git clone https://github.com/openliberty/guide-microprofile-rest-client.git
cd guide-microprofile-rest-client
Consuming RESTful services with template interfaces
Prerequisites:
Learn how to use MicroProfile Rest Client to invoke RESTful microservices over HTTP in a type-safe way.
What you’ll learn
You will learn how to build a MicroProfile Rest Client to access remote RESTful services. You will create a template interface that maps to the remote service that you want to call. MicroProfile Rest Client automatically generates a client instance based on what is defined and annotated in the template interface. Thus, you don’t have to worry about all of the boilerplate code, such as setting up a client class, connecting to the remote server, or invoking the correct URI with the correct parameters.
The application that you will be working with is an inventory
service, which fetches and stores the system property information for different hosts. Whenever a request is made to retrieve the system properties of a particular host, the inventory
service will create a client to invoke the system
service on that host. The system
service simulates a remote service in the application.
You will instantiate the client and use it in the inventory
service. You can choose from two different approaches, Context and Dependency Injection (CDI) with the help of MicroProfile Config or the RestClientBuilder method. In this guide, you will explore both methods to handle scenarios for providing a valid base URL.
-
When the base URL of the remote service is static and known, define the default base URL in the configuration file. Inject the client with a CDI method.
-
When the base URL is not yet known and needs to be determined during the run time, set the base URL as a variable. Build the client with the more verbose
RestClientBuilder
method.
Getting started
The fastest way to work through this guide is to clone the Git repository and use the projects that are provided inside:
The start
directory contains the starting project that you will build upon.
The finish
directory contains the finished project that you will build.
Before you begin, make sure you have all the necessary prerequisites.
Try what you’ll build
The finish
directory in the root of this guide contains the finished 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:
cd finish
mvn liberty:run
After you see the following message, your Liberty instance is ready:
The defaultServer server is ready to run a smarter planet.
The system
microservice simulates a service that returns the system property information for the host. The system
service is accessible at the http://localhost:9080/system/properties URL. In this case, localhost
is the host name.
The inventory
microservice makes a request to the system
microservice and stores the system property information. To fetch and store your system information, visit the http://localhost:9080/inventory/systems/localhost URL.
You can also use the http://localhost:9080/inventory/systems/{your-hostname}
URL. In Windows, MacOS, and Linux, get your fully qualified domain name (FQDN) by entering hostname
into your command-line. Visit the URL by replacing {your-hostname}
with your FQDN.
After you are finished checking out the application, stop the Liberty instance by pressing CTRL+C
in the command-line session where you ran Liberty. Alternatively, you can run the liberty:stop
goal from the finish
directory in another shell session:
mvn liberty:stop
Writing the RESTful client interface
Now, navigate to the start
directory to begin.
When you run Open Liberty in dev mode, dev mode listens for file changes and automatically recompiles and deploys your updates whenever you save a new change. Run the following goal to start Open Liberty in dev mode:
mvn liberty:dev
After you see the following message, your Liberty instance is ready in dev mode:
************************************************************** * Liberty is running in dev mode.
Dev mode holds your command-line session to listen for file changes. Open another command-line session to continue, or open the project in your editor.
The MicroProfile Rest Client API is included in the MicroProfile dependency specified by your pom.xml
file. Look for the dependency with the microprofile
artifact ID.
pom.xml
1<?xml version='1.0' encoding='utf-8'?>
2<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
3 http://maven.apache.org/xsd/maven-4.0.0.xsd">
4 <modelVersion>4.0.0</modelVersion>
5
6 <groupId>io.openliberty.guides</groupId>
7 <artifactId>guide-microprofile-rest-client</artifactId>
8 <version>1.0-SNAPSHOT</version>
9 <packaging>war</packaging>
10
11 <properties>
12 <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
13 <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
14 <maven.compiler.source>11</maven.compiler.source>
15 <maven.compiler.target>11</maven.compiler.target>
16 <!-- Liberty configuration -->
17 <liberty.var.http.port>9080</liberty.var.http.port>
18 <liberty.var.https.port>9443</liberty.var.https.port>
19 </properties>
20
21 <dependencies>
22 <!-- Provided dependencies -->
23 <dependency>
24 <groupId>jakarta.platform</groupId>
25 <artifactId>jakarta.jakartaee-api</artifactId>
26 <version>10.0.0</version>
27 <scope>provided</scope>
28 </dependency>
29 <dependency>
30 <groupId>org.eclipse.microprofile</groupId>
31 <!-- tag::microprofile[] -->
32 <artifactId>microprofile</artifactId>
33 <!-- end::microprofile[] -->
34 <version>6.1</version>
35 <type>pom</type>
36 <scope>provided</scope>
37 </dependency>
38 <!-- Java utility classes -->
39 <dependency>
40 <groupId>org.apache.commons</groupId>
41 <artifactId>commons-lang3</artifactId>
42 <version>3.17.0</version>
43 </dependency>
44 <!-- For tests-->
45 <dependency>
46 <groupId>org.junit.jupiter</groupId>
47 <artifactId>junit-jupiter</artifactId>
48 <version>5.11.1</version>
49 <scope>test</scope>
50 </dependency>
51 <dependency>
52 <groupId>org.jboss.resteasy</groupId>
53 <artifactId>resteasy-client</artifactId>
54 <version>6.2.10.Final</version>
55 <scope>test</scope>
56 </dependency>
57 <dependency>
58 <groupId>org.jboss.resteasy</groupId>
59 <artifactId>resteasy-json-binding-provider</artifactId>
60 <version>6.2.10.Final</version>
61 <scope>test</scope>
62 </dependency>
63 <dependency>
64 <groupId>org.glassfish</groupId>
65 <artifactId>jakarta.json</artifactId>
66 <version>2.0.1</version>
67 <scope>test</scope>
68 </dependency>
69 </dependencies>
70
71 <build>
72 <finalName>${project.artifactId}</finalName>
73 <plugins>
74 <plugin>
75 <groupId>org.apache.maven.plugins</groupId>
76 <artifactId>maven-war-plugin</artifactId>
77 <version>3.4.0</version>
78 </plugin>
79 <!-- Plugin to run unit tests -->
80 <plugin>
81 <groupId>org.apache.maven.plugins</groupId>
82 <artifactId>maven-surefire-plugin</artifactId>
83 <version>3.5.0</version>
84 </plugin>
85 <!-- Enable liberty-maven plugin -->
86 <plugin>
87 <groupId>io.openliberty.tools</groupId>
88 <artifactId>liberty-maven-plugin</artifactId>
89 <version>3.10.3</version>
90 </plugin>
91 <!-- Plugin to run functional tests -->
92 <plugin>
93 <groupId>org.apache.maven.plugins</groupId>
94 <artifactId>maven-failsafe-plugin</artifactId>
95 <version>3.5.0</version>
96 <configuration>
97 <systemPropertyVariables>
98 <http.port>${liberty.var.http.port}</http.port>
99 </systemPropertyVariables>
100 </configuration>
101 </plugin>
102 </plugins>
103 </build>
104</project>
This dependency provides a library that is required to implement the MicroProfile Rest Client interface.
The mpRestClient
feature is also enabled in the src/main/liberty/config/server.xml
file. This feature enables your Open Liberty to use MicroProfile Rest Client to invoke RESTful microservices.
server.xml
1<server description="Sample Liberty server">
2
3 <featureManager>
4 <feature>restfulWS-3.1</feature>
5 <feature>jsonp-2.1</feature>
6 <feature>jsonb-3.0</feature>
7 <feature>cdi-4.0</feature>
8 <!-- tag::mpConfig[] -->
9 <feature>mpConfig-3.1</feature>
10 <!-- end::mpConfig[] -->
11 <!-- tag::mpRestClient[] -->
12 <feature>mpRestClient-3.0</feature>
13 <!-- end::mpRestClient[] -->
14 </featureManager>
15
16 <variable name="http.port" defaultValue="9080"/>
17 <variable name="https.port" defaultValue="9443"/>
18
19 <httpEndpoint host="*" httpPort="${http.port}" httpsPort="${https.port}"
20 id="defaultHttpEndpoint"/>
21
22 <webApplication location="guide-microprofile-rest-client.war" contextRoot="/"/>
23</server>
The code for the system
service in the src/main/java/io/openliberty/guides/system
directory is provided for you. It simulates a remote RESTful service that the inventory
service invokes.
Create a RESTful client interface for the system
service. Write a template interface that maps the API of the remote system
service. The template interface describes the remote service that you want to access. The interface defines the resource to access as a method by mapping its annotations, return type, list of arguments, and exception declarations.
Create theSystemClient
class.src/main/java/io/openliberty/guides/inventory/client/SystemClient.java
SystemClient.java
The MicroProfile Rest Client feature automatically builds and generates a client implementation based on what is defined in the SystemClient
interface. There is no need to set up the client and connect with the remote service.
Notice the SystemClient
interface inherits the AutoCloseable
interface. This allows the user to explicitly close the client instance by invoking the close()
method or to implicitly close the client instance using a try-with-resources block. When the client instance is closed, all underlying resources associated with the client instance are cleaned up. Refer to the MicroProfile Rest Client specification for more details.
When the getProperties()
method is invoked, the SystemClient
instance sends a GET request to the <baseUrl>/properties
endpoint, where <baseUrl>
is the default base URL of the system
service. You will see how to configure the base URL in the next section.
The @Produces
annotation specifies the media (MIME) type of the expected response. The default value is MediaType.APPLICATION_JSON
.
The @RegisterProvider
annotation tells the framework to register the provider classes to be used when the framework invokes the interface. You can add as many providers as necessary. In the SystemClient
interface, add a response exception mapper as a provider to map the 404
response code with the UnknownUriException
exception.
Handling exceptions through ResponseExceptionMappers
Error handling is an important step to ensure that the application can fail safely. If there is an error response such as 404 NOT FOUND
when invoking the remote service, you need to handle it. First, define an exception, and map the exception with the error response code. Then, register the exception mapper in the client interface.
Look at the client interface again, the @RegisterProvider
annotation registers the UnknownUriExceptionMapper
response exception mapper. An exception mapper maps various response codes from the remote service to throwable exceptions.
SystemClient.java
Implement the actual exception class and the mapper class to see how this mechanism works.
Create theUnknownUriException
class.src/main/java/io/openliberty/guides/inventory/client/UnknownUriException.java
UnknownUriException.java
Now, link the UnknownUriException
class with the corresponding response code through a ResponseExceptionMapper
mapper class.
Create theUnknownUriExceptionMapper
class.src/main/java/io/openliberty/guides/inventory/client/UnknownUriExceptionMapper.java
UnknownUriExceptionMapper.java
The handles()
method inspects the HTTP response code to determine whether an exception is thrown for the specific response, and the toThrowable()
method returns the mapped exception.
Injecting the client with dependency injection
Now, instantiate the SystemClient
interface and use it in the inventory
service. If you want to connect only with the default host name, you can easily instantiate the SystemClient
with CDI annotations. CDI injection simplifies the process of bootstrapping the client.
First, you need to define the base URL of the SystemClient
instance. Configure the default base URL with the MicroProfile Config feature. This feature is enabled for you in the server.xml
file.
Create the configuration file.
src/main/resources/META-INF/microprofile-config.properties
microprofile-config.properties
The mp-rest/uri
base URL config property is configured to the default http://localhost:9080/system
URL.
This configuration is automatically picked up by the MicroProfile Config API.
Look at the annotations in the SystemClient
interface again.
SystemClient.java
The @RegisterRestClient
annotation registers the interface as a RESTful client. The runtime creates a CDI managed bean for every interface that is annotated with the @RegisterRestClient
annotation.
The configKey
value in the @RegisterRestClient
annotation replaces the fully-qualified classname of the properties in the microprofile-config.properties
configuration file. For example, the <fully-qualified classname>/mp-rest/uri
property becomes systemClient/mp-rest/uri
. The benefit of using Config Keys is when multiple client interfaces have the same configKey
value, the interfaces can be configured with a single MP config property.
The baseUri
value can also be set in the @RegisterRestClient
annotation. However, this value will be overridden by the base URI property defined in the microprofile-config.properties
configuration file, which takes precedence. In a production environment, you can use the baseUri
variable to specify a different URI for development and testing purposes.
The @RegisterRestClient
annotation, which is a bean defining annotation implies that the interface is manageable through CDI. You must have this annotation in order to inject the client.
Inject the SystemClient
interface into the InventoryManager
class, which is another CDI managed bean.
Replace theInventoryManager
class.src/main/java/io/openliberty/guides/inventory/InventoryManager.java
InventoryManager.java
@Inject
and @RestClient
annotations inject an instance of the SystemClient
called defaultRestClient
to the InventoryManager
class.
Because the InventoryManager
class is @ApplicationScoped
, and the SystemClient
CDI bean maintains the same scope through the default dependent scope, the client is initialized once per application.
If the hostname
parameter is localhost
, the service runs the getPropertiesWithDefaultHostName()
helper function to fetch system properties. The helper function invokes the system
service by calling the defaultRestClient.getProperties()
method.
server.xml
1<server description="Sample Liberty server">
2
3 <featureManager>
4 <feature>restfulWS-3.1</feature>
5 <feature>jsonp-2.1</feature>
6 <feature>jsonb-3.0</feature>
7 <feature>cdi-4.0</feature>
8 <!-- tag::mpConfig[] -->
9 <feature>mpConfig-3.1</feature>
10 <!-- end::mpConfig[] -->
11 <!-- tag::mpRestClient[] -->
12 <feature>mpRestClient-3.0</feature>
13 <!-- end::mpRestClient[] -->
14 </featureManager>
15
16 <variable name="http.port" defaultValue="9080"/>
17 <variable name="https.port" defaultValue="9443"/>
18
19 <httpEndpoint host="*" httpPort="${http.port}" httpsPort="${https.port}"
20 id="defaultHttpEndpoint"/>
21
22 <webApplication location="guide-microprofile-rest-client.war" contextRoot="/"/>
23</server>
Building the client with RestClientBuilder
The inventory
service can also connect with a host other than the default localhost
host, but you cannot configure a base URL that is not yet known. In this case, set the host name as a variable and build the client by using the RestClientBuilder
method. You can customize the base URL from the host name attribute.
Look at the getPropertiesWithGivenHostName()
method in the src/main/java/io/openliberty/guides/inventory/InventoryManager.java
file.
InventoryManager.java
The host name is provided as a parameter. This method first assembles the base URL that consists of the new host name. Then, the method instantiates a RestClientBuilder
builder with the new URL, registers the response exception mapper, and builds the SystemClient
instance.
Similarly, call the customRestClient.getProperties()
method to invoke the system
service.
Running the application
You started the Open Liberty in dev mode at the beginning of the guide, so all the changes were automatically picked up.
When the Liberty instance is running, select either approach to fetch your system properties:
-
Visit the http://localhost:9080/inventory/systems/localhost URL. The URL retrieves the system property information for the
localhost
host name by making a request to thesystem
service athttp://localhost:9080/system/properties
.
-
Get your FQDN first. Then, visit the
http://localhost:9080/inventory/systems/{your-hostname}
URL by replacing{your-hostname}
with your FQDN, which retrieves your system properties by making a request to thesystem
service athttp://{your-hostname}:9080/system/properties
.
Testing the application
Create theRestClientIT
class.src/test/java/it/io/openliberty/guides/client/RestClientIT.java
RestClientIT.java
Each test case tests one of the methods for instantiating a RESTful client.
The testDefaultLocalhost()
test fetches and compares system properties from the http://localhost:9080/inventory/systems/localhost URL.
The testRestClientBuilder()
test gets your IP address. Then, use your IP address as the host name to fetch your system properties and compare them.
In addition, a few endpoint tests are provided for you to test the basic functionality of the inventory
and system
services. If a test failure occurs, you might have introduced a bug into the code.
Running the tests
Because you started Open Liberty in dev mode, you can run the tests by pressing the enter/return
key from the command-line session where you started dev mode.
-------------------------------------------------------
T E S T S
-------------------------------------------------------
Running it.io.openliberty.guides.system.SystemEndpointIT
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 1.377 sec - in it.io.openliberty.guides.system.SystemEndpointIT
Running it.io.openliberty.guides.inventory.InventoryEndpointIT
Interceptor for {http://client.inventory.guides.openliberty.io/}SystemClient has thrown exception, unwinding now
Could not send Message.
[err] The specified host is unknown.
Tests run: 3, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.379 sec - in it.io.openliberty.guides.inventory.InventoryEndpointIT
Running it.io.openliberty.guides.client.RestClientIT
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.121 sec - in it.io.openliberty.guides.client.RestClientIT
Results :
Tests run: 5, Failures: 0, Errors: 0, Skipped: 0
The warning and error messages are expected and result from a request to a bad or an unknown hostname. This request is made in the testUnknownHost()
test from the InventoryEndpointIT
integration test.
To see whether the tests detect a failure, change the base URL in the configuration file so that when the inventory
service tries to access the invalid URL, an UnknownUriException
is thrown. Rerun the tests to see a test failure occur.
When you are done checking out the service, exit dev mode by pressing CTRL+C
in the command-line session where you ran Liberty.
Great work! You’re done!
You just invoked a remote service by using a template interface with MicroProfile Rest Client in Open Liberty.
MicroProfile Rest Client also provides a uniform way to configure SSL for the client. You can learn more in the Hostname verification with SSL on Open Liberty and MicroProfile Rest Client blog and the MicroProfile Rest Client specification.
Feel free to try one of the related guides where you can learn more technologies and expand on what you built here.
Guide Attribution
Consuming RESTful services with template interfaces by Open Liberty is licensed under CC BY-ND 4.0
Prerequisites:
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