How it works...

The key code line in this recipe for JTA is right here:

<persistence-unit name="ch02-jta-pu" transaction-type="JTA">

When you use transaction-type='JTA', you are saying to the server that it should take care of all transactions made under this context. If you use RESOURCE-LOCAL instead, you are saying that you are taking care of the transactions:

    @Test
    public void validTransaction() throws Exception{
        User user = new User(null, "Elder Moraes", 
        "elder@eldermoraes.com");
        
        userBean.add(user);
        user.setName("John Doe");
        userBean.update(user);
        
        User userDb = userBean.findById(1L);
        assertEquals(userDb.getName(), "John Doe");
        
    }

Each called method of UserBean starts a transaction to be completed and would run into a rollback if there's any issue while the transaction is alive would commit to the end of it.