SimpleDateFormat-多线程问题

SimpleDateFormat-多线程问题：

　　SimpleDateFormat类在多线程环境下中处理日期，极易出现日期转换错误的情况

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

/**
 *    线程类
 */
public class MyThread extends Thread {
    
    private SimpleDateFormat sdf;
    private String dateString;
    
    public MyThread(SimpleDateFormat sdf,String dateString) {
        this.sdf = sdf;
        this.dateString = dateString;
    }
    
    @Override
    public void run() {
        try {
            Date dateRef = sdf.parse(dateString);
            String newDateString = sdf.format(dateRef).toString();
            if(!newDateString.equals(dateString)) {
                System.out.println("ThreadName = " + Thread.currentThread().getName() 
                        + "报错了  日期字符串：" + dateString + "转换成日期为：" + newDateString);
            }
        } catch (ParseException e) {
            e.printStackTrace();
        }
    }
}

import java.text.SimpleDateFormat;

public class Test {

    /**
     *    测试单例的SimpleDateFormat类在多线程环境下中处理日期，极易出现日期转换错误的情况
     */
    public static void main(String[] args) {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        String[] dateStringArray = new String[] {
                "2000-01-01","2000-01-02","2000-01-03",
                "2000-01-04","2000-01-05","2000-01-06",
                "2000-01-07","2000-01-08","2000-01-09",
                "2000-01-10"
        };
        
        MyThread[] threadArray = new MyThread[10];
        for (int i = 0; i < 10; i++) {
            threadArray[i] = new MyThread(sdf, dateStringArray[i]);
        }
        for (int i = 0; i < 10; i++) {
            threadArray[i].start();
        }
    }
}

运行之后会输出很多的错误信息！

　　解决多线程出现的问题-为每个线程实例一个SimpleDateFormat：

import java.text.ParseException;
import java.util.Date;

/**
 *    线程类
 */
public class MyThread extends Thread {
    
    private String dateString;
    public MyThread(String dateString) {
        this.dateString = dateString;
    }
    
    @Override
    public void run() {
        try {
            Date dateRef = DateTools.parse("yyyy-MM-dd", dateString);
            String newDateString = DateTools.format("yyyy-MM-dd",dateRef).toString();
            if(!newDateString.equals(dateString)) {
                System.out.println("ThreadName = " + Thread.currentThread().getName() 
                        + "报错了  日期字符串：" + dateString + "转换成日期为：" + newDateString);
            }
        } catch (ParseException e) {
            e.printStackTrace();
        }
    }
}

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

/**
 *    日期格式化工具类
 */
public class DateTools {
    
    public static Date parse(String formatPattern,String dateString) throws ParseException {
        return new SimpleDateFormat(formatPattern).parse(dateString);
    }

    public static String format(String formatPattern,Date date) throws ParseException {
        return new SimpleDateFormat(formatPattern).format(date).toString();
    }
}

public class Test {

    /**
     *    测试,运行程序后，控制台没有任何输出，也就是转换没有任何异常，
     *    原理：创建了多个SimpleDateFormat实例
     */
    public static void main(String[] args) {
        String[] dateStringArray = new String[] {
                "2000-01-01","2000-01-02","2000-01-03",
                "2000-01-04","2000-01-05","2000-01-06",
                "2000-01-07","2000-01-08","2000-01-09",
                "2000-01-10"
        };
        
        MyThread[] threadArray = new MyThread[10];
        for (int i = 0; i < 10; i++) {
            threadArray[i] = new MyThread(dateStringArray[i]);
        }
        for (int i = 0; i < 10; i++) {
            threadArray[i].start();
        }
    }
}

　　解决多线程出现的问题-使用ThreadLocal：

import java.text.ParseException;
import java.util.Date;

/**
 *    线程类
 */
public class MyThread extends Thread {
    
    private String dateString;
    public MyThread(String dateString) {
        this.dateString = dateString;
    }
    
    @Override
    public void run() {
        try {
            Date dateRef = DateTools.getSimpleDateFormat("yyyy-MM-dd").parse(dateString);
            String newDateString = DateTools.getSimpleDateFormat("yyyy-MM-dd").format(dateRef).toString();
            if(!newDateString.equals(dateString)) {
                System.out.println("ThreadName = " + Thread.currentThread().getName() 
                        + "报错了  日期字符串：" + dateString + "转换成日期为：" + newDateString);
            }
        } catch (ParseException e) {
            e.printStackTrace();
        }
    }
}

import java.text.SimpleDateFormat;

/**
 *    日期格式化工具类,使用ThreadLocal解决SimpleDateFormat非线程安全问题
 */
public class DateTools {
    
    private static ThreadLocal<SimpleDateFormat> t1 = new ThreadLocal<>();
    
    public static SimpleDateFormat getSimpleDateFormat(String datePattern) {
        SimpleDateFormat sdf = null;
        sdf = t1.get();
        if(sdf == null) {
            sdf = new SimpleDateFormat(datePattern);
            t1.set(sdf);
        }
        return sdf;
    }
}

public class Test {

    /**
     *    测试,运行程序后，控制台没有任何输出，也就是转换没有任何异常
     *    原理：每个线程都会有自己的ThreadLocal存储全局变量，也就是每个线程都有自己的SimpleDateFormat实例
     */
    public static void main(String[] args) {
        String[] dateStringArray = new String[] {
                "2000-01-01","2000-01-02","2000-01-03",
                "2000-01-04","2000-01-05","2000-01-06",
                "2000-01-07","2000-01-08","2000-01-09",
                "2000-01-10"
        };
        
        MyThread[] threadArray = new MyThread[10];
        for (int i = 0; i < 10; i++) {
            threadArray[i] = new MyThread(dateStringArray[i]);
        }
        for (int i = 0; i < 10; i++) {
            threadArray[i].start();
        }
    }
}