hibernate Load 代码分析

一直以来都想好好研究下hibernate代码，每次都是看一小段有点理解过段时间回来有忘光了，所以这次做下记录，免得重复劳动.



发现hibernate的代码还是挺复杂的，比spring ，struts要复杂的多，所以就一段段看。


今天主要分析一下hibernate如何来更新数据做CURP操作。


先看看load 和　get

public Object load(String entityName, Serializable id) throws HibernateException {

        LoadEvent event = new LoadEvent(id, entityName, false, this);

        boolean success = false;

        try {

            fireLoad( event, LoadEventListener.LOAD );

            if ( event.getResult() == null ) {

                getFactory().getEntityNotFoundDelegate().handleEntityNotFound( entityName, id );

            }

            success = true;

            return event.getResult();

        }

        finally {

            afterOperation(success);

        }

    }

get:

public Object get(String entityName, Serializable id) throws HibernateException {
		LoadEvent event = new LoadEvent(id, entityName, false, this);
		boolean success = false;
		try {
			fireLoad(event, LoadEventListener.GET);
			success = true;
			return event.getResult();
		}
		finally {
			afterOperation(success);
		}
	}

对比这两个的代码发现只有

 if ( event.getResult() == null ) {

                getFactory().getEntityNotFoundDelegate().handleEntityNotFound( entityName, id );

            }

差别，其余都几乎一样，这行也没起多大作用，　仔细看发现调用fireLoad时的第２个参数不一样一个是LoadEventListener.GET，　LoadEventListener.LOAD

跟踪

public static final LoadType GET = new LoadType("GET")
			.setAllowNulls(true)
			.setAllowProxyCreation(false)
			.setCheckDeleted(true)
			.setNakedEntityReturned(false);
	
	public static final LoadType LOAD = new LoadType("LOAD")
			.setAllowNulls(false)
			.setAllowProxyCreation(true)
			.setCheckDeleted(true)
			.setNakedEntityReturned(false);

这里看名字就可以看出get允许为null, load不允许，　另外setAllowProxyCreation　值不一样，其他都一样。


下面看下fireLoad方法：

private void fireLoad(LoadEvent event, LoadType loadType) {
		errorIfClosed();
		checkTransactionSynchStatus();
		LoadEventListener[] loadEventListener = listeners.getLoadEventListeners();
		for ( int i = 0; i < loadEventListener.length; i++ ) {
			loadEventListener[i].onLoad(event, loadType);
		}
	}

这里是对LoadEvent进行处理，调用已经注册好的LoadEventListener　去处理这个onLoad事件

这里的LoadEventListener的实现类是DefaultLoadEventListener

要弄明白hibernate是怎么获得一个数据的，就要看onLoad　方法了

/**
	 * Handle the given load event.
	 *
	 * @param event The load event to be handled.
	 * @throws HibernateException
	 */
	public void onLoad(LoadEvent event, LoadEventListener.LoadType loadType) throws HibernateException {

		final SessionImplementor source = event.getSession();

		EntityPersister persister;
　　　　　　　　　//获得当前pojo 的　EntityPersister  
		if ( event.getInstanceToLoad() != null ) {
			persister = source.getEntityPersister( null, event.getInstanceToLoad() ); //the load() which takes an entity does not pass an entityName
			event.setEntityClassName( event.getInstanceToLoad().getClass().getName() );
		}
		else {
			persister = source.getFactory().getEntityPersister( event.getEntityClassName() );
		}

		if ( persister == null ) {
			throw new HibernateException(
					"Unable to locate persister: " +
					event.getEntityClassName()
				);
		}

		final Class idClass = persister.getIdentifierType().getReturnedClass();
		if ( persister.getIdentifierType().isComponentType() && EntityMode.DOM4J == event.getSession().getEntityMode() ) {
			// skip this check for composite-ids relating to dom4j entity-mode;
			// alternatively, we could add a check to make sure the incoming id value is
			// an instance of Element...
		}
		else {
			if ( idClass != null && ! idClass.isInstance( event.getEntityId() ) ) {
				// we may have the kooky jpa requirement of allowing find-by-id where
				// "id" is the "simple pk value" of a dependent objects parent.  This
				// is part of its generally goofy "derived identity" "feature"
				if ( persister.getEntityMetamodel().getIdentifierProperty().isEmbedded() ) {
					final EmbeddedComponentType dependentIdType =
							(EmbeddedComponentType) persister.getEntityMetamodel().getIdentifierProperty().getType();
					if ( dependentIdType.getSubtypes().length == 1 ) {
						final Type singleSubType = dependentIdType.getSubtypes()[0];
						if ( singleSubType.isEntityType() ) {
							final EntityType dependentParentType = (EntityType) singleSubType;
							final Type dependentParentIdType = dependentParentType.getIdentifierOrUniqueKeyType( source.getFactory() );
							if ( dependentParentIdType.getReturnedClass().isInstance( event.getEntityId() ) ) {
								// yep that's what we have...
								loadByDerivedIdentitySimplePkValue(
										event,
										loadType,
										persister,
										dependentIdType,
										source.getFactory().getEntityPersister( dependentParentType.getAssociatedEntityName() )
								);
								return;
							}
						}
					}
				}
				throw new TypeMismatchException(
						"Provided id of the wrong type for class " + persister.getEntityName() + ". Expected: " + idClass + ", got " + event.getEntityId().getClass()
				);
			}
		}

		EntityKey keyToLoad = new EntityKey( event.getEntityId(), persister, source.getEntityMode()  );

		try {
			if ( loadType.isNakedEntityReturned() ) {
				//do not return a proxy!
				//(this option indicates we are initializing a proxy)
				event.setResult( load(event, persister, keyToLoad, loadType) );
			}
			else {
				//return a proxy if appropriate
				if ( event.getLockMode() == LockMode.NONE ) {
					event.setResult( proxyOrLoad(event, persister, keyToLoad, loadType) );
				}
				else {
					event.setResult( lockAndLoad(event, persister, keyToLoad, loadType, source) );
				}
			}
		}
		catch(HibernateException e) {
			log.info("Error performing load command", e);
			throw e;
		}
	}

	

这里最后调用event.setResult( proxyOrLoad(event, persister, keyToLoad, loadType) );　在默认无锁的情况下。

protected Object proxyOrLoad(
		final LoadEvent event,
		final EntityPersister persister,
		final EntityKey keyToLoad,
		final LoadEventListener.LoadType options) {

		if ( log.isTraceEnabled() ) {
			log.trace(
					"loading entity: " +
					MessageHelper.infoString( persister, event.getEntityId(), event.getSession().getFactory() )
				);
		}

		if ( !persister.hasProxy() ) {
			// this class has no proxies (so do a shortcut)
			return load(event, persister, keyToLoad, options);
		}
		else {
			final PersistenceContext persistenceContext = event.getSession().getPersistenceContext();

			// look for a proxy
			Object proxy = persistenceContext.getProxy(keyToLoad);
			if ( proxy != null ) {
				return returnNarrowedProxy( event, persister, keyToLoad, options, persistenceContext, proxy );
			}
			else {
				if ( options.isAllowProxyCreation() ) {
					return createProxyIfNecessary( event, persister, keyToLoad, options, persistenceContext );
				}
				else {
					// return a newly loaded object
					return load(event, persister, keyToLoad, options);
				}
			}

		}
	}

protected Object doLoad(
			final LoadEvent event,
			final EntityPersister persister,
			final EntityKey keyToLoad,
			final LoadEventListener.LoadType options) {

		if ( log.isTraceEnabled() ) {
			log.trace(
					"attempting to resolve: " +
					MessageHelper.infoString( persister, event.getEntityId(), event.getSession().getFactory() )
				);
		}

		Object entity = loadFromSessionCache( event, keyToLoad, options );
		if ( entity == REMOVED_ENTITY_MARKER ) {
			log.debug( "load request found matching entity in context, but it is scheduled for removal; returning null" );
			return null;
		}
		if ( entity == INCONSISTENT_RTN_CLASS_MARKER ) {
			log.debug( "load request found matching entity in context, but the matched entity was of an inconsistent return type; returning null" );
			return null;
		}
		if ( entity != null ) {
			if ( log.isTraceEnabled() ) {
				log.trace(
						"resolved object in session cache: " +
						MessageHelper.infoString( persister, event.getEntityId(), event.getSession().getFactory()  )
					);
			}
			return entity;
		}

		entity = loadFromSecondLevelCache(event, persister, options);
		if ( entity != null ) {
			if ( log.isTraceEnabled() ) {
				log.trace(
						"resolved object in second-level cache: " +
						MessageHelper.infoString( persister, event.getEntityId(), event.getSession().getFactory() )
					);
			}
			return entity;
		}

		if ( log.isTraceEnabled() ) {
			log.trace(
					"object not resolved in any cache: " +
					MessageHelper.infoString( persister, event.getEntityId(), event.getSession().getFactory() )
				);
		}

		return loadFromDatasource(event, persister, keyToLoad, options);
	}

	private Object createProxyIfNecessary(
			final LoadEvent event,
			final EntityPersister persister,
			final EntityKey keyToLoad,
			final LoadEventListener.LoadType options,
			final PersistenceContext persistenceContext) {
		Object existing = persistenceContext.getEntity( keyToLoad );
		if ( existing != null ) {
			// return existing object or initialized proxy (unless deleted)
			log.trace( "entity found in session cache" );
			if ( options.isCheckDeleted() ) {
				EntityEntry entry = persistenceContext.getEntry( existing );
				Status status = entry.getStatus();
				if ( status == Status.DELETED || status == Status.GONE ) {
					return null;
				}
			}
			return existing;
		}
		else {
			log.trace( "creating new proxy for entity" );
			// return new uninitialized proxy
			Object proxy = persister.createProxy( event.getEntityId(), event.getSession() );
			persistenceContext.getBatchFetchQueue().addBatchLoadableEntityKey(keyToLoad);
			persistenceContext.addProxy(keyToLoad, proxy);
			return proxy;
		}
	}

hibernate 是采用事件回调模式去处理一个数据库的操作，通过load返回的实际上是一个proxy的对象, 在debug模式下查看这个proxy 对象得到

"proxy"= Event_$$_javassist_0  (id=78)

看来这个好像是用javassist类库来增强过的pojo对象，load返回的就是这个对象了。

如果是get方法将会调用　doLoad 方法去获得

if ( options.isAllowProxyCreation() ) {
					return createProxyIfNecessary( event, persister, keyToLoad, options, persistenceContext );
				}
				else {
					// return a newly loaded object
					return load(event, persister, keyToLoad, options);
				}

get 方式的时候　isAllowProxyCreation　值为false的，参看前面的代码

doLoad方法先去sessionCache然后再去secondCache去查找　给定对象，如果没有的话就调用

return loadFromDatasource(event, persister, keyToLoad, options);

从数据库找，代码：

/**
	 * Performs the process of loading an entity from the configured
	 * underlying datasource.
	 *
	 * @param event The load event
	 * @param persister The persister for the entity being requested for load
	 * @param keyToLoad The EntityKey representing the entity to be loaded.
	 * @param options The load options.
	 * @return The object loaded from the datasource, or null if not found.
	 */
	protected Object loadFromDatasource(
			final LoadEvent event,
			final EntityPersister persister,
			final EntityKey keyToLoad,
			final LoadEventListener.LoadType options) {
		final SessionImplementor source = event.getSession();
		Object entity = persister.load(
				event.getEntityId(),
				event.getInstanceToLoad(),
				event.getLockOptions(),
				source
		);

		if ( event.isAssociationFetch() && source.getFactory().getStatistics().isStatisticsEnabled() ) {
			source.getFactory().getStatisticsImplementor().fetchEntity( event.getEntityClassName() );
		}

		return entity;
	}

这里最终调用　AbstractEntityPersister.load 方法，代码：

public Object load(Serializable id, Object optionalObject, LockOptions lockOptions, SessionImplementor session)
			throws HibernateException {

		if ( log.isTraceEnabled() ) {
			log.trace(
					"Fetching entity: " +
					MessageHelper.infoString( this, id, getFactory() )
				);
		}

		final UniqueEntityLoader loader = getAppropriateLoader(lockOptions, session );
		return loader.load( id, optionalObject, session, lockOptions );
	}

这里会得到一个合适的loader, 我测试的时候得到的是　org.hibernate.loader.entity.EntityLoader
它继承与 AbstractEntityLoader

public Object load(Serializable id, Object optionalObject, SessionImplementor session, LockOptions lockOptions) {
		return load( session, id, optionalObject, id, lockOptions );
	}

	protected Object load(
			SessionImplementor session,
			Object id,
			Object optionalObject,
			Serializable optionalId,
			LockOptions lockOptions) {
		
		List list = loadEntity(
				session, 
				id, 
				uniqueKeyType, 
				optionalObject, 
				entityName, 
				optionalId, 
				persister,
				lockOptions
			);
		
		if ( list.size()==1 ) {
			return list.get(0);
		}
		else if ( list.size()==0 ) {
			return null;
		}
		else {
			if ( getCollectionOwners()!=null ) {
				return list.get(0);
			}
			else {
				throw new HibernateException(
						"More than one row with the given identifier was found: " +
						id +
						", for class: " +
						persister.getEntityName()
					);
			}
		}
		
	}

调用　loadEntity　方法，　代码:

protected final List loadEntity(
			final SessionImplementor session,
			final Object id,
			final Type identifierType,
			final Object optionalObject,
			final String optionalEntityName,
			final Serializable optionalIdentifier,
			final EntityPersister persister,
			LockOptions lockOptions) throws HibernateException {
		
		if ( log.isDebugEnabled() ) {
			log.debug( 
					"loading entity: " + 
					MessageHelper.infoString( persister, id, identifierType, getFactory() ) 
				);
		}

		List result;
		try {
			QueryParameters qp = new QueryParameters();
			qp.setPositionalParameterTypes( new Type[] { identifierType } );
			qp.setPositionalParameterValues( new Object[] { id } );
			qp.setOptionalObject( optionalObject );
			qp.setOptionalEntityName( optionalEntityName );
			qp.setOptionalId( optionalIdentifier );
			qp.setLockOptions( lockOptions );
			result = doQueryAndInitializeNonLazyCollections( session, qp, false );
		}
		catch ( SQLException sqle ) {
			final Loadable[] persisters = getEntityPersisters();
			throw JDBCExceptionHelper.convert(
			        factory.getSQLExceptionConverter(),
			        sqle,
			        "could not load an entity: " + 
			        MessageHelper.infoString( persisters[persisters.length-1], id, identifierType, getFactory() ),
			        getSQLString()
				);
		}

		log.debug("done entity load");
		
		return result;
		
	}

继续追踪，跟到方法

private List doQuery(
			final SessionImplementor session,
			final QueryParameters queryParameters,
			final boolean returnProxies) throws SQLException, HibernateException {

		final RowSelection selection = queryParameters.getRowSelection();
		final int maxRows = hasMaxRows( selection ) ?
				selection.getMaxRows().intValue() :
				Integer.MAX_VALUE;

		final int entitySpan = getEntityPersisters().length;

		final ArrayList hydratedObjects = entitySpan == 0 ? null : new ArrayList( entitySpan * 10 );
		final PreparedStatement st = prepareQueryStatement( queryParameters, false, session );
		final ResultSet rs = getResultSet( st, queryParameters.hasAutoDiscoverScalarTypes(), queryParameters.isCallable(), selection, session );

// would be great to move all this below here into another method that could also be used
// from the new scrolling stuff.
//
// Would need to change the way the max-row stuff is handled (i.e. behind an interface) so
// that I could do the control breaking at the means to know when to stop

		final EntityKey optionalObjectKey = getOptionalObjectKey( queryParameters, session );
		final LockMode[] lockModesArray = getLockModes( queryParameters.getLockOptions() );
		final boolean createSubselects = isSubselectLoadingEnabled();
		final List subselectResultKeys = createSubselects ? new ArrayList() : null;
		final List results = new ArrayList();

		try {

			handleEmptyCollections( queryParameters.getCollectionKeys(), rs, session );

			EntityKey[] keys = new EntityKey[entitySpan]; //we can reuse it for each row

			if ( log.isTraceEnabled() ) log.trace( "processing result set" );

			int count;
			for ( count = 0; count < maxRows && rs.next(); count++ ) {
				
				if ( log.isTraceEnabled() ) log.debug("result set row: " + count);

				Object result = getRowFromResultSet( 
						rs,
						session,
						queryParameters,
						lockModesArray,
						optionalObjectKey,
						hydratedObjects,
						keys,
						returnProxies 
				);
				results.add( result );

				if ( createSubselects ) {
					subselectResultKeys.add(keys);
					keys = new EntityKey[entitySpan]; //can't reuse in this case
				}
				
			}

			if ( log.isTraceEnabled() ) {
				log.trace( "done processing result set (" + count + " rows)" );
			}

		}
		finally {
			session.getBatcher().closeQueryStatement( st, rs );
		}

		initializeEntitiesAndCollections( hydratedObjects, rs, session, queryParameters.isReadOnly( session ) );

		if ( createSubselects ) createSubselects( subselectResultKeys, queryParameters, session );

		return results; //getResultList(results);

	}

这里进行最终的数据库读取，