How to implement mustache lambda in java

I would like to implement a lambda in java that can be called from a mustache template. So far, I've only been able to find a poorly written blog article about this subject: https://spring.io/blog/2016/11/21/the-joy-of-mustache-server-side-templates-for-the-jvm. Essentially, I have a template file

{{#getContentType}}
     {{http-response.headers}}
{{/getContentType}}

The method name is getContentType it takes a list object http-response.headers and returns the value from the key-value pair

{
    "name": "Content-Type",
	"value": "application/json"
}

I have even looked at the documentation: https://github.com/spullara/mustache.java#readmehttps://github.com/spullara/mustache.java#readme, but I am still struggling to figure this out. Any help would be greatly appreciated!

For reference an example input for the getContentType lambda would like

[
			{
				"name": "Content-Type",
				"value": "application/json"
			},
			{
				"name": "Content-Length",
				"value": "363"
			},
			{
				"name": "Authorization",
				"value": "Bearer bearerToken"
			},
			{
				"name": "Host",
				"value": "host"
			}
		]

There aren't any examples of using TemplateFunctions, but the documentation is here:http://spullara.github.io/mustache/apidocs/. It's pretty hard for a new person to "just get it".

I was able to figure this out by referring to the Spring Docs on lambda functions. The problem is that they have a typo, it says "lamba" not lambda: https://docs.spring.io/spring-restdocs/docs/current/reference/html5/#working-with-asciidoctor-customizing-tables-formatting-problems.

Going off of the tabelCellContent lambda function. I found an example in the tests for StandardWriterReslover. https://www.codota.com/code/java/classes/org.springframework.restdocs.snippet.StandardWriterResolver.

The lambda function is added to the map templateContext:

Map<String, Object> templateContext = new HashMap<>();
    templateContext.put("tableCellContent",
        new AsciidoctorTableCellContentLambda());

The class for AsciidoctorTableCellContentLambda looks like:

/*
 * Copyright 2014-2016 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.restdocs.templates.mustache;

import java.io.IOException;
import java.io.Writer;

import org.springframework.restdocs.mustache.Mustache.Lambda;
import org.springframework.restdocs.mustache.Template.Fragment;

/**
 * A {@link Lambda} that escapes {@code |} characters so that the do not break the table's
 * formatting.
 *
 * @author Andy Wilkinson
 * @since 1.1.0
 */
public final class AsciidoctorTableCellContentLambda implements Lambda {

	@Override
	public void execute(Fragment fragment, Writer writer) throws IOException {
		String output = fragment.execute();
		for (int i = 0; i < output.length(); i++) {
			char current = output.charAt(i);
			if (current == '|' && (i == 0 || output.charAt(i - 1) != '\\')) {
				writer.append('\\');
			}
			writer.append(current);
		}
	}

}