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/*
* CycleStrategy.java April 2007
*
* Copyright (C) 2007, Niall Gallagher
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General
* Public License along with this library; if not, write to the
* Free Software Foundation, Inc., 59 Temple Place, Suite 330,
* Boston, MA 02111-1307 USA
*/
package org.simpleframework.xml.strategy;
import org.simpleframework.xml.stream.NodeMap;
import java.util.Map;
/**
* The CycleStrategy represents a strategy that is used
* to augment the deserialization and serialization process such that
* cycles in an object graph can be supported. This adds additional
* attributes to the serialized XML elements so that during the
* deserialization process an objects cycles can be created. Without
* the use of a strategy such as this, cycles could cause an infinite
* loop during the serialization process while traversing the graph.
*
* In the above serialized XML there is a circular reference, where
* the XML element with id "2" contains a reference to itself. In
* most data binding frameworks this will cause an infinite loop,
* or in some cases will just fail to represent the references well.
* With this strategy you can ensure that cycles in complex object
* graphs will be maintained and can be serialized safely.
*
* @author Niall Gallagher
*
* @see org.simpleframework.xml.core.Persister
* @see org.simpleframework.xml.strategy.Strategy
*/
public class CycleStrategy implements Strategy {
/**
* The default name of the attribute used for circular references.
*/
private static final String REFER = "reference";
/**
* The default name of the attribute used to identify an object.
*/
private static final String MARK = "id";
/**
* The default name of the attribute used to specify the length.
*/
private static final String LENGTH = "length";
/**
* The default name of the attribute used to specify the class.
*/
private static final String LABEL = "class";
/**
* This is used to maintain session state for writing the graph.
*/
private WriteState write;
/**
* This is used to maintain session state for reading the graph.
*/
private ReadState read;
/**
* This is used to provide the names of the attributes to use.
*/
private Contract contract;
/**
* Constructor for the CycleStrategy object. This is
* used to create a strategy with default values. By default the
* values used are "id" and "reference". These values will be
* added to XML elements during the serialization process. And
* will be used to deserialize the object cycles fully.
*/
public CycleStrategy() {
this(MARK, REFER);
}
/**
* Constructor for the CycleStrategy object. This is
* used to create a strategy with the specified attributes, which
* will be added to serialized XML elements. These attributes
* are used to serialize the objects in such a way the cycles in
* the object graph can be deserialized and used fully.
*
* @param mark this is used to mark the identity of an object
* @param refer this is used to refer to an existing object
*/
public CycleStrategy(String mark, String refer) {
this(mark, refer, LABEL);
}
/**
* Constructor for the CycleStrategy object. This is
* used to create a strategy with the specified attributes, which
* will be added to serialized XML elements. These attributes
* are used to serialize the objects in such a way the cycles in
* the object graph can be deserialized and used fully.
*
* @param mark this is used to mark the identity of an object
* @param refer this is used to refer to an existing object
* @param label this is used to specify the class for the field
*/
public CycleStrategy(String mark, String refer, String label){
this(mark, refer, label, LENGTH);
}
/**
* Constructor for the CycleStrategy object. This is
* used to create a strategy with the specified attributes, which
* will be added to serialized XML elements. These attributes
* are used to serialize the objects in such a way the cycles in
* the object graph can be deserialized and used fully.
*
* @param mark this is used to mark the identity of an object
* @param refer this is used to refer to an existing object
* @param label this is used to specify the class for the field
* @param length this is the length attribute used for arrays
*/
public CycleStrategy(String mark, String refer, String label, String length){
this.contract = new Contract(mark, refer, label, length);
this.write = new WriteState(contract);
this.read = new ReadState(contract);
}
/**
* This method is used to read an object from the specified node.
* In order to get the root type the field and node map are
* specified. The field represents the annotated method or field
* within the deserialized object. The node map is used to get
* the attributes used to describe the objects identity, or in
* the case of an existing object it contains an object reference.
*
* @param field the method or field in the deserialized object
* @param node this is the XML element attributes to read
* @param map this is the session map used for deserialization
*
* @return this returns an instance to insert into the object
*/
public Value getRoot(Class field, NodeMap node, Map map) throws Exception {
return getElement(field, node, map);
}
/**
* This method is used to read an object from the specified node.
* In order to get the root type the field and node map are
* specified. The field represents the annotated method or field
* within the deserialized object. The node map is used to get
* the attributes used to describe the objects identity, or in
* the case of an existing object it contains an object reference.
*
* @param field the method or field in the deserialized object
* @param node this is the XML element attributes to read
* @param map this is the session map used for deserialization
*
* @return this returns an instance to insert into the object
*/
public Value getElement(Class field, NodeMap node, Map map) throws Exception {
ReadGraph graph = read.find(map);
if(graph != null) {
return graph.getElement(field, node);
}
return null;
}
/**
* This is used to set the reference in to the XML element that
* is to be written. This will either insert an object identity if
* the object has not previously been written, or, if the object
* has already been written in a previous element, this will write
* the reference to that object. This allows all cycles within the
* graph to be serialized so that they can be fully deserialized.
*
* @param field the type of the field or method in the object
* @param value this is the actual object that is to be written
* @param node this is the XML element attribute map to use
* @param map this is the session map used for the serialization
*
* @return returns true if the object has been fully serialized
*/
public boolean setRoot(Class field, Object value, NodeMap node, Map map){
return setElement(field, value, node, map);
}
/**
* This is used to set the reference in to the XML element that
* is to be written. This will either insert an object identity if
* the object has not previously been written, or, if the object
* has already been written in a previous element, this will write
* the reference to that object. This allows all cycles within the
* graph to be serialized so that they can be fully deserialized.
*
* @param field the type of the field or method in the object
* @param value this is the actual object that is to be written
* @param node this is the XML element attribute map to use
* @param map this is the session map used for the serialization
*
* @return returns true if the object has been fully serialized
*/
public boolean setElement(Class field, Object value, NodeMap node, Map map){
WriteGraph graph = write.find(map);
if(graph != null) {
return graph.setElement(field, value, node);
}
return false;
}
}
