APIs as OSGi Services

APIs as OSGi Services

After you’ve learned what a module is and how to deploy one, you can use modules to define APIs and implement them. Liferay APIs are OSGi services, defined by Java interfaces and implemented by concrete Java classes.

Liferay exposes APIs, implementations, and clients as components. OSGi Declarative Services (DS) annotations define components and their relationships.

  • @ProviderType defines an interface that components can provide (implement) or consume.
  • @Component declares the class a component, providing a particular capability.
  • @Reference wires another component to a class member (typically a field).

You can separate API and implementation concerns into different modules.

  • API modules define capabilities using Java interfaces. The modules export the interface packages.
  • Implementation modules provide capabilities using concrete Java classes.

Here you’ll deploy API and implementation modules that create a simple greeter OSGi service. You’ll also examine the implementation module and its JAR to learn how the implementation provides the greeter service capability. In the next tutorial, you’ll create the client—the part that you can invoke in the UI.

Deploy a Simple API and Implementation

Start a new Liferay instance by running

docker run -it -m 8g -p 8080:8080 liferay/portal:

Sign in to Liferay at http://localhost:8080. Use the email address [email protected] and the password test. When prompted, change the password to learn.

Then, follow these steps to start the example modules:

  1. Download and unzip liferay-p9g2.zip.

    curl https://resources.learn.liferay.com/dxp/latest/en/liferay-internals/fundamentals/liferay-p9g2.zip -O
    unzip liferay-p9g2.zip
  2. From the project root folder, deploy the modules.

    cd liferay-p9g2
    ./gradlew deploy -Ddeploy.docker.container.id=$(docker ps -lq)
  3. Confirm module startup in the Docker container console.

    STARTED com.acme.p9g2.api_1.0.0
    STARTED com.acme.p9g2.impl_1.0.0
  4. Go to http://localhost:8080 and sign in.

  5. Go to the Gogo Shell.

  6. Get the module IDs using the lb Gogo Shell command.

    g! lb | grep -i "Acme P9G2"


    1150|Active     |   15|Acme P9G2 API (1.0.0)|1.0.0
    1151|Active     |   15|Acme P9G2 Implementation (1.0.0)|1.0.0
  7. List the implementation module service capabilities by executing the following command, replacing the number with your module’s ID.

    g! inspect capability service 1195


    com.acme.p9g2.impl_1.0.0 [1151] provides:
    service; com.acme.p9g2.Greeter with properties:
       service.id = 22933
       service.bundleid = 1151
       service.scope = bundle
       component.name = com.acme.p9g2.internal.P9G2Greeter
       component.id = 8462

The Acme P9G2 Implementation module provides one service: com.acme.p9g2.Greeter. The component.name property indicates that the module’s com.acme.p9g2.internal.P9G2Greeter component implements the service.

You verified that the P9G2Greeter component provides the Greeter service.

Next you’ll learn how the API module defines the greeter capability and the implementation module provides the greeter capability as an OSGi service. Start with creating the API.

Create the API

An API is created in just two steps:

Define the Capability

The example API module Greeter class is a Java interface.

public interface Greeter {

The @ProviderType annotation registers Greeter as a type that a component can implement or consume.

The greet method takes a name String as input.

public void greet(String name);

The Greeter capability is defined.

Export the Interface Package

The API module bnd.bnd file describes the module and exports the com.acme.p9g2 interface package.

Bundle-Name: Acme P9G2 API
Bundle-SymbolicName: com.acme.p9g2.api
Bundle-Version: 1.0.0
Export-Package: com.acme.p9g2

The package export shares the Greeter interface with other modules.

The Greeter service type is available to implement and use.

Create the Implementation

The example implementation module contains a concrete Java class that provides the Greeter capability. Here are the implementation steps.

Add the Component Annotation

The P9G2Greeter class implements the Greeter interface:

@Component(service = Greeter.class)
public class P9G2Greeter implements Greeter {

The @Component annotation and its service = Greeter.class attribute make the P9G2Greeter class a Greeter service provider.

Implement the Interface

The Greeter interface defines a method greet(String) with a void return value.

public void greet(String name) {
	System.out.println("Hello " + name + "!");

The example greet method prints an enthusiastic greeting using the given name.

Add a Dependency on the API

Here is the implementation module build.gradle file.

dependencies {
	compileOnly group: "com.liferay.portal", name: "release.portal.api"
	compileOnly project(":p9g2-api")

It includes a compile-time dependency on the p9g2-api module project because it requires the module’s Greeter class.

Examine the Module JAR

When you built the p9g2-impl/build/libs/com.acme.p9g2.impl-1.0.0.jar implementation module JAR, Bnd generated the JAR’s /META-INF/MANIFEST.MF file.

Here are key service-related headers that Bnd generates in the manifest:

Import-Package: com.acme.p9g2;version="[1.0,2)"

The Import-Package header imports the API module’s public package that contains the Greeter service definition.

Provide-Capability: osgi.service;objectClass:List<String>="com.acme.p9

The Provide-Capability header configures the P9G2Greeter component service.

Service-Component: OSGI-INF/com.acme.p9g2.internal.P9G2Greeter.xml

The Service-Component header lists a configuration file (.xml) for each of the module’s service components.

When you deployed the module, the Service Component Runtime registered the P9G2Greeter service component as providing the Greeter service.


You have defined a service capability called Greeter and provided it in a service component called P9G2Greeter. The Greeter service is in place. How do clients access the service and use it? That’s demonstrated in Using an OSGi Service.