Gson 问题汇总

使用Gson

gson是google推出的json解析框架,相较于其他json解析框架,它速度更快也更安全(网上有很多资料,这里就不赘述了),在maven项目中使用gson,只需要引入以下配置即可,这里使用的2.8.5版本

 <dependency> <groupId>com.google.code.gson</groupId><artifactId>gson</artifactId><version>2.8.5</version></dependency>

springboot中指定Gson为默认消息转换器

在springBoot中注入定义的HttpMessageConverter 即可(若使用的其他json框架,注入相应的HttpMessageConverter),例如:

@Configuration
public class WebConfig implements WebMvcConfigurer {@Beanpublic HttpMessageConverter GsonHttpMessageConverter() {GsonHttpMessageConverter gsonHttpMessageConverter = new GsonHttpMessageConverter();gsonHttpMessageConverter.setGson(new Gson());//放入Gson实例return gsonHttpMessageConverter;}}

自定义Gson实例

在Gson使用过程中,可能遇到多种情况,例如:

• 需要设定日期类型的格式
• 处理特殊格式数据
• 在解析serialVersionUID字段时发生 class declares multiple JSON fields named serialVersionUid
• json字符串中的int类型,转换后变成了double类型

解决方法

设定日期类型的格式

第一种方式:
使用JsonDeserializer来指定反序列化(解析)方式,与此对应的还有JsonSerializer接口,用来指定对象序列化方式:

//设定Date类型的反序列化(解析)规则private static final com.google.gson.JsonDeserializer<Date> deser = new com.google.gson.JsonDeserializer<Date>() {@Overridepublic Date deserialize(JsonElement json, Type typeOfT,JsonDeserializationContext context) throws JsonParseException {String date = json.getAsString();SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss");format.setTimeZone(TimeZone.getTimeZone("GMT"));try {return format.parse(date);} catch (ParseException exp) {System.err.println(exp.getMessage());return null;}}};private final static Gson INSTANCE = new GsonBuilder().serializeNulls().registerTypeAdapter(Date.class, deser).create();//制定Date类型的转换器

第二种方式:

   private static final String DATE_FORMAT = "yyyy-MM-dd HH:mm:ss";private final static Gson INSTANCE = new GsonBuilder().serializeNulls().setDateFormat(DATE_FORMAT).create();//此规则 将用于序列化与反序列化Date对象,如果多次指定此规则,将以最后一次为准

第二种方式=JsonDeserializer+JsonSerializer
如果项目中已经约定好使用统一的日期格式,使用第二种方式更简便易读

处理特殊格式数据

在某场景中,原本是double类型的数据,在上百条数据中,有几条数据因为数据为空,我接收到是"-“,一转换就报类型无法转换错误(因为是爬取数据),我只能自己想办法把”-"转为——(double)0

    @Testvoid contextLoads() {String dataJson = "{\"number\" : 30.2, \"amount\" : \"-\"}";Gson gson = GsonUtils.getSingleton();Map map = gson.fromJson(dataJson, Map.class);System.out.println(map);}

解决:

        public static final TypeAdapter<Number> NumberFormat = new TypeAdapter<Number>() {@Overridepublic Number read(JsonReader in) throws IOException {if (in.peek() == JsonToken.NULL) {in.nextNull();return 0;}try {double i = in.nextDouble();return  i;} catch (NumberFormatException e) {//如果报数字格式化异常,判断是否为目标字符串if (e.getMessage().contains("-")){Class<JsonReader> jsonReaderClass = JsonReader.class;try {Field peekedString = jsonReaderClass.getDeclaredField("peekedString");peekedString.setAccessible(true);peekedString.set(in,"0.0");//通过反射更改了json字符串中原"-"为0.0double i = in.nextDouble();//更改值后重新计算return  i;} catch (NoSuchFieldException ex) {ex.printStackTrace();} catch (IllegalAccessException ex) {ex.printStackTrace();}return  0.0;}else {throw new JsonSyntaxException(e);}}}@Overridepublic void write(JsonWriter out, Number value) throws IOException {out.value(value);}};private final static Gson INSTANCE = new GsonBuilder().serializeNulls().disableHtmlEscaping()//默认情况下，Gson会转义HTML字符，例如<>等。使用此选项将 Gson配置为直接传递HTML字符.registerTypeAdapter(Date.class, deser).registerTypeAdapter(Double.class, NumberFormat).registerTypeAdapter(double.class, NumberFormat).create();

由于registerTypeAdapter只注册指定的类型,不会注册相关类型,所以注册Double时,也需要注册double类型,贴出registerTypeAdapter方法源码以及注释:

 /*** Configures Gson for custom serialization or deserialization. This method combines the* registration of an {@link TypeAdapter}, {@link InstanceCreator}, {@link JsonSerializer}, and a* {@link JsonDeserializer}. It is best used when a single object {@code typeAdapter} implements* all the required interfaces for custom serialization with Gson. If a type adapter was* previously registered for the specified {@code type}, it is overwritten.** <p>This registers the type specified and no other types: you must manually register related* types! For example, applications registering {@code boolean.class} should also register {@code* Boolean.class}.** @param type the type definition for the type adapter being registered* @param typeAdapter This object must implement at least one of the {@link TypeAdapter},* {@link InstanceCreator}, {@link JsonSerializer}, and a {@link JsonDeserializer} interfaces.* @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern*/@SuppressWarnings({"unchecked", "rawtypes"})public GsonBuilder registerTypeAdapter(Type type, Object typeAdapter) {$Gson$Preconditions.checkArgument(typeAdapter instanceof JsonSerializer<?>|| typeAdapter instanceof JsonDeserializer<?>|| typeAdapter instanceof InstanceCreator<?>|| typeAdapter instanceof TypeAdapter<?>);if (typeAdapter instanceof InstanceCreator<?>) {instanceCreators.put(type, (InstanceCreator) typeAdapter);}if (typeAdapter instanceof JsonSerializer<?> || typeAdapter instanceof JsonDeserializer<?>) {TypeToken<?> typeToken = TypeToken.get(type);factories.add(TreeTypeAdapter.newFactoryWithMatchRawType(typeToken, typeAdapter));}if (typeAdapter instanceof TypeAdapter<?>) {factories.add(TypeAdapters.newFactory(TypeToken.get(type), (TypeAdapter)typeAdapter));}return this;}

大家不一定会遇到一摸一样的场景,这里只是提供一个思路

解析serialVersionUID字段报错

在项目中,出现:class declares multiple JSON fields named serialVersionUid异常
解决:

  private static final List<String> EXCLUDE = new ArrayList<String>() {{add("serialVersionUID");}};private final static Gson INSTANCE = new GsonBuilder().serializeNulls().setExclusionStrategies(new ExclusionStrategy() {@Overridepublic boolean shouldSkipField(FieldAttributes f) {boolean exclude = false;try {exclude = EXCLUDE.contains(f.getName());} catch (Exception ignore) {}return exclude;}@Overridepublic boolean shouldSkipClass(Class<?> aClass) {return false;}})//制定排除策略.disableHtmlEscaping()//默认情况下，Gson会转义HTML字符，例如<>等。使用此选项将 Gson配置为直接传递HTML字符.registerTypeAdapter(Date.class, deser).registerTypeAdapter(Double.class, NumberFormat).registerTypeAdapter(double.class, NumberFormat).create();

int类型,转换后变成了double类型

参考
Error: Class declares multiple JSON fields named serialVersionUid