Flattening a 3 level nested JSON string in java

Question

The requirement is to create a generic flattening utility for an input JSON object to a flattened JSON object.

The sample JSON looks like the below

Answer 1

Try this code:

public static void flattenJson(JsonNode node, String parent, Map<String, ValueNode> map) {
    if (node instanceof ValueNode) {
        map.put(parent, (ValueNode)node);
    } else {
        String prefix = parent == null ? "" : parent + ".";
        if (node instanceof ArrayNode) {
            ArrayNode arrayNode = (ArrayNode)node;
            for(int i = 0; i < arrayNode.size(); i++) {
                flattenJson(arrayNode.get(i), prefix + i, map);
            }
        } else if (node instanceof ObjectNode) {
            ObjectNode objectNode = (ObjectNode) node;
            for (Iterator<Map.Entry<String, JsonNode>> it = objectNode.fields(); it.hasNext(); ) {
                Map.Entry<String, JsonNode> field = it.next();
                flattenJson(field.getValue(), prefix + field.getKey(), map);
            }
        } else {
            throw new RuntimeException("unknown json node");
        }
    }
}

public static Map<String, ValueNode> flattenJson(JsonNode input) {
    Map<String, ValueNode> map = new LinkedHashMap<>();
    flattenJson(input, null, map);
    return map;
}

Then you can call

    ObjectMapper om = new ObjectMapper();
    JsonNode jsonNode = om.readTree(json);
    Map<String, ValueNode> m = flattenJson(jsonNode);
    for (Map.Entry<String, ValueNode> kv : m.entrySet()) {
        System.out.println(kv.getKey() + "=" + kv.getValue().asText());
    }

Output:

Source=source-1
Rows.0.Keys.device-id=BC04-EBH-N3K-01
Rows.0.Keys.interface-name=TenGigE0/0/0/39
Rows.0.Keys.node-name=0/0/CPU0
Rows.0.Timestamp=1567621527656
Rows.0.inner.donm.id=0062
Rows.0.inner.donm.mol.rem=30
Rows.0.inner.donm.mol.len=11
Rows.0.inner.donm.mol.org.ldp.0.t=486
Rows.0.inner.donm.mol.org.ldp.0.o=322
Rows.0.inner.donm.mol.org.ldp.1.t=487
Rows.0.inner.donm.mol.org.ldp.1.o=32
Rows.0.inner.donm.mol.org.ldp.1.twss=1
Rows.0.inner.donm.mol.org.ldp.1.tlv=00:01
Rows.0.inner.donm.mol.chlen=14
Rows.0.inner.donm.mol.poe=5
Rows.0.inner.donm.mol.combs=10
Rows.0.inner.donm.mol.chaype=4
Rows.0.inner.donm.mol.rek=0
Rows.0.inner.donm.mol.rem-um=67
Rows.0.inner.donm.detail.enas=B,R
Rows.0.inner.donm.detail.systes=B,R
Rows.0.inner.donm.detail.timng=91
Rows.0.inner.donm.detail.syn=C
Rows.0.inner.donm.detail.met-type=0
Rows.0.inner.donm.detail.neses.lldEDIT.0.ium=830
Rows.0.inner.donm.detail.neses.lldEDIT.0.m=1
Rows.0.inner.donm.detail.neses.lldEDIT.0.ass.ape=ipv4
Rows.0.inner.donm.detail.neses.lldEDIT.0.ass.ipvs=94
Rows.0.inner.donm.detail.pess=0008
Rows.0.inner.donm.detail.por]d=0
Rows.0.inner.donm.detail.pon=BCtive
Rows.0.inner.donm.detail.sysme=BC1
Rows.0.inner.donm.reme=Bu1
Rows.0.inner.donm.hean=0
Rows.0.inner.donm.porl=Et1
Tey.epath=Cgetail
Tey.sustr=MX
Tey.coime=1567621527653
Tey.msp=1567621527653
Tey.come=1567621527660
Tey.nor=BC5
Tey.cid=14789654

Answer 2

To avoid conflicts with key names you can use JSON Pointer specification to create them. It is also supported by Jackson library, so you can use them later to traverse the JsonNode node.

Simple implementation could look like below:

import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.node.ArrayNode;
import com.fasterxml.jackson.databind.node.ObjectNode;
import java.io.File;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Objects;
import java.util.concurrent.atomic.AtomicInteger;

public class JsonApp {

  public static void main(String[] args) throws Exception {
    File jsonFile = new File("./test.json");

    ObjectMapper mapper = new ObjectMapper();
    JsonNode root = mapper.readTree(jsonFile);
    Map<String, JsonNode> map = new JsonFlattener(root).flatten();

    System.out.println("Use key-value pairs:");
    map.forEach(
        (k, v) -> {
          System.out.println(k + " => " + v);
        });

    System.out.println();
    System.out.println("Use pointers:");
    map.forEach(
        (k, v) -> {
          System.out.println(k + " => " + root.at(k));
        });
  }
}

class JsonFlattener {

  private final Map<String, JsonNode> json = new LinkedHashMap<>(64);
  private final JsonNode root;

  JsonFlattener(JsonNode node) {
    this.root = Objects.requireNonNull(node);
  }

  public Map<String, JsonNode> flatten() {
    process(root, "");
    return json;
  }

  private void process(JsonNode node, String prefix) {
    if (node.isObject()) {
      ObjectNode object = (ObjectNode) node;
      object
          .fields()
          .forEachRemaining(
              entry -> {
                  process(entry.getValue(), prefix + "/" + entry.getKey());
              });
    } else if (node.isArray()) {
      ArrayNode array = (ArrayNode) node;
      AtomicInteger counter = new AtomicInteger();
      array
          .elements()
          .forEachRemaining(
              item -> {
                process(item, prefix + "/" + counter.getAndIncrement());
              });
    } else {
      json.put(prefix, node);
    }
  }
}

Above code prints:

Use key-value pairs:
/Source => "source-1"
/Rows/0/Keys/device-id => "BC04-EBH-N3K-01"
/Rows/0/Keys/interface-name => "TenGigE0/0/0/39"
/Rows/0/Keys/node-name => "0/0/CPU0"
/Rows/0/Timestamp => 1567621527656
/Rows/0/inner/donm/id => "0062"
/Rows/0/inner/donm/mol/rem => 30
/Rows/0/inner/donm/mol/len => 11
/Rows/0/inner/donm/mol/org/ldp/0/t => 486
/Rows/0/inner/donm/mol/org/ldp/0/o => 322
/Rows/0/inner/donm/mol/org/ldp/1/t => 487
/Rows/0/inner/donm/mol/org/ldp/1/o => 32
/Rows/0/inner/donm/mol/org/ldp/1/twss => 1
/Rows/0/inner/donm/mol/org/ldp/1/tlv => "00:01"
/Rows/0/inner/donm/mol/chlen => 14
/Rows/0/inner/donm/mol/poe => 5
/Rows/0/inner/donm/mol/combs => 10
/Rows/0/inner/donm/mol/chaype => 4
/Rows/0/inner/donm/mol/rek => 0
/Rows/0/inner/donm/mol/rem-um => 67
/Rows/0/inner/donm/detail/enas => "B,R"
/Rows/0/inner/donm/detail/systes => "B,R"
/Rows/0/inner/donm/detail/timng => 91
/Rows/0/inner/donm/detail/syn => "C"
/Rows/0/inner/donm/detail/met-type => 0
/Rows/0/inner/donm/detail/neses/lldEDIT/0/ium => 830
/Rows/0/inner/donm/detail/neses/lldEDIT/0/m => 1
/Rows/0/inner/donm/detail/neses/lldEDIT/0/ass/ape => "ipv4"
/Rows/0/inner/donm/detail/neses/lldEDIT/0/ass/ipvs => "94"
/Rows/0/inner/donm/detail/pess => "0008"
/Rows/0/inner/donm/detail/por]d => 0
/Rows/0/inner/donm/detail/pon => "BCtive"
/Rows/0/inner/donm/detail/sysme => "BC1"
/Rows/0/inner/donm/reme => "Bu1"
/Rows/0/inner/donm/hean => 0
/Rows/0/inner/donm/porl => "Et1"
/Tey/epath => "Cgetail"
/Tey/sustr => "MX"
/Tey/coime => 1567621527653
/Tey/msp => 1567621527653
/Tey/come => 1567621527660
/Tey/nor => "BC5"
/Tey/cid => 14789654

Use pointers:
/Source => "source-1"
/Rows/0/Keys/device-id => "BC04-EBH-N3K-01"
/Rows/0/Keys/interface-name => "TenGigE0/0/0/39"
/Rows/0/Keys/node-name => "0/0/CPU0"
/Rows/0/Timestamp => 1567621527656
/Rows/0/inner/donm/id => "0062"
/Rows/0/inner/donm/mol/rem => 30
/Rows/0/inner/donm/mol/len => 11
/Rows/0/inner/donm/mol/org/ldp/0/t => 486
/Rows/0/inner/donm/mol/org/ldp/0/o => 322
/Rows/0/inner/donm/mol/org/ldp/1/t => 487
/Rows/0/inner/donm/mol/org/ldp/1/o => 32
/Rows/0/inner/donm/mol/org/ldp/1/twss => 1
/Rows/0/inner/donm/mol/org/ldp/1/tlv => "00:01"
/Rows/0/inner/donm/mol/chlen => 14
/Rows/0/inner/donm/mol/poe => 5
/Rows/0/inner/donm/mol/combs => 10
/Rows/0/inner/donm/mol/chaype => 4
/Rows/0/inner/donm/mol/rek => 0
/Rows/0/inner/donm/mol/rem-um => 67
/Rows/0/inner/donm/detail/enas => "B,R"
/Rows/0/inner/donm/detail/systes => "B,R"
/Rows/0/inner/donm/detail/timng => 91
/Rows/0/inner/donm/detail/syn => "C"
/Rows/0/inner/donm/detail/met-type => 0
/Rows/0/inner/donm/detail/neses/lldEDIT/0/ium => 830
/Rows/0/inner/donm/detail/neses/lldEDIT/0/m => 1
/Rows/0/inner/donm/detail/neses/lldEDIT/0/ass/ape => "ipv4"
/Rows/0/inner/donm/detail/neses/lldEDIT/0/ass/ipvs => "94"
/Rows/0/inner/donm/detail/pess => "0008"
/Rows/0/inner/donm/detail/por]d => 0
/Rows/0/inner/donm/detail/pon => "BCtive"
/Rows/0/inner/donm/detail/sysme => "BC1"
/Rows/0/inner/donm/reme => "Bu1"
/Rows/0/inner/donm/hean => 0
/Rows/0/inner/donm/porl => "Et1"
/Tey/epath => "Cgetail"
/Tey/sustr => "MX"
/Tey/coime => 1567621527653
/Tey/msp => 1567621527653
/Tey/come => 1567621527660
/Tey/nor => "BC5"
/Tey/cid => 14789654

Answer 3

Just create namespace of the keys by appending each level key into the key from higher level. In other words your flattened JSON's keys would be:

{
 "L1key::L2key": "L2val",
 "L1key": "L1val",
 "L1key::L2key::L3key": "L3val"
}

This way you guarantee uniqueness but also you can create the original json from this as well. Lastly be sure that the level separator (here ::) will not be present in your key.

HTH