Jackson JSON: get node name from json-tree

Question

How can I receive the node names from a JSON tree using Jackson? The JSON-File looks something like this:

{  
    node1:"value1",
    node2:"value2",
    node3:{  
        node3.1:"value3.1",
        node3.2:"value3.2"
    }
}

I have

JsonNode rootNode = mapper.readTree(fileReader);

and need something like

for (JsonNode node : rootNode){
    if (node.getName().equals("foo"){
        //bar
  }
}

thanks.

Answer 1

For Jackson 2+ ( com.fasterxml.jackson ), the methods are little bit different:

Iterator<Entry<String, JsonNode>> nodes = rootNode.get("foo").fields();

while (nodes.hasNext()) {
  Map.Entry<String, JsonNode> entry = (Map.Entry<String, JsonNode>) nodes.next();

  logger.info("key --> " + entry.getKey() + " value-->" + entry.getValue());
}

Answer 2

This answer applies to Jackson versions prior to 2+ (originally written for 1.8). See @SupunSameera's answer for a version that works with newer versions of Jackson.

---

The JSON terms for "node name" is "key." Since JsonNode#iterator() does not include keys, you need to iterate differently :

for (Map.Entry<String, JsonNode> elt : rootNode.fields())
{
    if ("foo".equals(elt.getKey()))
    {
        // bar
    }
}

If you only need to see the keys, you can simplify things a bit with JsonNode#fieldNames() :

for (String key : rootNode.fieldNames())
{
    if ("foo".equals(key))
    {
        // bar
    }
}

---

And if you just want to find the node with key "foo" , you can access it directly . This will yield better performance (constant-time lookup) and cleaner/clearer code than using a loop:

JsonNode foo = rootNode.get("foo");
if (foo != null)
{
    // frob that widget
}

Answer 3

fields() and fieldNames() both were not working for me. And I had to spend quite sometime to find a way to iterate over the keys. There are two ways by which it can be done.

One is by converting it into a map (takes up more space):

ObjectMapper mapper = new ObjectMapper();
Map<String, Object> result = mapper.convertValue(jsonNode, Map.class);
for (String key : result.keySet())
{
    if(key.equals(foo))
    {
        //code here
    }
}

Another, by using a String iterator:

Iterator<String> it = jsonNode.getFieldNames();
while (it.hasNext())
{
    String key = it.next();
    if (key.equals(foo))
    {
         //code here
    }
}

Answer 4

Clarification Here:

While this will work:

 JsonNode rootNode = objectMapper.readTree(file);
 Iterator<Map.Entry<String, JsonNode>> fields = rootNode.fields();
 while (fields.hasNext()) {
    Map.Entry<String, JsonNode> entry = fields.next();
    log.info(entry.getKey() + ":" + entry.getValue())
 }

This will not:

JsonNode rootNode = objectMapper.readTree(file);

while (rootNode.fields().hasNext()) {
    Map.Entry<String, JsonNode> entry = rootNode.fields().next();
    log.info(entry.getKey() + ":" + entry.getValue())
}

So be careful to declare the Iterator as a variable and use that.

Be sure to use the fasterxml library rather than codehaus .

Answer 5

JsonNode root = mapper.readTree(json);
root.at("/some-node").fields().forEachRemaining(e -> {
                              System.out.println(e.getKey()+"---"+ e.getValue());

        });

In one line Jackson 2+