JDO (1.0.1) expects the Meta-Data to be specified in a file or files in particular positions in the file system. For example, if you have a class com.mycompany.sample.myexample, JDO will look for any of the following files until it finds one (in the order stated) :-

META-INF/package.jdo
WEB-INF/package.jdo
package.jdo
com/package.jdo
com/mycompany/package.jdo
com/mycompany/sample/package.jdo
com/mycompany/sample/myexample.jdo
                

In addition, for this example, JPOX allows the previous JDO 1.0.0 alternatives of

com.jdo
com/mycompany.jdo
com/mycompany/sample.jdo
                

In addition to the above, you can split your MetaData definitions between JDO MetaData files. For example if you have the following classes

com/mycompany/A.java
com/mycompany/B.java
com/mycompany/C.java
com/mycompany/app1/D.java
com/mycompany/app1/E.java
                

You could define the MetaData for these 5 classes in many ways -- for example put all definitions in com/mycompany/package.jdo, or put the definitions for D and E in com/mycompany/app1/package.jdo and the definitions for A, B, C in com/mycompany/package.jdo, or have some in their class named MetaData files e.g com/mycompany/app1/A.jdo, or a mixture of the above. JPOX will always search for the MetaData file containing the class definition for the class that it requires.

The contents in your XML files must conform to the Document Type Definition (DTD) for the jdo file you provide. It is mandatory to include the correct DOCTYPE header for each jdo file, and either SYSTEM or PUBLIC forms can be used. The DOCTYPE header information must include a valid URL syntax in order to avoid parser errors.

• jdo
• package
• class
• field
• collection
• extension
• map
• extension
• array
• extension
• extension
• extension
• extension

These are attributes within the <package> tag (jdo/package).

These are attributes within the <class> tag (jdo/package/class).

These are attributes within the <extension> tag (jdo/package/class/extension).

These are attributes within the <field> tag (jdo/package/class/field).

These are attributes within the <extension> tag (jdo/package/class/field/extension).

<!-- RDMBS database column type: VARCHAR 100 -->
<extension vendor-name="jpox" key="length" value="max 100" />
                            

<!-- RDMBS database column type: CHAR 50 -->
<extension vendor-name="jpox" key="length" value="50" />
                            

<extension vendor-name="jpox" key="length" value="unlimited" />
                            

<extension vendor-name="jpox" key="precision" value="1" />
                            

<extension vendor-name="jpox" key="scale" value="10" />
                            

<!-- RDMBS database column type: VARCHAR 100 -->
<extension vendor-name="jpox" key="key-length" value="max 100" />
                            

<!-- RDMBS database column type: CHAR 50 -->
<extension vendor-name="jpox" key="key-length" value="50" />
                            

<extension vendor-name="jpox" key="key-length" value="unlimited" />
                            

<extension vendor-name="jpox" key="key-precision" value="1" />
                            

<extension vendor-name="jpox" key="key-scale" value="10" />
                            

<!-- RDMBS database column type: VARCHAR 100 -->
<extension vendor-name="jpox" key="value-length" value="max 100" />
                            

<!-- RDMBS database column type: CHAR 50 -->
<extension vendor-name="jpox" key="value-length" value="50" />
                            

<extension vendor-name="jpox" key="value-length" value="unlimited" />
                            

<extension vendor-name="jpox" key="value-precision" value="1" />
                            

<extension vendor-name="jpox" key="value-scale" value="10" />
                            

These are attributes within the <collection> tag (jdo/package/class/field/collection).

These are attributes within the <extension> tag (jdo/package/class/field/collection/extension).

These are attributes within the <map> tag (jdo/package/class/field/map).

These are attributes within the <extension> tag (jdo/package/class/field/map/extension).

The only arrays currently supported by JPOX are byte arrays, and these are always stored as embedded.

Let us take a sample class and generate MetaData for it. Suppose I have a class as follows

package org.jpox.mysample;

public class Person
{
    /** Title of the Person. */
    String title=null;

    /** Forename of the Person. */
    String forename=null;

    /** Surname of the Person. */
    String surname=null;

    ...
}
                

and I want to use an existing schema. With this case I need to define the table and column names that it maps to. To do this I need to use JPOX extension tags. So I come up with MetaData as follows

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE jdo PUBLIC
    "-//Sun Microsystems, Inc.//DTD Java Data Objects Metadata 1.0//EN"
    "http://java.sun.com/dtd/jdo_1_0.dtd">
<jdo>
<package name="org.jpox.mysample">
    <class name="Person"
        identity-type="datastore">
        <extension vendor-name="jpox" key="table-name" value="PERSON"/>
        <field name="title">
            <extension vendor-name="jpox" key="column-name" value="TITLE"/>
        </field>
        <field name="forename">
            <extension vendor-name="jpox" key="column-name" value="FORENAME"/>
            <extension vendor-name="jpox" key="length" value="max 100"/>
        </field>
        <field name="surname">
            <extension vendor-name="jpox" key="column-name" value="SURNAME"/>
            <extension vendor-name="jpox" key="length" value="max 100"/>
        </field>
    </class>
</package>
</jdo>
                

So you see that our class is being mapped across to a table "PERSON" in the datastore, with columns "TITLE", "FORENAME", "SURNAME". We have also specified that the upper size limit on the forename and surname fields is 100.