解决XStream对于Null值toXML时不输出标签的问题

说明：XStream在toXML的时候对于Null值不进行标签输出，如下转换器修改了此内容。

main方法中有使用例子。

import java.lang.reflect.Method;

import java.util.ArrayList;

import java.util.HashMap;

import java.util.List;

import java.util.Map;

import tellhow.ner.vo.BrakeVo;

import tellhow.ner.vo.TransferComponentVo;

import tellhow.ner.vo.Unit;

import com.thoughtworks.xstream.XStream;

import com.thoughtworks.xstream.converters.Converter;

import com.thoughtworks.xstream.converters.MarshallingContext;

import com.thoughtworks.xstream.converters.UnmarshallingContext;

import com.thoughtworks.xstream.io.HierarchicalStreamReader;

import com.thoughtworks.xstream.io.HierarchicalStreamWriter;

/**

* 自定义XStream转化器，

* 使null值标签可以输出到XML

*/

public class NullConverter implements Converter

{

private Map<Class<?>, List<String>> attributes = null;

public void regAttribute(Class<?> type, String attribute)

{

if (null == attributes)

{

attributes = new HashMap<Class<?>, List<String>>();

}

List value = attributes.get(type);

if (null == value)

{

value = new ArrayList<String>();

attributes.put(type, value);

}

value.add(attribute);

}

/**

* 是否是属性（是属性的不用以单独标签实现）

* @param type

* @param attribute

* @return

*/

private boolean isClassAttribute(Class<?> type, String attribute)

{

List<String> value = getAttributes(type);

if (null == value)

return false;

if (value.contains(attribute))

{

return true;

}

return false;

}

/**

* 获取注册的属性

* @param type

* @return

*/

private List<String> getAttributes(Class<?> type)

{

if (null != attributes)

{

return attributes.get(type);

}

return null;

}

/**

* 输出对象的属性标签

* @param source

* @param writer

*/

private void writerAttribute(Object source, HierarchicalStreamWriter writer)

{

Class cType = source.getClass();

List<String> value = getAttributes(cType);

if ((null != value) && (value.size() > 0))

{

Method[] methods = cType.getMethods();

for (Method method : methods)

{

String methodName = method.getName();

if (methodName.indexOf("get") != -1 && methodName != "getClass")

{

String name = methodName.substring(3);

name = name.toLowerCase();

if (value.contains(name))

{

Object o = null;

try {

o = method.invoke(source, null);

} catch (Exception e) {

e.printStackTrace();

}

writer.addAttribute(name, o==null?"":o.toString());

}

}

}

}

}

public void marshal(Object source, HierarchicalStreamWriter writer,

MarshallingContext context)

{

if (null == source)

return;

Class cType = source.getClass();

Method[] methods = cType.getMethods();

writerAttribute(source, writer);

for(Method m : methods)

{

String methodName = m.getName();

if (methodName.indexOf("get") != -1 && methodName != "getClass")

{

if (source instanceof List)

{

List list = (List)source;

for (Object obj: list)

{

String name = obj.getClass().toString();

name = name.substring(name.lastIndexOf(".") + 1);

writer.startNode(name);

marshal(obj, writer, context);

writer.endNode();

}

}

else

{

boolean isBaseType = isBaseType(m.getReturnType());

String name = methodName.substring(3);

if (isBaseType)

{

name = name.toLowerCase();

}

Object o = null;

try

{

o = m.invoke(source, null);

} catch (Exception e) {

e.printStackTrace();

}

//如果是基本类型调用toString，否则递归

if (isBaseType)

{

if (!isClassAttribute(cType, name))

{

writer.startNode(name);

writer.setValue(o==null?"":o.toString());

writer.endNode();

}

}

else

{

writer.startNode(name);

marshal(o, writer, context);

writer.endNode();

}

}

}

}

}

public Object unmarshal(HierarchicalStreamReader reader,

UnmarshallingContext context) {

return null;

}

public boolean canConvert(Class type) {

return true;

}

private boolean isBaseType(Class<?> type)

{

if (type.equals(Integer.class)

|| type.equals(Double.class)

|| type.equals(String.class)

|| type.equals(Boolean.class)

|| type.equals(Long.class)

||type.equals(Short.class)

||type.equals(Byte.class)

||type.equals(Float.class))

{

return true;

}

return false;

}

public static void main(String[] args)

{

XStream xs= ExtHelper.initXStream();

NullConverter v = new NullConverter();

v.regAttribute(Unit.class, "unittype");

xs.registerConverter(v);

Unit unit = new Unit();

unit.setUnitcomment("aaaaaaaa1");

List<TransferComponentVo> transfer = new ArrayList<TransferComponentVo>();

transfer.add(new TransferComponentVo());

unit.setTransferComponentVoList(transfer);

BrakeVo brake = new BrakeVo();

brake.setAsetid("1111111111111100000000001");

unit.setBrakeVo(brake);

String xml = xs.toXML(unit);

System.out.println(xml);

}