Java中的时间处理

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。

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

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

View Code

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：