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This is the full Active Objects library, if you don't know which one to use, you probably want this one.

There is a newer version: 6.1.1

/*
 * Copyright 2007 Daniel Spiewak
 * 
 * Licensed under the Apache License, Version 2.0 (the "License"); 
 * you may not use this file except in compliance with the License. 
 * You may obtain a copy of the License at
 * 
 *	    http://www.apache.org/licenses/LICENSE-2.0 
 * 
 * Unless required by applicable law or agreed to in writing, software 
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package net.java.ao;

/**
 * A super-interface designed to allow auto-generated values
 * to be inserted into database fields on INSERT.  Potentially,
 * this could be expanded to also include on UPDATE
 * functionality, but for the moment it only handles on entity
 * creation (INSERT).  Implementations are assumed to be
 * completely stateless and thus thread-safe.
 *
 * The type parameter represents the return type of the
 * method which does the actual generation.
 *
 * @author Daniel Spiewak
 * @see net.java.ao.Generator
 */
public interface ValueGenerator {

    /**
     * Generate a new value for an arbitrary field.  No write
     * database operations should be performed within this
     * method.  The EntityManager instance is only
     * intended to ensure various field constraints (such as
     * uniqueness for primary keys).  Great care should be
     * taken with performing such operations, as the value
     * generation method needs to be extremely efficient.
     *
     * @param manager An instance to gain read access to the database.
     * @return A new value of the relevant type for INSERT.
     */
    public T generateValue(EntityManager manager);
}