我知道如何在 Jackson 中使用自定义序列化程序（通过扩展JsonSerializer），但我希望默认序列化程序适用于所有字段，除了 1 个字段，我想使用自定义序列化程序覆盖它。
注释不是一个选项，因为我正在序列化一个生成的类（来自 Thrift）。
编写自定义杰克逊序列化程序时，如何仅指定要覆盖的某些字段？
更新：
这是我要序列化的类：
class Student {
int age;
String firstName;
String lastName;
double average;
int numSubjects
// .. more such properties ...
}
上面的类有很多属性，其中大部分使用原生类型。我只想覆盖自定义序列化程序中的一些属性，让 Jackson 像往常一样处理其余的。例如，我只想将“年龄”字段转换为自定义输出。
假设您的目标类是
public class Student {
int age;
String firstName;
String lastName;
double average;
int numSubjects;
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public double getAverage() {
return average;
}
public void setAverage(double average) {
this.average = average;
}
public int getNumSubjects() {
return numSubjects;
}
public void setNumSubjects(int numSubjects) {
this.numSubjects = numSubjects;
}
}
您需要编写一个自定义序列化程序，如下所示
public class MyCustomSerializer extends JsonSerializer<Student> {
@Override
public void serialize(Student value, JsonGenerator jgen,
SerializerProvider provider) throws IOException,
JsonProcessingException {
if (value != null) {
jgen.writeStartObject();
jgen.writeStringField("age", "Age: " + value.getAge()); //Here a custom way to render age field is used
jgen.writeStringField("firstName", value.getFirstName());
jgen.writeStringField("lastName", value.getLastName());
jgen.writeNumberField("average", value.getAverage());
jgen.writeNumberField("numSubjects", value.getNumSubjects());
//Write other properties
jgen.writeEndObject();
}
}
}
然后将其添加到 ObjectMapper
ObjectMapper mapper = new ObjectMapper();
SimpleModule module = new SimpleModule("custom",
Version.unknownVersion());
module.addSerializer(Student.class, new MyCustomSerializer());
mapper.registerModule(module);
然后像使用它
Student s = new Student();
s.setAge(2);
s.setAverage(3.4);
s.setFirstName("first");
s.setLastName("last");
s.setNumSubjects(3);
StringWriter sw = new StringWriter();
mapper.writeValue(sw, s);
System.out.println(sw.toString());
它会产生 ao/p 之类的
{"age":"Age: 2","firstName":"first","lastName":"last","average":3.4,"numSubjects":3}
仅仅因为你不能修改类并不意味着你不能使用注释：只需使用混合注释。例如，请参阅此博客条目（或使用“jackson mixin annotations”获取更多信息）以了解如何使用它。
我专门将 Jackson 与 protobuf 和 thrift 生成的类一起使用，它们工作得很好。对于较早的 Thrift 版本，我必须禁用“is-setters”的发现，Thrift 生成的方法以查看是否已明确设置特定属性，但除此之外一切正常。
我遇到了同样的问题，我用CustomSerializerFactory解决了它。
这种方法允许您忽略所有对象或特定类型的某些特定字段。
public class EntityCustomSerializationFactory extends CustomSerializerFactory {
//ignored fields
private static final Set<String> IGNORED_FIELDS = new HashSet<String>(
Arrays.asList(
"class",
"value",
"some"
)
);
public EntityCustomSerializationFactory() {
super();
}
public EntityCustomSerializationFactory(Config config) {
super(config);
}
@Override
protected void processViews(SerializationConfig config, BeanSerializerBuilder builder) {
super.processViews(config, builder);
//ignore fields only for concrete class
//note, that you can avoid or change this check
if (builder.getBeanDescription().getBeanClass().equals(Entity.class)){
//get original writer
List<BeanPropertyWriter> originalWriters = builder.getProperties();
//create actual writers
List<BeanPropertyWriter> writers = new ArrayList<BeanPropertyWriter>();
for (BeanPropertyWriter writer: originalWriters){
String propName = writer.getName();
//if it isn't ignored field, add to actual writers list
if (!IGNORED_FIELDS.contains(propName)){
writers.add(writer);
}
}
builder.setProperties(writers);
}
}
}
之后你可以使用它，如下所示：
objectMapper.setSerializerFactory(new EntityCustomSerializationFactory());
objectMapper.writeValueAsString(new Entity());//response will be without ignored fields
如果你不想用注释污染你的模型，你可以使用 mixins。
ObjectMapper mapper = new ObjectMapper();
SimpleModule simpleModule = new SimpleModule();
simpleModule.setMixInAnnotation(Student.class, StudentMixin.class);
mapper.registerModule(simpleModule);
并且您想覆盖 id 字段，例如：
public abstract class StudentMixin {
@JsonSerialize(using = StudentIdSerializer.class)
public String id;
}
做任何你需要的领域：
public class StudentIdSerializer extends JsonSerializer<Integer> {
@Override
public void serialize(Integer integer, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
jsonGenerator.writeString(String.valueOf(integer * 2));
}
}
在@JsonView 的帮助下，我们可以决定要序列化的模型类的字段满足最低标准（我们必须定义标准），就像我们可以拥有一个具有 10 个属性的核心类，但只能序列化 5 个客户需要的属性只要
通过简单地创建以下类来定义我们的视图：
public class Views
{
static class Android{};
static class IOS{};
static class Web{};
}
带视图的注释模型类：
public class Demo
{
public Demo()
{
}
@JsonView(Views.IOS.class)
private String iosField;
@JsonView(Views.Android.class)
private String androidField;
@JsonView(Views.Web.class)
private String webField;
// getters/setters
...
..
}
现在我们必须通过简单地从 spring 扩展 HttpMessageConverter 类来编写自定义 json 转换器：
public class CustomJacksonConverter implements HttpMessageConverter<Object>
{
public CustomJacksonConverter()
{
super();
//this.delegate.getObjectMapper().setConfig(this.delegate.getObjectMapper().getSerializationConfig().withView(Views.ClientView.class));
this.delegate.getObjectMapper().configure(MapperFeature.DEFAULT_VIEW_INCLUSION, true);
this.delegate.getObjectMapper().setSerializationInclusion(Include.NON_NULL);
}
// a real message converter that will respond to methods and do the actual work
private MappingJackson2HttpMessageConverter delegate = new MappingJackson2HttpMessageConverter();
@Override
public boolean canRead(Class<?> clazz, MediaType mediaType) {
return delegate.canRead(clazz, mediaType);
}
@Override
public boolean canWrite(Class<?> clazz, MediaType mediaType) {
return delegate.canWrite(clazz, mediaType);
}
@Override
public List<MediaType> getSupportedMediaTypes() {
return delegate.getSupportedMediaTypes();
}
@Override
public Object read(Class<? extends Object> clazz,
HttpInputMessage inputMessage) throws IOException,
HttpMessageNotReadableException {
return delegate.read(clazz, inputMessage);
}
@Override
public void write(Object obj, MediaType contentType, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException
{
synchronized(this)
{
String userAgent = ((ServletRequestAttributes) RequestContextHolder.getRequestAttributes()).getRequest().getHeader("userAgent");
if ( userAgent != null )
{
switch (userAgent)
{
case "IOS" :
this.delegate.getObjectMapper().setConfig(this.delegate.getObjectMapper().getSerializationConfig().withView(Views.IOS.class));
break;
case "Android" :
this.delegate.getObjectMapper().setConfig(this.delegate.getObjectMapper().getSerializationConfig().withView(Views.Android.class));
break;
case "Web" :
this.delegate.getObjectMapper().setConfig(this.delegate.getObjectMapper().getSerializationConfig().withView( Views.Web.class));
break;
default:
this.delegate.getObjectMapper().setConfig(this.delegate.getObjectMapper().getSerializationConfig().withView( null ));
break;
}
}
else
{
// reset to default view
this.delegate.getObjectMapper().setConfig(this.delegate.getObjectMapper().getSerializationConfig().withView( null ));
}
delegate.write(obj, contentType, outputMessage);
}
}
}
现在需要告诉spring使用这个自定义的json转换，只需将它放在dispatcher-servlet.xml中
<mvc:annotation-driven>
<mvc:message-converters register-defaults="true">
<bean id="jsonConverter" class="com.mactores.org.CustomJacksonConverter" >
</bean>
</mvc:message-converters>
</mvc:annotation-driven>
这就是您能够决定要序列化哪些字段的方式。
谢谢