所有分类
  • 所有分类
  • 未分类

SimpleDateFormat–线程安全的操作方法(有实例)

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
1

评论2

请先

  1. threadlocal 线程变量 用完不需要释放吗?
    花生 2024-04-11 0
    • 需要释放,已修复
      自学精灵 2024-04-11 0
显示验证码
没有账号?注册  忘记密码?

社交账号快速登录