Apache Hop
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Development Documentation 2.20.0

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Extension Points

An extension point is an event-driven hook in Apache Hop. Hop announces specific lifecycle events (such as HopGuiStart, PipelinePrepareExecution, TransformBeforeStart, or PipelineCompleted), and any registered plugin listening for that event is invoked synchronously.

Implementing an Extension Point

Annotate your class with @ExtensionPoint and implement IExtensionPoint<T>:

java
import org.apache.hop.core.extension.ExtensionPoint;
import org.apache.hop.core.extension.IExtensionPoint;
import org.apache.hop.core.logging.ILogChannel;
import org.apache.hop.core.variables.IVariables;
import org.apache.hop.pipeline.PipelineMeta;
import org.apache.hop.pipeline.engine.IPipelineEngine;

@ExtensionPoint(
    id = "AuditPipelineCompleted",
    extensionPointId = "PipelineCompleted",
    description = "Logs auditing information when any pipeline finishes execution"
)
public class AuditPipelineCompletedXp implements IExtensionPoint<IPipelineEngine<PipelineMeta>> {

  @Override
  public void callExtensionPoint(ILogChannel log, IVariables variables,
                                 IPipelineEngine<PipelineMeta> pipeline) throws HopException {
    log.logBasic("Pipeline completed: " + pipeline.getPipelineMeta().getName() +
                 " with status: " + pipeline.getStatusDescription());
  }
}

Key Architectural Rules

1. Bind to Interfaces, Not Implementations

Always bind to the engine interface (IPipelineEngine or IWorkflowEngine), never concrete classes (Pipeline or Workflow). Declaring IExtensionPoint<Pipeline> will function on the local engine but crash with a ClassCastException on remote engines or Apache Beam engines.

2. Inline Synchronous Execution

Extension points execute on the thread that triggered the event. Keep your logic fast and non-blocking. If your extension point needs to perform heavy I/O, network requests, or database writes, offload the workload to a background worker queue.

3. Discovering Available Extension Points

All core extension point IDs are defined as constants in the org.apache.hop.core.extension.HopExtensionPoint enum.

Concrete Codebase Examples

  • engine/src/main/java/org/apache/hop/lineage/xp/LineageHubPipelineCompletedXp.java: Emits lineage events upon pipeline completion.

  • ui/src/main/java/org/apache/hop/ui/hopgui/notifications/NotificationSystemInitializer.java: Initializes notifications on the HopGuiStart extension point.

  • plugins/misc/projects/src/main/java/org/apache/hop/projects/xp/PipelineStartCheckProjectExtensionPoint.java: Hooks PipelinePrepareExecution to check the active project.

See also