What is lazy loading in Hibernate?

What is lazy loading in Java? I don't understand the process. Can anybody help me to understand the process of lazy loading?

asked Jun 30, 2015 in hibernate by rahulgii
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2 Answers

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Say you have a parent and that parent has a collection of children. Hibernate now can "lazy-load" the children, which means that it does not actually load all the children when loading the parent. Instead, it loads them when requested to do so. You can either request this explicitly or, and this is far more common, hibernate will load them automatically when you try to access a child.

Lazy-loading can help improve the performance significantly since often you won't need the children and so they will not be loaded.

Also beware of the n+1-problem. Hibernate will not actually load all children when you access the collection. Instead, it will load each child individually. When iterating over the collection, this causes a query for every child. In order to avoid this, you can trick hibernate into loading all children simultaneously, e.g. by calling parent.getChildren().size().
answered Jun 30, 2015 by rahulgii
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Lazy fetching decides whether to load child objects while loading the Parent Object. You need to do this setting respective hibernate mapping file of the parent class. Lazy = true (means not to load child) By default the lazy loading of the child objects is true.

This make sure that the child objects are not loaded unless they are explicitly invoked in the application by calling getChild() method on parent.In this case hibernate issues a fresh database call to load the child when getChild() is actully called on the Parent object.

But in some cases you do need to load the child objects when parent is loaded. Just make the lazy=false and hibernate will load the child when parent is loaded from the database.

Example : If you have a TABLE ? EMPLOYEE mapped to Employee object and contains set of Address objects. Parent Class : Employee class, Child class : Address Class

public class Employee { 
private Set address = new HashSet(); // contains set of child Address objects 
public Set getAddress () { 
return address; 
public void setAddresss(Set address) { 
this. address = address; 

In the Employee.hbm.xml file

<set name="address" inverse="true" cascade="delete" lazy="false"> 
<key column="a_id" /> 
<one-to-many class="beans Address"/> 

In the above configuration. If lazy="false" : - when you load the Employee object that time child object Address is also loaded and set to setAddresss() method. If you call employee.getAdress() then loaded data returns.No fresh database call.

If lazy="true" :- This the default configuration. If you don?t mention then hibernate consider lazy=true. when you load the Employee object that time child object Adress is not loaded. You need extra call to data base to get address objects. If you call employee.getAdress() then that time database query fires and return results. Fresh database call.

answered Jun 30, 2015 by rahulgii