SimpleDateFormat非线程安全

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类SimpleDateFormat主要负责日期的装换和格式化，但在多线程的环境中，使用此类容易造成数据转换及处理的不准确，因为SimpleDateFormat类并不是线程安全的。

package me.mymilkbottles.Study11;import java.text.ParseException;import java.text.SimpleDateFormat;import java.util.Date;/** * Created by Administrator on 2017/07/17 17:08. */public class SimpleDateFormatStudy {    public static void main(String[] args) {        SimpleDateFormat sdf = new SimpleDateFormat("yyyy");        String[] strings =                new String[]{"2017", "2016", "2014", "2012",                             "2010", "2017", "1997", "1990",                             "1999", "1998", "1875", "1987",                             "1991", "1993", "1994", "1777",                             "1853", "1879", "1888", "1889"};        for (String date : strings) {            new MyThread(sdf, date).start();        }    }}class MyThread extends Thread {    private SimpleDateFormat sdf;    private String date;    public MyThread(SimpleDateFormat sdf, String date) {        this.sdf = sdf;        this.date = date;    }    @Override    public void run() {        Date parseDate = null;        try {            parseDate = sdf.parse(date);        } catch (ParseException e) {            e.printStackTrace();        }        if (!sdf.format(parseDate).toString().equals(date)) {            System.out.println(sdf.format(parseDate).toString() + " != " + date);        }    }}

这是因为单例的SimpleDateFormat是不安全的，所以我们可以考虑使用多例。

package me.mymilkbottles.Study11;import java.text.ParseException;import java.text.SimpleDateFormat;import java.util.Date;/** * Created by Administrator on 2017/07/17 17:08. */public class SimpleDateFormatStudy {    public static void main(String[] args) {        String[] strings =                new String[]{"2017", "2016", "2014", "2012",                             "2010", "2017", "1997", "1990",                             "1999", "1998", "1875", "1987",                             "1991", "1993", "1994", "1777",                             "1853", "1879", "1888", "1889"};        for (String date : strings) {            new MyThread(date).start();        }    }}class MyThread extends Thread {    private String date;    public MyThread(String date) {        this.date = date;    }    @Override    public void run() {        SimpleDateFormat sdf = new SimpleDateFormat("yyyy");        Date parseDate = null;        try {            parseDate = sdf.parse(date);        } catch (ParseException e) {            e.printStackTrace();        }        if (!sdf.format(parseDate).toString().equals(date)) {            System.out.println(sdf.format(parseDate).toString() + " != " + date);        }    }}

这种方法会创建多个SimpleDateFormat对象，如果我们希望少创建对象，我们可以考虑使用ThreadLocal。

package me.mymilkbottles.Study11;import java.text.ParseException;import java.text.SimpleDateFormat;import java.util.Date;/** * Created by Administrator on 2017/07/17 17:08. */public class SimpleDateFormatStudy {    public static void main(String[] args) {        String[] strings =                new String[]{"2017", "2016", "2014", "2012",                        "2010", "2017", "1997", "1990",                        "1999", "1998", "1875", "1987",                        "1991", "1993", "1994", "1777",                        "1853", "1879", "1888", "1889"};        for (String date : strings) {            Thread thread = new MyThread(date);            thread.start();        }    }}class MyThread extends Thread {    public static ThreadLocal<SimpleDateFormat> threadLocal = new ThreadLocal<>();    private String date;    public MyThread(String date) {        this.date = date;    }    @Override    public void run() {        SimpleDateFormat sdf = threadLocal.get();        if (sdf == null) {            sdf = new SimpleDateFormat("yyyy");            threadLocal.set(sdf);        }        Date parseDate = null;        try {            parseDate = sdf.parse(date);        } catch (ParseException e) {            e.printStackTrace();        }        if (!sdf.format(parseDate).toString().equals(date)) {            System.out.println(sdf.format(parseDate).toString() + " != " + date);        }    }}

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