SimpleDateFormat非线程安全

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);
}
}
}