java， volatile 静态和同步 - 线程中的空异常

Question

我定义了一个公共(public) volatile LinkedList 与某些线程共享，并在静态的 main() 中同步它......所以我得到一个编译错误:“无法引用非静态字段”

如果我将定义修改为 public volatile static LinkedList，则会出现异常:

 Exception in thread "filteringThread" java.lang.NullPointerException
at com.swimtechtest.swimmerbench.FilteringThread.run(FilteringThread.java:39)

这是我的代码的一部分

public class SwimmerBench{
    ...
    public volatile LinkedList<InterpolatedEvent> samplings = new LinkedList<InterpolatedEvent>();
    ...
    public static void main(String args[]) throws IOException {
       ...
       InterpolatedEvent event = createInterpolatedEvent(fields);
   synchronized(samplings){
        samplings.add(event);
    samplings.notifyAll();
   }
       ...
    }
}

  ======

public class FilteringThread extends Thread {

    LinkedList<InterpolatedEvent> samplings;
       public void run() {

   System.out.println("Running " +  threadName );

  while(running ) {
      try {
        synchronized(samplings) { // <=  exception error line 39
            InterpolatedEvent event = samplings.getLast();
            samplings.notifyAll();
       }
  }
}

Answer 1

I defined a public volatile LinkedList to be shared with some threads, and I synchronize it within main() which is static... so I get a compile error : " cannot make a reference to a non static field"

这与 volatile 或同步无关，这是因为您正在静态 main() 方法中访问实例变量 samplings。

If I modify the definition to be public volatile static LinkedList, then I get an exception :

 Exception in thread "filteringThread" java.lang.NullPointerException
at com.swimtechtest.swimmerbench.FilteringThread.run(FilteringThread.java:39)

这是因为，FilteringThread 中的实例变量 samplings 从未分配给 List 类型的对象。默认分配为 null