oracle.toplink.essentials.queryframework.FetchGroup Maven / Gradle / Ivy
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*
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// Copyright (c) 1998, 2007, Oracle. All rights reserved.
package oracle.toplink.essentials.queryframework;
import java.util.*;
import oracle.toplink.essentials.expressions.Expression;
/**
* Purpose: A fetch group is a performance enhancement that allows a group of
* attributes of an object to be loaded on demand, which means that the data for an attribute
* might not loaded from the underlying data source until an explicit access call for the
* attribute first occurs. It avoids the wasteful practice of loading up all data of the object?s
* attributes, in which the user is interested in only partial of them.
*
* A great deal of caution and careful system use case analysis should be use when using the fetch
* group feature, as the extra round-trip would well offset the gain from the deferred loading in
* many cases.
*
* TopLink fetch group support is twofold: the pre-defined fetch groups at the descriptor level; and
* dynamic (use case) fetch groups at the query level.
*
* TopLink fetch group support is only on CMP project.
*
* Every query can has at most one fetch group. There is an optional pre-defined default fetch group
* at the descriptor level. If set, and the query has no fetch group being set, the default fetch group
* would be used, unless query.setShouldUseDefaultFetchGroup(false) is also called. In the latter case,
* the full object will be fetched after the query execution.
*
* @see oracle.toplink.essentials.queryframework.FetchGroup
* @see oracle.toplink.essentials.queryframework.FetchGroupTracker
*
* @author King Wang
* @since TopLink 10.1.3.
*/
public class FetchGroup implements java.io.Serializable {
//fetch group name, default is empty if not set
private String name = "";
//all attributes in the group
private Set attributes = new TreeSet();
//attibute expression list used for the query preparation
private List fetchGroupAttributeExpressions;
/**
* Constructor.
*/
public FetchGroup() {
this("");
}
/**
* Constructor with a group name.
*/
public FetchGroup(String name) {
this.name = name;
this.fetchGroupAttributeExpressions = new ArrayList();
}
/**
* Return all attributes defined in the group
*/
public Set getAttributes() {
return attributes;
}
/**
* Add an attribute to the group
*/
public void addAttribute(String attrName) {
attributes.add(attrName);
}
/**
* Add a set of attributes to the group
*/
public void addAttributes(Set newAttributes) {
attributes.addAll(newAttributes);
}
/**
* Remove an attribute from the group
*/
public void removeAttribute(String attrName) {
attributes.remove(attrName);
}
/**
* Return the group name
*/
public String getName() {
return name;
}
/**
* Set the group name
*/
public void setName(String name) {
this.name = name;
}
/**
* INTERNAL:
* Return true if this fetch group is a super-set of the passed in fetch group
*/
public boolean isSupersetOf(FetchGroup anotherGroup) {
return (anotherGroup != null) && getAttributes().containsAll(anotherGroup.getAttributes());
}
/**
* INTERNAL:
* Return the attibute expression list.
*/
public List getFetchGroupAttributeExpressions() {
return fetchGroupAttributeExpressions;
}
/**
* INTERNAL:
* Set the attibute expression list.
*/
public void setFetchGroupAttributeExpressions(List fetchGroupAttributeExpressions) {
this.fetchGroupAttributeExpressions = fetchGroupAttributeExpressions;
}
/**
* INTERNAL:
* Return if fetch group attributes.
*/
public boolean hasFetchGroupAttributeExpressions() {
return !fetchGroupAttributeExpressions.isEmpty();
}
/**
* INTERNAL:
* Specify that only a subset of the class' attributes be fetched in this query.
*
This allows for the query to be optimized through selecting less data.
*
Partial objects will be returned from the query, where the unspecified attributes will be left in their default value.
* The primary key, and optimistic locking version (version or timestamp) if defined, will always be fetched.
* The partial object is cached and can be modified.
* An access through getter to an un-fetched attribute will trigger another SELECT to load the whole object.
*
Note: Modifying un-fetched attribute is not supported and exception would thrown.
*/
public void addFetchGroupAttribute(Expression attributeExpression) {
getFetchGroupAttributeExpressions().add(attributeExpression);
}
}