小编带你了解SimpleDateFormat-多线程问题

SimpleDateFormat-多线程问题：
　　SimpleDateFormat类在多线程环境下中处理日期，极易出现日期转换错误的情况
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

/**

• 线程类
  */
  public class MyThread extends Thread {

  private SimpleDateFormat sdf;
  private String dateString;

  public MyThread(SimpleDateFormat sdf,String dateString) {
  this.sdf = sdf;
  this.dateString = dateString;
  }

  @Override
  public void run() {
  try {
  Date dateRef = sdf.parse(dateString);
  String newDateString = sdf.format(dateRef).toString();
  if(!newDateString.equals(dateString)) {
  System.out.println("ThreadName = " + Thread.currentThread().getName()

  • "报错了 日期字符串：" + dateString + "转换成日期为：" + newDateString);
    }
    } catch (ParseException e) {
    e.printStackTrace();
    }
    }
    }
    import java.text.SimpleDateFormat;

public class Test {

/**
 *    测试单例的SimpleDateFormat类在多线程环境下中处理日期，极易出现日期转换错误的情况
 */
public static void main(String[] args) {
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
    String[] dateStringArray = new String[] {
            "2000-01-01","2000-01-02","2000-01-03",
            "2000-01-04","2000-01-05","2000-01-06",
            "2000-01-07","2000-01-08","2000-01-09",
            "2000-01-10"
    };

    MyThread[] threadArray = new MyThread[10];
    for (int i = 0; i < 10; i++) {
        threadArray[i] = new MyThread(sdf, dateStringArray[i]);
    }
    for (int i = 0; i < 10; i++) {
        threadArray[i].start();
    }
}

}
运行之后会输出很多的错误信息！
解决多线程出现的问题-为每个线程实例一个SimpleDateFormat：
import java.text.ParseException;
import java.util.Date;

/**

• 线程类
  */
  public class MyThread extends Thread {

  private String dateString;
  public MyThread(String dateString) {
  this.dateString = dateString;
  }

  @Override
  public void run() {
  try {
  Date dateRef = DateTools.parse("yyyy-MM-dd", dateString);
  String newDateString = DateTools.format("yyyy-MM-dd",dateRef).toString();
  if(!newDateString.equals(dateString)) {
  System.out.println("ThreadName = " + Thread.currentThread().getName()

  • "报错了 日期字符串：" + dateString + "转换成日期为：" + newDateString);
    }
    } catch (ParseException e) {
    e.printStackTrace();
    }
    }
    }
    import java.text.ParseException;
    import java.text.SimpleDateFormat;
    import java.util.Date;

/**

• 日期格式化工具类
  */
  public class DateTools {

  public static Date parse(String formatPattern,String dateString) throws ParseException {
  return new SimpleDateFormat(formatPattern).parse(dateString);
  }

  public static String format(String formatPattern,Date date) throws ParseException {
  return new SimpleDateFormat(formatPattern).format(date).toString();
  }
  }
  public class Test {

  /**

  • 测试,运行程序后，控制台没有任何输出，也就是转换没有任何异常，
  • 原理：创建了多个SimpleDateFormat实例
    */
    public static void main(String[] args) {
    String[] dateStringArray = new String[] {
    "2000-01-01","2000-01-02","2000-01-03",
    "2000-01-04","2000-01-05","2000-01-06",
    "2000-01-07","2000-01-08","2000-01-09",
    "2000-01-10"
    };

    MyThread[] threadArray = new MyThread[10];
    for (int i = 0; i < 10; i++) {
    threadArray[i] = new MyThread(dateStringArray[i]);
    }
    for (int i = 0; i < 10; i++) {
    threadArray[i].start();
    }
    }
    }
    解决多线程出现的问题-使用ThreadLocal：
    import java.text.ParseException;
    import java.util.Date;

/**

• 线程类
  */
  public class MyThread extends Thread {

  private String dateString;
  public MyThread(String dateString) {
  this.dateString = dateString;
  }

  @Override
  public void run() {
  try {
  Date dateRef = DateTools.getSimpleDateFormat("yyyy-MM-dd").parse(dateString);
  String newDateString = DateTools.getSimpleDateFormat("yyyy-MM-dd").format(dateRef).toString();
  if(!newDateString.equals(dateString)) {
  System.out.println("ThreadName = " + Thread.currentThread().getName()

  • "报错了 日期字符串：" + dateString + "转换成日期为：" + newDateString);
    }
    } catch (ParseException e) {
    e.printStackTrace();
    }
    }
    }
    import java.text.SimpleDateFormat;

/**

• 日期格式化工具类,使用ThreadLocal解决SimpleDateFormat非线程安全问题
  */
  public class DateTools {

  private static ThreadLocal<SimpleDateFormat> t1 = new ThreadLocal<>();

  public static SimpleDateFormat getSimpleDateFormat(String datePattern) {
  SimpleDateFormat sdf = null;
  sdf = t1.get();
  if(sdf == null) {
  sdf = new SimpleDateFormat(datePattern);
  t1.set(sdf);
  }
  return sdf;
  }
  }
  public class Test {

  /**

  • 测试,运行程序后，控制台没有任何输出，也就是转换没有任何异常
  • 原理：每个线程都会有自己的ThreadLocal存储全局变量，也就是每个线程都有自己的SimpleDateFormat实例
    */
    public static void main(String[] args) {
    String[] dateStringArray = new String[] {
    "2000-01-01","2000-01-02","2000-01-03",
    "2000-01-04","2000-01-05","2000-01-06",
    "2000-01-07","2000-01-08","2000-01-09",
    "2000-01-10"
    };

    MyThread[] threadArray = new MyThread[10];
    for (int i = 0; i < 10; i++) {
    threadArray[i] = new MyThread(dateStringArray[i]);
    }
    for (int i = 0; i < 10; i++) {
    threadArray[i].start();
    }
    }
    }

原文地址：http://blog.51cto.com/13954634/2170816