Java – why doesn’t this code throw a NullPointerException?

background

I want to know why a piece of code does not throw NullPointerException

source code

Consider the following codes:

public class Agent {
  public List files = new ArrayList();

  public void deliver() {
    if( files != null && files.iterator().hasNext() ) {
      File file = (File)files.iterator().next();
    }

    files = new ArrayList();
  }
}

The pass method is called repeatedly, and the following code runs in a separate thread:

public void run() {
    agent.files = null;
  }

There is only one agent instance

problem

NullPointerException will never be thrown

However, when the delivery method is paused, NullPointerException will be thrown as expected even if it is 0 milliseconds:

public void deliver() {
    if( files != null ) {
      Thread.currentThread().sleep( 0 );

      if( files.iterator().hasNext() ) {
        File file = (File)files.iterator().next();
      }
    }

    files = new ArrayList();
  }

My understanding is that, in theory, after checking the file = = null and calling files iterator(). There is a competition condition between hasnext() In practice, I cannot trigger race conditions without introducing pauses (that is, splitting null checks from subsequent method calls)

topic

Why doesn't the first pass method throw an exception when it combines null checking and use in the same statement?

Solution

Two things:

> Thread. Sleep (0) still stops execution (possibly more than 0 milliseconds) Basically, even 0 sleep will cause execution on the thread to stop temporarily and then restart This gives another thread a chance to run and complete, which is why you can trigger a race condition. > Files should be volatile, otherwise allow the JVM to optimize in such a way that you may never notice that it is changing values because it does not think it needs to maintain consistency between threads