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【JAVA8新的时间与日期 API】- 传统时间格式化的线程安全问题

Java8之前的日期和时间API,存在一些问题,最重要的就是线程安全的问题。这些问题都在Java8中的日期和时间API中得到了解决,而且Java8中的日期和时间API更加强大。

传统时间格式化的线程安全问题

示例:

import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.concurrent.*;

public class TestOldSimpleDateFormat {
    public static void main(String[] args) throws Exception {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        Callable<Date> task = new Callable<Date>() {
            @Override
            public Date call() throws Exception {
                return sdf.parse("2020-01-01");
            }
        };
        ExecutorService pool = Executors.newFixedThreadPool(10);
        List<Future<Date>> list = new ArrayList<>();
        for (int i=0;i<10;i++){
            Future<Date> future = pool.submit(task);
            list.add(future);
        }
        for (Future<Date> future : list){
            System.out.println(future.get());
        }

     pool.shutdown();
    }
}

以上代码运行会报错:

【JAVA8新的时间与日期 API】- 传统时间格式化的线程安全问题

报错缘由:取部分源码解释

    /**
     * SimpleDateFormat 类的 parse 方法 部分源码
     */
    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;
    }

public Date parse(String text, ParsePosition pos)
    {
        // 省略上面诸多代码
        Date parsedDate;

        CalendarBuilder calb = new CalendarBuilder();
        try {
            //这里这个 calendar 对象是 SimpleDateFormat 类的父类 DateFormat 中的属性  : protected Calendar calendar;
            parsedDate = calb.establish(calendar).getTime();//这个 calb.establish(calendar) 方法中,这个方法中的主要步骤不是原子操作,并且会对 calendar 对象进行修改,所以在多线程环境下就会出现线程安全问题。
            // 省略下面面诸多代码
        }
        catch (IllegalArgumentException e) {
            //省略.........................
            return null;
        }
        return parsedDate;
    }
 Calendar establish(Calendar cal) {
        boolean weekDate = isSet(WEEK_YEAR)
                            && field[WEEK_YEAR] > field[YEAR];
        if (weekDate && !cal.isWeekDateSupported()) {
            // Use YEAR instead
            if (!isSet(YEAR)) {
                set(YEAR, field[MAX_FIELD + WEEK_YEAR]);
            }
            weekDate = false;
        }

        cal.clear();
        // Set the fields from the min stamp to the max stamp so that
        // the field resolution works in the Calendar.
        for (int stamp = MINIMUM_USER_STAMP; stamp < nextStamp; stamp++) {
            for (int index = 0; index <= maxFieldIndex; index++) {
                if (field[index] == stamp) {
                    cal.set(index, field[MAX_FIELD + index]);
                    break;
                }
            }
        }

        if (weekDate) {
            int weekOfYear = isSet(WEEK_OF_YEAR) ? field[MAX_FIELD + WEEK_OF_YEAR] : 1;
            int dayOfWeek = isSet(DAY_OF_WEEK) ?
                                field[MAX_FIELD + DAY_OF_WEEK] : cal.getFirstDayOfWeek();
            if (!isValidDayOfWeek(dayOfWeek) && cal.isLenient()) {
                if (dayOfWeek >= 8) {
                    dayOfWeek--;
                    weekOfYear += dayOfWeek / 7;
                    dayOfWeek = (dayOfWeek % 7) + 1;
                } else {
                    while (dayOfWeek <= 0) {
                        dayOfWeek += 7;
                        weekOfYear--;
                    }
                }
                dayOfWeek = toCalendarDayOfWeek(dayOfWeek);
            }
            cal.setWeekDate(field[MAX_FIELD + WEEK_YEAR], weekOfYear, dayOfWeek);
        }
        return cal;
    }

综上,我们可以看到 SimpleDateFormat 类中的parse 方法,调用了 CalendarBuilder 的 establish(calendar) 方法,并在方法中,对 calendar 对象进行了各种判断及修改,并且这些操作都不是原子操作或同步操作,而这个calendar 对象又是 SimpleDateFormat 的父类 DateFormat 的一个实例变量,所以,在多线程同时调用SimpleDateFormat 的 parse 方法的时候,就会出现线程安全问题。

针对以上异常,JAVA8之前的解决办法:

1. 将 SimpleDateFormat 对象定义成局部变量。

2. 加锁。

3. 使用ThreadLocal,每个线程都拥有自己的SimpleDateFormat对象副本。

示例(加锁):

        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        Callable<Date> task = new Callable<Date>() {
            @Override
            public synchronized Date call() throws Exception {//加个同步,解决问题
                return sdf.parse("2020-01-01");
//                return DateFormatThreadLocal.convert("2020-01-01");
            }
        };
        ExecutorService pool = Executors.newFixedThreadPool(10);
        List<Future<Date>> list = new ArrayList<>();
        for (int i=0;i<10;i++){
            Future<Date> future = pool.submit(task);
            list.add(future);
        }
        for (Future<Date> future : list){
            System.out.println(future.get());
        }
        pool.shutdown();

示例(ThreadLocal):

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

public class DateFormatThreadLocal {
    private static final ThreadLocal<DateFormat> df = new ThreadLocal<DateFormat>() {
        protected DateFormat initialValue() {
            return new SimpleDateFormat("yyyy-MM-dd");
        }
    };
    public static Date convert(String source) throws Exception {
        return df.get().parse(source);
    }
}


////////////////////////////////////////////////////////////////

public class TestOldSimpleDateFormat {
    public static void main(String[] args) throws Exception {
//        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        Callable<Date> task = new Callable<Date>() {
            @Override
            public Date call() throws Exception {
//                return sdf.parse("2020-01-01");
                return DateFormatThreadLocal.convert("2020-01-01");
            }
        };
        ExecutorService pool = Executors.newFixedThreadPool(10);
        List<Future<Date>> list = new ArrayList<>();
        for (int i=0;i<10;i++){
            Future<Date> future = pool.submit(task);
            list.add(future);
        }
        for (Future<Date> future : list){
            System.out.println(future.get());
        }
     pool.shutdown();
} }

JAVA8的解决办法:使用新的API(DateTimeFormatter  和 LocalDate )

示例:

import java.time.LocalDate;
import java.time.format.DateTimeFormatter;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.*;

public class TestOldSimpleDateFormat {
    public static void main(String[] args) throws Exception {
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");

        Callable<LocalDate> task = new Callable<LocalDate>() {
            @Override
            public LocalDate call() throws Exception {
//                return sdf.parse("2020-01-01");
                return LocalDate.parse("2020-01-01",formatter);
            }
        };
        ExecutorService pool = Executors.newFixedThreadPool(10);
        List<Future<LocalDate>> list = new ArrayList<>();
        for (int i=0;i<10;i++){
            Future<LocalDate> future = pool.submit(task);
            list.add(future);
        }
        for (Future<LocalDate> future : list){
            System.out.println(future.get());
        }
        pool.shutdown();
    }
}
原文  http://www.cnblogs.com/y3blogs/p/13172896.html
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