Hessian序列化Hibernate的延迟加载Set等集合的解决方案

假如有如下类，Customer和Order,在使用Hessian序列化Customer时，如果orders延迟加载，并且序列化时Hibernate的session已经关闭，则会抛出Hibernate的LazyInitializationException.

@Entity
public class Customer implements java.io.Serializable {

    @Id
    private long id;

    
    private String name;

    //一对多，延迟加载
    @OneToMany(fetch=FetchType.LAZY)
    @JoinColumn(name="CUSTOMER_ID")
    private Set<Order> orders;
    ...........


@Entity
public class Order implements java.io.Serializable{

    @Id
    private long id;

    .......

该Set为Hibernate的PersistentSet,是Hibernate的PersistentCollection的子类。原因是Hessian序列化时，会调用PersistentSet的方法导致Hibernate从session里加载数据，如果session已经关闭，就会抛出异常，导致序列化出错。不仅Set会出现，PersistentCollection的其他子类也会出现该现象，如PersistentList,PersistentBag,PersistentMap等。使用JDK的序列化不会出现该现象。

解决方案如下，可以为Hibernate的PersistentColletion定制一个Serializer,代码如下：

import com.caucho.hessian.io.AbstractHessianOutput;
import com.caucho.hessian.io.AbstractSerializer;
import com.caucho.hessian.io.CollectionSerializer;
import com.caucho.hessian.io.MapSerializer;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import org.hibernate.Hibernate;
import org.hibernate.collection.PersistentMap;


public class HibernateSerializer extends AbstractSerializer {

    CollectionSerializer collectionSeiralizer = new CollectionSerializer();
    
    MapSerializer mapSerializer = new MapSerializer();

    @Override
    public void writeObject(Object obj, AbstractHessianOutput out) throws IOException {

        boolean init = Hibernate.isInitialized(obj);

        if (init) {
            out.writeObject(obj);
            out.flush();
            return;
        }

        if (PersistentMap.class.isAssignableFrom(obj.getClass())) {
            //将没有初始化的Map序列化空的HashMap
            mapSerializer.writeObject(new HashMap(), out);
        } else {
            //将没有初始化的List,Set等序列化为空的ArrayList
            collectionSeiralizer.writeObject(new ArrayList(), out);
        }
    }

    

}

同时还需要定制一个序列化工厂SerializerFactory,代码如下：

import com.caucho.hessian.io.HessianProtocolException;
import com.caucho.hessian.io.Serializer;
import com.caucho.hessian.io.SerializerFactory;
import org.hibernate.collection.PersistentCollection;


public class HibernateSerializerFactory  extends SerializerFactory {

    @Override
    public Serializer getSerializer(Class cl) throws HessianProtocolException {

        //Hibernate的集合API使用为HibernateSerializer序列化
        if (PersistentCollection.class.isAssignableFrom(cl)) {
            return new HibernateSerializer();
        }
        return super.getSerializer(cl);
    }

}

可以调用HessianServlet.getSerializerFactory().addFactory()添加HibernateSerializerFactory来使用该HibernateSerializer序列化Hibernate的PersistentCollection.

或者写一个serializers文件放到classpath的META-INF/hessian/目录下，该文件的内容为

org.hibernate.collection.PersistentCollection=HibernateSerializer