What is @Component annotation

The @Component annotation is fundamental in Liferay’s OSGi (Open Service Gateway Initiative) module system. It declares a Java class as an OSGi component that can provide or consume services. Below is a comprehensive breakdown of its attributes and usage.

---

1. What is @Component?

• Purpose: Marks a class as an OSGi Declarative Services (DS) component.
• Usage: Used to define components that can be managed by the OSGi container.
• Location: From the org.osgi.service.component.annotations package.

---

2. Full Attribute Reference

Here’s a table of all possible @Component attributes in Liferay 7.4:

Attribute	Type	Default	Description
immediate	boolean	false	If true, the component is activated immediately after registration.
enabled	boolean	true	If false, the component is disabled and won’t start.
service	Class<?>[]	{}	Specifies the service interfaces this component provides.
property	String[]	{}	OSGi properties for the component (key-value pairs).
properties	Property[]	{}	Alternative to property for more structured properties.
configurationPid	String[]	{}	Associates the component with a Configuration PID.
configurationPolicy	ConfigurationPolicy	ConfigurationPolicy.OPTIONAL	Defines how the component reacts to missing configurations.
scope	ServiceScope	ServiceScope.SINGLETON	Defines the component’s lifecycle (SINGLETON, PROTOTYPE, or BUNDLE).
factory	boolean	false	If true, the component is a factory component.
name	String	Generated	Explicitly sets the component name.

---

3. Common Use Cases with Examples

1. Basic Component Registration

@Component(
    immediate = true,
    service = MyService.class
)
public class MyServiceImpl implements MyService {
    // Implementation
}

• Explanation: Registers MyServiceImpl as an OSGi service under MyService.

2. Adding Custom Properties

@Component(
    property = {
        "key1=value1",
        "key2=value2",
        "service.ranking:Integer=100"
    },
    service = MyService.class
)
public class MyServiceImpl implements MyService {
    // Implementation
}

• Explanation: Adds metadata (key1, key2) and sets service ranking.

3. Configuration-PID Based Component

@Component(
    configurationPid = "com.example.MyConfig",
    configurationPolicy = ConfigurationPolicy.REQUIRE
)
public class ConfigurableComponent {
    // Requires "com.example.MyConfig" to activate
}

• Explanation: Only activates if com.example.MyConfig is available.

4. Factory Component (Multiple Instances)

@Component(
    factory = true,
    scope = ServiceScope.PROTOTYPE
)
public class PrototypeComponent {
    // New instance created per request
}

5. Sample portlet with @Component

@Component(property = {
    "com.liferay.portlet.display-category=category.sample",
    "com.liferay.portlet.header-portlet-css=/css/main.css",
    "com.liferay.portlet.instanceable=true",
    "javax.portlet.display-name=My",
    "javax.portlet.init-param.template-path=/",
    "javax.portlet.init-param.view-template=/view.jsp",
    "javax.portlet.name=" + MyPortletKeys.MY,
    "javax.portlet.resource-bundle=content.Language",
    "javax.portlet.security-role-ref=power-user,user"
}, service = Portlet.class)

• Explanation: Creates a new instance each time the service is requested.

---

4. Key Properties in Liferay

Liferay uses special properties in @Component for framework integration:

Property	Meaning	Example
"osgi.jaxrs.name=..."	Registers a JAX-RS resource	"osgi.jaxrs.name=my.rest"
"auth.verifier.guest.allowed=true"	Allows guest access	"auth.verifier.guest.allowed=true"
"service.ranking:Integer=100"	Sets service priority	"service.ranking:Integer=100"

---

5. Best Practices

1. Use immediate = true for components that must start right away.
2. Avoid ConfigurationPolicy.REQUIRE unless mandatory (can prevent startup).
3. Prefer @Reference for service dependencies instead of manual lookups.
4. Use service.ranking to control service override priority.

---

6. Troubleshooting

Issue	Solution
Component not starting	Check immediate=true and dependencies.
Missing service	Verify @Reference or service= declaration.
Configuration not applying	Ensure configurationPid matches .cfg file.

---

7. Conclusion

• @Component is the backbone of OSGi services in Liferay.
• It supports flexible registration, configuration, and lifecycle control.
• Proper use of attributes ensures modularity and maintainability.

Next Steps

• Explore @Reference for dependency injection.
• Learn about OSGi Configuration Admin for dynamic settings.
• Check Liferay’s official docs for advanced patterns.