SimpleDateFormat并发场景测试与验证

前几天工作中，遇到一个并发环境下有人写了SimpleDateFormat的场景，印象中这个是不能支持多线程的，应该使用ThreadLocal作为每个线程局部变量使用，今天有空，试了下SimpleDateFormat多线程使用，多个线程并发打印对象，代码如下：

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/**
 * TestDateFormat.java
 * zhm.test.dateFormat
 * 2018年5月2日下午9:02:07
 *
 */
package zhm.test.dateFormat;

import java.sql.Timestamp;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.Random;

/**
 * @author zhuheming TestDateFormat 2018年5月2日下午9:02:07
 */
public class TestDateFormat {

    public static TestDateFormat tdf = null;

    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");

    public static void main(String[] args) {
        tdf = new TestDateFormat();
        for (int i = 1; i <= 9; i++) {

            Thread th = new Thread(new Runnable() {
                @Override
                public void run() {
                    while (true) {
                        Random r = new Random();
                        int k = r.nextInt(100);

                        int randomSwitch = k % 3;

                        Calendar c = Calendar.getInstance();
                        c.add(Calendar.DATE, k);
                        try {
                            Thread.sleep(100);
                            switch (randomSwitch) {
                            case 0:
                                String s1 = c.get(Calendar.YEAR) + "-"
                                        + TestDateFormat.formatNumber((c.get(Calendar.MONTH) + 1)) + "-"
                                        + TestDateFormat.formatNumber((c.get(Calendar.DAY_OF_MONTH)));
                                Date date1 = tdf.printDateFormat(s1);
                                String d1 = new Timestamp(date1.getTime()).toString();
                                d1 = d1.substring(0,d1.indexOf(" "));
                                if(!s1.equals(d1)){
                                    System.out.println(s1+"     "+d1);
                                }
                                break;
                            case 1:
                                String s2 = c.get(Calendar.YEAR) + "-"
                                        + TestDateFormat.formatNumber((c.get(Calendar.MONTH) + 1)) + "-"
                                        + TestDateFormat.formatNumber((c.get(Calendar.DAY_OF_MONTH)));
                                Date date2 = TestDateFormat.printStaticDateFormat(s2);
                                String d2 = new Timestamp(date2.getTime()).toString();
                                d2 = d2.substring(0,d2.indexOf(" "));
                                if(!s2.equals(d2)){
                                    System.out.println(s2+"     "+d2);
                                }
                                break;
                            case 2:
                                String s = c.get(Calendar.YEAR) + "-"
                                        + TestDateFormat.formatNumber((c.get(Calendar.MONTH) + 1)) + "-"
                                        + TestDateFormat.formatNumber((c.get(Calendar.DAY_OF_MONTH)));
                                Date date3 = tdf.printReferenceDateFormat(s);
                                String d3 = new Timestamp(date3.getTime()).toString();
                                d3 = d3.substring(0,d3.indexOf(" "));
                                if(!s.equals(d3)){
                                    System.out.println(s+"     "+d3);
                                }
                                break;
                            default:
                                return;

                            }
                        } catch (Exception e) {

                        }
                    }
                }
            });
            th.start();
            try {
                th.join();
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }

    }

    public Date printReferenceDateFormat(String str) {
        System.out.println("get the ReferenceDateFormat:" + sdf);
        try {
            return sdf.parse(str);
        } catch (ParseException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            return null;
        }
    }

    public static Date printStaticDateFormat(String str) {
        SimpleDateFormat df1 = new SimpleDateFormat("yyyy-MM-dd");
        System.out.println("get static method the SimpleDateFormat:" + df1);
        try {
            return df1.parse(str);
        } catch (ParseException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            return null;
        }
    }

    public Date printDateFormat(String str) {
        SimpleDateFormat df2 = new SimpleDateFormat("yyyy-MM-dd");

        try {

            System.out.println("get the SimpleDateFormat:" + df2);
            return df2.parse(str);
        } catch (ParseException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            return null;
        }
    }

    private static String formatNumber(int n) {
        String s = "0" + n;
        return s.substring(s.length() - 2);
    }
}

按照设想，应该是sdf打印出的对象是同一个，后面两个打印出的对象因为每次需要新建对象，打印出来的都不相同才对。

但是结果如下：
get static method the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get static method the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the ReferenceDateFormat:java.text.SimpleDateFormat@f67a0200
get the ReferenceDateFormat:java.text.SimpleDateFormat@f67a0200
get static method the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get static method the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the ReferenceDateFormat:java.text.SimpleDateFormat@f67a0200
get the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the ReferenceDateFormat:java.text.SimpleDateFormat@f67a0200
get the ReferenceDateFormat:java.text.SimpleDateFormat@f67a0200
get the ReferenceDateFormat:java.text.SimpleDateFormat@f67a0200
get the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get static method the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get static method the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get static method the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get static method the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get static method the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get static method the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the ReferenceDateFormat:java.text.SimpleDateFormat@f67a0200
get the ReferenceDateFormat:java.text.SimpleDateFormat@f67a0200
get the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the ReferenceDateFormat:java.text.SimpleDateFormat@f67a0200
get the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get static method the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the ReferenceDateFormat:java.text.SimpleDateFormat@f67a0200
get the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get static method the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get static method the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get static method the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the ReferenceDateFormat:java.text.SimpleDateFormat@f67a0200
get the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get static method the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the ReferenceDateFormat:java.text.SimpleDateFormat@f67a0200
get the SimpleDateFormat:java.text.SimpleDateFormat@f67a0200
get the ReferenceDateFormat:java.text.SimpleDateFormat@f67a0200

对象居然是同一个，这就很奇怪了，谁知道原因呢？

看了下SimpleDateFormat源码，里面根据pattern从static的concurrentHashMap中取对应的信息，对于不同的SimpleDateFormat对象，只要定义的格式相同，pattern对象是同一个。

后来换了种方式测试，直接粗暴的写五个进程跑，证明确实会出现并发问题：
2018-12-25 2203-01-24
2498-10-10 2203-01-24
2018-12-25 0001-10-10
2998-01-02 3004-12-01
1698-09-22 3004-12-01
且只有共享的static会有这个问题，去掉共享的static，那么在方法内调用的就不存在并发问题。

/**
 * TestDateFormat.java
 * zhm.test.dateFormat
 * 2018年5月2日下午9:02:07
 *
 */
package zhm.test.dateFormat;

import java.sql.Timestamp;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.Random;

/**
 * @author zhuheming TestDateFormat 2018年5月2日下午9:02:07
 */
public class TestDateFormat {

    public static TestDateFormat tdf = null;

    //SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");

    public static void main(String[] args) {
        tdf = new TestDateFormat();


            Thread th2 = new Thread(new Runnable() {
                @Override
                public void run() {
                    while (true) {
                        Random r = new Random();
                        int k = r.nextInt(100);

                        int randomSwitch = k % 3;

                        //Calendar c = Calendar.getInstance();
                        //c.add(Calendar.DATE, k);
                        try {
                            Thread.sleep(100);
                            switch (randomSwitch) {
                            case 0:
                                String s1 = "2018-12-25";
                                String d1 = tdf.printDateFormat(s1);
                                //String d1 = new Timestamp(date1.getTime()).toString();
                                //d1 = d1.substring(0,d1.indexOf(" "));
                                if(!s1.equals(d1)){
                                    System.out.println(s1+"     "+d1);
                                }
                                break;
                            case 1:
                                String s2 = "2018-12-25";
                                String d2 = TestDateFormat.printStaticDateFormat(s2);
                                //String d2 = new Timestamp(date2.getTime()).toString();
                                //d2 = d2.substring(0,d2.indexOf(" "));
                                if(!s2.equals(d2)){
                                    System.out.println(s2+"     "+d2);
                                }
                                break;
                            case 2:
                                String s ="2018-12-25";
                                String d3 = tdf.printReferenceDateFormat(s);
                                //String d3 = new Timestamp(date3.getTime()).toString();
                                //d3 = d3.substring(0,d3.indexOf(" "));
                                if(!s.equals(d3)){
                                    System.out.println(s+"     "+d3);
                                }
                                break;
                            default:
                                return;

                            }
                        } catch (Exception e) {

                        }
                    }
                }
            });

    Thread th3 = new Thread(new Runnable() {
        @Override
        public void run() {
            while (true) {
                Random r = new Random();
                int k = r.nextInt(3);
                int randomSwitch = k % 3;

                //Calendar c = Calendar.getInstance();
                //c.add(Calendar.DATE, k);
                try {
                    Thread.sleep(100);
                    switch (randomSwitch) {
                    case 0:
                        String s1 = "1998-11-12";
                        String d1 = tdf.printDateFormat(s1);
                        //String d1 = new Timestamp(date1.getTime()).toString();
                        //d1 = d1.substring(0,d1.indexOf(" "));
                         if(!s1.equals(d1)){
                            System.out.println(s1+"     "+d1);
                         }
                        break;
                    case 1:
                        String s2 = "1998-11-12";
                        String d2 = TestDateFormat.printStaticDateFormat(s2);
                        //String d2 = new Timestamp(date2.getTime()).toString();
                        //d2 = d2.substring(0,d2.indexOf(" "));
                        if(!s2.equals(d2)){
                            System.out.println(s2+"     "+d2);
                        }
                        break;
                    case 2:
                        String s = "1998-11-12";
                        String d3 = tdf.printReferenceDateFormat(s);
                        //String d3 = new Timestamp(date3.getTime()).toString();
                        //d3 = d3.substring(0,d3.indexOf(" "));
                        if(!s.equals(d3)){
                            System.out.println(s+"     "+d3);
                        }
                        break;
                    default:
                        return;

                    }
                } catch (Exception e) {

                }
            }
        }
    });

    Thread th4 = new Thread(new Runnable() {
        @Override
        public void run() {
            while (true) {
                Random r = new Random();
                int k = r.nextInt(3);
                int randomSwitch = k % 3;

                //Calendar c = Calendar.getInstance();
                //c.add(Calendar.DATE, k);
                try {
                    Thread.sleep(100);
                    switch (randomSwitch) {
                    case 0:
                        String s1 = "1698-09-22";
                        String d1 = tdf.printDateFormat(s1);
                        //String d1 = new Timestamp(date1.getTime()).toString();
                        //d1 = d1.substring(0,d1.indexOf(" "));
                         if(!s1.equals(d1)){
                            System.out.println(s1+"     "+d1);
                         }
                        break;
                    case 1:
                        String s2 = "1698-09-22";
                        String d2 = TestDateFormat.printStaticDateFormat(s2);
                        //String d2 = new Timestamp(date2.getTime()).toString();
                        //d2 = d2.substring(0,d2.indexOf(" "));
                        if(!s2.equals(d2)){
                            System.out.println(s2+"     "+d2);
                        }
                        break;
                    case 2:
                        String s = "1698-09-22";
                        String d3 = tdf.printReferenceDateFormat(s);
                        //String d3 = new Timestamp(date3.getTime()).toString();
                        //d3 = d3.substring(0,d3.indexOf(" "));
                        if(!s.equals(d3)){
                            System.out.println(s+"     "+d3);
                        }
                        break;
                    default:
                        return;

                    }
                } catch (Exception e) {

                }
            }
        }
    });

    Thread th5 = new Thread(new Runnable() {
        @Override
        public void run() {
            while (true) {
                Random r = new Random();
                int k = r.nextInt(3);
                int randomSwitch = k % 3;

                //Calendar c = Calendar.getInstance();
                //c.add(Calendar.DATE, k);
                try {
                    Thread.sleep(100);
                    switch (randomSwitch) {
                    case 0:
                        String s1 = "2498-10-10";
                        String d1 = tdf.printDateFormat(s1);
                        //String d1 = new Timestamp(date1.getTime()).toString();
                        //d1 = d1.substring(0,d1.indexOf(" "));
                         if(!s1.equals(d1)){
                            System.out.println(s1+"     "+d1);
                         }
                        break;
                    case 1:
                        String s2 = "2498-10-10";
                        String d2 = TestDateFormat.printStaticDateFormat(s2);
                        //String d2 = new Timestamp(date2.getTime()).toString();
                        //d2 = d2.substring(0,d2.indexOf(" "));
                        if(!s2.equals(d2)){
                            System.out.println(s2+"     "+d2);
                        }
                        break;
                    case 2:
                        String s = "2498-10-10";
                        String d3 = tdf.printReferenceDateFormat(s);
                        //String d3 = new Timestamp(date3.getTime()).toString();
                        //d3 = d3.substring(0,d3.indexOf(" "));
                        if(!s.equals(d3)){
                            System.out.println(s+"     "+d3);
                        }
                        break;
                    default:
                        return;

                    }
                } catch (Exception e) {

                }
            }
        }
    });

    Thread th6 = new Thread(new Runnable() {
        @Override
        public void run() {
            while (true) {
                Random r = new Random();
                int k = r.nextInt(3);
                int randomSwitch = k % 3;

                //Calendar c = Calendar.getInstance();
                //c.add(Calendar.DATE, k);
                try {
                    Thread.sleep(100);
                    switch (randomSwitch) {
                    case 0:
                        String s1 = "2998-01-02";
                        String d1 = tdf.printDateFormat(s1);
                        //String d1 = new Timestamp(date1.getTime()).toString();
                        //d1 = d1.substring(0,d1.indexOf(" "));
                         if(!s1.equals(d1)){
                            System.out.println(s1+"     "+d1);
                         }
                        break;
                    case 1:
                        String s2 = "2998-01-02";
                        String d2 = TestDateFormat.printStaticDateFormat(s2);
                        //String d2 = new Timestamp(date2.getTime()).toString();
                        //d2 = d2.substring(0,d2.indexOf(" "));
                        if(!s2.equals(d2)){
                            System.out.println(s2+"     "+d2);
                        }
                        break;
                    case 2:
                        String s = "2998-01-02";
                        String d3 = tdf.printReferenceDateFormat(s);
                        //String d3 = new Timestamp(date3.getTime()).toString();
                        //d3 = d3.substring(0,d3.indexOf(" "));
                        if(!s.equals(d3)){
                            System.out.println(s+"     "+d3);
                        }
                        break;
                    default:
                        return;

                    }
                } catch (Exception e) {

                }
            }
        }
    });
    th2.start();
    th3.start();
    th4.start();
    th5.start();
    th6.start();
    try {
        th2.join();
        th3.join();
        th4.join();
        th5.join();
        th6.join();
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}


    public String printReferenceDateFormat(String str) {
        //Date dt= sdf.parse(str);
        //return sdf.format(dt);
        return str;
    }

    public static String printStaticDateFormat(String str) {
        SimpleDateFormat df1 = new SimpleDateFormat("yyyy-MM-dd");
        //System.out.println("get static method the SimpleDateFormat:" + df1);
        try {
            Thread.sleep(100);
            Date dt=df1.parse(str);
            str=df1.format(dt);
            //df1.getCalendar().clear();
            return str;
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            return null;
        }
    }

    public String printDateFormat(String str) {
        SimpleDateFormat df2 = new SimpleDateFormat("yyyy-MM-dd");

        try {
            Thread.sleep(100);
            Date dt=df2.parse(str);
            str=df2.format(dt);
            //df2.getCalendar().clear();
            //System.out.println("get the SimpleDateFormat:" + df2);
            return str;
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            return null;
        }
    }

}

从结果来看，确实不是同一个对象，只是看起来像。

那么只有最后一个问题了，为什么看起来是同一个对象呢：
元芳，你怎么看？
大人，属下这样认为：

大家都知道system.out.prinl(object)其实是打印的String.valueof的对象值；valueof是什么呢？

public static String valueOf(Object obj) { 
    return (obj == null) ? “null” : obj.toString(); 
}

就是obj的toString的值，那么obj的toString由Object类定义，各个子类自己重写，在Object中，定义如下：

public String toString() { 
    return getClass().getName() + “@” + Integer.toHexString(hashCode()); 
}

显然就是Object的hashCode的十六进制数值，那么在SimpleDateFormat中hashcode方法是如何呢

public int hashCode() 
{ 
    return pattern.hashCode(); 
    // just enough fields for a reasonable distribution 
}

从这里可以看出，SimpleDateFormat没有返回自己的对象的地址等，而是返回了pattern的hashcode，前面已经说过，输入的格式相同，pattern是同一个，所以，此处SimpleDateFormat对象打印出来是同一个，但其实不是。

狄仁杰：元芳，我这个宰相的位置，以后要不你来做吧。。。

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之前我写在CSDN上的文章，搬到平台上来：

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原文链接：https://blog.csdn.net/qq_35039122/article/details/80172714