简介
本文用示例介绍Java中的SimpleDateFormat的线程安全的操作方法，也介绍SimpleDateFormat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：new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>() {
@Override
protected 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();
Date date = dateFormat.parse("2016-12-18 15:00:34");
// 移除ThreadLocal的值，防止内存溢出
DateFormatThreadSafe.THREAD_LOCAL.remove();
return date;
}
};
// 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（推荐）
说明
如果是JDK8的应用，可以使用Instant代替Date，LocalDateTime代替Calendar，DateTimeFormatter代替Simpledateformatter。
为什么DateTimeFormatter线程安全？
依据1：官方给出的解释：simple beautiful strong immutable thread-safe。
依据2：看DateTimeFormatter代码，它所有字段都是final类型。
实例
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
请先 ！