SimpleDateFormat-多線程問題：

　　SimpleDateFormat類在多線程環境下中處理日期，極易出現日期轉換錯誤的情況

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

/**
 *    線程類
 */
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() 
                        + "報錯了  日期字符串：" + dateString + "轉換成日期為：" + newDateString);
            }
        } catch (ParseException e) {
            e.printStackTrace();
        }
    }
}

import java.text.SimpleDateFormat;

public class Test {

    /**
     *    測試單例的SimpleDateFormat類在多線程環境下中處理日期，極易出現日期轉換錯誤的情況
     */
    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();
        }
    }
}

運行之后會輸出很多的錯誤信息！

 

　　解決多線程出現的問題-為每個線程實例一個SimpleDateFormat：

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

/**
 *    線程類
 */
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() 
                        + "報錯了  日期字符串：" + dateString + "轉換成日期為：" + newDateString);
            }
        } catch (ParseException e) {
            e.printStackTrace();
        }
    }
}

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

/**
 *    日期格式化工具類
 */
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 {

    /**
     *    測試,運行程序后，控制台沒有任何輸出，也就是轉換沒有任何異常，
     *    原理：創建了多個SimpleDateFormat實例
     */
    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();
        }
    }
}

　　解決多線程出現的問題-使用ThreadLocal：

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

/**
 *    線程類
 */
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() 
                        + "報錯了  日期字符串：" + dateString + "轉換成日期為：" + newDateString);
            }
        } catch (ParseException e) {
            e.printStackTrace();
        }
    }
}

import java.text.SimpleDateFormat;

/**
 *    日期格式化工具類,使用ThreadLocal解決SimpleDateFormat非線程安全問題
 */
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 {

    /**
     *    測試,運行程序后，控制台沒有任何輸出，也就是轉換沒有任何異常
     *    原理：每個線程都會有自己的ThreadLocal存儲全局變量，也就是每個線程都有自己的SimpleDateFormat實例
     */
    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();
        }
    }
}