关于ThreadLocal的问题

猎祖猎宗 发表于 2020-11-26 15:41

private static ThreadLocal<Project> project = ThreadLocal.withInitial(()->new Project());public static void main(String[] args) throws ParseException, InterruptedException {
new Thread(()-> {
   try {
         Project sdf = project.get();
         //Thread.sleep(10L);
         sdf.setCompany("啊");
         System.out.println(sdf+"----" + Thread.currentThread().getId());
         sdf.setCompany("C");
         System.out.println(sdf+"----" + Thread.currentThread().getId());
   } catch (Exception e) {
   }
}).start();
new Thread(()-> {
   try {
         Project sdf = project.get();
         sdf.setCompany("啊");
         System.out.println(sdf+"----" + Thread.currentThread().getId());
   } catch (Exception e) {
   }
}).start();
new Thread(()-> {
   try {
         Project sdf = project.get();
         System.out.println(sdf+"----" + Thread.currentThread().getId());
   } catch (Exception e) {
   }
}).start();

while (true);

}输出结果com.lzy.demo.ProjectDto.Project@141f979companynull----14com.lzy.demo.ProjectDto.Project@1424e98company啊----13com.lzy.demo.ProjectDto.Project@1424e98company啊----12com.lzy.demo.ProjectDto.Project@141f991companyC----12
请问为什么当我Project实体里面的值相同时，对象地址相同，而company发生改变是线程12里面指向的地址发生了变化，这块搞不太懂，有大佬帮忙解答一下吗

猎祖猎宗 发表于 2020-11-26 15:42

输出结果

Vvvvvoid 发表于 2020-11-26 17:46

ThreadLocal 的作用是单个线程内执行多个方法可以做到参数共享

你创建了三个线程, 每个线程里的 ThreadLocal 初始化都会创建一个 Project 对象, 所以一共会创建三个对象

每个线程的 id 都是不同的 , 所以有三个线程ID , 而且每个线程ID 对应的 Project 对象ID 是不会变的

不夏流年 发表于 2020-11-26 19:03

问题都没说清楚，线程起个名字很难么

猎祖猎宗 发表于 2020-11-27 10:12

private static ThreadLocal<SimpleDateFormat> threadLocal = ThreadLocal.withInitial(()->new SimpleDateFormat("yyyy"));
private static Object a,b,c;
public static void main(String[] args) throws ParseException, InterruptedException {
   Thread threadA = new Thread(()->{
         Object object = threadLocal.get();
         a= object;
         System.out.println("线程名:"+Thread.currentThread().getName()+"，输出结果"+object);
   });
   threadA.setName("A");

   Thread threadB = new Thread(()->{
         Object object = threadLocal.get();
         b = object;
         System.out.println("线程名:"+Thread.currentThread().getName()+"，输出结果"+object);
   });
   threadB.setName("B");

   Thread threadC = new Thread(()->{
         Object object = threadLocal.get();
         c=object;
         System.out.println("线程名:"+Thread.currentThread().getName()+"，输出结果"+object);
   });
   threadC.setName("C");
   threadA.start();
   threadB.start();
   threadC.start();
   threadA.join();
   threadB.join();
   threadC.join();
   System.out.println(b==c);
   System.out.println(c==a);
   System.out.println(a==b);
}
输出结果线程名:A，输出结果java.text.SimpleDateFormat@38d640
线程名:C，输出结果java.text.SimpleDateFormat@38d640
线程名:B，输出结果java.text.SimpleDateFormat@38d640
false
false
false

输出的结果调用toString，由于重写了SimpleDateFormat(问题中提到的Project对象同理)hashcode，看到线程ABC输出结果java.text.SimpleDateFormat@38d640都是一样的，
但是通过把对象赋值 abc三个静态变量，然后发现全是false，
//SimpleDateFormat的hashCode
//实际为调用String的hashCode
public class SimpleDateFormat extends DateFormat {
private String pattern;
@Override
public int hashCode()
{
   return pattern.hashCode();
   // just enough fields for a reasonable distribution
}
}

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关于ThreadLocal的问题