FreeMarker: Interpolations

Interpolations are used to insert the value of an expression converted to text.

 

Syntax

${expression}

 

Interpolation can be used in two places.

a.   Inside the text: Hello ${name}!

b.   Inside String literal: <#include "/header/${org}.txt">

 

The result of interpolated string must be one of these values.

a.   String

b.   Number

c.    Date-time value

 

If you want to use other values like Boolean in interpolation, you should convert them explicitly.

 

Example

${b?string("yes", "no")}

 

Above statement evaluate to "yes" if the variable ‘b’ is true, else  "no".

 

Find the below working application.

 

Step 1: Create ‘interpolations.ftl’ file under src/main/resources/templates folder.

 

interpolations.ftl

<#assign a = 10, b = true, c = false, d = 123.45>

a : ${a}
b : ${b?string("yes", "no")}
c : ${c?string("yes", "no")}
d : ${d}


"a : ${a}"
"b : ${b?string("yes", "no")}"
"c : ${c?string("yes", "no")}"
"d : ${d}"

Step 2: Define ‘FreeMarkerUtil’ class that take model class and template file as input and merge them.

 

FreeMarkerUtil.java

package com.sample.app.util;

import java.io.StringWriter;
import java.util.Locale;

import freemarker.template.Configuration;
import freemarker.template.Template;
import freemarker.template.TemplateExceptionHandler;

public class FreeMarkerUtil {

	private static final Configuration FREE_MARKER_CONFIGURATION = new Configuration(Configuration.VERSION_2_3_30);

	static {
		FREE_MARKER_CONFIGURATION.setClassForTemplateLoading(FreeMarkerUtil.class, "/templates/");
		FREE_MARKER_CONFIGURATION.setDefaultEncoding("UTF-8");
		FREE_MARKER_CONFIGURATION.setLocale(Locale.US);
		FREE_MARKER_CONFIGURATION.setTemplateExceptionHandler(TemplateExceptionHandler.RETHROW_HANDLER);
		FREE_MARKER_CONFIGURATION.setFallbackOnNullLoopVariable(false);
	}

	public static StringWriter mergeModelAndTemplate(Object modelObject, String ftlFile) throws Exception {
		StringWriter stringWriter = new StringWriter();

		Template template = FREE_MARKER_CONFIGURATION.getTemplate(ftlFile);

		template.process(modelObject, stringWriter);

		return stringWriter;
	}

}

Step 3: Define InterpolationPopulator.

 

InterpolationPopulator.java

package com.sample.app;

import java.io.StringWriter;
import java.util.HashMap;
import java.util.Map;

import com.sample.app.util.FreeMarkerUtil;

public class InterpolationPopulator {
  public static void main(String args[]) throws Exception {

    Map<String, Object> modelObject = new HashMap<String, Object>();

    StringWriter stringWriter = FreeMarkerUtil.mergeModelAndTemplate(modelObject, "interpolations.ftl");
    System.out.println(stringWriter.toString().trim());

  }
}

Output

a : 10
b : yes
c : no
d : 123.45


"a : 10"
"b : yes"
"c : no"
"d : 123.45"

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