This was a bit of an exercise in frustration made worse by the fact that I couldn't find any answers to this question. So I'm going to answer the question here.
What I find most difficult to understand was that JsonNode doesn't have any getName() or similar method which I was expecting given that JSON is a name:value data type. Though I realised after working out this solution that arrays aren't name:value.
See answer below.
package treeWalker;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.Map;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.google.common.collect.Lists;
public class TreeWalker
{
public JsonNode convertJSONToNode(String json) throws JsonProcessingException, IOException
{
ObjectMapper mapper = new ObjectMapper();
JsonNode jsonNode = mapper.readTree(json);
return jsonNode;
}
public void walkTree(JsonNode root)
{
walker(null, root);
}
private void walker(String nodename, JsonNode node)
{
String nameToPrint = nodename != null ? nodename : "must_be_root";
System.out.println("walker - node name: " + nameToPrint);
if (node.isObject())
{
Iterator<Map.Entry<String, JsonNode>> iterator = node.fields();
ArrayList<Map.Entry<String, JsonNode>> nodesList = Lists.newArrayList(iterator);
System.out.println("Walk Tree - root:" + node + ", elements keys:" + nodesList);
for (Map.Entry<String, JsonNode> nodEntry : nodesList)
{
String name = nodEntry.getKey();
JsonNode newNode = nodEntry.getValue();
// System.out.println(" entry - key: " + name + ", value:" + node);
walker(name, newNode);
}
}
else if (node.isArray())
{
Iterator<JsonNode> arrayItemsIterator = node.elements();
ArrayList<JsonNode> arrayItemsList = Lists.newArrayList(arrayItemsIterator);
for (JsonNode arrayNode : arrayItemsList)
{
walker("array item", arrayNode);
}
}
else
{
if (node.isValueNode())
{
System.out.println(" valueNode: " + node.asText());
}
else
{
System.out.println(" node some other type");
}
}
}
}
And a Unit test to exercise (with no asserts! Sorry).
package treeWalker;
import java.io.IOException;
import org.junit.Test;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonNode;
public class TreeWalkerTest
{
TreeWalker treeWalker = new TreeWalker();
private String getJSON()
{
String json = "{\"a\":\"val_a\",\"b\":\"val_b\",\"c\":[1,2,3]}";
return json;
}
@Test
public void testConvertJSONToNode() throws JsonProcessingException, IOException
{
String json = getJSON();
JsonNode jNode = treeWalker.convertJSONToNode(json);
System.out.println("jnode:" + jNode);
}
@Test
public void testWalkTree() throws JsonProcessingException, IOException
{
JsonNode jNode = treeWalker.convertJSONToNode(getJSON());
treeWalker.walkTree(jNode);
}
}
Oh, and the build.gradle:
apply plugin: 'java'
apply plugin: 'eclipse'
repositories {
mavenCentral()
}
dependencies {
compile 'org.slf4j:slf4j-api:1.7.5'
compile 'com.fasterxml.jackson.core:jackson-core:2.4.3'
compile 'com.fasterxml.jackson.core:jackson-databind:2.4.3'
compile 'com.fasterxml.jackson.core:jackson-annotations:2.4.3'
compile 'com.google.guava:guava:18.0'
testCompile "junit:junit:4.11"
}
This is just an updated variation on HankCa's excellent answer. Under Java 8 and later, you can use Iterator.forEachRemaining() to loop through and you can externalize the consumer. Here's an updated version:
package treeWalker;
import java.io.IOException;
import java.util.function.BiConsumer;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
public class TreeWalker
{
public JsonNode convertJSONToNode(String json) throws JsonProcessingException, IOException
{
ObjectMapper mapper = new ObjectMapper();
JsonNode jsonNode = mapper.readTree(json);
return jsonNode;
}
public void walkTree(JsonNode root, BiConsumer<String, String> consumer)
{
walker(null, root, consumer);
}
private void walker(String nodename, JsonNode node, BiConsumer<String, String> consumer)
{
String nameToPrint = nodename != null ? nodename : "must_be_root";
if (node.isObject())
{
node.fields().forEachRemaining(e->walker(e.getKey(), e.getValue(), consumer));
}
else if (node.isArray())
{
node.elements().forEachRemaining(n->walker("array item of '" + nameToPrint + "'", n, consumer));
}
else
{
if (node.isValueNode())
{
consumer.accept(nameToPrint, node.asText());
}
else
{
throw new IllegalStateException("Node must be one of value, array or object.");
}
}
}
}
Here's the unit test class (Now in Junit5):
package treeWalker;
import java.io.IOException;
import org.junit.jupiter.api.Test;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonNode;
public class TreeWalkerTest
{
TreeWalker treeWalker = new TreeWalker();
private String getJSON()
{
String json = "{\"a\":\"val_a\",\"b\":\"val_b\",\"c\":[1,2,3]}";
return json;
}
@Test
public void testConvertJSONToNode() throws JsonProcessingException, IOException
{
String json = getJSON();
JsonNode jNode = treeWalker.convertJSONToNode(json);
System.out.println("jnode:" + jNode);
}
@Test
public void testWalkTree() throws JsonProcessingException, IOException
{
JsonNode jNode = treeWalker.convertJSONToNode(getJSON());
treeWalker.walkTree(jNode, (a,b)->System.out.println("name='" + a + "', value='" + b + "'."));
}
}
