extension.md

Extension System

Based on the Java SPI mechanism, it provides annotation, dependency injection, and configuration functionalities.

1. Define Extension

Define an extension interface and use the @Extensible annotation to declare the extension.

The following defines an extension interface for AgentService:

@Extensible("AgentService")
public interface AgentService {
    
    CompletableFuture<Void> start();
    
    CompletableFuture<Void> stop();
    
}

2. Extension Implementation

Implement the extension interface and use the @Extension annotation to declare the extension implementation.

The following implements a file synchronization service LiveSpaceFileSyncer:

@Injectable
@Extension("LiveSpaceFileSyncer")
@ConditionalOnProperty(name = SyncConfig.SYNC_LIVE_SPACE_TYPE, value = "file")
@ConditionalOnProperty(name = GovernanceConfig.CONFIG_LIVE_ENABLED, matchIfMissing = true)
public class LiveSpaceFileSyncer extends AbstractFileSyncer<List<LiveSpace>> {
    
    private static final String CONFIG_LIVE_SPACE = "livespaces.json";

    @Inject(PolicySupervisor.COMPONENT_POLICY_SUPERVISOR)
    private PolicySupervisor policySupervisor;

    @Config(SyncConfig.SYNC_LIVE_SPACE)
    private SyncConfig syncConfig = new SyncConfig();
}

Annotations on this class:

1. The @Extension annotation declares the extension implementation and provides a name.
2. The @ConditionalOnProperty annotation declares the conditions for enabling, allowing multiple conditions to be combined.
3. The @Injectable annotation declares that the type requires automatic injection.
4. The @Inject annotation declares that the field requires automatic injection.
5. The @Config annotation declares that the field requires automatic configuration.

3. Enable Extension

1. Configure the full path name of the extension in the SPI file META-INF/services/com.jd.live.agent.core.service.AgentService: com.jd.live.agent.implement.service.policy.file.LiveSpaceFileSyncer
2. Configure the live space synchronization type as file.

4. Annotation Description

4.1 @Extensible

This annotation is declared on the extension interface with the following configuration parameters:

Parameter	Type	Default Value	Description
value	String		Extension interface name

4.2 @Extension

This annotation is declared on the extension implementation with the following configuration parameters:

Parameter	Type	Default Value	Description
value	String		Extension implementation name
order	int	Short.MAX_VALUE	Priority, the smaller the value, the higher the priority
singleton	boolean	true	Whether it is a singleton

4.3 Enabling Conditions

Conditions can be configured on the extension implementation.

4.3.1 @ConditionalOnProperty

This annotation declares the configuration item switch for enabling. Multiple @ConditionalOnProperty annotations can be configured, with an AND relationship between each.

Parameter	Type	Default Value	Description
value	String		Configuration value
name	String[]		Configuration name array
matchIfMissing	boolean	true	Whether to match if not configured
relation	ConditionalRelation	OR	Relation OR or AND
caseSensitive	boolean	false	Case sensitivity
comparison	enum	EQUAL	comparison EQUAL or NOT_EQAUL

4.3.2 @ConditionalOnProperties

This annotation declares multiple configuration item switches, consisting of multiple @ConditionalOnProperty annotations, with a relationship between each.

Parameter	Type	Default Value	Description
value	ConditionalOnProperty[]		Configuration item array
relation	ConditionalRelation	AND	Relation OR or AND

4.3.3 @ConditionalOnClass

This annotation declares the type condition for enabling. If the type exists, it matches. Multiple @ConditionalOnClass annotations can be configured, with an AND relationship between each.

Parameter	Type	Default Value	Description
value	String		Full path class name

4.3.4 @ConditionalOnMissingClass

This annotation declares the type condition for enabling. If the type does not exist, it matches. Multiple @ConditionalOnMissingClass annotations can be configured, with an AND relationship between each.

Parameter	Type	Default Value	Description
value	String		Full path class name

4.4 @Injectable

This annotation is declared on the extension implementation with the following configuration parameters:

Parameter	Type	Default Value	Description
value	boolean	true	Whether to enable injection

4.4.1 @Inject

This annotation is declared on fields of the extension implementation with the following configuration parameters:

Parameter	Type	Default Value	Description
value	String	""	Component name
nullable	boolean	false	Whether the value can be null

4.4.1.1 Injecting System Components

Declare on a field to inject the system component with the specified name:

@Inject(Application.COMPONENT_APPLICATION)
private Application application;

Common system components include:

Name	Type	Description
AgentConfig.COMPONENT_AGENT_CONFIG	AgentConfig	Agent configuration
EnhanceConfig.COMPONENT_ENHANCE_CONFIG	EnhanceConfig	Enhancement configuration
AgentPath.COMPONENT_AGENT_PATH	AgentPath	Agent path
Application.COMPONENT_APPLICATION	Application	Application instance
Timer.COMPONENT_TIMER	Timer	Timer wheel
ClassLoaderConfig.COMPONENT_CLASSLOADER_CONFIG	ClassLoaderConfig	Class loader configuration
AgentLifecycle.COMPONENT_AGENT_LIFECYCLE	AgentLifecycle	Agent lifecycle management
Registry.COMPONENT_REGISTRY	Registry	Registry
PolicySupervisor.COMPONENT_POLICY_SUPERVISOR	PolicySupervisor	Policy supervisor
InvocationContext.COMPONENT_INVOCATION_CONTEXT	InvocationContext	Flow control intercept context

4.4.1.2 Injecting Extension Implementations

Declare on an extension interface field to inject the highest priority implementation of that extension:

@Inject
private LoadBalancer loadBalancer;

Declare on a map field of the extension interface to inject all extension implementations:

@Inject
private Map<String, LoadBalancer> loadBalancers;

4.4.1.3 Injecting Event Publishers

Declare on an event publisher field to inject the event publisher with the specified name:

@Inject(Publisher.POLICY_SUBSCRIBER)
private Publisher<PolicySubscriber> policyPublisher;