SimpleDateFormat线程不安全及解决办法

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一. 为什么SimpleDateFormat不是线程安全的？
源码：

public class SimpleDateFormat extends DateFormat {public Date parse(String text, ParsePosition pos){          calendar.clear(); // Clears all the time fields          // other logic ...          Date parsedDate = calendar.getTime();      }   }

public abstract class DateFormat extends Format {         protected Calendar calendar;          public Date parse(String source) throws ParseException{          ParsePosition pos = new ParsePosition(0);          Date result = parse(source, pos);          if (pos.index == 0)              throw new ParseException("Unparseable date: \"" + source + "\"" ,                  pos.errorIndex);          return result;      }  }

如果我们把SimpleDateFormat定义成static成员变量，那么多个thread之间会共享这个sdf对象，所以Calendar对象也会共享。
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。
二. 问题重现：

    public class DateFormatTest {          private static SimpleDateFormat sdf = new SimpleDateFormat("dd-MMM-yyyy", Locale.US);          private static String date[] = { "01-Jan-1999", "01-Jan-2000", "01-Jan-2001" };          public static void main(String[] args) {              for (int i = 0; i < date.length; i++) {                  final int temp = i;                  new Thread(new Runnable() {                      @Override                      public void run() {                          try {                              while (true) {                                  String str1 = date[temp];                                  String str2 = sdf.format(sdf.parse(str1));                                  System.out.println(Thread.currentThread().getName() + ", " + str1 + "," + str2);                                  if(!str1.equals(str2)){                                      throw new RuntimeException(Thread.currentThread().getName()                                               + ", Expected " + str1 + " but got " + str2);                                  }                              }                          } catch (Exception e) {                              throw new RuntimeException("parse failed", e);                          }                      }                  }).start();              }          }      }

创建三个进程，使用静态成员变量SimpleDateFormat的parse和format方法，然后比较经过这两个方法折腾后的值是否相等：

程序如果出现以下错误，说明传入的日期就串掉了，即SimpleDateFormat不是线程安全的：

    Exception in thread "Thread-0" java.lang.RuntimeException: parse failed          at cn.test.DateFormatTest$1.run(DateFormatTest.java:27)          at java.lang.Thread.run(Thread.java:662)      Caused by: java.lang.RuntimeException: Thread-0, Expected 01-Jan-1999 but got 01-Jan-2000          at cn.test.DateFormatTest$1.run(DateFormatTest.java:22)          ... 1 more

三. 解决方案：

解决方案a：

将SimpleDateFormat定义成局部变量：

    SimpleDateFormat sdf = new SimpleDateFormat("dd-MMM-yyyy", Locale.US);      String str1 = "01-Jan-2010";      String str2 = sdf.format(sdf.parse(str1));

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。
2. 解决方案b:
加一把线程同步锁：synchronized(lock)

    public class SyncDateFormatTest {          private static SimpleDateFormat sdf = new SimpleDateFormat("dd-MMM-yyyy", Locale.US);          private static String date[] = { "01-Jan-1999", "01-Jan-2000", "01-Jan-2001" };          public static void main(String[] args) {              for (int i = 0; i < date.length; i++) {                  final int temp = i;                  new Thread(new Runnable() {                      @Override                      public void run() {                          try {                              while (true) {                                  synchronized (sdf) {                                      String str1 = date[temp];                                      Date date = sdf.parse(str1);                                      String str2 = sdf.format(date);                                      System.out.println(Thread.currentThread().getName() + ", " + str1 + "," + str2);                                      if(!str1.equals(str2)){                                          throw new RuntimeException(Thread.currentThread().getName()                                                   + ", Expected " + str1 + " but got " + str2);                                      }                                  }                              }                          } catch (Exception e) {                              throw new RuntimeException("parse failed", e);                          }                      }                  }).start();              }          }      }

缺点：性能较差，每次都要等待锁释放后其他线程才能进入
3. 解决方案c: (推荐)
使用ThreadLocal: 每个线程都将拥有自己的SimpleDateFormat对象副本。
写一个工具类：

    public class DateUtil {          private static ThreadLocal<SimpleDateFormat> local = new ThreadLocal<SimpleDateFormat>();          public static Date parse(String str) throws Exception {              SimpleDateFormat sdf = local.get();              if (sdf == null) {                  sdf = new SimpleDateFormat("dd-MMM-yyyy", Locale.US);                  local.set(sdf);              }              return sdf.parse(str);          }          public static String format(Date date) throws Exception {              SimpleDateFormat sdf = local.get();              if (sdf == null) {                  sdf = new SimpleDateFormat("dd-MMM-yyyy", Locale.US);                  local.set(sdf);              }              return sdf.format(date);          }      }

测试代码：

    public class ThreadLocalDateFormatTest {          private static String date[] = { "01-Jan-1999", "01-Jan-2000", "01-Jan-2001" };          public static void main(String[] args) {              for (int i = 0; i < date.length; i++) {                  final int temp = i;                  new Thread(new Runnable() {                      @Override                      public void run() {                          try {                              while (true) {                                  String str1 = date[temp];                                  Date date = DateUtil.parse(str1);                                  String str2 = DateUtil.format(date);                                  System.out.println(str1 + "," + str2);                                  if(!str1.equals(str2)){                                      throw new RuntimeException(Thread.currentThread().getName()                                               + ", Expected " + str1 + " but got " + str2);                                  }                              }                          } catch (Exception e) {                              throw new RuntimeException("parse failed", e);                          }                      }                  }).start();              }          }      }

补充：java.util.Random是线程安全的，不过多个线程的时候要同时公用一个生成器，这里可以使用ThreadLocalRandom为每个线程生成一个随机生成器

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