SimpleDateFormat非线程安全

为了渐少new 的次数而把SimpleDateFormat做成成员或者静态成员，但这样的做法是隐含着错误的，是不安全的。

对于这句话我写了个testcase，感觉单个进程下是没有问题的（网上大部分说这么写是有问题的，隐藏问题是什么，知道的同学说下？）。

public class hello{
     public static void main(String[] arg) throws InterruptedException {
        Date date1 = new Date();
        Date date2 = new Date(date1.getTime() + 1000 * 60 * 60 * 24);
        while (1 == 1) {
            if (!"2013-01-11".equals(method1(date1))) {
                System.out.println("error today:" + date1);
            }
            if (!"2013-01-12".equals(method2(date2))) {
                System.out.println("error tomorrow:" + date2);
            }
            Thread.sleep(1);
        }
    }

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

    public static String method1(Date date) {
        return sdf.format(date);
    }

    public static String method2(Date date) {
        return sdf.format(date);
    }
}

开一个线程也是没有问题的，只有开多个线程的情况下有问题。

public class hello{
     public static void main(String[] arg) throws InterruptedException {

         SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
         Date date1 = new Date();
         Date date2 = new Date(date1.getTime()+1000*60*60*24);
         System.out.println("date1:" + date1);
         System.out.println("date2:" + date2);
         Thread thread1 = new Thread(new Thread1(sdf,date1));
         Thread thread2 = new Thread(new Thread2(sdf,date2));
         thread1.start();
         thread2.start(); //如果注释掉这行，是没有问题的        
     }
}

class Thread1 implements Runnable {
    private SimpleDateFormat sdf;
    private Date date;
    public Thread1(SimpleDateFormat sdf, Date date) {
        this.sdf = sdf;
        this.date = date;
    }
    public void run() {
        while(1==1) {
            String temp = sdf.format(date);
            if(!"2013-01-11".equals(temp)) {
                System.out.println("error today:" +date);
                System.exit(0);
            } else {
                System.out.println("ok1");
            }
        }
    }
}

class Thread2 implements Runnable {
    private SimpleDateFormat sdf;
    private Date date;
    public Thread2(SimpleDateFormat sdf, Date date) {
        this.sdf = sdf;
        this.date = date;
    }
    public void run() {
        while(1==1) {
            String temp = sdf.format(date);
            if(!"2013-01-12".equals(temp)) {
                System.out.println("error tomorrow:" +date);
                System.exit(0);
            } else {
                System.out.println("ok2");
            }
        }
    }
}

运行结果是（出错）：

date1:Fri Jan 11 11:26:29 CST 2013
date2:Sat Jan 12 11:26:29 CST 2013
ok1
ok1
ok1
ok1
error tomorrow:Sat Jan 12 11:26:29 CST 2013
ok1
ok1
ok1
ok1
ok1
ok1
ok1
ok1
ok1
ok1
ok1