Jackson Tree Model

Internal

• Jackson

Overview

This method converts a JSON document into a mutable in-memory tree representation of the JSON document. Tree model is arguably the most flexible of all three methods. The tree model is similar to the XML DOM.

JSON to Java with Tree Model

org.codehaus.jackson.map.ObjectMapper is the main API that builds trees from JSON content.

InputStream is = ...

ObjectMapper om = new ObjectMapper();

JsonNode root = om.readTree(is);

The trees consists of JsonNodes. Each node has one of the following types, coded as the JsonNodeType enum:

• NULL
• BOOLEAN
• NUMBER
• STRING
• OBJECT
• ARRAY
• BINARY
• POJO
• MISSING

There is no "FIELD" node. If the node is an OBJECT, we get its fields with:

for(Iterator<Map.Entry<String, JsonNode>> i = node.fields(); i.hasNext(); ) {

    Map.Entry<String, JsonNode> field = i.next();
    String fieldName = field.getKey();
    JsonNode value = field.getValue();
    ...
}

This is a standard tree traversing method:

public static void traverse(int depth, boolean indent, JsonNode node) {

    JsonNodeType type = node.getNodeType();

    if (JsonNodeType.OBJECT.equals(type)) {

        System.out.println("\n" + indentation(depth) + "{");

        for(Iterator<Map.Entry<String, JsonNode>> i = node.fields(); i.hasNext(); ) {

            Map.Entry<String, JsonNode> field = i.next();
            String fieldName = field.getKey();
            JsonNode value = field.getValue();

            System.out.print(indentation(depth + 1) + "\"" + fieldName + "\": ");

            traverse(depth + 1, false, value);

            if (i.hasNext()) {
                System.out.print(",");
            }
            System.out.println();
        }
        System.out.print(indentation(depth) + "}");
    }
    else if (JsonNodeType.ARRAY.equals(type)) {

        System.out.println("\n" + indentation(depth) + "[");

        for(Iterator<JsonNode> i = node.iterator(); i.hasNext(); ) {

           traverse(depth + 1, true, i.next());

           if (i.hasNext()) {
               System.out.print(",");
           }
           System.out.println();
       }
       System.out.print(indentation(depth) + "]");
    }
    else {
        displayLeaf(depth, indent, node);
    }
}

public static void displayLeaf(int depth, boolean indent, JsonNode node) {

    JsonNodeType type = node.getNodeType();

    if (JsonNodeType.OBJECT.equals(type) || JsonNodeType.ARRAY.equals(type)) {
        throw new IllegalArgumentException(node + " not a leaf");
    }

    if (indent) {
        System.out.print(indentation(depth));
    }

    if (JsonNodeType.NULL.equals(type)) {
         System.out.print("null");
    }
    else if (JsonNodeType.BOOLEAN.equals(type)) {
        System.out.print(node.booleanValue());
    }
    else if (JsonNodeType.NUMBER.equals(type)) {
        System.out.print(node.numberValue());
    }
    else if (JsonNodeType.STRING.equals(type)) {
        System.out.print("\"" + node.textValue() + "\"");
    }
    else {
        throw new RuntimeException("NOT YET IMPLEMENTED");
    }
}

Access Methods

For Objects

get(String fieldName) 

returns the value of the property if exists, or null if it does not, or if the node is not an Object node.

path(String fieldName)

is similar with get(...), with the difference that it returns MissingNode instead of null for missing values. MissingNode implements all JsonNode methods, but they return another MissingNode, making for a safe traversal, that will never throw NullPointerException. The result will at worst MissingNode.

with(String fieldName)

is similar with with(...), but instead of returning MissingNode, will actually create and add new ObjectNode. The method is useful for safe modifications: you can "materialize" sub-trees as necessary.

For Arrays

get(int index) 

returns the value of the array element at "index" (0-based), or null the index is out of bounds, or the node is not an Array node.

path(int index)

See path(String fieldName).

with(int index)

See with(String fieldName).

JSON to Java with Tree Model Code Example

https://github.com/NovaOrdis/playground/blob/master/json/jackson/tree-model-json-to-java/src/main/java/io/novaordis/playground/json/jackson/tree/json2java/Main.java

Java to JSON with Tree Model

ObjectMapper will introspect a Java object coded following Java Beans conventions and output it as JSON content.

The method names (without the "get") will be used as field names.

Example:

ByteArrayOutputStream baos = new ByteArrayOutputStream();

ObjectMapper om = new ObjectMapper();

Object root = ... // some hierarchical object

om.writeValue(baos, root);

System.out.println(new String(baos.toByteArray()));

Java to JSON with Tree Model Code Example

https://github.com/NovaOrdis/playground/blob/master/json/jackson/tree-model-java-to-json/src/main/java/io/novaordis/playground/json/jackson/tree/java2json/Main.java