SimpleDateFormat 的线程安全问题

1、问题：

SimpleDateFormat 的非线程安全问题:

package testDate;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Locale;

public class DateTester {

        static SimpleDateFormat df = new SimpleDateFormat("dd-MMM-yyyy", Locale.US);
        static String testdata[] = { "01-Jan-1999", "14-Feb-2001", "31-Dec-2007" };

        public static void main(String[] args) {
            Runnable r[] = new Runnable[testdata.length];
            for (int i = 0; i < r.length; i++) {
                final int i2 = i;
                r[i] = new Runnable() {
                    public void run() {
                        try {
                            for (int j = 0; j < 2000; j++) {
                                String str = testdata[i2];
                                String str2 = null;
                                /* synchronized(df) */{
                                    Date d = df.parse(str);
                                    str2 = df.format(d);
                                    System.out.println("i: " + i2 + "\tj: " + j + "\tThreadID: "
                                            + Thread.currentThread().getId() + "\tThreadName: "
                                            + Thread.currentThread().getName() + "\t" + str + "\t" + str2);
                                }
                                if (!str.equals(str2)) {
                                    throw new RuntimeException("date conversion failed after " + j
                                            + " iterations. Expected " + str + " but got " + str2);
                                }
                            }
                        } catch (ParseException e) {
                            throw new RuntimeException("parse failed");
                        }
                    }
                };
                new Thread(r[i]).start();
            }
        }

}

异常:

i: 1    j: 0    ThreadID: 19    ThreadName: Thread-6    14-Feb-2001 14-Jan-1999
i: 2    j: 0    ThreadID: 20    ThreadName: Thread-7    31-Dec-2007 14-Jan-1999
i: 0    j: 0    ThreadID: 18    ThreadName: Thread-5    01-Jan-1999 14-Jan-1999
Exception in thread "Thread-7" Exception in thread "Thread-5" Exception in thread "Thread-6" java.lang.RuntimeException: date conversion failed after 0 iterations. Expected 31-Dec-2007 but got 14-Jan-1999java.lang.RuntimeException: date conversion failed after 0 iterations. Expected 01-Jan-1999 but got 14-Jan-1999

java.lang.RuntimeException: date conversion failed after 0 iterations. Expected 14-Feb-2001 but got 14-Jan-1999
    at testDate.DateTester$1.run(DateTester.java:31)
    at testDate.DateTester$1.run(DateTester.java:31)	at testDate.DateTester$1.run(DateTester.java:31)    at java.lang.Thread.run(Thread.java:785)

    at java.lang.Thread.run(Thread.java:785)

    at java.lang.Thread.run(Thread.java:785)

恩，原因你是知道了，这是由于 SimpleDateFormat 的非线程安全问题引起的，

我们现在简化下问题，错误的代码应该是这样的：

public class DateUtil {

 SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");

 public String formatDate(Date input) {
      return sdf.format(input);
 }

}

这里写图片描述

2 解决方案

（1）使用局部变量：

import java.text.SimpleDateFormat;
import java.util.Date;

public class DateUtil {

 private static final String SIMPLE_FORMAT = "dd/MM/yyyy";

 public String formatDate(Date input) {

  if(input == null){
   return null;
  }

  SimpleDateFormat sdf = new SimpleDateFormat(SIMPLE_FORMAT);//local variable
  return sdf.format(input);
 }
}

这里写图片描述

（2）使用 ThreadLocal
这里每个线程将有它自己的 SimpleDateFormat 副本。

import java.text.SimpleDateFormat;
import java.util.Date;

public class DateUtil {

 // anonymous inner class. Each thread will have its own copy of the SimpleDateFormat
 private final static ThreadLocal<simpledateformat> tl = new ThreadLocal<simpledateformat>() {
  protected SimpleDateFormat initialValue() {
   return new SimpleDateFormat("dd/MM/yyyy");
  }
 } 
public String formatDate(Date input) {
  if (input == null) {
   return null;
  }

  return tl.get().format(input);
 }
}