How to handle Signals in an Workflow in Java

Use the @SignalMethod annotation to handle Signals in the Workflow interface.

The Signal type defaults to the name of the method. In the following example, the Signal type defaults to retryNow.

@WorkflowInterface
public interface FileProcessingWorkflow {

   @WorkflowMethod
   String processFile(Arguments args);

   @SignalMethod
   void retryNow();
}

To overwrite this default naming and assign a custom Signal type, use the @SignalMethod annotation with the name parameter. In the following example, the Signal type is set to retrysignal.

@WorkflowInterface
public interface FileProcessingWorkflow {

   @WorkflowMethod
   String processFile(Arguments args);

   @SignalMethod(name = "retrysignal")
   void retryNow();
}

A Workflow interface can define any number of methods annotated with @SignalMethod, but the method names or the name parameters for each must be unique.

In the following example, we define a Signal method updateGreeting to update the greeting in the Workflow. We set a Workflow.await in the Workflow implementation to block the current Workflow Execution until the provided unblock condition is evaluated to true. In this case, the unblocking condition is evaluated to true when the Signal to update the greeting is received.

@WorkflowInterface
public interface HelloWorld {
    @WorkflowMethod
    void sayHello(String name);

    @SignalMethod
    void updateGreeting(String greeting);
}

public class HelloWorldImpl implements HelloWorld {
    private final Logger workflowLogger = Workflow.getLogger(HelloWorldImpl.class);
    private String greeting;

    @Override
    public void sayHello(String name) {
        int count = 0;
        while (!"Bye".equals(greeting)) {
            String oldGreeting = greeting;
            Workflow.await(() -> !Objects.equals(greeting, oldGreeting));
        }
        workflowLogger.info(++count + ": " + greeting + " " + name + "!");
    }

    @Override
    public void updateGreeting(String greeting) {
        this.greeting = greeting;
    }
}

This Workflow completes when the Signal updates the greeting to Bye.

Dynamic Signal Handler You can also implement Signal handlers dynamically. This is useful for library-level code and implementation of DSLs.

Use Workflow.registerListener(Object) to register an implementation of the DynamicSignalListener in the Workflow implementation code.

      Workflow.registerListener(
        (DynamicSignalHandler)
            (signalName, encodedArgs) -> name = encodedArgs.get(0, String.class));

When registered, any Signals sent to the Workflow without a defined handler will be delivered to the DynamicSignalHandler. Note that you can only register one Workflow.registerListener(Object) per Workflow Execution. DynamicSignalHandler can be implemented in both regular and dynamic Workflow implementations.