[hibernate-issues] [Hibernate-JIRA] Closed: (HHH-2842) Problem with instantiating components

[Steve Ebersole (JIRA)]

     [ http://opensource.atlassian.com/projects/hibernate/browse/HHH-2842?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Steve Ebersole closed HHH-2842.
-------------------------------

    Resolution: Incomplete

This is a usage question, which should be directed at the user forum, irc, etc.

But I will give you a hint: check out tuplizers...

> Problem with instantiating components
> -------------------------------------
>
>                 Key: HHH-2842
>                 URL: http://opensource.atlassian.com/projects/hibernate/browse/HHH-2842
>             Project: Hibernate3
>          Issue Type: Improvement
>          Components: core
>    Affects Versions: 3.2.5
>         Environment: Hibernate 3.2.5ga, DB2 8
>            Reporter: Lanna Kim
>         Attachments: quickfix.zip, school.zip
>
>
> Hi,
> I am working on a project migrating a legacy application into J2EE environment using Hibernate. 
> Although it has fairly complicated class hierarchy and table structures, we have managed to migrate most of features successfully, thanks to the flexibility of Hibernate. (Thank you for the great work!)
> However, we have one issue with instantiating dependent objects.
> Sometimes only containing class knows how to instantiate its dependent objects properly, so these objects need to be created by the containing class.
> For example, the following persistent class Student has a dependent object studentInfo, which is mapped as as component in the mapping file.
> --------------------------------------------------
> public class Student {
>     private Long id;
>     private StudentInfo studentInfo;
>     
>     public Student() {
>         // instantiate StudentInfo object here with some additional information.
>         studentInfo = new StudentInfo(SchoolHelper.lookupType("C"));
>     }
>    // ... 
> }
> --------------------------------------------------
> public class StudentInfo {
>     private String description;
>     private transient String typeA;
>     private transient String typeB;
>     
>     public StudentInfo() {
>     }
>     
>     public StudentInfo(String type) {
>         this.typeA = SchoolHelper.getTypeA(type);
>         this.typeB = SchoolHelper.getTypeB(type);
>     }
>    // ...
> --------------------------------------------------
>     <class name="school.Student" table="STUDENT">
>         <id name="id" column="STUDENT_ID">
>             <generator class="native"/>
>         </id>
>         <component name="studentInfo" class="school.StudentInfo">
>             <property name="description" column="SI_DESC" />
>         </component>
>     </class>
> --------------------------------------------------
> Only 'description' member is mapped to a database column and other transient members of StudentInfo aren't. These transient members can be set only by using the information passed from its parent class, Student.
> However, when a Student object is loaded from database, this information (typeA and typeB) becomes null, because the default constructor StudentInfo() is used by Hibernate, and then Student.setStudentInfo() is called to replace the existing (but properly instantiated) studentInfo object.
> I know it is a Hibernate requirement for all persistent classes to have no-arg default constructors.
> This is fine, but is it true for all dependent objects as well? 
> Some of our depedent objects are contained in both persistent classes and business logic classes and it is very difficult to separate them.
> It would be great if Hibernate supports either of following features:
>   1) <property> can accept a dotted notaton for its name, so that we can access directly the member of the depedent object without having <component>.
>      i.e. 
>      <property name="studentInfo.description" column="SI_DESC" />
>      <!-- this will call getStudentInfo().setDescription(). -->
> or,
>   2) <component> has an option to indicate to reuse the existing instance instead of always creating a new one.
>     i.e.
>      <component name="studentInfo" reuse="true" class="school.StudentInfo>
>          ...
>      </component>
>      <!-- the existing studentInfo object will be used (unless it is null), instead of creating one with its default constructor -->
> I have attached student.zip to indicate this issue and quickfix.zip for the (rough) hibernate fix so that ComponentType reuses the existing value unless it is null.
> Any advice or comment would be appreciated.

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