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.
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
}
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
}
}
JsonNode root = mapper.readTree(json);
root.at("/some-node").fields().forEachRemaining(e -> {
System.out.println(e.getKey()+"---"+ e.getValue());
});
In one line Jackson 2+
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.
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());
}
Source: Stackoverflow.com