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

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
}