题 Jackson - 自定义序列化程序,仅覆盖特定字段


我知道如何在杰克逊中使用自定义序列化器(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 ...
}

上面的类有很多特性,大多数都使用本机类型。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。


17
2018-03-13 06:59


起源


使用哪个杰克逊版本 - Arun P Johny
看看这个(stackoverflow.com/questions/7161638/...你可能会有所了解。 - Krushna


答案:


我遇到了同样的问题,我用它解决了 CustomSerializerFactory

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

5
2018-03-13 09:21





假设您的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);

然后像它一样使用它

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

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


12
2018-03-13 07:24



假设我为T类定义了一个自定义序列化器,它有3个字段,我只在序列化器中做一个write()(对于我想要覆盖的唯一字段),其他2个字段是否会使用默认值进行序列化? - jeffreyveon
是的,但被覆盖类型的所有字段都将使用自定义序列化程序 - Arun P Johny
@jeffreyveon看到了编辑 - Arun P Johny
得到它 - 在我的情况下,我有一个类具有本机类型的字段..例如。 int count,long total等。在这种情况下看起来我不能使用这种方法。 - jeffreyveon
如果你可以分享课程,我们可以看看可以做些什么 - Arun P Johny


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。看到 这个 例如博客条目(或谷歌更多与“杰克逊mixin注释”)如何使用它。

我特意将杰克逊用于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,但是其他方法工作正常。


11
2018-03-13 23:57



看起来很整洁 - 谢谢。我也会尝试这种方法。 - jeffreyveon
是的,知道选项很好,有些在某些情况下效果更好,在其他情况下则有效。 - StaxMan


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,这对于客户端是必需的只要

只需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中即可

<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的方法。

感谢名单


0
2017-07-12 06:28