.example.a;import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.concurrent.*;class DateFormatThreadSafe {public static final ThreadLocal<DateFormat> THREAD_LOCAL = new ThreadLocal<DateFormat>() {@Overrideprotected DateFormat initialValue() {return new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");}};
}public class Demo{public static void main(String[] args) {Callable<Date> task = new Callable<Date>() {public Date call() throws Exception {DateFormat dateFormat = DateFormatThreadSafe.THREAD_LOCAL.get();return dateFormat.parse("2016-12-18 15:00:34");}};// 创建5个线程的线程池ExecutorService exec = Executors.newFixedThreadPool(5);List<Future<Date>> results = new ArrayList<Future<Date>>();for (int i = 0; i < 10; i++) {results.add(exec.submit(task));}exec.shutdown();// 输出结果for (Future<Date> result : results) {try {System.out.println(result.get());} catch (InterruptedException e) {e.printStackTrace();} catch (ExecutionException e) {e.printStackTrace();}}}
}

执行结果（正常）

Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016
Sun Dec 18 15:00:34 CST 2016

方案4：针对JDK1.8

        如果是JDK8的应用，可以使用Instant代替Date，LocalDateTime代替Calendar，DateTimeFormatter代替Simpledateformatter，官方给出的解释：simple beautiful strong immutable thread-safe。

package org.example.a;import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.*;public class Demo{public static void main(String[] args) {DateTimeFormatter dtf=DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");Callable<LocalDateTime> task = new Callable<LocalDateTime>() {public LocalDateTime call() throws Exception {return LocalDateTime.parse("2016-12-18 15:00:34", dtf);}};// 创建5个线程的线程池ExecutorService exec = Executors.newFixedThreadPool(5);List<Future<LocalDateTime>> results = new ArrayList<Future<LocalDateTime>>();for (int i = 0; i < 10; i++) {results.add(exec.submit(task));}exec.shutdown();// 输出结果for (Future<LocalDateTime> result : results) {try {System.out.println(result.get());} catch (InterruptedException e) {e.printStackTrace();} catch (ExecutionException e) {e.printStackTrace();}}}
}

执行结果（正常）

2016-12-18T15:00:34
2016-12-18T15:00:34
2016-12-18T15:00:34
2016-12-18T15:00:34
2016-12-18T15:00:34
2016-12-18T15:00:34
2016-12-18T15:00:34
2016-12-18T15:00:34
2016-12-18T15:00:34
2016-12-18T15:00:34

其他网址

SimpleDateFormat线程不安全原因及解决方案 - 杨七 - 博客园
SimpleDateFormat线程不安全及解决办法_java_诚-CSDN博客