http://www.blogjava.net/bulktree/archive/2008/08/10/221122.html
dom4j遍历xml文档树有种很特别的方式就是访问者(Visitor)模式,初次接触Visitor模式，写出个人理解大家交流！
Visitor访问者模式定义：作用于某个对象树中各个对象的操作. 它可以使你在不改变这些对象树本身的情况下,定义作用于这些对象树各个节点的新操作。
先看以下代码：Person为简单的vo类
package org.bulktree.visitor;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.List;
/**
*
* @author bulktree Email: laoshulin@gmail.com
* @date Aug 10, 2008
*/
public class ReadCollection {
private Collection c = null;
ReadCollection() {
/*
* 准备数据-String对象-Person对象-Integer对象-List对象
*/
String str = "bulktree.laoshulin";
Person person = new Person("bulktree", "22", "M");
Integer a = new Integer(99);
/*
* 使用范型
*/
List<String> list = new ArrayList<String>();
list.add("BULKTREE");
list.add("LAOSHULIN");
list.add("OAKERTREE");
c = new ArrayList();
c.add(str);
c.add(person);
c.add(a);
c.add(list);
}
/**
* 遍历Collection中的每一个对象并打印
*/
public void testCollection() {
Iterator iter = getCollection().iterator();
while (iter.hasNext()) {
Object o = iter.next();
if (o instanceof String) {
System.out.println("String--> " + o.toString());
} else if (o instanceof Person) {
readPerson((Person) o);
} else if (o instanceof Integer) {
Integer inta = (Integer) o;
System.out.println(inta.intValue());
} else if (o instanceof List) {
readList((List) o);
}
}
}
public Collection getCollection() {
return c;
}
private void readPerson(Person person) {
System.out.println("person-name-> " + person.getName());
System.out.println("person-age-> " + person.getAge());
System.out.println("person-sex-> " + person.getSex());
}
private void readList(List<String> list) {
/*
* 增强的for循环
*/
for (String s : list) {
System.out.println(s);
}
}
public static void main(String[] args) {
new ReadCollection().testCollection();
}
}
我们使用了 instanceof来判断 Object对象 o 的类型，这样做的缺点是代码中If/else if 很繁琐，而JDK中的范型又限制了只能使用相同的类型，这时Vistor访问模式派上用场了。
当我们要访问Collection的每一个Element(被访问者)时，定义一个accept操作使其具有可被访问性，我们定义一个Visiable接口，使Collection的每一个Element继承这个接口，实现自身的访问操作
package org.bulktree.visitor;
/**
* 可访问性--接收一个访问者
* @author bulktree Email: laoshulin@gmail.com
* @date Aug 10, 2008
*/
public interface Visitable {
public void accept(Visitor visitor);
}
下来是四个被访问的类型String,Integer,Person,Collection的实现类
package org.bulktree.visitor;
/**
* 被访问者--String对象
* @author bulktree Email: laoshulin@gmail.com
* @date Aug 10, 2008
*/
public class StringElement implements Visitable {
private String str;
public StringElement(String str) {
this.str = str;
}
public String getStr() {
return str;
}
public void accept(Visitor visitor) {
visitor.visitString(this);
}
}
package org.bulktree.visitor;
/**
* 被访问者--Integer对象
* @author bulktree Email: laoshulin@gmail.com
* @date Aug 10, 2008
*/
public class IntegerElement implements Visitable {
private Integer i;
public IntegerElement(Integer i) {
this.i = i;
}
public Integer getI() {
return i;
}
public void accept(Visitor visitor) {
visitor.visitInteger(this);
}
}
package org.bulktree.visitor;
import java.util.Collection;
/**
* 被访问者--Person对象
* @author bulktree Email: laoshulin@gmail.com
* @date Aug 10, 2008
*/
public class PersonElement implements Visitable{
private Person p;
public PersonElement(Person p) {
this.p = p;
}
public Person getP() {
return p;
}
public void accept(Visitor visitor) {
visitor.visitPerson(this);
}
}
package org.bulktree.visitor;
import java.util.Collection;
import java.util.List;
/**
* 被访问者--Collection对象
* @author bulktree Email: laoshulin@gmail.com
* @date Aug 10, 2008
*/
public class CollectionElement implements Visitable {
private Collection collection;
public CollectionElement(Collection collection) {
this.collection = collection;
}
public Collection getCollection() {
return collection;
}
public void accept(Visitor visitor) {
visitor.visitCollection(collection);
}
}
下来定义一个访问者Visitor接口，它可以访问Integer,String,Person(VO对象),Collection类型
package org.bulktree.visitor;
import java.util.Collection;
/**
* 访问者接口
* @author bulktree Email: laoshulin@gmail.com
* @date Aug 10, 2008
*/
public interface Visitor {
public void visitString(StringElement str);
public void visitInteger(IntegerElement i);
public void visitCollection(Collection collection);
public void visitPerson(PersonElement perE);
}
关键的Visitor实现类
package org.bulktree.visitor;
import java.util.Collection;
import java.util.Iterator;
/**
* 访问者实现类
* @author bulktree Email: laoshulin@gmail.com
* @date Aug 10, 2008
*/
public class VisitorImpl implements Visitor {
/*
*访问字符串，仅对字符串输出
*/
public void visitString(StringElement str) {
System.out.println("*******************字符串输出*************************");
System.out.println(str.getStr());
}
/**
* 访问Integer类型
*/
public void visitInteger(IntegerElement i) {
System.out.println("*******************整型输出*************************");
System.out.println(i.getI());
}
/**
* 访问Collection对象，遍历每一个元素
* 使用了一个if语句判断属于Visitable哪一个被访问对象，然后调用相应的accept方法
* 实现递归调用
*/
public void visitCollection(Collection collection) {
Iterator iter = collection.iterator();
while (iter.hasNext()) {
Object o = iter.next();
if (o instanceof Visitable) {
((Visitable) o).accept(this);
}
}
}
/**
* 访问单个Person对象
*/
public void visitPerson(PersonElement perE) {
System.out.println("*******************Person对象输出*************************");
Person person = perE.getP();
System.out.println("person-name-> " + person.getName());
System.out.println("person-age-> " + person.getAge());
System.out.println("person-sex-> " + person.getSex());
}
}
客户端测试：
package org.bulktree.visitor;
import java.util.ArrayList;
import java.util.Collection;
/**
* Visitor模式客户端
* @author bulktree Email: laoshulin@gmail.com
* @date Aug 10, 2008
*/
public class VisitorMain {
public static void main(String[] args) {
Visitor visitor = new VisitorImpl();
/*
* 访问字符串
*/
System.out.println("======================访问字符串=========================");
StringElement stringE = new StringElement(
"bulktree.laoshulin.oakertree");
visitor.visitString(stringE);
/*
* 访问集合
*/
System.out.println("=======================访问集合========================");
Collection list = new ArrayList();
StringElement str1 = new StringElement("aaa");
StringElement str2 = new StringElement("bbb");
list.add(str1);
list.add(str2);
PersonElement perE1 = new PersonElement(new Person("LAOSHULIN", "22", "M"));
PersonElement perE2 = new PersonElement(new Person("BULKTREE", "21", "W"));
list.add(perE1);
list.add(perE2);
IntegerElement intE1 = new IntegerElement(new Integer(99));
IntegerElement intE2 = new IntegerElement(new Integer(100));
list.add(intE1);
list.add(intE2);
visitor.visitCollection(list);
/*
* 访问Person
*/
System.out.println("======================访问Person=========================");
Person p = new Person("BULKTREE", "22", "M");
PersonElement perE = new PersonElement(p);
visitor.visitPerson(perE);
/*
* 访问Integer
*/
System.out.println("=====================访问Integer==========================");
IntegerElement intE = new IntegerElement(new Integer(77));
visitor.visitInteger(intE);
}
}
使用访问者模式的前提是对象群结构中(Collection) 中的对象类型很少改变，在两个接口Visitor(访问)和Visitable（可访问）中,确保Visitable很少变化,也就是说，确保不能有新的元素类型加进来，可以变化的是访问者行为或操作，也就是Visitor的不同子类可以有多种,这样使用访问者模式最方便,当系统中存在着固定的数据结构,且有着不同的行为，访问者模式也许是个不错的选择
package org.bulktree.xml;
import java.io.File;
import org.dom4j.Attribute;
import org.dom4j.Document;
import org.dom4j.DocumentException;
import org.dom4j.Element;
import org.dom4j.VisitorSupport;
import org.dom4j.io.SAXReader;
/**
* dom4j访问者模式解析xml文档
* @author bulktree Email: laoshulin@gmail.com
* @date Aug 10, 2008
*/
public class ReadXmlVisitor {
ReadXmlVisitor() {
File file = new File("student.xml");
SAXReader saxReader = new SAXReader();
try {
Document doc = saxReader.read(file);
doc.accept(new MyVisitor());
} catch (DocumentException e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
new ReadXmlVisitor();
}
}
/*
* org.dom4j 包里有Visitor接口，VisitorSupport是它的实现类，定义了多个重载的visit方法
*/
class MyVisitor extends VisitorSupport {
public void visit(Attribute attr) {
String name = attr.getName();
String value = attr.getValue();
System.out.println("Attribute--> " + name + " : " + value);
}
public void visit(Element element) {
String name = element.getName();
if (element.isTextOnly()) {
System.out
.println("Element--> " + name + " : " + element.getText());
} else {
System.out.println("Element-->" + name);
}
}
}
