String Interpolator (OGNL) Library

This module (isis-module-stringinterpolator) provides a mechanism to interpolate string templates with either Isis system properties or values obtained from a domain object. It uses OGNL under the covers.

One use case for this service is in building URLs based on an object’s state, parameterized by environment (prod/test/dev etc). These URLs could be anything; for example, to a reporting service:



  • ${property['reportServerBase']} is an Isis system property

  • ${} is an expression that is evaluated in the context of a domain object (this), returning this.getProperty().getId()

Apache Isis system properties are exposed as the properties map, while the target object is exposed as the this object.


The module’s functionality can be explored by running the quickstart with example usage using the

Install example fixtures

A home page is displayed when the app is run:

010 install fixtures

... returning an example entity:

020 example entity

The Open (contributed) action is:

public URL open(ToDoItem toDoItem) throws MalformedURLException {
    String urlStr = stringInterpolatorService.interpolate(
        toDoItem, "${properties['']}/${this.documentationPage}");
    return new URL(urlStr);

where the configuration property "" has been set (eg in WEB-INF/

and where (as the screenshot shows) ToDoItem entity has the structure:

public class ToDoItem ... {

    private String description;
    private String documentationPage;

    // getters and setters omitted

Invoking the Open action computes the urlStr local variable, and then (because the action returns a URL), results in the browser opening the appropriate web page:

030 opened page

How to configure/use


Update your classpath by adding this dependency in your dom project’s pom.xml:


Check for later releases by searching Maven Central Repo.

For instructions on how to use the latest -SNAPSHOT, see the contributors guide.


In the AppManifest, update its getModules() method, eg:

public List<Class<?>> getModules() {
    return Arrays.asList(

API and Usage

The module consists of a single domain service, StringInterpolatorService.

The interpolation replaces each occurrence of ${…​} with its interpolated value. The expression in within the braces is interpreted using OGNL.

Object graph interpolation

The main API exposed by this service provides object-graph interpolation:

public class StringInterpolatorService {

    public void init(                                       (1)
        Map<String,String> properties) { ... }

    public String interpolate(                              (2)
        Object domainObject, String template) { ... }

1 called by Isis (which passes in all Isis properties)
2 public API

Using this API makes domainObject available as this in the template.

For example, assuming an instance of the Customer class:

public class Customer {
    private String firstName;
    private String lastName;
    private Address address;
    private Address billingAddress;

    // getters and setters omitted

... that in turn has relationships to the Address class

public class Address {
    private int houseNumber;
    private String town;
    private String postalCode;

    // getters and setters omitted

then the following are valid expressions:

  • ${this.firstName}

  • ${this.lastName != null? this.lastName : ''}

  • ${this.address.houseNumber}

Object graph interpolation (using the lower-level API)

The service also offers a lower-level API which allows multiple objects to be made accessible from the context:

public class StringInterpolatorService {

    public static class Root {
        public Root(final Object context) {
            this._this = context;
        public Object getThis() { return _this; }

    // public API
    public String interpolate(Root root, String template) { ... }


The Root class can be extended as necessary.

For example, create a custom subclass of the Root class:

final class CustomRoot extends StringInterpolatorService.Root {
    private Customer customer;
    public CustomRoot(Object context, Customer customer) {
        this.customer = customer;
    public Customer getCustomer() {
        return customer;

The example above exposes the customer property. This can then be used in the template, eg:

public void simple() throws Exception {
    String interpolated = service.interpolate(
        new CustomRoot(null, customer), "${customer.firstName}");
    assertThat(interpolated, is("Fred"));

Strict Mode (applies to both APIs)

By default, any expression that cannot be parsed or would generate an exception (eg null pointer exception) is instead returned unchanged in the interpolated string.

The service also provides a "strict" mode, which is useful for testing expressions:

StringInterpolatorService service = new StringInterpolatorService().withStrict(true);

If enabled, then an exception is thrown instead.

Known issues

None known at this time.


In addition to Apache Isis, this module depends on:

  • ognl:ognl (ASL v2.0 License)

Maven can report modules dependencies using:

mvn dependency:list -o -pl modules/lib/stringinterpolator/impl -D excludeTransitive=true

which, excluding Apache Isis itself, returns these compile/runtime dependencies:


For further details on 3rd-party dependencies, see: