SimpleDateFormat非线程不安全

1 单线程环境

类SimpleDateFormate常用于时间格式转换，在单线程环境下运行结果正常，如下所示

public class MyThread extends Thread {   private SimpleDateFormatsdf;   private StringdateString;    publicMyThread(SimpleDateFormatsdf, String dateString) {      this.sdf =sdf;      this.dateString =dateString;   }    Public void run() {      try {        Date date = sdf.parse(dateString);        String newDateString = sdf.format(date).toString();        if (!newDateString.equals(dateString)) {           System.out.println("日期字符串： " + dateString + "转换成日期为：" +newDateString);        }      } catch(ParseExceptione) {        e.printStackTrace();      }   }}

public class SingleThreadTest{   public static void main(String[]args) {      SimpleDateFormat sdf = newSimpleDateFormat("yyyy-MM-dd");      String[] dateStringArray = new String[]{"2017-01-01","2017-01-02", "2017-01-03","2017-01-04", "2000-01-05",           "2017-01-06", "2017-01-07", "2017-01-08", "2017-01-09", "2017-01-10"};      MyThread[] threadArray = newMyThread[10];      for(inti = 0; i < 10;i++){        threadArray[i] = new MyThread(sdf,dateStringArray[i]);      }      for(inti = 0; i < 10;i++){       threadArray[i].run(); //单线程情况下      }   }}

运行结果：

2 并发环境

在并发环境下，使用类SimpleDateFormate进行日期格式转换会出现数据处理不正确，根本原因是存在共享变量calendar。

public class MutiThreadTest{   public static void main(String[]args) {      SimpleDateFormat sdf = newSimpleDateFormat("yyyy-MM-dd");      String[] dateStringArray = new String[]{"2017-01-01","2017-01-02", "2017-01-03","2017-01-04", "2000-01-05",           "2017-01-06", "2017-01-07", "2017-01-08", "2017-01-09", "2017-01-10"};      MyThread[] threadArray = newMyThread[10];      for(inti = 0; i < 10;i++){        threadArray[i] = new MyThread(sdf,dateStringArray[i]);      }      for(inti = 0; i < 10;i++){        threadArray[i].start(); //多线程条件下      }   }}

运行结果：

 

3  解决方法

3.1使用 LocalDate类

 JDK8时间新特性类LocalDate是线程安全，而且使用方法更简单

public class Test {   public static void main(String[]args) {      String[] dateStringArray = new String[]{"2017-01-01","2017-01-02", "2017-01-03","2017-01-04", "2000-01-05",           "2017-01-06", "2017-01-07", "2017-01-08", "2017-01-09", "2017-01-10"};      MyThread[] threadArray = newMyThread[10];      for(inti = 0; i < 10;i++){        threadArray[i] = new MyThread(dateStringArray[i]);      }      for(inti = 0; i < 10;i++){        threadArray[i].start();  //多线程条件下             }   }}

public class MyThread extends Thread {   private StringdateString;   public MyThread(StringdateString) {      super();      this.dateString =dateString;   }    public void run() {      LocalDate date = LocalDate.parse(dateString);// JDK8新特性      if (!date.toString().equals(dateString)) {        System.out.println("日期字符串： " +dateString + "转换成日期为：" +date);      }   }} 

3.2 使用ThreadLocal类

类ThreadLocal解决的是变量在不同线程间的隔离性，也就是不同线程拥有自己的隔离值，不同线程中的值可以放入ThreadLocal类中进行保存。

public class DateTools{   privatestaticThreadLocal<SimpleDateFormat> t1 = newThreadLocal<SimpleDateFormat>();   publicstaticSimpleDateFormat getSimpleDateFormate(StringdatePattern){      SimpleDateFormat sdf = null;      sdf = t1.get();      if(sdf ==null){        sdf = newSimpleDateFormat(datePattern);        t1.set(sdf);      }      returnsdf;   }}

public class MyThread extends Thread {   private StringdateString;     public MyThread(StringdateString) {      super();      this.dateString =dateString;   }    publicvoid run(){      try {           Date dateRef =DateTools.getSimpleDateFormate("yyyy-MM-dd").parse(dateString);        String newDateString =  DateTools.getSimpleDateFormate("yyyy-MM-dd").format(dateRef).toString();        if (!newDateString.equals(dateString)) {           System.out.println("日期字符串： " + dateString + "转换成日期为：" +newDateString);        }      } catch(ParseExceptione) {        e.printStackTrace();      }       }}

public class Test {   publicstatic voidmain(String[]args) {      String[] dateStringArray = new String[]{"2017-01-01","2017-01-02", "2017-01-03","2017-01-04", "2000-01-05",           "2017-01-06", "2017-01-07", "2017-01-08", "2017-01-09", "2017-01-10"};      MyThread[] threadArray = newMyThread[10];      for(inti = 0; i < 10;i++){        threadArray[i] = newMyThread(dateStringArray[i]);      }      for(inti = 0; i < 10;i++){        threadArray[i].start();  //多线程条件下             }   }}

4 参考文献

[1]  高洪岩, Java多线程编程核心技术. 2015.

[2]  Goetz, B.等, Java 并发编程实战. 2012.

[3]  https://docs.oracle.com/javase/8/docs/api/index.html