How to recursively Print data from template data Map Object in Freemarker?

We have a requirement where we need to pass object of type Map<String,List> to freemarker template. Here the issue is the Object inside a list can be a List, a Map or custom object or just a simple string. List and Map type can be further nested.something like below.

Map<String,Object> templateData = new HashMap<>();
templateData.put("complexKey","ABC");
        //or
templateData.put("complexKey",new List<String>());
       //or
templateData.put("complexKey",new List<Map<String,List<String>>>());

I need to find a way to identify the type of Object and apply some recursive solution until I find the suitable object to print.

I need to know if there is a way we can achieve this in free marker directly or through providing custom implementation of any class/interface from freemarker or through some configuration changes.

I really don't know anything about freemarker, but if you can get your hooks into it to process via straight Java you can try this:

package com.jdluke.treewalker;

import java.util.*;
import java.util.function.Consumer;

public class TreeWalker {
    public static void main(String[] args) {
        Map<String, Object> mainMap = new HashMap();
        mainMap.put("stringNode", "This is just a String");
        mainMap.put("listNode", Arrays.asList("one", "two"));
        Map mapNode1 = new HashMap();
        Map mapNode2 = new HashMap();
        Map mapNode3 = new HashMap();
        mapNode1.put("mapnode1.1", "first element");
        mapNode1.put("mapnode1.2", "second element");
        mapNode2.put("mapnode2.1", Arrays.asList("three", "four"));
        mapNode3.put("mapnode3.1", "map node 3, element 1");
        mapNode3.put("mapnode3.2", "map node 3, element 2");
        mainMap.put("listNode2", Arrays.asList(mapNode1, mapNode2));
        mainMap.put("mapNode", mapNode3);
        int count = 0;
        walk(mainMap, (node) -> System.out.println("Visiting " + node.toString()));
    }

    public static void walk(Map node, Consumer lambda) {
        System.out.println("Map object: " + node);
        node.forEach((k, v) -> handleNode(v, lambda));
    }

    public static void walk(Collection node, Consumer lambda) {
        System.out.println("Iterable object: " + node);
        node.forEach(v -> handleNode(v, lambda));
    }

    public static void walk(Object catchall, Consumer lambda) {
        System.out.println("Catchall object: " + catchall);
        lambda.accept(catchall);
    }

    private static void handleNode(Object v, Consumer lambda) {
        System.out.println("Handling object of type" + v.getClass().toString());
        lambda.accept(v);
        if (v instanceof Collection) {
            walk((Collection) v, lambda);
        } else if (v instanceof Map) {
            walk((Map) v, lambda);
        } else {
            walk(v, lambda);
        }
    }
}

What's happening here is that the various overloaded 'walk' methods have their own ways of dealing with their current location in the mixed tree. The lambda (which can gather stats, print stuff, or whatever) is applied to all nodes. This could be enhanced if necessary by having both a pre- and a post- processing Consumer for each node.

Since we lose cast information when we 'handle' an object we need to sort of reinject that in handleNode. I was hoping for something cleaner but may need more coffee.

Looks doable inside the template based on what I see.

Recursion is supported for FreeMarker macros and functions.

To check the type of a value you can use value?is_sequence (for List), value?is_hash_ex (for Map), and ?is_string (for String). (Actually this depends on the ObjectWrapper set inside the Configuration, but on almost all real world configuration this will work.)