通過注冊這個NullConverter（需要將這個轉換器的優先級調到最低）輸出Null值對應的標簽:

public static void main(String[] args) {Student stu = new Student();stu.setAge(1);stu.setName(null);XStream xStream = new XStream();// 使用注解xStream.autodetectAnnotations(true);// 不輸出class信息xStream.aliasSystemAttribute(null, "class");NullConverter nullConverter = new NullConverter(xStream.getMapper(), new SunUnsafeReflectionProvider());// 將轉換器注冊到非常低的位置非常重要xStream.registerConverter(nullConverter,XStream.PRIORITY_VERY_LOW);System.out.println(xStream.toXML(stu)); }

NullConverter全部的代碼：

import com.thoughtworks.xstream.converters.ConversionException; import com.thoughtworks.xstream.converters.MarshallingContext; import com.thoughtworks.xstream.converters.SingleValueConverter; import com.thoughtworks.xstream.converters.reflection.ReflectionConverter; import com.thoughtworks.xstream.converters.reflection.ReflectionProvider; import com.thoughtworks.xstream.core.ReferencingMarshallingContext; import com.thoughtworks.xstream.core.util.ArrayIterator; import com.thoughtworks.xstream.io.ExtendedHierarchicalStreamWriterHelper; import com.thoughtworks.xstream.io.HierarchicalStreamWriter; import com.thoughtworks.xstream.mapper.Mapper;import java.lang.reflect.Field; import java.util.*; /*** 在xstream中注冊這個converter，可以輸出值為null的標簽* 注意在使用的時候，記得將轉換器注冊到非常低的位置：xStream.registerConverter(nullConverter, XStream.PRIORITY_VERY_LOW);* @author : yuxia* @date : 2020/3/19*/ public class NullConverter extends ReflectionConverter {public NullConverter(Mapper mapper, ReflectionProvider reflectionProvider) {super(mapper, reflectionProvider);}public NullConverter(Mapper mapper, ReflectionProvider reflectionProvider, Class type) {super(mapper, reflectionProvider, type);}@Overrideprotected void doMarshal(final Object source, final HierarchicalStreamWriter writer,final MarshallingContext context) {final List fields = new ArrayList();final Map defaultFieldDefinition = new HashMap();final Class sourceType = source.getClass();// Attributes might be preferred to child elements ...reflectionProvider.visitSerializableFields(source, new ReflectionProvider.Visitor() {final Set writtenAttributes = new HashSet();@Overridepublic void visit(String fieldName, Class type, Class definedIn, Object value) {if (!mapper.shouldSerializeMember(definedIn, fieldName)) {return;}if (!defaultFieldDefinition.containsKey(fieldName)) {Class lookupType = source.getClass();// See XSTR-457 and OmitFieldsTestif (definedIn != sourceType&& !mapper.shouldSerializeMember(lookupType, fieldName)) {lookupType = definedIn;}defaultFieldDefinition.put(fieldName, reflectionProvider.getField(lookupType, fieldName));}SingleValueConverter converter = mapper.getConverterFromItemType(fieldName, type, definedIn);if (converter != null) {final String attribute = mapper.aliasForAttribute(mapper.serializedMember(definedIn, fieldName));if (value != null) {if (writtenAttributes.contains(fieldName)) {ConversionException exception =new ConversionException("Cannot write field as attribute for object, attribute name already in use");exception.add("field-name", fieldName);exception.add("object-type", sourceType.getName());throw exception;}final String str = converter.toString(value);if (str != null) {writer.addAttribute(attribute, str);}}writtenAttributes.add(fieldName);} else {fields.add(new FieldInfo(fieldName, type, definedIn, value));}}});new Object() {{final Map hiddenMappers = new HashMap();for (Iterator fieldIter = fields.iterator(); fieldIter.hasNext(); ) {FieldInfo info = (FieldInfo) fieldIter.next();if (info.value != null) {final Field defaultField = (Field) defaultFieldDefinition.get(info.fieldName);Mapper.ImplicitCollectionMapping mapping = mapper.getImplicitCollectionDefForFieldName(defaultField.getDeclaringClass() == info.definedIn ? sourceType : info.definedIn,info.fieldName);if (mapping != null) {Set mappings = (Set) hiddenMappers.get(info.fieldName);if (mappings == null) {mappings = new HashSet();mappings.add(mapping);hiddenMappers.put(info.fieldName, mappings);} else {if (!mappings.add(mapping)) {mapping = null;}}}if (mapping != null) {if (context instanceof ReferencingMarshallingContext) {if (info.value != Collections.EMPTY_LIST&& info.value != Collections.EMPTY_SET&& info.value != Collections.EMPTY_MAP) {ReferencingMarshallingContext refContext = (ReferencingMarshallingContext) context;refContext.registerImplicit(info.value);}}final boolean isCollection = info.value instanceof Collection;final boolean isMap = info.value instanceof Map;final boolean isEntry = isMap && mapping.getKeyFieldName() == null;final boolean isArray = info.value.getClass().isArray();for (Iterator iter = isArray? new ArrayIterator(info.value): isCollection ? ((Collection) info.value).iterator() : isEntry? ((Map) info.value).entrySet().iterator(): ((Map) info.value).values().iterator(); iter.hasNext(); ) {Object obj = iter.next();final String itemName;final Class itemType;if (obj == null) {itemType = Object.class;itemName = mapper.serializedClass(null);} else if (isEntry) {final String entryName = mapping.getItemFieldName() != null? mapping.getItemFieldName(): mapper.serializedClass(Map.Entry.class);Map.Entry entry = (Map.Entry) obj;ExtendedHierarchicalStreamWriterHelper.startNode(writer, entryName, entry.getClass());writeItem(entry.getKey(), context, writer);writeItem(entry.getValue(), context, writer);writer.endNode();continue;} else if (mapping.getItemFieldName() != null) {itemType = mapping.getItemType();itemName = mapping.getItemFieldName();} else {itemType = obj.getClass();itemName = mapper.serializedClass(itemType);}writeField(info.fieldName, itemName, itemType, info.definedIn, obj);}} else {writeField(info.fieldName, null, info.type, info.definedIn, info.value);}}else{// 處理null值的標簽也輸出writeField(info.fieldName,null,info.type,info.definedIn,"");}}}void writeField(String fieldName, String aliasName, Class fieldType,Class definedIn, Object newObj) {Class actualType = newObj != null ? newObj.getClass() : fieldType;ExtendedHierarchicalStreamWriterHelper.startNode(writer, aliasName != null? aliasName: mapper.serializedMember(sourceType, fieldName), actualType);if (newObj != null) {Class defaultType = mapper.defaultImplementationOf(fieldType);if (!actualType.equals(defaultType)) {String serializedClassName = mapper.serializedClass(actualType);if (!serializedClassName.equals(mapper.serializedClass(defaultType))) {String attributeName = mapper.aliasForSystemAttribute("class");if (attributeName != null) {writer.addAttribute(attributeName, serializedClassName);}}}final Field defaultField = (Field) defaultFieldDefinition.get(fieldName);if (defaultField.getDeclaringClass() != definedIn) {String attributeName = mapper.aliasForSystemAttribute("defined-in");if (attributeName != null) {writer.addAttribute(attributeName, mapper.serializedClass(definedIn));}}Field field = reflectionProvider.getField(definedIn, fieldName);marshallField(context, newObj, field);}writer.endNode();}void writeItem(Object item, MarshallingContext context,HierarchicalStreamWriter writer) {if (item == null) {String name = mapper.serializedClass(null);ExtendedHierarchicalStreamWriterHelper.startNode(writer, name, Mapper.Null.class);writer.endNode();} else {String name = mapper.serializedClass(item.getClass());ExtendedHierarchicalStreamWriterHelper.startNode(writer, name, item.getClass());context.convertAnother(item);writer.endNode();}}};}private static class FieldInfo extends FieldLocation {final Class type;final Object value;FieldInfo(final String fieldName, final Class type, final Class definedIn, final Object value) {super(fieldName, definedIn);this.type = type;this.value = value;}}private static class FieldLocation {final String fieldName;final Class definedIn;FieldLocation(final String fieldName, final Class definedIn) {this.fieldName = fieldName;this.definedIn = definedIn;}@Overridepublic int hashCode() {final int prime = 7;int result = 1;result = prime * result + (definedIn == null ? 0 : definedIn.getName().hashCode());result = prime * result + (fieldName == null ? 0 : fieldName.hashCode());return result;}@Overridepublic boolean equals(final Object obj) {if (this == obj) {return true;}if (obj == null) {return false;}if (getClass() != obj.getClass()) {return false;}final NullConverter.FieldLocation other = (NullConverter.FieldLocation) obj;if (definedIn != other.definedIn) {return false;}if (fieldName == null) {if (other.fieldName != null) {return false;}} else if (!fieldName.equals(other.fieldName)) {return false;}return true;}} }

總結

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