Injecting dependencies into microservices
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Learn how to use Contexts and Dependency Injection (CDI) to manage scopes and inject dependencies into microservices.
What you’ll learn
You will learn how to use Contexts and Dependency Injection (CDI) to manage scopes and inject dependencies in a simple inventory management application.
The application that you will be working with is an
which stores the information about various JVMs that run on different systems.
Whenever a request is made to the
inventory service to retrieve the JVM
system properties of a particular host, the
inventory service communicates with the
service on that host to get these system properties. The system properties are then stored and returned.
You will use scopes to bind objects in this application to their well-defined contexts. CDI provides a variety of scopes for you to work with and while you will not use all of them in this guide, there is one for almost every scenario that you may encounter. Scopes are defined by using CDI annotations. You will also use dependency injection to inject one bean into another to make use of its functionalities. This enables you to inject the bean in its specified context without having to instantiate it yourself.
The implementation of the application and its services are provided for you in the
system service can be found in the
inventory service can be found in the
If you want to learn more about RESTful web services and how to build them, see
Creating a RESTful web service for details about how to build the
inventory service is built in a similar way.
What is CDI?
Contexts and Dependency Injection (CDI) defines a rich set of complementary services that improve the application structure. The most fundamental services that are provided by CDI are contexts that bind the lifecycle of stateful components to well-defined contexts, and dependency injection that is the ability to inject components into an application in a typesafe way. With CDI, the container does all the daunting work of instantiating dependencies, and controlling exactly when and how these components are instantiated and destroyed.
Try what you’ll build
finish directory in the root of this guide contains the finished inventory application. Give it a try before you proceed.
To try out the application, first go to the
finish directory and run the following
Maven goal to build the application and deploy it to Open Liberty:
Point your browser to the http://localhost:9080/inventory/systems URL. This is the starting point of the
service and it displays the current contents of the inventory. As you might expect, these are empty since
nothing is stored in the inventory yet. Next, point your browser to the http://localhost:9080/inventory/systems/localhost URL.
You see a result in JSON format with the system properties of your local JVM. When you visit this URL, these system
properties are automatically stored in the inventory. Go back to http://localhost:9080/inventory/systems and
you see a new entry for
localhost. For simplicity, only the OS name and username are shown here for
each host. You can repeat this process for your own hostname or any other machine that is running
After you are finished checking out the application, stop the Open Liberty server by pressing
in the shell session where you ran the server. Alternatively, you can run the
finish directory in another shell session:
Handling dependencies in the application
You will use CDI to inject dependencies into the inventory manager application and learn how to manage the life cycles of your objects.
Managing scopes and contexts
Navigate to the
start directory to begin.
Start Open Liberty in development mode, which starts the Open Liberty server and listens for file changes:
This bean contains two simple functions.
add() function is for adding entries to the inventory.
list() function is for listing all the entries currently stored in the inventory.
This bean must be persistent between all of the clients, which means multiple clients need to share the same instance.
To achieve this by using CDI, you can simply add the
@ApplicationScoped annotation onto the class.
This annotation indicates that this particular bean is to be initialized once per application. By making it application-scoped, the container ensures that the same instance of the bean is used whenever it is injected into the application.
The inventory resource is a RESTful service that is served at the
Annotating a class with the
@ApplicationScoped annotation indicates that the bean is initialized once and is shared between all requests while the application runs.
If you want this bean to be initialized once for every request, you can annotate the class with the
@RequestScoped annotation instead. With the
@RequestScoped annotation, the bean is instantiated when the request is received and destroyed when a response is sent back to the client. A request scope is short-lived.
Injecting a dependency
Refer to the
InventoryResource class you created above.
@Inject annotation indicates a dependency injection.
You are injecting your
SystemClient beans into the
This injects the beans in their specified context and makes all of their functionalities
available without the need of instantiating them yourself.
The injected bean
InventoryManager can then be invoked directly through the
manager.list() function calls. The injected bean
SystemClient can be invoked through the
systemClient.getProperties(hostname) function call.
Finally, you have a client component
SystemClient that can be found in the
src/main/java/io/openliberty/guides/inventory/client directory. This class communicates
system service to retrieve the JVM system properties for a particular host
that exposes them. This class also contains detailed Javadocs that you can read for reference.
Your inventory application is now completed.
Building and running the application
Testing the inventory application
While you can test your application manually, you should rely on automated tests since they trigger a failure whenever a code change introduces a defect. Since the application is a RESTful web service application, you can use JUnit and the RESTful web service Client API to write tests. In testing the functionality of the application, the scopes and dependencies are being tested.
@BeforeAll annotation is placed on a method that runs before any of the test cases.
In this case, the
oneTimeSetup() method retrieves the port number for the Open Liberty server and builds
a base URL string that is used throughout the tests.
@AfterEach annotations are placed on methods that run before and after every test case.
These methods are generally used to perform any setup and teardown tasks. In this case, the
creates a JAX-RS client, which makes HTTP requests to the
inventory service. This client must
also be registered with a JSON-P provider (
JsrJsonpProvider) to process JSON resources. The
method simply destroys this client instance.
See the following descriptions of the test cases:
testHostRegistration()verifies that a host is correctly added to the inventory.
testSystemPropertiesMatch()verifies that the JVM system properties returned by the
systemservice match the ones stored in the
testUnknownHost()verifies that an unknown host or a host that does not expose their JVM system properties is correctly handled as an error.
To force these test cases to run in a particular order, annotate your
InventoryEndpointIT test class with the
OrderAnnotation.class runs test methods in numerical order,
according to the values specified in the
You can also create a custom
MethodOrderer class or use built-in
Random.class. Label your test cases
@Test annotation so that they automatically run when your test class runs.
is included for you to test the basic functionality of the
If a test failure occurs, then you might have introduced a bug into the code.
Running the tests
Since you started Open Liberty in development mode at the start of the guide, press the
enter/return key to run the tests.
If the tests pass, you see a similar output to the following example:
------------------------------------------------------- T E S T S ------------------------------------------------------- Running it.io.openliberty.guides.system.SystemEndpointIT Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.99 sec - in it.io.openliberty.guides.system.SystemEndpointIT Running it.io.openliberty.guides.inventory.InventoryEndpointIT Tests run: 3, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.325 sec - in it.io.openliberty.guides.inventory.InventoryEndpointIT Results : Tests run: 4, Failures: 0, Errors: 0, Skipped: 0
To see whether the tests detect a failure, change the
endpoint for the
inventory service in
src/main/java/io/openliberty/guides/inventory/InventoryResource.java file to something else. Then,
run the tests again to see that a test failure occurs.
When you are done checking out the service, exit development mode by typing
q in the shell session where you ran the server, and then press the
Great work! You’re done!
You just used CDI services in Open Liberty to build a simple inventory application.