Hibernate 다음 객체 상태들을 정의하고 지원한다:

We will now discuss the states and state transitions (and the Hibernate methods that trigger a transition) in more detail.

하나의 영속 클래스의 새로이 초기화 된 인스턴스들은 Hibernate에 의해 transient로 간주된다. 우리는 그것을 세션과 연관지어서 transient 인스턴스를 영속화 시킬 수 있다:

DomesticCat fritz = new DomesticCat();
fritz.setColor(Color.GINGER);
fritz.setSex('M');
fritz.setName("Fritz");
Long generatedId = (Long) sess.save(fritz);

If Cat has a generated identifier, the identifier is generated and assigned to the cat when save() is called. If Cat has an assigned identifier, or a composite key, the identifier should be assigned to the cat instance before calling save(). You can also use persist() instead of save(), with the semantics defined in the EJB3 early draft.

Alternatively, you can assign the identifier using an overloaded version of save().

DomesticCat pk = new DomesticCat();
pk.setColor(Color.TABBY);
pk.setSex('F');
pk.setName("PK");
pk.setKittens( new HashSet() );
pk.addKitten(fritz);
sess.save( pk, new Long(1234) );

If the object you make persistent has associated objects (e.g. the kittens collection in the previous example), these objects can be made persistent in any order you like unless you have a NOT NULL constraint upon a foreign key column. There is never a risk of violating foreign key constraints. However, you might violate a NOT NULL constraint if you save() the objects in the wrong order.

Usually you do not bother with this detail, as you will normally use Hibernate's transitive persistence feature to save the associated objects automatically. Then, even NOT NULL constraint violations do not occur - Hibernate will take care of everything. Transitive persistence is discussed later in this chapter.

The load() methods of Session provide a way of retrieving a persistent instance if you know its identifier. load() takes a class object and loads the state into a newly instantiated instance of that class in a persistent state.

Cat fritz = (Cat) sess.load(Cat.class, generatedId);

// you need to wrap primitive identifiers
long id = 1234;
DomesticCat pk = (DomesticCat) sess.load( DomesticCat.class, new Long(id) );

다른 방법으로 당신은 주어진 인스턴스 속으로 상태를 로드시킬 수 있다:

Cat cat = new DomesticCat();
// load pk's state into cat
sess.load( cat, new Long(pkId) );
Set kittens = cat.getKittens();

Be aware that load() will throw an unrecoverable exception if there is no matching database row. If the class is mapped with a proxy, load() just returns an uninitialized proxy and does not actually hit the database until you invoke a method of the proxy. This is useful if you wish to create an association to an object without actually loading it from the database. It also allows multiple instances to be loaded as a batch if batch-size is defined for the class mapping.

If you are not certain that a matching row exists, you should use the get() method which hits the database immediately and returns null if there is no matching row.

Cat cat = (Cat) sess.get(Cat.class, id);
if (cat==null) {
    cat = new Cat();
    sess.save(cat, id);
}
return cat;

You can even load an object using an SQL SELECT ... FOR UPDATE, using a LockMode. See the API documentation for more information.

Cat cat = (Cat) sess.get(Cat.class, id, LockMode.UPGRADE);

Any associated instances or contained collections will not be selected FOR UPDATE, unless you decide to specify lock or all as a cascade style for the association.

refresh() 메소드를 사용하여, 아무때나 하나의 객체와 모든 그것의 콜렉션들을 다시 로드시키는 것이 가능하다. 데이터베이스 트리거들이 그 객체의 프로퍼티들 중 어떤 것을 초기화 시키는데 사용될 때 이것이 유용하다.

sess.save(cat);
sess.flush(); //force the SQL INSERT
sess.refresh(cat); //re-read the state (after the trigger executes)

How much does Hibernate load from the database and how many SQL SELECTs will it use? This depends on the fetching strategy. This is explained in 19.1절. “페칭 방도들”.

If you do not know the identifiers of the objects you are looking for, you need a query. Hibernate supports an easy-to-use but powerful object oriented query language (HQL). For programmatic query creation, Hibernate supports a sophisticated Criteria and Example query feature (QBC and QBE). You can also express your query in the native SQL of your database, with optional support from Hibernate for result set conversion into objects.

List cats = session.createQuery(
    "from Cat as cat where cat.birthdate < ?")
    .setDate(0, date)
    .list();

List mothers = session.createQuery(
    "select mother from Cat as cat join cat.mother as mother where cat.name = ?")
    .setString(0, name)
    .list();

List kittens = session.createQuery(
    "from Cat as cat where cat.mother = ?")
    .setEntity(0, pk)
    .list();

Cat mother = (Cat) session.createQuery(
    "select cat.mother from Cat as cat where cat = ?")
    .setEntity(0, izi)
    .uniqueResult();]]

Query mothersWithKittens = (Cat) session.createQuery(
    "select mother from Cat as mother left join fetch mother.kittens");
Set uniqueMothers = new HashSet(mothersWithKittens.list());

// fetch ids
Iterator iter = sess.createQuery("from eg.Qux q order by q.likeliness").iterate();
while ( iter.hasNext() ) {
    Qux qux = (Qux) iter.next();  // fetch the object
    // something we couldnt express in the query
    if ( qux.calculateComplicatedAlgorithm() ) {
        // delete the current instance
        iter.remove();
        // dont need to process the rest
        break;
    }
}

Iterator kittensAndMothers = sess.createQuery(
            "select kitten, mother from Cat kitten join kitten.mother mother")
            .list()
            .iterator();

while ( kittensAndMothers.hasNext() ) {
    Object[] tuple = (Object[]) kittensAndMothers.next();
    Cat kitten = (Cat) tuple[0];
    Cat mother = (Cat) tuple[1];
    ....
}

Iterator results = sess.createQuery(
        "select cat.color, min(cat.birthdate), count(cat) from Cat cat " +
        "group by cat.color")
        .list()
        .iterator();

while ( results.hasNext() ) {
    Object[] row = (Object[]) results.next();
    Color type = (Color) row[0];
    Date oldest = (Date) row[1];
    Integer count = (Integer) row[2];
    .....
}

//named parameter (preferred)
Query q = sess.createQuery("from DomesticCat cat where cat.name = :name");
q.setString("name", "Fritz");
Iterator cats = q.iterate();

//positional parameter
Query q = sess.createQuery("from DomesticCat cat where cat.name = ?");
q.setString(0, "Izi");
Iterator cats = q.iterate();

//named parameter list
List names = new ArrayList();
names.add("Izi");
names.add("Fritz");
Query q = sess.createQuery("from DomesticCat cat where cat.name in (:namesList)");
q.setParameterList("namesList", names);
List cats = q.list();

Query q = sess.createQuery("from DomesticCat cat");
q.setFirstResult(20);
q.setMaxResults(10);
List cats = q.list();

Query q = sess.createQuery("select cat.name, cat from DomesticCat cat " +
                            "order by cat.name");
ScrollableResults cats = q.scroll();
if ( cats.first() ) {

    // find the first name on each page of an alphabetical list of cats by name
    firstNamesOfPages = new ArrayList();
    do {
        String name = cats.getString(0);
        firstNamesOfPages.add(name);
    }
    while ( cats.scroll(PAGE_SIZE) );

    // Now get the first page of cats
    pageOfCats = new ArrayList();
    cats.beforeFirst();
    int i=0;
    while( ( PAGE_SIZE > i++ ) && cats.next() ) pageOfCats.add( cats.get(1) );

}
cats.close()

<query name="ByNameAndMaximumWeight"><![CDATA[
    from eg.DomesticCat as cat
        where cat.name = ?
        and cat.weight > ?
] ]></query>

Query q = sess.getNamedQuery("ByNameAndMaximumWeight");
q.setString(0, name);
q.setInt(1, minWeight);
List cats = q.list();

Collection blackKittens = session.createFilter(
    pk.getKittens(), 
    "where this.color = ?")
    .setParameter( Color.BLACK, Hibernate.custom(ColorUserType.class) )
    .list()
);

Collection blackKittenMates = session.createFilter(
    pk.getKittens(), 
    "select this.mate where this.color = eg.Color.BLACK.intValue")
    .list();

Collection tenKittens = session.createFilter(
    mother.getKittens(), "")
    .setFirstResult(0).setMaxResults(10)
    .list();

Criteria crit = session.createCriteria(Cat.class);
crit.add( Restrictions.eq( "color", eg.Color.BLACK ) );
crit.setMaxResults(10);
List cats = crit.list();

List cats = session.createSQLQuery("SELECT {cat.*} FROM CAT {cat} WHERE ROWNUM<10")
    .addEntity("cat", Cat.class)
.list();

List cats = session.createSQLQuery(
    "SELECT {cat}.ID AS {cat.id}, {cat}.SEX AS {cat.sex}, " +
           "{cat}.MATE AS {cat.mate}, {cat}.SUBCLASS AS {cat.class}, ... " +
    "FROM CAT {cat} WHERE ROWNUM<10")
    .addEntity("cat", Cat.class)
.list()

Transactional persistent instances (i.e. objects loaded, saved, created or queried by the Session) can be manipulated by the application, and any changes to persistent state will be persisted when the Session is flushed. This is discussed later in this chapter. There is no need to call a particular method (like update(), which has a different purpose) to make your modifications persistent. The most straightforward way to update the state of an object is to load() it and then manipulate it directly while the Session is open:

DomesticCat cat = (DomesticCat) sess.load( Cat.class, new Long(69) );
cat.setName("PK");
sess.flush();  // changes to cat are automatically detected and persisted

Sometimes this programming model is inefficient, as it requires in the same session both an SQL SELECT to load an object and an SQL UPDATE to persist its updated state. Hibernate offers an alternate approach by using detached instances.

많은 어플리케이션들은 하나의 트랜잭션 내에서 하나의 객체를 검색하고, 처리를 위한 UI 계층으로 그것을 전송하고, 그런 다음 새로운 트랜잭션 내에서 변경들을 저장할 필요가 있다. 고도의-동시성 환경에서 이런 종류의 접근법을 사용하는 어플리케이션들은 대개 작업의 "긴" 단위를 확실히 격리시키기 위해 버전화 된 데이터를 사용한다.

Hibernate는 Session.update() 메소드 또는 Session.merge() 메소드를 사용하여 detached 인스턴스들의 재첨부를 제공함으로써 이 모형을 지원한다:

// in the first session
Cat cat = (Cat) firstSession.load(Cat.class, catId);
Cat potentialMate = new Cat();
firstSession.save(potentialMate);

// in a higher layer of the application
cat.setMate(potentialMate);

// later, in a new session
secondSession.update(cat);  // update cat
secondSession.update(mate); // update mate

만일 catId 식별자를 가진 Cat이 secondSession에 의해 이미 로드되었을 경우에 어플리케이션이 그것을 다시 재첨부하려고 시도할 때, 예외상황이 던져졌을 것이다.

Use update() if you are certain that the session does not contain an already persistent instance with the same identifier. Use merge() if you want to merge your modifications at any time without consideration of the state of the session. In other words, update() is usually the first method you would call in a fresh session, ensuring that the reattachment of your detached instances is the first operation that is executed.

The application should individually update() detached instances that are reachable from the given detached instance only if it wants their state to be updated. This can be automated using transitive persistence. See 10.11절. “Transitive persistence(전이 영속)” for more information.

The lock() method also allows an application to reassociate an object with a new session. However, the detached instance has to be unmodified.

//just reassociate:
sess.lock(fritz, LockMode.NONE);
//do a version check, then reassociate:
sess.lock(izi, LockMode.READ);
//do a version check, using SELECT ... FOR UPDATE, then reassociate:
sess.lock(pk, LockMode.UPGRADE);

Note that lock() can be used with various LockModes. See the API documentation and the chapter on transaction handling for more information. Reattachment is not the only usecase for lock().

긴 작업 단위에 대한 다른 모형들은 11.3절. “Optimistic 동시성 제어”에서 논의된다.

Hibernate 사용자들은 새로운 식별자를 생성시켜서 transient 인스턴스를 저장하거나 그것의 현재 식별자와 연관된 detached 인스턴스들을 업데이트/재첨부 시키는 일반적인 용도의 메소드를 요청했다. saveOrUpdate() 메소드는 이 기능을 구현한다.

// in the first session
Cat cat = (Cat) firstSession.load(Cat.class, catID);

// in a higher tier of the application
Cat mate = new Cat();
cat.setMate(mate);

// later, in a new session
secondSession.saveOrUpdate(cat);   // update existing state (cat has a non-null id)
secondSession.saveOrUpdate(mate);  // save the new instance (mate has a null id)

saveOrUpdate()의 사용 예제와 의미는 초심자들에게는 혼동스러워 보인다. 먼저, 하나의 세션에서 온 인스턴스를 또 다른 새로운 세션 내에서 사용하려고 시도하지 않는 한, 당신은 update(), saveOrUpdate(), 또는 merge()를 사용할 필요는 없을 것이다. 몇몇 전체 어플리케이션들은 이들 메소드들 중 어느 것도 결코 사용하지 않을 것이다.

대개 update() 또는 saveOrUpdate()는 다음 시나리오에서 사용된다:

saveOrUpdate()는 다음을 행한다:

그리고 merge()는 매우 다르다:

Session.delete() will remove an object's state from the database. Your application, however, can still hold a reference to a deleted object. It is best to think of delete() as making a persistent instance, transient.

sess.delete(cat);

You can delete objects in any order, without risk of foreign key constraint violations. It is still possible to violate a NOT NULL constraint on a foreign key column by deleting objects in the wrong order, e.g. if you delete the parent, but forget to delete the children.

It is sometimes useful to be able to take a graph of persistent instances and make them persistent in a different datastore, without regenerating identifier values.

//retrieve a cat from one database
Session session1 = factory1.openSession();
Transaction tx1 = session1.beginTransaction();
Cat cat = session1.get(Cat.class, catId);
tx1.commit();
session1.close();

//reconcile with a second database
Session session2 = factory2.openSession();
Transaction tx2 = session2.beginTransaction();
session2.replicate(cat, ReplicationMode.LATEST_VERSION);
tx2.commit();
session2.close();

The ReplicationMode determines how replicate() will deal with conflicts with existing rows in the database:

이 특징의 쓰임새들은 다른 데이터베이스 인스턴스들 속으로 입력된 데이터 일치시키기, 제품 업그레이드 동안에 시스템 구성 정보 업데이트 하기, non-ACID 트랜잭션들 동안에 행해진 변경들을 롤백시키기 등을 포함한다.

Sometimes the Session will execute the SQL statements needed to synchronize the JDBC connection's state with the state of objects held in memory. This process, called flush, occurs by default at the following points:

The SQL statements are issued in the following order:

An exception is that objects using native ID generation are inserted when they are saved.

Except when you explicitly flush(), there are absolutely no guarantees about when the Session executes the JDBC calls, only the order in which they are executed. However, Hibernate does guarantee that the Query.list(..) will never return stale or incorrect data.

It is possible to change the default behavior so that flush occurs less frequently. The FlushMode class defines three different modes: only flush at commit time when the Hibernate Transaction API is used, flush automatically using the explained routine, or never flush unless flush() is called explicitly. The last mode is useful for long running units of work, where a Session is kept open and disconnected for a long time (see 11.3.2절. “확장된 세션과 자동적인 버전화”).

sess = sf.openSession();
Transaction tx = sess.beginTransaction();
sess.setFlushMode(FlushMode.COMMIT); // allow queries to return stale state

Cat izi = (Cat) sess.load(Cat.class, id);
izi.setName(iznizi);

// might return stale data
sess.find("from Cat as cat left outer join cat.kittens kitten");

// change to izi is not flushed!
...
tx.commit(); // flush occurs
sess.close();

flush 동안에, 하나의 예외상황이 발생할 수도 있다(예를 들면. 만일 DML 오퍼레이션이 컨스트레인트를 위반할 경우). 예외상황들을 처리하는 것은 Hibernatem의 트랜잭션 특징에 관한 어떤 이해를 수반하며, 우리는 11장. Transactions and Concurrency에서 그것을 논의한다.

특히 당신이 연관된 객체들의 그래프를 다룰 경우에, 특히 개별 객체들을 저장하고, 삭제하거나, 재첨부시키는 것이 꽤 번거롭다. 공통된 경우는 하나의 부모/자식 관계이다. 다음 예제를 검토하자:

If the children in a parent/child relationship would be value typed (e.g. a collection of addresses or strings), their life cycle would depend on the parent and no further action would be required for convenient "cascading" of state changes. When the parent is saved, the value-typed child objects are saved and when the parent is deleted, the children will be deleted, etc. This works for operations such as the removal of a child from the collection. Since value-typed objects cannot have shared references, Hibernate will detect this and delete the child from the database.

Now consider the same scenario with parent and child objects being entities, not value-types (e.g. categories and items, or parent and child cats). Entities have their own life cycle and support shared references. Removing an entity from the collection does not mean it can be deleted), and there is by default no cascading of state from one entity to any other associated entities. Hibernate does not implement persistence by reachability by default.

- persist(), merge(), saveOrUpdate(), delete(), lock(), refresh(), evict(), replicate()를 포함하는- Hibernate 세션에 대한 각각의 기본 오퍼레이션에 대해서 하나의 대응하는 케스케이딩 스타일이 존재한다. 케스케이드 스타일들 각각은 create, merge, save-update, delete, lock, refresh, evict, replicate로 명명된다. 만일 당신이 하나의 오퍼레이션이 하나의 연관에 따라 케스케이딩되는 것을 원할 경우, 당신은 매핑 문서 내에 그것을 지시해야 한다. 예를 들면:

<one-to-one name="person" cascade="persist"/>

케스케이딩 스타일들이 결합될 수도 있다:

<one-to-one name="person" cascade="persist,delete,lock"/>

You can even use cascade="all" to specify that all operations should be cascaded along the association. The default cascade="none" specifies that no operations are to be cascaded.

특정한 케스케이드 스타일인, delete-orphan은 오직 one-to-many 연관들에만 적용되고, delete() 오퍼레이션이 그 연관으로부터 제거되는 임의의 자식 객체에 적용되어야 함을 나타낸다.

권장사항들 :

cascade="all"을 가진 (단일 값 연관이든 하나의 콜렉션이든) 하나의 연관을 매핑시키는 것은 그 연관을 부모의 저장/업데이트/삭제가 자식 또는 자식들의 저장/업데이트/삭제로 귀결되는 부모/자식 스타일의 관계로 마크한다.

Furthermore, a mere reference to a child from a persistent parent will result in save/update of the child. This metaphor is incomplete, however. A child which becomes unreferenced by its parent is not automatically deleted, except in the case of a <one-to-many> association mapped with cascade="delete-orphan". The precise semantics of cascading operations for a parent/child relationship are as follows:

Finally, note that cascading of operations can be applied to an object graph at call time or at flush time. All operations, if enabled, are cascaded to associated entities reachable when the operation is executed. However, save-update and delete-orphan are transitive for all associated entities reachable during flush of the Session.

Hibernate requires a rich meta-level model of all entity and value types. This model can be useful to the application itself. For example, the application might use Hibernate's metadata to implement a "smart" deep-copy algorithm that understands which objects should be copied (eg. mutable value types) and which objects that should not (e.g. immutable value types and, possibly, associated entities).

Hibernate exposes metadata via the ClassMetadata and CollectionMetadata interfaces and the Type hierarchy. Instances of the metadata interfaces can be obtained from the SessionFactory.

Cat fritz = ......;
ClassMetadata catMeta = sessionfactory.getClassMetadata(Cat.class);

Object[] propertyValues = catMeta.getPropertyValues(fritz);
String[] propertyNames = catMeta.getPropertyNames();
Type[] propertyTypes = catMeta.getPropertyTypes();

// get a Map of all properties which are not collections or associations
Map namedValues = new HashMap();
for ( int i=0; i<propertyNames.length; i++ ) {
    if ( !propertyTypes[i].isEntityType() && !propertyTypes[i].isCollectionType() ) {
        namedValues.put( propertyNames[i], propertyValues[i] );
    }
}