JsonMapper 把空值，空数组转为 空字符串和空数组

import java.io.IOException;import java.util.List;import java.util.Map;import java.util.TimeZone;import org.apache.commons.lang3.StringEscapeUtils;import org.apache.commons.lang3.StringUtils;import org.slf4j.Logger;import org.slf4j.LoggerFactory;import com.fasterxml.jackson.annotation.JsonInclude.Include;import com.fasterxml.jackson.core.JsonGenerator;import com.fasterxml.jackson.core.JsonProcessingException;import com.fasterxml.jackson.core.JsonParser.Feature;import com.fasterxml.jackson.databind.DeserializationFeature;import com.fasterxml.jackson.databind.JavaType;import com.fasterxml.jackson.databind.JsonSerializer;import com.fasterxml.jackson.databind.ObjectMapper;import com.fasterxml.jackson.databind.SerializationFeature;import com.fasterxml.jackson.databind.SerializerProvider;import com.fasterxml.jackson.databind.module.SimpleModule;import com.fasterxml.jackson.databind.util.JSONPObject;import com.fasterxml.jackson.module.jaxb.JaxbAnnotationModule;import com.google.common.collect.Lists;import com.google.common.collect.Maps;/** * 简单封装Jackson，实现JSON String<->Java Object的Mapper. * 封装不同的输出风格, 使用不同的builder函数创建实例. * @author ThinkGem * @version 2013-11-15 */public class JsonMapper extends ObjectMapper {private static final long serialVersionUID = 1L;private static Logger logger = LoggerFactory.getLogger(JsonMapper.class);private static JsonMapper mapper;public JsonMapper() {this(Include.NON_EMPTY);}public JsonMapper(Include include) {// 设置输出时包含属性的风格if (include != null) {this.setSerializationInclusion(include);}// 允许单引号、允许不带引号的字段名称this.enableSimple();// 设置输入时忽略在JSON字符串中存在但Java对象实际没有的属性this.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);// 为mapper注册一个带有SerializerModifier的Factory，此modifier主要做的事情为：当序列化类型为array，list、set时，当值为空时，序列化成[]        this.setSerializerFactory(this.getSerializerFactory().withSerializerModifier(new SerializerModifier()));                // 空值处理为空串this.getSerializerProvider().setNullValueSerializer(new JsonSerializer<Object>(){@Overridepublic void serialize(Object value, JsonGenerator jgen,SerializerProvider provider) throws IOException,JsonProcessingException {jgen.writeString("");}        });// 进行HTML解码。this.registerModule(new SimpleModule().addSerializer(String.class, new JsonSerializer<String>(){@Overridepublic void serialize(String value, JsonGenerator jgen,SerializerProvider provider) throws IOException,JsonProcessingException {jgen.writeString(StringEscapeUtils.unescapeHtml4(value));}        }));// 设置时区this.setTimeZone(TimeZone.getDefault());//getTimeZone("GMT+8:00")}/** * 创建只输出非Null且非Empty(如List.isEmpty)的属性到Json字符串的Mapper,建议在外部接口中使用. */public static JsonMapper getInstance() {if (mapper == null){mapper = new JsonMapper(Include.ALWAYS).enableSimple();}return mapper;}/** * 创建只输出初始值被改变的属性到Json字符串的Mapper, 最节约的存储方式，建议在内部接口中使用。 */public static JsonMapper nonDefaultMapper() {if (mapper == null){mapper = new JsonMapper(Include.NON_DEFAULT);}return mapper;}/** * Object可以是POJO，也可以是Collection或数组。 * 如果对象为Null, 返回"null". * 如果集合为空集合, 返回"[]". */public String toJson(Object object) {try {return this.writeValueAsString(object);} catch (IOException e) {logger.warn("write to json string error:" + object, e);return null;}}/** * 反序列化POJO或简单Collection如List<String>. *  * 如果JSON字符串为Null或"null"字符串, 返回Null. * 如果JSON字符串为"[]", 返回空集合. *  * 如需反序列化复杂Collection如List<MyBean>, 请使用fromJson(String,JavaType) * @see #fromJson(String, JavaType) */public <T> T fromJson(String jsonString, Class<T> clazz) {if (StringUtils.isEmpty(jsonString)) {return null;}try {return this.readValue(jsonString, clazz);} catch (IOException e) {logger.warn("parse json string error:" + jsonString, e);return null;}}/** * 反序列化复杂Collection如List<Bean>, 先使用函數createCollectionType构造类型,然后调用本函数. * @see #createCollectionType(Class, Class...) */@SuppressWarnings("unchecked")public <T> T fromJson(String jsonString, JavaType javaType) {if (StringUtils.isEmpty(jsonString)) {return null;}try {return (T) this.readValue(jsonString, javaType);} catch (IOException e) {logger.warn("parse json string error:" + jsonString, e);return null;}}/** * 構造泛型的Collection Type如: * ArrayList<MyBean>, 则调用constructCollectionType(ArrayList.class,MyBean.class) * HashMap<String,MyBean>, 则调用(HashMap.class,String.class, MyBean.class) */public JavaType createCollectionType(Class<?> collectionClass, Class<?>... elementClasses) {return this.getTypeFactory().constructParametricType(collectionClass, elementClasses);}/** * 當JSON裡只含有Bean的部分屬性時，更新一個已存在Bean，只覆蓋該部分的屬性. */@SuppressWarnings("unchecked")public <T> T update(String jsonString, T object) {try {return (T) this.readerForUpdating(object).readValue(jsonString);} catch (JsonProcessingException e) {logger.warn("update json string:" + jsonString + " to object:" + object + " error.", e);} catch (IOException e) {logger.warn("update json string:" + jsonString + " to object:" + object + " error.", e);}return null;}/** * 輸出JSONP格式數據. */public String toJsonP(String functionName, Object object) {return toJson(new JSONPObject(functionName, object));}/** * 設定是否使用Enum的toString函數來讀寫Enum, * 為False時時使用Enum的name()函數來讀寫Enum, 默認為False. * 注意本函數一定要在Mapper創建後, 所有的讀寫動作之前調用. */public JsonMapper enableEnumUseToString() {this.enable(SerializationFeature.WRITE_ENUMS_USING_TO_STRING);this.enable(DeserializationFeature.READ_ENUMS_USING_TO_STRING);return this;}/** * 支持使用Jaxb的Annotation，使得POJO上的annotation不用与Jackson耦合。 * 默认会先查找jaxb的annotation，如果找不到再找jackson的。 */public JsonMapper enableJaxbAnnotation() {JaxbAnnotationModule module = new JaxbAnnotationModule();this.registerModule(module);return this;}/** * 允许单引号 * 允许不带引号的字段名称 */public JsonMapper enableSimple() {this.configure(Feature.ALLOW_SINGLE_QUOTES, true);this.configure(Feature.ALLOW_UNQUOTED_FIELD_NAMES, true);return this;}/** * 取出Mapper做进一步的设置或使用其他序列化API. */public ObjectMapper getMapper() {return this;}/** * 对象转换为JSON字符串 * @param object * @return */public static String toJsonString(Object object){return JsonMapper.getInstance().toJson(object);}/** * JSON字符串转换为对象 * @param jsonString * @param clazz * @return */public static Object fromJsonString(String jsonString, Class<?> clazz){return JsonMapper.getInstance().fromJson(jsonString, clazz);}/** * 测试 */public static void main(String[] args) {List<Map<String, Object>> list = Lists.newArrayList();Map<String, Object> map = Maps.newHashMap();map.put("id", 1);map.put("pId", -1);map.put("name", "根节点");list.add(map);map = Maps.newHashMap();map.put("id", 2);map.put("pId", 1);map.put("name", "你好");map.put("open", true);list.add(map);String json = JsonMapper.getInstance().toJson(list);System.out.println(json);}}

import java.util.List;import java.util.Set;import com.fasterxml.jackson.databind.BeanDescription;import com.fasterxml.jackson.databind.JsonSerializer;import com.fasterxml.jackson.databind.SerializationConfig;import com.fasterxml.jackson.databind.ser.BeanPropertyWriter;import com.fasterxml.jackson.databind.ser.BeanSerializerModifier;public class SerializerModifier extends BeanSerializerModifier {    private JsonSerializer<Object> _nullArrayJsonSerializer = new ArrayJsonSerializer();    @Override    public List<BeanPropertyWriter> changeProperties(SerializationConfig config, BeanDescription beanDesc,            List<BeanPropertyWriter> beanProperties) {        // 循环所有的beanPropertyWriter        for (int i = 0; i < beanProperties.size(); i++) {            BeanPropertyWriter writer = beanProperties.get(i);            // 判断字段的类型，如果是array，list，set则注册nullSerializer            if (isArrayType(writer)) {                 //给writer注册一个自己的nullSerializer                writer.assignNullSerializer(this.defaultNullArrayJsonSerializer());            }        }        return beanProperties;    }    // 判断是什么类型    protected boolean isArrayType(BeanPropertyWriter writer) {        Class<?> clazz = writer.getPropertyType();        return clazz.isArray() || clazz.equals(List.class) || clazz.equals(Set.class);    }    protected JsonSerializer<Object> defaultNullArrayJsonSerializer() {        return _nullArrayJsonSerializer;    }}

import java.io.IOException;import com.fasterxml.jackson.core.JsonGenerator;import com.fasterxml.jackson.core.JsonProcessingException;import com.fasterxml.jackson.databind.JsonSerializer;import com.fasterxml.jackson.databind.SerializerProvider;public class ArrayJsonSerializer extends JsonSerializer<Object> {    @Override    public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonProcessingException {        if (value == null) {            jgen.writeStartArray();            jgen.writeEndArray();        } else {            jgen.writeObject(value);        }    }}