使用JAXB映射HashMap
JAXB是很强大的XML <--> Java Class映射工具。很可惜它默认不支持对Hashmap的映射。但我们可以通过使用XmlJavaTypeAdapter来扩展实现，本文介绍详细方法。

首先创建一个带有HashMap的Class：

package net.bluedash;import java.util.HashMap;import javax.xml.bind.annotation.XmlAccessType;import javax.xml.bind.annotation.XmlAccessorType;import javax.xml.bind.annotation.XmlRootElement;import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;@XmlRootElement@XmlAccessorType(XmlAccessType.FIELD)public class Foo {@XmlJavaTypeAdapter(MyHashMapAdapter.class)HashMap hashmap;public Foo() {hashmap = new HashMap();}public HashMap getHashmap() {return hashmap;}public void setHashmap(HashMap hashmap) {this.hashmap = hashmap;}}

我们使用自己的MyHashMapAdapter.class来映射HashMap：

package net.bluedash;import java.util.HashMap;import java.util.List;import java.util.Set;import java.util.Map.Entry;import javax.xml.bind.annotation.adapters.XmlAdapter;public final class MyHashMapAdapter extends XmlAdapter<MyHashMapType, HashMap> {@Overridepublic MyHashMapType marshal(HashMap arg0) throws Exception {MyHashMapType myHashMapType = new MyHashMapType();for (Entry entry : (Set<Entry>) arg0.entrySet()) {MyHashEntryType myHashEntryType = new MyHashEntryType();myHashEntryType.key = (Integer) entry.getKey();myHashEntryType.value = (String) entry.getValue();myHashMapType.entries.add(myHashEntryType);// myHashMapType = myHashEntryType;}return myHashMapType;}@Overridepublic HashMap unmarshal(MyHashMapType arg0) throws Exception {HashMap hashMap = new HashMap();for (MyHashEntryType myHashEntryType : (List<MyHashEntryType>) arg0.entries) {hashMap.put(myHashEntryType.key, myHashEntryType.value);}return hashMap;}}

里面使用了自定义的MyHashMapType：

package net.bluedash;import java.util.ArrayList;import java.util.List;import javax.xml.bind.annotation.XmlElement;import javax.xml.bind.annotation.XmlRootElement;@XmlRootElementpublic class MyHashMapType {    @XmlElement(required = true)public List<MyHashEntryType> entries = new ArrayList<MyHashEntryType>();}

使用MyHashEntryType来实现对HashMap每一条数据的映射：

package net.bluedash;import javax.xml.bind.annotation.XmlAccessType;import javax.xml.bind.annotation.XmlAccessorType;import javax.xml.bind.annotation.XmlAttribute;import javax.xml.bind.annotation.XmlType;import javax.xml.bind.annotation.XmlValue;@XmlAccessorType(XmlAccessType.FIELD)@XmlTypepublic class MyHashEntryType {@XmlAttributepublic Integer key;@XmlValuepublic String value;}

下面做一个Demo来使用一下我们的Adapter：

package net.bluedash;import java.io.File;import javax.xml.bind.JAXBContext;import javax.xml.bind.JAXBException;import javax.xml.bind.Marshaller;import javax.xml.bind.Unmarshaller;public class App {public static void main(String[] args) throws JAXBException {JAXBContext jaxbContext = JAXBContext.newInstance(Foo.class);Foo foo = new Foo();foo.getHashmap().put(1, "One");foo.getHashmap().put(2, "Two");foo.getHashmap().put(3, "Three");Marshaller marshaller = jaxbContext.createMarshaller();marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);// Output the generated XML:marshaller.marshal(foo, System.out);// Save the output to a foo.xmlFile xmlFile = new File("foo.xml");marshaller.marshal(foo, xmlFile);// Restore the Foo class from xml fileUnmarshaller unmarshaller = jaxbContext.createUnmarshaller();Foo createdFoo = (Foo) unmarshaller.unmarshal(xmlFile);// See the resultSystem.out.println(createdFoo.hashmap);}}

代码运行结果如下：

<?xml version="1.0" encoding="UTF-8" standalone="yes"?><foo>    <hashmap>        <entries key="1">One</entries>        <entries key="2">Two</entries>        <entries key="3">Three</entries>    </hashmap></foo>{1=One, 2=Two, 3=Three}

我将上面的例子放在了github上面，可以取下来动手玩玩看：

git clone git://github.com/liweinan/jaxb-hashmap.git

下载完成后，首先需要编译代码：

mvn install

然后执行下述Maven命令运行：

mvn exec:java -Dexec.mainClass="net.bluedash.jaxb.App"