Creating a RESTful web service

Note	This repository contains the guide documentation source. To view the guide in published form, view it on the Open Liberty website.

Learn how to create a RESTful service with Jakarta Restful Web Services, JSON-B, and Open Liberty.

What you’ll learn

You will learn how to build and test a simple RESTful service with Jakarta Restful Web Services and JSON-B, which will expose the JVM’s system properties. The RESTful service responds to GET requests made to the http://localhost:9080/LibertyProject/system/properties URL.

The service responds to a GET request with a JSON representation of the system properties, where each property is a field in a JSON object, like this:

{
  "os.name":"Mac",
  "java.version": "1.8"
}

The design of an HTTP API is an essential part of creating a web application. The REST API is the go-to architectural style for building an HTTP API. The Jakarta Restful Web Services API offers functions to create, read, update, and delete exposed resources. The Jakarta Restful Web Services API supports the creation of RESTful web services that are performant, scalable, and modifiable.

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Check out the service at the http://localhost:9080/LibertyProject/system/properties URL.

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Creating a RESTful application

Navigate to the start directory to begin.

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Jakarta Restful Web Services defines two key concepts for creating REST APIs. The most obvious one is the resource itself, which is modelled as a class. The second is a RESTful application, which groups all exposed resources under a common path. You can think of the RESTful application as a wrapper for all of your resources.

Replace the SystemApplication class.
src/main/java/io/openliberty/guides/rest/SystemApplication.java

SystemApplication.java

link:finish/src/main/java/io/openliberty/guides/rest/SystemApplication.java[role=include]

The SystemApplication class extends the Application class, which associates all RESTful resource classes in the WAR file with this RESTful application. These resources become available under the common path that’s specified with the @ApplicationPath annotation. The @ApplicationPath annotation has a value that indicates the path in the WAR file that the RESTful application accepts requests from.

Creating the RESTful resource

In a RESTful application, a single class represents a single resource, or a group of resources of the same type. In this application, a resource might be a system property, or a set of system properties. A single class can easily handle multiple different resources, but keeping a clean separation between types of resources helps with maintainability in the long run.

Create the PropertiesResource class.
src/main/java/io/openliberty/guides/rest/PropertiesResource.java

PropertiesResource.java

link:finish/src/main/java/io/openliberty/guides/rest/PropertiesResource.java[role=include]

SystemApplication.java

link:finish/src/main/java/io/openliberty/guides/rest/SystemApplication.java[role=include]

The @Path annotation on the class indicates that this resource responds to the properties path in the RESTful Web Services application. The @ApplicationPath annotation in the SystemApplication class together with the @Path annotation in this class indicates that the resource is available at the system/properties path.

Jakarta Restful Web Services maps the HTTP methods on the URL to the methods of the class by using annotations. Your application uses the GET annotation to map an HTTP GET request to the system/properties path.

The @GET annotation on the method indicates that this method is called for the HTTP GET method. The @Produces annotation indicates the format of the content that is returned. The value of the @Produces annotation is specified in the HTTP Content-Type response header. This application returns a JSON structured. The desired Content-Type for a JSON response is application/json, with MediaType.APPLICATION_JSON instead of the String content type. Using a constant such as MediaType.APPLICATION_JSON is better because a spelling error results in a compile failure.

Jakarta Restful Web Services supports a number of ways to marshal JSON. The Jakarta Restful Web Services specification mandates JSON-Binding (JSON-B). The method body returns the result of System.getProperties(), which is of type java.util.Properties. The method is annotated with @Produces(MediaType.APPLICATION_JSON) so Jakarta Restful Web Services uses JSON-B to automatically convert the returned object to JSON data in the HTTP response.

Configuring Liberty

To get the service running, the Liberty server.xml configuration file needs to be correctly configured.

Replace the Liberty server.xml configuration file.
src/main/liberty/config/server.xml

server.xml

link:finish/src/main/liberty/config/server.xml[role=include]

The configuration does the following actions:

• Configures Liberty to enable Jakarta Restful Web Services. This is specified in the featureManager element.

• Configures Liberty to resolve the HTTP port numbers from variables, which are then specified in the Maven pom.xml file. This is specified in the httpEndpoint element. Variables use the ${variableName} syntax.

• Configures Liberty to run the produced web application on a context root specified in the pom.xml file. This is specified in the webApplication element.

pom.xml

link:finish/pom.xml[role=include]

The variables that are being used in the server.xml file are provided by the properties set in the Maven pom.xml file. The properties must be formatted as liberty.var.variableName.

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Check out the service that you created at the http://localhost:9080/LibertyProject/system/properties URL.

Testing the service

You can test this service manually by starting Liberty and pointing a web browser at the http://localhost:9080/LibertyProject/system/properties URL. However, automated tests are a much better approach because they trigger a failure if a change introduces a bug. JUnit and the Jakarta Restful Web Services Client API provide a simple environment to test the application.

You can write tests for the individual units of code outside of a running Liberty instance, or they can be written to call the Liberty instance directly. In this example, you will create a test that does the latter.

Create the EndpointIT class.
src/test/java/it/io/openliberty/guides/rest/EndpointIT.java

EndpointIT.java

link:finish/src/test/java/it/io/openliberty/guides/rest/EndpointIT.java[role=include]

This test class has more lines of code than the resource implementation. This situation is common. The test method is indicated with the @Test annotation.

pom.xml

link:finish/pom.xml[role=include]

The test code needs to know some information about the application to make requests. The server port and the application context root are key, and are dictated by the Liberty’s configuration. While this information can be hardcoded, it is better to specify it in a single place like the Maven pom.xml file. Refer to the pom.xml file to see how the application information such as the http.port, https.port and app.context.root elements are provided in the file.

These Maven properties are then passed to the Java test program as the systemPropertyVariables element in the pom.xml file.

Getting the values to create a representation of the URL is simple. The test class uses the getProperty method to get the application details.

To call the RESTful service using the Jakarta Restful Web Services client, first create a WebTarget object by calling the target method that provides the URL. To cause the HTTP request to occur, the request().get() method is called on the WebTarget object. The get method call is a synchronous call that blocks until a response is received. This call returns a Response object, which can be inspected to determine whether the request was successful.

The first thing to check is that a 200 response was received. The JUnit assertEquals method can be used for this check.

Check the response body to ensure it returned the right information. The client and the server are running on the same machine so it is reasonable to expect that the system properties for the local and remote JVM would be the same. In this case, an assertEquals assertion is made so that the os.name system property for both JVMs is the same. You can write additional assertions to check for more values.

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You will see the following output:

-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running it.io.openliberty.guides.rest.EndpointIT
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 2.884 sec - in it.io.openliberty.guides.rest.EndpointIT

Results :

Tests run: 1, Failures: 0, Errors: 0, Skipped: 0

To see whether the tests detect a failure, add an assertion that you know fails, or change the existing assertion to a constant value that doesn’t match the os.name system property.

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Great work! You’re done!

You just developed a RESTful service in Open Liberty by using Jakarta Restful Web Services and JSON-B.

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