20

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。

4

5 回答 5

14

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}

于 2013-03-13T07:24:09.200 回答
14

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。

于 2013-03-13T23:57:10.903 回答
6

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
于 2013-03-13T09:21:31.220 回答
1

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));
    }
}
于 2019-04-05T09:27:37.677 回答
0

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

于 2016-07-12T06:28:36.837 回答