Enterprise Java Development@TOPIC@

Chapter 58. Relationship Capabilities

58.1. Fetching
58.1.1. fetch=LAZY
58.1.2. fetch=EAGER
58.2. Cascades
58.3. Orphan Removal
58.4. Summary

58.1. Fetching

Figure 58.1. fetch=LAZY Example Declaration

@OneToMany(mappedBy="borrower", //this relationship is owned by Checkout

        fetch=FetchType.LAZY)

private Collection<Checkout> checkouts = new ArrayList<Checkout>();

  • @XxxToXxx Relationship annotated with fetch=LAZY property

Figure 58.2. fetch=LAZY Example Use

Borrower borrower2 = em.find(Borrower.class, borrower.getId());

logger.info("found borrower: {}", borrower.getName());

assertEquals(6, borrower2.getCheckouts().size());

  • Parent accessed, debug printed, and then child collection accessed

Figure 58.3. fetch=LAZY SQL Output

-
    select
        borrower0_.BORROWER_ID as BORROWER1_15_0_,
        borrower0_.endDate as endDate2_15_0_,
        borrower0_.startDate as startDat3_15_0_
    from
        ORMREL_BORROWER borrower0_
    where
        borrower0_.BORROWER_ID=?
-Borrower@7cfb0c4c, ctor()
...
-found borrower: john smith
-
    select
        checkouts0_.CHECKOUT_BID as CHECKOUT4_16_0_,
        checkouts0_.CHECKOUT_ID as CHECKOUT1_16_0_,
        checkouts0_.CHECKOUT_ID as CHECKOUT1_16_1_,
        checkouts0_.CHECKOUT_BID as CHECKOUT4_16_1_,
        checkouts0_.outDate as outDate2_16_1_,
        checkouts0_.returnDate as returnDa3_16_1_
    from
        ORMREL_CHECKOUT checkouts0_
    where
        checkouts0_.CHECKOUT_BID=?

  • Debug for parent printed before child rows retrieved

  • Child rows retrieved when parent collection accessed

  • Other SQL details of Borrower left out of example

Figure 58.4. fetch=EAGER Example Declaration

@OneToMany(mappedBy="borrower", //this relationship is owned by Checkout

        fetch=FetchType.EAGER)

private Collection<Checkout> checkouts = new ArrayList<Checkout>();

Figure 58.5. fetch=EAGER Example Use

Borrower borrower3 = em.find(Borrower.class, borrowerId);

logger.info("found borrower: {}", borrower.getName());

assertEquals(0,borrower3.getCheckouts().size());        

  • Same as fetch=LAZY case

Figure 58.6. fetch=EAGER SQL Output

-
    select
        borrower0_.BORROWER_ID as BORROWER1_15_0_,
        borrower0_.endDate as endDate2_15_0_,
        borrower0_.startDate as startDat3_15_0_,
        checkouts1_.CHECKOUT_BID as CHECKOUT4_16_1_,
        checkouts1_.CHECKOUT_ID as CHECKOUT1_16_1_,
        checkouts1_.CHECKOUT_ID as CHECKOUT1_16_2_,
        checkouts1_.CHECKOUT_BID as CHECKOUT4_16_2_,
        checkouts1_.outDate as outDate2_16_2_,
        checkouts1_.returnDate as returnDa3_16_2_
    from
        ORMREL_BORROWER borrower0_
    left outer join
        ORMREL_CHECKOUT checkouts1_
            on borrower0_.BORROWER_ID=checkouts1_.CHECKOUT_BID
    where
        borrower0_.BORROWER_ID=?
-found borrower: john smith

  • Child objects fetched with parent

  • Access to child collection occurs after all children fetched


fetch=EAGER does not always mean "done "efficiently"

fetch=EAGER is only a promise to fetch the related object before returning from the call. It does not always mean that a SQL JOIN was performed. There are times (e.g., fetch=EAGER on multiple collections within parent) when the provider will perform a SQL JOIN for some of the relationships and the functional equivalent of a fetch=LAZY for the remaining relationships.

Don't over user fetch=EAGER

Although useful at times, avoid haphazard use of fetch=EAGER when defining relationships and look to rely on custom queries to do this behavior instead. It is very easy to turn a fetch=LAZY into EAGER at query time. It is very difficult to do the opposite. Custom queries also allow us to incrementally build a complex parent object tree an optimized query at a time. fetch=EAGER will do the same, but may perform 10s, 100s, 1000s of more unoptimized queries of child rows we may never need.

58.2. Cascades

Figure 58.7. 

@Entity

@Table(name="RELATIONEX_LICAPP")

public class LicenseApplication {

    @Id @GeneratedValue

    private int id;

    @Temporal(TemporalType.TIMESTAMP)

    private Date updated;

    

    @OneToOne(optional=false, fetch=FetchType.EAGER, 

            cascade={

                CascadeType.PERSIST,

                CascadeType.DETACH,

                CascadeType.REMOVE,

                CascadeType.REFRESH,

                CascadeType.MERGE

            })

    private License license;

PERSIST

Related entities persisted when this entity is passed to em.persist()

DETACH

Related entities detached from persistence unit when this entity passed to em.detach()

REMOVE

Related entities deleted from database when this entity passed to em.remove()

REFRESH

Related entities refreshed with state of database when this entity passed to em.refresh()

MERGE

Related entities update state of database when this entity is passed to em.merge()

58.3. Orphan Removal

Figure 58.8. OrphanRemoval Example Declaration



@Entity

@Table(name="RELATIONEX_ATTENDEE")

public class Attendee {

    @Id @GeneratedValue

    private int id;

    

    //orphanRemoval will take care of dereference and DELETE from dependent Attendee 

    @OneToOne(cascade=CascadeType.PERSIST, orphanRemoval=true)

    private Residence residence;

Figure 58.9. Triggering OrphanRemoval

attendee.setResidence(null);

em.flush();

Figure 58.10. OrphanRemoval Database Interaction

Hibernate: 
    update RELATIONEX_ATTENDEE 
    set name=?, residence_id=? 
    where id=?
Hibernate: 
    delete from RELATIONEX_RESIDENCE 
    where id=?

  1. Attendee.residence_id set to null

  2. Orphaned residence deleted

58.4. Summary