Defining Data Classes with JDO

You can use JDO to store plain Java data objects (sometimes referred to as "Plain Old Java Objects" or "POJOs") in the datastore. Each object that is made persistent with the PersistenceManager becomes an entity in the datastore. You use annotations to tell JDO how to store and recreate instances of your data classes.

Note: Earlier versions of JDO use .jdo XML files instead of Java annotations. These still work with JDO 2.3. This documentation only covers using Java annotations with data classes.

  1. Class and Field Annotations
  2. Core Value Types
  3. Serializable Objects
  4. Child Objects and Relationships
  5. Embedded Classes
  6. Collections
  7. Object Fields and Entity Properties
  8. Inheritance

Class and Field Annotations

Each object saved by JDO becomes an entity in the App Engine datastore. The entity's kind is derived from the simple name of the class (inner classes use the $ path without the package name). Each persistent field of the class represents a property of the entity, with the name of the property equal to the name of the field (with case preserved).

To declare a Java class as capable of being stored and retrieved from the datastore with JDO, give the class a @PersistenceCapable annotation. For example:

import javax.jdo.annotations.PersistenceCapable;

public class Employee {
    // ...

Fields of the data class that are to be stored in the datastore must be declared as persistent fields. To declare a field as persistent, give it a @Persistent annotation:

import java.util.Date;
import javax.jdo.annotations.Persistent;

// ...
    private Date hireDate;

To declare a field as not persistent (it does not get stored in the datastore, and is not restored when the object is retrieved), give it a @NotPersistent annotation.

Tip: JDO specifies that fields of certain types are persistent by default if neither the @Persistent nor @NotPersistent annotations are specified, and fields of all other types are not persistent by default. See