Jackson - 自定义序列化程序，仅覆盖特定字段

时间：2013-03-13 06:59:05

标签： java serialization jackson

我知道如何在Jackson中使用自定义序列化程序（通过扩展JsonSerializer），但我希望默认的序列化程序适用于所有字段，只有1个字段，我想使用自定义序列化程序覆盖。

注释不是一个选项，因为我正在序列化生成的类（来自Thrift）。

在编写自定义jackson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 ...
}

上面的类有许多特性，其中大多数使用本机类型。我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属性。对于例如我只想将“age”字段转换为自定义输出。

5 个答案:

答案 0 :(得分：12)

假设你的Target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

一样使用它 
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());

它会产生类似

的o / p
  

{ “年龄”：“年龄：   2" ， “名字”： “第一”， “姓氏”： “最后”， “平均”：3.4 “numSubjects”：3}

答案 1 :(得分：12)

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this博客条目（或google了解更多“jackson mixin annotations”）了解如何使用此功能。

我特意使用了Jackson和protobuf和thrift生成的类，并且它们工作得非常好。对于早期的Thrift版本，我不得不禁用“is-setters”的发现，Thrift生成的方法是为了查看是否已经明确设置了特定属性，但是其他方法都运行正常。

答案 2 :(得分：5)

我遇到了同样的问题，我用CustomSerializerFactory解决了这个问题。

此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);
        }

    }
}

之后您可以使用以下内容：

objectMapper.setSerializerFactory(new EntityCustomSerializationFactory());
objectMapper.writeValueAsString(new Entity());//response will be without ignored fields

答案 3 :(得分：0)

在@JsonView的帮助下，我们可以决定要序列化的模型类的字段，它们满足最小标准（我们必须定义标准），就像我们可以有一个具有10个属性的核心类，但只有5个属性可以序列化只需要客户

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{};
}

带注释的带注释的模型类：

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>

这就是你如何决定序列化哪些字段。

感谢名单

答案 4 :(得分：0)

如果您不想使用注释污染模型，则可以使用mixins。

ObjectMapper mapper = new ObjectMapper();
SimpleModule simpleModule = new SimpleModule();
simpleModule.setMixInAnnotation(Student.class, StudentMixin.class);
mapper.registerModule(simpleModule);

例如，您想覆盖id字段：

public abstract class StudentMixin {
    @JsonSerialize(using = StudentIdSerializer.class)
    public String id;
}

在字段上做任何您需要的事情

public class StudentIdSerializer extends JsonSerializer<Integer> {
    @Override
    public void serialize(Integer integer, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
        jsonGenerator.writeString(String.valueOf(integer * 2));
    }
}
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