Write a JSON File using FreeMarker Templates

This example shows how to write records to a JSON file using FreeMarker templates. It provides a customizable approach to generating JSON output based on templates.

FreeMarker template follows the MVC pattern to write dynamic content to JSON without manually changing the template. By implementing this example you can perform a variety of JSON-based transformations, some of which include:

  • dynamically updating data models
  • customizing output structures
  • seamlessly integrating with other JSON-based systems or APIs.


Input Files

The following files are being used as templates:


  "actors": {
    "north-america": [



  "stageName": "${stageName}",
  "realName": "${realName}",
  "gender": "${gender}",
  "city": "${city}"
  <#if (balance >= 0)>
  ,"balance": "${balance}"





Java Code Listing

package com.northconcepts.datapipeline.examples.cookbook;

import com.northconcepts.datapipeline.core.*;
import com.northconcepts.datapipeline.job.Job;
import com.northconcepts.datapipeline.memory.MemoryReader;
import com.northconcepts.datapipeline.template.TemplateWriter;

import java.io.File;
import java.io.FileWriter;
import java.io.IOException;

public class WriteAJsonFileUsingFreeMarkerTemplates {
    public static void main(String[] args) throws IOException {
        Record record1 = new Record();
        record1.getField("stageName", true).setValue("John Wayne");
        record1.getField("realName", true).setValue("Marion Robert Morrison");
        record1.getField("gender", true).setValue("male");
        record1.getField("city", true).setValue("Winterset");
        record1.getField("balance", true).setValue(156.35);

        Record record2 = new Record();
        record2.getField("stageName", true).setValue("Spiderman");
        record2.getField("realName", true).setValue("Peter Parker");
        record2.getField("gender", true).setValue("male");
        record2.getField("city", true).setValue("New York");
        record2.getField("balance", true).setValue(-0.96);

        RecordList recordList = new RecordList(record1, record2);
        DataReader reader = new MemoryReader(recordList);

        TemplateWriter writer = new TemplateWriter(new FileWriter("example/data/output/credit-balance-04.json"));
        writer.getConfiguration().setDirectoryForTemplateLoading(new File("example/data/input"));

        Job.run(reader, writer);


Code Walkthrough

  1. Beginning the execution, two Record instances are created and aggregated into a RecordList.
  2. MemoryReader is used to initialize the reader object with the prepared record list.
  3. Then, a TemplateWriter is configured with the output file path, template directory, and template files header.jsonfooter.jsondetail.json sections.
  4. Finally, Job.run() method is invoked with reader and writer objects to store the output to credit-balance-04.json file.


Output File

The following JSON is populated in the credit-balance-04.json file:

  "actors": {
    "north-america": [{
      "stageName": "John Wayne",
      "realName": "Marion Robert Morrison",
      "gender": "male",
      "city": "Winterset"
      ,"balance": "156.35"
      "stageName": "Spiderman",
      "realName": "Peter Parker",
      "gender": "male",
      "city": "New York"
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