Jackson JSON: get node name from json-tree

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.