Examples for thread divergence actions and external actions in the Java Memory Model

The existing answer already correctly defines what these actions are.

As for the reordering: look at the following example of a thread-divergence action:

class ThreadDivergence { 

  int foo = 0; 

  void thread1() { 
    while (true); // thread-divergence action
    foo = 42; 
  } 

  void thread2() { 
    assert foo == 0; 
  } 
}

You would expect this assertion to never fail. Without thread-divergence actions, this could not be guaranteed because the unreachable definition of setting foo = 42 could be reordered to be executed first. This is forbidden by the JMM.

Similarly, external actions are added to avoid surprising outcomes:

class Externalization { 

  int foo = 0; 

  void method() { 
    jni(); // external action
    foo = 42; 
  } 

  native void jni(); /* { 
    assert foo == 0; 
  } */ 
}

Assuming that the JNI method was implemented to run the same assertion, you would not expect this to fail. The JIT compiler cannot determine the outcome of anything external such that the JMM forbidds such reorderings, too.