zeppelin pyspark doesn't print java output

Problem description:

i created a custom jar file my-test-1.0-SNAPSHOT.jar with following java code:

package com.mycompany.app; 
 import java.util.LinkedList; import java.util.List; 
 public class Stack { 
     public void my_open() {         System.out.println("calling open file");     } 
     public void my_close() {         System.out.println("calling close file");     } 
 }

 
when i use pyspark shell, it works well.
pyspark --driver-class-path my-test-1.0-SNAPSHOT.jar

 
but when i use pyspark in Zeppelin, it works only for the first time, with proper output "calling open file". But after then, as following shows the method 'my_open' failed to execute with 'None' returned.

 
more actions:
1. when i tried to use Scala in Zeppelin to do the same, it worked well.

 
  2. i restarted the spark interpreter, then it worked again for the first time, and further running failed.