【Java线程】SimpleDateFormat的线程安全性实验

【代码】

package unsafesdf;

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Collections;
import java.util.HashSet;
import java.util.Set;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;

public class Test {
    private static SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd");
    private static final int COUNT=100;
    private static final Executor exec=Executors.newFixedThreadPool(COUNT);
    
    public static void main(String[] args) throws Exception{
        Set<String> set=Collections.synchronizedSet(new HashSet<String>());
        CountDownLatch cdl=new CountDownLatch(COUNT);
        
        for(int i=0;i<COUNT;i++) {
            Calendar clder=Calendar.getInstance();
            final int ct=i;
            
            exec.execute(()->{
                clder.add(Calendar.DATE, ct);
                String dateStr=sdf.format(clder.getTime());
                set.add(dateStr);
                cdl.countDown();
            });
        }
        
        cdl.await();
        
        // 预期100，但实际小于100
        System.out.println(set.size());
    }
}