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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 all of the registered hosts. Each host has a collection of HATEOAS links:
{
"foo": [
{
"href": "http://localhost:9080/inventory/hosts/foo",
"rel": "self"
}
],
"bar": [
{
"href": "http://localhost:9080/inventory/hosts/bar",
"rel": "self"
}
],
"*": [
{
"href": "http://localhost:9080/inventory/hosts/*",
"rel": "self"
}
]
}HATEOAS is a constraint of REST application architectures. 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.
In the context of HATEOAS, each resource must contain a link reference to itself, which is commonly referred to as self. In this guide, the JSON structure features a mapping between the hostname and its corresponding list of HATEOAS links:
"*": [
{
"href": "http://localhost:9080/inventory/hosts/*",
"rel": "self"
}
]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"
}After the Liberty instance runs, you can find your hypermedia-driven inventory service at the following URL:
Navigate to the start directory.
Begin by building your response JSON, which is composed of the name of the host machine and its list of HATEOAS links.
As mentioned before, your starting point is an existing simple inventory REST service.
Look at the request handlers in the InventoryResource.java file.
The …/inventory/hosts/ URL will no longer respond with a JSON representation of your inventory contents, so you can discard the listContents method and integrate it into the getPropertiesForHost method.
Replace theInventoryResourceclass.src/main/java/io/openliberty/guides/microprofile/InventoryResource.java
InventoryResource.java
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 http://localhost:9080/inventory/hosts/* URL.
The GET request handler is responsible for handling all GET requests that are made to the target URL. This method responds with a JSON that contains HATEOAS links.
The UriInfo object is what will be used to build your HATEOAS links.
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 replaced. Next, you will implement the getSystems method and build the response JSON object.
InventoryManager.java
link:finish/src/main/java/io/openliberty/guides/microprofile/InventoryManager.java[role=include]Take a look at your InventoryManager and InventoryUtil files.
Replace theInventoryManagerclass.src/main/java/io/openliberty/guides/microprofile/InventoryManager.java
InventoryManager.java
link:finish/src/main/java/io/openliberty/guides/microprofile/InventoryManager.java[role=include]The getSystems method accepts a target URL as an argument and returns a JSON object that contains HATEOAS links.
Replace theInventoryUtilclass.src/main/java/io/openliberty/guides/microprofile/util/InventoryUtil.java
InventoryUtil.java
link:finish/src/main/java/io/openliberty/guides/microprofile/util/InventoryUtil.java[role=include]The helper builds a link that points to the inventory entry with a self relationship. The helper also builds a link that points to the system service with a properties relationship:
-
http://localhost:9080/inventory/hosts/<hostname>
-
http://<hostname>:9080/system/properties
InventoryUtil.java
link:finish/src/main/java/io/openliberty/guides/microprofile/util/InventoryUtil.java[role=include]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 added in the HATEOAS links array of the hostname.
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.
After the Liberty instance updates, 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:
If the Liberty instances 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 because they are more reliable and trigger a failure if a change introduces a defect.
Create theEndpointITclass.src/test/java/it/io/openliberty/guides/hateoas/EndpointIT.java
EndpointIT.java
link:finish/src/test/java/it/io/openliberty/guides/hateoas/EndpointIT.java[role=include]The @BeforeEach and @AfterEach annotations are placed on setup and teardown tasks that are run for each individual test.
Each test method must be marked with the @Test annotation. The execution order of test methods is controlled by marking them with the @Order annotation. The value that is passed into the annotation denotes the order in which the methods are run.
The testLinkForInventoryContents test is responsible for asserting that the correct HATEOAS link is created for the inventory contents.
Finally, the testLinksForSystem 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, which is running on the localhost system.
https://raw.githubusercontent.com/OpenLiberty/guides-common/prod/devmode-test.adoc You will see the following output:
-------------------------------------------------------
T E S T S
-------------------------------------------------------
Running it.io.openliberty.guides.hateoas.EndpointIT
Tests run: 2, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.951 s - in it.io.openliberty.guides.hateoas.EndpointIT
Results:
Tests run: 2, Failures: 0, Errors: 0, Skipped: 0
Integration tests finished.You’ve just built and tested a hypermedia-driven RESTful web service on top of Open Liberty.