Java中的时间处理

1. 日期时间组件使用

   java.util.Date：实现类，其对象具有时间、日期组件。
   java.util.Calendar：抽象类，其对象具有时间、日期组件。
   java.sql.Date：实现类，其对象具有日期组件。
   java.sql.Time：实现类，其对象具有时间组件。
   java.sql.Timestamp：实现类，其对象具有时间日期组件。
   java.text.DateFormat：抽象类，其对象格式化时间日期。
   java.text.DateFormatSymbols：实现类，其对象为格式化时间日期提供参数。

2. long currentTime = System.currentTimeMillis(); //获取当前时间戳，从1970-01-01以来的毫秒数
   LOGGER.info("{}", currentTime); java.util.Date utilDate = new java.util.Date(currentTime);
   LOGGER.info("{}|{}", utilDate, utilDate.getTime()); Calendar calendar = Calendar.getInstance(); //除某些特殊地区外，默认等效于new GregorianCalendar()
   calendar.setTimeInMillis(currentTime);
   LOGGER.info("{}", calendar); java.sql.Date sqlDate = new java.sql.Date(currentTime);
   LOGGER.info("{}|{}", sqlDate, sqlDate.getTime()); java.sql.Time sqlTime = new java.sql.Time(currentTime);
   LOGGER.info("{}|{}", sqlTime, sqlTime.getTime()); java.sql.Timestamp sqlTimestamp = new java.sql.Timestamp(currentTime);
   LOGGER.info("{}|{}", sqlTimestamp, sqlTimestamp.getTime());

   DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); //默认是SimpleDateFormat
   LOGGER.info("{}", dateFormat.format(utilDate));
   LOGGER.info("{}", dateFormat.parse("2017-05-25 23:00:00"));

   输出：

   1495724884755
   Thu May 25 23:08:04 CST 2017|1495724884755
   java.util.GregorianCalendar[time=1495724884755,...,zone=sun.util.calendar.ZoneInfo[id="Asia/Shanghai"...]
   2017-05-25|1495724884755
   23:08:04|1495724884755
   2017-05-25 23:08:04.755|1495724884755
   2017-05-25 23:08:04
   Thu May 25 23:00:00 CST 2017

3. java.text.SimpleDateFormat的线程不安全问题
   问题重现：

   //final SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd"); Callable<Date> task = new Callable<Date>() {
   @Override
   public Date call() throws Exception {
   LOGGER.info("executing task...");
   try{
   return new SimpleDateFormat("yyyy-MM-dd").parse("2017-05-25");
   //return format.parse("2017-05-25"); //if use shared SimpleDateFormat, executing result is not expected.
   }catch (Exception ex){
   LOGGER.error("Exception：", ex);
   return new Date(0);
   }
   }
   };
   // pool with 5 threads
   ExecutorService exec = Executors.newFixedThreadPool(5);
   List<Future<Date>> results = new ArrayList<Future<Date>>();
   // perform 10 date conversions
   for (int i = 0; i < 10; i++) {
   LOGGER.info("submit task[{}]", i);
   results.add(exec.submit(task));
   }
   exec.shutdown();
   LOGGER.info("shutdown");
   // look at the results
   DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
   for (Future<Date> result : results) {
   LOGGER.info("{}", dateFormat.format(result.get()));
   }
   LOGGER.info("completed");

   输出（使用共享的SimpleDateFormat时，执行结果并不是预期结果）：

   submit task[0]
   submit task[1]
   submit task[2]
   executing task...
   submit task[3]
   executing task...
   submit task[4]
   executing task...
   executing task...
   submit task[5]
   executing task...
   submit task[6]
   submit task[7]
   submit task[8]
   submit task[9]
   shutdown
   executing task...
   executing task...
   executing task...
   executing task...
   executing task...
   Exception：
   java.lang.NumberFormatException: multiple points
   at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1890)
   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:1869)
   at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
   at java.text.DateFormat.parse(DateFormat.java:364)
   at com.liq.DateTimeTest$1.call(DateTimeTest.java:65)
   at com.liq.DateTimeTest$1.call(DateTimeTest.java:59)
   at java.util.concurrent.FutureTask.run(FutureTask.java:266)
   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
   at java.lang.Thread.run(Thread.java:745)
   Exception：
   java.lang.NumberFormatException: empty String
   at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1842)
   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:1869)
   at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
   at java.text.DateFormat.parse(DateFormat.java:364)
   at com.liq.DateTimeTest$1.call(DateTimeTest.java:65)
   at com.liq.DateTimeTest$1.call(DateTimeTest.java:59)
   at java.util.concurrent.FutureTask.run(FutureTask.java:266)
   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
   at java.lang.Thread.run(Thread.java:745)
   Exception：
   java.lang.NumberFormatException: For input string: "E.2505E2"
   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:1869)
   at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
   at java.text.DateFormat.parse(DateFormat.java:364)
   at com.liq.DateTimeTest$1.call(DateTimeTest.java:65)
   at com.liq.DateTimeTest$1.call(DateTimeTest.java:59)
   at java.util.concurrent.FutureTask.run(FutureTask.java:266)
   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
   at java.lang.Thread.run(Thread.java:745)
   Exception：
   java.lang.NumberFormatException: For input string: "E.2505"
   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:1869)
   at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
   at java.text.DateFormat.parse(DateFormat.java:364)
   at com.liq.DateTimeTest$1.call(DateTimeTest.java:65)
   at com.liq.DateTimeTest$1.call(DateTimeTest.java:59)
   at java.util.concurrent.FutureTask.run(FutureTask.java:266)
   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
   at java.lang.Thread.run(Thread.java:745)
   1970-01-01 08:00:00
   1970-01-01 08:00:00
   1970-01-01 08:00:00
   1970-01-01 08:00:00
   2017-05-25 00:00:00
   2017-05-25 00:00:00
   2017-05-25 00:00:00
   2017-05-25 00:00:00
   2017-05-25 00:00:00
   2017-05-25 00:00:00
   completed

   产生原因：
   SimpleDateFormat维护一个共享的Calendar对象，如果共享SimpleDateFomat时这个Calendar也是共享的。由多个线程并发进行操作就会有问题了。

   protected Calendar calendar;

   解决：
   （1）使用局部变量
   （2）使用 ThreadLocal

   final ThreadLocal<DateFormat> df = new ThreadLocal<DateFormat>() {
   @Override
   protected DateFormat initialValue() {
   return new SimpleDateFormat("yyyy-MM-dd");
   }
   };
   //final SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd"); Callable<Date> task = new Callable<Date>() {
   @Override
   public Date call() throws Exception {
   LOGGER.info("executing task...");
   try{
   //return new SimpleDateFormat("yyyy-MM-dd").parse("2017-05-25");
   //return format.parse("2017-05-25"); //if use shared SimpleDateFormat, executing result is not expected.
   return df.get().parse("2017-05-25");
   }catch (Exception ex){
   LOGGER.error("Exception：", ex);
   return new Date(0);
   }
   }
   };

   （3）同步代码块 synchronized(code)
   （4）使用第三方的日期处理函数：比如使用 commons-lang 包中的 FastDateFormat 工具类

参考：

1. 如何在Java 8中愉快地处理日期和时间
2. Java中的时间日期处理
3. 详解SimpleDateFormat的线程安全问题与解决方案
4. java中关于时间的处理 *
5. 日历字段之间的转换