Hibernate:拉动所有懒惰集合的最佳实践

Question

是)我有的:

@Entity
public class MyEntity {
  @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, orphanRemoval = true)
  @JoinColumn(name = "myentiy_id")
  private List<Address> addreses;

  @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, orphanRemoval = true)
  @JoinColumn(name = "myentiy_id")
  private List<Person> persons;

  //....
}

public void handle() {

   Session session = createNewSession();
   MyEntity entity = (MyEntity) session.get(MyEntity.class, entityId);
   proceed(session); // FLUSH, COMMIT, CLOSE session!

   Utils.objectToJson(entity); //TROUBLES, because it can't convert to json lazy collections
}

真是个问题:

问题是我不能在会话关闭后拉懒惰集合.但我也不能在proceed方法中关闭会话.

什么解决方案(粗解决方案):

a)在会话关闭之前,强制hibernate拉取惰性集合

entity.getAddresses().size();
entity.getPersons().size();

....

b)使用@Fetch(FetchMode.SUBSELECT)注释可能更为优雅的方式

题:

什么是最佳实践/常用方式/更优雅的方式来做到这一点？意味着将我的对象转换为JSON.

Answer 1

使用Hibernate.initialize()内@Transactional初始化懒对象.

 start Transaction 
      Hibernate.initialize(entity.getAddresses());
      Hibernate.initialize(entity.getPersons());
 end Transaction 

现在,在事务的一边,你可以得到懒惰的对象.

entity.getAddresses().size();
entity.getPersons().size();

Answer 2

您可以在同一事务中遍历Hibernate对象的Getters,以确保使用以下通用帮助程序类急切地获取所有延迟子对象:

HibernateUtil.initializeObject(myObject,"my.app.model");

package my.app.util;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.HashSet;
import java.util.Set;

import org.aspectj.org.eclipse.jdt.core.dom.Modifier;
import org.hibernate.Hibernate;

public class HibernateUtil {

public static byte[] hibernateCollectionPackage = "org.hibernate.collection".getBytes();

public static void initializeObject( Object o, String insidePackageName ) {
    Set<Object> seenObjects = new HashSet<Object>();
    initializeObject( o, seenObjects, insidePackageName.getBytes() );
    seenObjects = null;
}

private static void initializeObject( Object o, Set<Object> seenObjects, byte[] insidePackageName ) {

    seenObjects.add( o );

    Method[] methods = o.getClass().getMethods();
    for ( Method method : methods ) {

        String methodName = method.getName();

        // check Getters exclusively
        if ( methodName.length() < 3 || !"get".equals( methodName.substring( 0, 3 ) ) )
            continue;

        // Getters without parameters
        if ( method.getParameterTypes().length > 0 )
            continue;

        int modifiers = method.getModifiers();

        // Getters that are public
        if ( !Modifier.isPublic( modifiers ) )
            continue;

        // but not static
        if ( Modifier.isStatic( modifiers ) )
            continue;

        try {

            // Check result of the Getter
            Object r = method.invoke( o );

            if ( r == null )
                continue;

            // prevent cycles
            if ( seenObjects.contains( r ) )
                continue;

            // ignore simple types, arrays und anonymous classes
            if ( !isIgnoredType( r.getClass() ) && !r.getClass().isPrimitive() && !r.getClass().isArray() && !r.getClass().isAnonymousClass() ) {

                // ignore classes out of the given package and out of the hibernate collection
                // package
                if ( !isClassInPackage( r.getClass(), insidePackageName ) && !isClassInPackage( r.getClass(), hibernateCollectionPackage ) ) {
                    continue;
                }

                // initialize child object
                Hibernate.initialize( r );

                // traverse over the child object
                initializeObject( r, seenObjects, insidePackageName );
            }

        } catch ( InvocationTargetException e ) {
            e.printStackTrace();
            return;
        } catch ( IllegalArgumentException e ) {
            e.printStackTrace();
            return;
        } catch ( IllegalAccessException e ) {
            e.printStackTrace();
            return;
        }
    }

}

private static final Set<Class<?>> IGNORED_TYPES = getIgnoredTypes();

private static boolean isIgnoredType( Class<?> clazz ) {
    return IGNORED_TYPES.contains( clazz );
}

private static Set<Class<?>> getIgnoredTypes() {
    Set<Class<?>> ret = new HashSet<Class<?>>();
    ret.add( Boolean.class );
    ret.add( Character.class );
    ret.add( Byte.class );
    ret.add( Short.class );
    ret.add( Integer.class );
    ret.add( Long.class );
    ret.add( Float.class );
    ret.add( Double.class );
    ret.add( Void.class );
    ret.add( String.class );
    ret.add( Class.class );
    ret.add( Package.class );
    return ret;
}

private static Boolean isClassInPackage( Class<?> clazz, byte[] insidePackageName ) {

    Package p = clazz.getPackage();
    if ( p == null )
        return null;

    byte[] packageName = p.getName().getBytes();

    int lenP = packageName.length;
    int lenI = insidePackageName.length;

    if ( lenP < lenI )
        return false;

    for ( int i = 0; i < lenI; i++ ) {
        if ( packageName[i] != insidePackageName[i] )
            return false;
    }

    return true;
}
}

Answer 3

放置 Utils.objectToJson(entity); 在会话结束前调用。

或者您可以尝试设置获取模式并使用这样的代码

Session s = ...
DetachedCriteria dc = DetachedCriteria.forClass(MyEntity.class).add(Expression.idEq(id));
dc.setFetchMode("innerTable", FetchMode.EAGER);
Criteria c = dc.getExecutableCriteria(s);
MyEntity a = (MyEntity)c.uniqueResult();

Answer 4

不是最好的解决方案，但这是我得到的：

1）使用此注释注释要初始化的getter：

@Retention(RetentionPolicy.RUNTIME)
public @interface Lazy {

}

2）从数据库读取对象后，对对象使用此方法（可以放在通用类中，或者可以用Object类更改T）：

    public <T> void forceLoadLazyCollections(T entity) {

    Session session = getSession().openSession();
    Transaction tx = null;
    try {

        tx = session.beginTransaction();
        session.refresh(entity);
        if (entity == null) {
            throw new RuntimeException("Entity is null!");
        }
        for (Method m : entityClass.getMethods()) {

            Lazy annotation = m.getAnnotation(Lazy.class);
            if (annotation != null) {
                m.setAccessible(true);
                logger.debug(" method.invoke(obj, arg1, arg2,...); {} field", m.getName());
                try {
                    Hibernate.initialize(m.invoke(entity));
                }
                catch (Exception e) {
                    logger.warn("initialization exception", e);
                }
            }
        }

    }
    finally {
        session.close();
    }
}

Answer 5

当必须获取多个集合时，您需要：

1. JOIN FETCH 一个集合
2. Hibernate.initialize对剩余的集合使用。

因此，就您的情况而言，您需要第一个 JPQL 查询，如下所示：

MyEntity entity = session.createQuery("select e from MyEntity e join fetch e.addreses where e.id 
= :id", MyEntity.class)
.setParameter("id", entityId)
.getSingleResult();

Hibernate.initialize(entity.persons);

这样，您可以通过 2 个 SQL 查询来实现您的目标，并避免笛卡尔积。