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Note
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This repository contains the guide documentation source. To view the guide in published form, view it on the Open Liberty website. |
You’ll explore how to use Hypermedia as the Engine of Application State (HATEOAS) to drive your RESTful web service on Open Liberty.
You will learn how to use hypermedia to create a specific style of a response JSON, which has contents that you can use to navigate your REST service. You’ll build on top of a simple inventory REST service that you can develop with MicroProfile technologies. You can find the service at the following URL:
http://localhost:9080/inventory/hostsThe service responds with a JSON file that contains an array of registered hosts. Each host has a collection of HATEOAS links:
[
{
"hostname": "foo",
"_links": [
{
"href": "http://localhost:9080/inventory/hosts/foo",
"rel": "self"
}
]
},
{
"hostname": "bar",
"_links": [
{
"href": "http://localhost:9080/inventory/hosts/bar",
"rel": "self"
}
]
},
{
"hostname": "*",
"_links": [
{
"href": "http://localhost:9080/inventory/hosts/*",
"rel": "self"
}
]
}
]HATEOAS is a constrained form of REST application architecture. With HATEOAS, the client receives information about the available resources from the REST application. The client does not need to be hardcoded to a fixed set of resources, and the application and client can evolve independently. In other words, the application tells the client where it can go and what it can access by providing it with a simple collection of links to other available resources.
When you build a RESTful web service, consider the style of your response files.
Whether they are JSON files, XML files, or in some other format, a good practice is to always have them in
a clean and organized form. In the context of HATEOAS, each resource must contain a link reference to
itself, commonly referred to as self. Each link also needs a relationship to be associated with it,
although no strict rules exist as to how you need to format this relationship. Contain the collection
of such links within a _links array, which itself must be a direct property of the
resource object. The underscore in the _links property is used so that the property does not collide with any
existing fields that are named links. In this guide, you will use the following structure of HATEOAS links:
"_links": [
{
"href": ,
"rel":
}
]The following example shows two different links. The first link has a self relationship with the
resource object and is generated whenever you register a host. The link points to that host
entry in the inventory:
{
"href": "http://localhost:9080/inventory/hosts/<hostname>",
"rel": "self"
}The second link has a properties relationship with the resource object and is generated
if the host system service is running. The link points to the properties resource on the host:
{
"href": "http://<hostname>:9080/system/properties",
"rel": "properties"
}Although you should stick to the previous format for the purpose of this guide, another common convention has the link as the value of the relationship:
"_links": {
"self": "http://localhost:9080/inventory/hosts/<hostname>",
"properties": "http://<hostname>:9080/system/properties"
}Begin by building your response JSON, which is composed of the _links array, as well as the name of
the host machine.
As mentioned before, your starting point is an existing simple inventory REST service. You can learn more about this service and how to build it by visiting Creating a MicroProfile application.
First, tweak the request handlers in the InventoryResource class. Since the …/inventory/hosts/
URL will no longer respond with a JSON representation of the contents of your inventory, you can discard the listContents
method and integrate it into the getPropertiesForHost method:
link:finish/src/main/java/io/openliberty/guides/microprofile/InventoryResource.java[role=include]The contents of your inventory are now under the asterisk (*) wildcard and reside at the following URL:
http://localhost:9080/inventory/hosts/*Next, add a simple GET request handler that is responsible for handling all GET requests that are
made to the target URL. This method responds with a JSON that contains HATEOAS links:
link:finish/src/main/java/io/openliberty/guides/microprofile/InventoryResource.java[role=include]You also need a UriInfo object, which you use to build your HATEOAS links:
link:finish/src/main/java/io/openliberty/guides/microprofile/InventoryResource.java[role=include]The @Context annotation is a part of CDI and indicates that the UriInfo will be injected when the
resource is instantiated.
Your new InventoryResource class is now finished. Next, let’s implement the getSystems method and build
the response JSON object.
Let’s implement the getSystems method in your InventoryManager class. This method accepts a
target URL as an argument and returns a JSON object that contains HATEOAS links.
link:finish/src/main/java/io/openliberty/guides/microprofile/InventoryManager.java[role=include]The buildHostJson helper method builds the actual JSON object. Use this method together with a
simple map function to create an array of the JSON objects for each system.
Let’s implement the buildHostJson helper in the InventoryUtil class:
link:finish/src/main/java/io/openliberty/guides/microprofile/util/InventoryUtil.java[role=include]You’re creating a JSON object that contains the name of your host system and the _links array, which
is generated separately in another helper:
link:finish/src/main/java/io/openliberty/guides/microprofile/util/InventoryUtil.java[role=include]This helper accepts a host name and a target URL as arguments. The helper builds a link that points to the
inventory entry with a self relationship and also builds a link that points to the system service with
a properties relationship:
Consider what happens when one of the return links does not work or when a link should be available
for one object but not for another. In other words, it is important that a resource or service is
available and running before it is linked in the _links array.
Although this guide does not cover this case, always make sure that you receive a good response code from a service before you link that service. Similarly, make sure that it makes sense for a particular object to access a resource it is linked to. For instance, it doesn’t make sense for an account holder to be able to withdraw money from their account when their balance is 0. Hence, the account holder should not be linked to a resource that provides money withdrawal.
To see the new application in action, run the Maven liberty:start-server command from the start directory:
cd start
mvn liberty:start-serverAfter the server runs, you can find your new hypermedia-driven inventory service at the following URL:
At the following URLs, access the inventory service that is now driven by hypermedia:
The first URL returns the current contents of the inventory, and the second URL returns the system properties for
the host name. If the inventory does not contain an entry for the host name that is specified in the URL,
the system service that is running on the requested host name is called instead. The system
properties are retrieved from that system service and then stored in the inventory and returned.
If the servers are running, you can point your browser to each of the previous URLs to test the application manually. Nevertheless, you should rely on automated tests since they are more reliable and trigger a failure if a change introduces a defect.
Create a src/test/java/it/io/openliberty/guides/hateoas/EndpointTest.java test class.
You can use the @Before and @After annotations to perform any setup and teardown tasks for each
of your individual tests.
link:finish/src/test/java/it/io/openliberty/guides/hateoas/EndpointTest.java[role=include]Each test method must be marked with the @Test annotation. Typically, the execution order of test
methods is not controlled, but if control is required, you can place methods in a single container test
method. This container method is then the only one marked with the @Test annotation. The following
test suite contains two test methods, which run in the order they appear:
link:finish/src/test/java/it/io/openliberty/guides/hateoas/EndpointTest.java[role=include]Create a test called testLinkForInventoryContents. This test is responsible for asserting that
the correct HATEOAS link is created for the inventory contents.
link:finish/src/test/java/it/io/openliberty/guides/hateoas/EndpointTest.java[role=include]Write a getResponse helper method to reuse the same line of code for retrieving a response from a
specific URL. This technique helps keep your code neat and organized:
link:finish/src/test/java/it/io/openliberty/guides/hateoas/EndpointTest.java[role=include]Write another helper method called assertResponse. This method ensures that the response code that you receive is
valid (200):
link:finish/src/test/java/it/io/openliberty/guides/hateoas/EndpointTest.java[role=include]Create a test called testLinksForSystem. This test is responsible for asserting that the correct
HATEOAS links are created for the localhost system. This method checks for both the self link that points
to the inventory service and the properties link that points to the system service that is running on the
localhost system.
link:finish/src/test/java/it/io/openliberty/guides/hateoas/EndpointTest.java[role=include]Write a helper method called visitLocalhost. This method creates a GET request to the
system service, registering the localhost system:
link:finish/src/test/java/it/io/openliberty/guides/hateoas/EndpointTest.java[role=include]To rebuild and run the tests, navigate to the start directory and run the mvn clean install command
from the command line.
# If the server is still running from previous steps, stop it first with the following command:
mvn liberty:stop-server
# Then run this command:
mvn clean installSome time might elapse before the tests finish running. If the tests pass, you will see the following output:
-------------------------------------------------------
T E S T S
-------------------------------------------------------
Running it.io.openliberty.guides.hateoas.EndpointTest
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 1.086 sec - in it.io.openliberty.guides.hateoas.EndpointTest
Results :
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0You’ve just built and tested a hypermedia-driven RESTful web service on top of Open Liberty.