How to convert below JSON into POJO in java for deserialisation purpose

Question

How to write the equivalent POJO for below type JSON :

[
  {
    "oneImageBlock": [{
      "imagePath": "/abc",
      "mobileImageOverride": {
        "imagePath": "/abc"
      },
      "imageAltText": "image",
      "imageCaptionText": "caption",
      "captionTextReplacement": {
        "type": "overlay"
      },
      "urlPath": "/url",
      "displayURLInModal": true
    }]
  },
  {
    "fourImageBlock": [{
      "imagePath": "/abc",
      "mobileImageOverride": {
        "imagePath": "/abc"
      },
      "imageAltText": "image",
      "imageCaptionText": "caption",
      "captionTextReplacement": {
        "type": "overlay/Below Image"
      },
      "urlPath": "/l/123",
      "displayURLInModal": true
     },
     {
      "imagePath": "/abc",
      "mobileImageOverride": {
        "imagePath": "/abc"
      },
      "imageAltText": "image",
      "imageCaptionText": "caption",
      "captionTextReplacement": {
        "type": "overlay/Below Image"
      },
      "urlPath": "/l/123",
      "displayURLInModal": true
     },
     {
      "imagePath": "/abc",
      "mobileImageOverride": {
        "imagePath": "/abc"
      },
      "imageAltText": "image",
      "imageCaptionText": "caption",
      "captionTextReplacement": {
        "type": "overlay/Below Image"
      },
      "urlPath": "/l/123",
      "displayURLInModal": true
     },
     {
      "imagePath": "/abc",
      "mobileImageOverride": {
        "imagePath": "/abc"
      },
      "imageAltText": "image",
      "imageCaptionText": "caption",
      "captionTextReplacement": {
        "type": "overlay/Below Image"
      },
      "urlPath": "/l/123",
      "displayURLInModal": true
     } 
    ]
  },
  {
    "fourImageBlock": [
      
    ]
  },
  {
    "oneImageBlock": [
      
    ]
  },
  {
    "oneTextBlock": [
      "text": "test",
      "textAlignment": "centre",
      "mobileTextOverride": {
        "text": "test"
      },
      "altText": "alt",
      "urlPath": "/url"
    ]
  },
  {
    "fourImageBlock": [
      
    ]
  }
]

provided there can be multiple objects with the same key in the JSON like here in example it has 2 occurrence of oneImageBlock & 3 occurrence of fourImageBlock and also there is no fixed order as well i.e it's possible that in the JSON we have first 5 objects with fourImageBlock followed by 2 objects with oneImageBlock again followed by 2 objects with fourImageBlock.

Answer 1

The easiest way to deserialize your data would be to set up some POJOs that conform to your JSON data:

Since your top-level objects can contain the keys: oneImageBlock, fourImageBlock, or oneTextBlock, you need an object that defines all three of those as possibilities. They are defined in the Entry class below.

import java.io.File;
import java.io.IOException;
import com.fasterxml.jackson.core.exc.StreamReadException;
import com.fasterxml.jackson.databind.DatabindException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class BlockDeserializer {
    protected static ObjectMapper mapper = new ObjectMapper();

    public static void main(String[] args) throws StreamReadException, DatabindException, IOException {
        Entry[] entries = mapper.readValue(new File("src/main/resources/blocks.json"), Entry[].class);
        System.out.println(entries);
    }

    private static class Entry {
        public ImageBlock[] oneImageBlock;
        public ImageBlock[] fourImageBlock;
        public TextBlock[] oneTextBlock;
        public TextBlock[] fourTextBlock;
    }

    private static class ImageBlock {
        public String imagePath;
        public MobileImageOverride mobileImageOverride;
        public String imageAltText;
        public String imageCaptionText;
        public CaptionTextReplacement captionTextReplacement;
        public String urlPath;
        public boolean displayURLInModal;
    }

    private static class TextBlock {
        public String text;
        public String textAlignment;
        public MobileTextOverride mobileTextOverride;
        public String altText;
        public String urlPath;
    }

    private static class CaptionTextReplacement {
        public String type;
    }

    private static class MobileImageOverride {
        public String imagePath;
    }

    private static class MobileTextOverride {
        public String text;
    }
}

Also, make sure your oneTextBlock value is an array of objects. Looks like your JSON is invalid.

[
  //...
  {
    "oneTextBlock": [
      {
        "text": "test",
        "textAlignment": "centre",
        "mobileTextOverride": {
          "text": "test"
        },
        "altText": "alt",
        "urlPath": "/url"
      }
    ]
  },
  // ...
]

Alternatively, you can create your own deserializer so that you can correctly deserialize into the correct "block" type.

I removed the Entry concept and added a Block interface to all the sub-blocks.

import java.io.File;
import java.io.IOException;

import com.fasterxml.jackson.core.JacksonException;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.exc.StreamReadException;
import com.fasterxml.jackson.databind.DatabindException;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.deser.std.StdDeserializer;
import com.fasterxml.jackson.databind.module.SimpleModule;

public class BlockDeserializerDriver {
    protected static ObjectMapper mapper;
    protected static SimpleModule module;

    static {
        mapper = new ObjectMapper();
        module = new SimpleModule();

        module.addDeserializer(Block.class, new BlockDeserializer());
        mapper.registerModule(module);
    }

    public static void main(String[] args) throws StreamReadException, DatabindException, IOException {
        Block[] entries = loadBlocks("src/main/resources/blocks.json");

        System.out.println(entries[0] instanceof OneImageBlock); // true
        System.out.println(entries[1] instanceof FourImageBlock); // true
        System.out.println(entries[2] instanceof FourImageBlock); // true
        System.out.println(entries[3] instanceof OneImageBlock); // true
        System.out.println(entries[4] instanceof OneTextBlock); // true
        System.out.println(entries[5] instanceof FourImageBlock); // true
    }

    public static Block[] loadBlocks(String filePath) throws StreamReadException, DatabindException, IOException {
        return mapper.readValue(new File(filePath), Block[].class);
    }

    private static interface Block {
    }

    private static class OneImageBlock implements Block {
        public ImageBlock[] oneImageBlock;
    }

    private static class FourImageBlock implements Block {
        public ImageBlock[] fourImageBlock;
    }

    private static class OneTextBlock implements Block {
        public TextBlock[] oneTextBlock;
    }

    private static class FourTextBlock implements Block {
        public TextBlock[] fourTextBlock;
    }

    private static class ImageBlock {
        public String imagePath;
        public MobileImageOverride mobileImageOverride;
        public String imageAltText;
        public String imageCaptionText;
        public CaptionTextReplacement captionTextReplacement;
        public String urlPath;
        public boolean displayURLInModal;
    }

    private static class TextBlock {
        public String text;
        public String textAlignment;
        public MobileTextOverride mobileTextOverride;
        public String altText;
        public String urlPath;
    }

    private static class CaptionTextReplacement {
        public String type;
    }

    private static class MobileImageOverride {
        public String imagePath;
    }

    private static class MobileTextOverride {
        public String text;
    }

    // See: https://www.baeldung.com/jackson-deserialization
    private static class BlockDeserializer extends StdDeserializer<Block> {
        public BlockDeserializer() {
            this(null);
        }

        public BlockDeserializer(Class<?> vc) {
            super(vc);
        }

        @Override
        public Block deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JacksonException {
            JsonNode node = jp.getCodec().readTree(jp);

            if (node.has("oneImageBlock")) {
                return mapper.convertValue(node, OneImageBlock.class);
            } else if (node.has("fourImageBlock")) {
                return mapper.convertValue(node, FourImageBlock.class);
            } else if (node.has("oneTextBlock")) {
                return mapper.convertValue(node, OneTextBlock.class);
            } else if (node.has("fourTextBlock")) {
                return mapper.convertValue(node, FourTextBlock.class);
            }

            return null; // Could not parse Block
        }
    }
}