Why Try-With-Resources does not create a new instance every time

Question

I'm trying to write a Unit test using TestNG to save test data to a db. I'm using DataProvider for creating test data as below:

    @DataProvider
private Object[][] movieTitles() {
    return  new Object[][] { 
            new String[] { "Golden Eye" },
            new String[] { "Troy" }, new String[] { "Gladiator" },
            new String[] { "Ring" }, new String[] { "The Village" } };
}

And my test case is using try-with-resources as below:

    @Test(dataProvider = "movieTitles")
public void save(String title) {
         EntityTransaction tx = null;
     try(PersistenceUtil util = PersistenceUtil.getUtil()){ 
         Movie movie = new Movie(title);
     EntityManager em = util.getManager();
         tx = util.getTransaction();    
        tx.begin();
        em.persist(movie);
        tx.commit();
        Assert.assertTrue(movie.getId() > 0);
    } catch (Exception e) {
        if(tx.isActive())
            tx.rollback();
        e.fillInStackTrace();
        return;
    }
}

The first data is saved successfully. However, the PersistenceUtil in the try block is not created/initialized again and thus EntityTransaction is null. And thus a NPE exception occurs for the rest of the data. Why is PersistenceUtil not initialized again in the try-with-resources block. Is there anyway to achieve the same?

Answer 1

The bug has nothing to do with automatic resource management. The try block will execute getUtil() each time the save() method is called.

The problem lies in the PersistenceUtil class; getUtil() is returning a non-null PersistenceUtil, but it's not behaving properly, and returns null from getTransaction().

Answer 2

Check whether you persistence layer stays live after util.close() has been called at the end of the try-with-resources.

Note also that you can simplify the Object matrix creation:

public class Main {

   static Object[][] array = {  { "a", "b" } , { "c", "d" } } ;

   // test print
   public static void main(String[] argv) {
      for (int i=0;i<array.length;i++) {
         Object[] sub = array[i];
         for (int j=0;j<sub.length;j++) {
            System.out.println(sub[j]);
         }
      }
   }

}