Ignore array/collection properties of specific types using Jackson

This article describes how to use the Jackson library to ignore properties containing arrays or collections of specific types without modifying the DTO definition. Through the `addMixIn()` method of `ObjectMapper`, combined with the `@JsonIgnoreType` annotation, you can flexibly control the serialization and deserialization behavior and solve the problem of DTO containing fields that cannot be serialized. This article provides a solution for `MultipartFile[]` and discusses the handling of `List`.

When developing with Spring Boot, you often encounter scenarios where you need to clone a DTO (Data Transfer Object). If the DTO contains fields that cannot be serialized, such as `MultipartFile` (for file upload), directly using `ObjectOutputStream`/`ObjectInputStream` for serialization/deserialization will fail. Although it is possible to add the `transient` keyword to these fields, this approach does not work if the DTO definition cannot be modified. The Jackson library provides a flexible way to solve this problem, that is, through the `addMixIn()` method of `ObjectMapper`, combined with the `@JsonIgnoreType` annotation, fields of a specific type can be ignored without modifying the DTO definition. ### Ignoring fields of a specific type Suppose we have a DTO that contains a field of type `MultipartFile`: ```java public class MyDto { private String name; private MultipartFile file; // Getters and setters public String getName() { return name; } public void setName(String name) { this.name = name; } public MultipartFile getFile() { return file; } public void setFile(MultipartFile file) { this.file = file; } }

We can use the following code to ignore fields of type MultipartFile:

 import com.fasterxml.jackson.annotation.JsonIgnoreType;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.core.JsonProcessingException;
import org.springframework.web.multipart.MultipartFile;

public class JacksonIgnoreExample {

    private Object makeClone(Object obj) {
        ObjectMapper mapper = new ObjectMapper();
        mapper.addMixIn(MultipartFile.class, JacksonMixInForIgnoreType.class);
        try {
            return mapper.readValue(mapper.writeValueAsString(obj), obj.getClass());
        } catch (JsonProcessingException e) {
            throw new RuntimeException(e);
        }
    }

    @JsonIgnoreType
    class JacksonMixInForIgnoreType {}

    public static void main(String[] args) {
        JacksonIgnoreExample example = new JacksonIgnoreExample();
        MyDto dto = new MyDto();
        dto.setName("test");
        //Assume there is a MultipartFile file object //dto.setFile(file);

        MyDto clonedDto = (MyDto) example.makeClone(dto);
        System.out.println(clonedDto.getName());
        //clonedDto.getFile() will be null because it has been ignored}
}

This code first creates an ObjectMapper instance, and then uses the addMixIn() method to associate MultipartFile.class with JacksonMixInForIgnoreType.class. JacksonMixInForIgnoreType is an empty class marked with the @JsonIgnoreType annotation, indicating that all fields of this type should be ignored.

Ignore arrays of specific types

If the DTO contains an array of MultipartFile type, such as MultipartFile[], you need to add an additional addMixIn() call:

 import com.fasterxml.jackson.annotation.JsonIgnoreType;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.core.JsonProcessingException;
import org.springframework.web.multipart.MultipartFile;

public class JacksonIgnoreExample {

    private Object makeClone(Object obj) {
        ObjectMapper mapper = new ObjectMapper();
        mapper.addMixIn(MultipartFile.class, JacksonMixInForIgnoreType.class);
        mapper.addMixIn(MultipartFile[].class, JacksonMixInForIgnoreType.class);
        try {
            return mapper.readValue(mapper.writeValueAsString(obj), obj.getClass());
        } catch (JsonProcessingException e) {
            throw new RuntimeException(e);
        }
    }

    @JsonIgnoreType
    class JacksonMixInForIgnoreType {}

    public static void main(String[] args) {
        JacksonIgnoreExample example = new JacksonIgnoreExample();
        MyDto dto = new MyDto();
        dto.setName("test");
        //Assume there is an array of MultipartFile file objects //MultipartFile[] files = new MultipartFile[1];
        //dto.setFileArray(files);

        MyDto clonedDto = (MyDto) example.makeClone(dto);
        System.out.println(clonedDto.getName());
        //clonedDto.getFileArray() will be null because it has been ignored}
}

class MyDto {
    private String name;
    private MultipartFile[] fileArray;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public MultipartFile[] getFileArray() {
        return fileArray;
    }

    public void setFileArray(MultipartFile[] fileArray) {
        this.fileArray = fileArray;
    }
}

In this way, fields of type MultipartFile and MultipartFile[] will be ignored.

Ignore List

Currently, it seems that fields of type List cannot be ignored directly using the ObjectMapper#addMixIn() method. You may want to consider other approaches, such as a custom serializer, or using other data structures in the DTO to store file information.

Summarize

Using Jackson's ObjectMapper and addMixIn() methods, you can flexibly ignore specific type fields in a DTO, including array types. This is useful when working with DTOs that contain fields that cannot be serialized, especially if the DTO definition cannot be modified. Although it is currently not possible to directly ignore List, similar effects can be achieved in other ways.

Things to note:

• Make sure the Jackson Databind version is above 2.x.
• The @JsonIgnoreType annotation ignores all fields of the type, so it needs to be used with caution.
• For complex types, a custom serializer may be required for finer control.

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