javadoc

spring-webflow/src/main/java/org/springframework/webflow/engine/FlowExecutionExceptionHandler.java

@@ -15,11 +15,26 @@
  */
 package org.springframework.webflow.engine;
 
+import org.springframework.webflow.engine.support.TransitionExecutingFlowExecutionExceptionHandler;
 import org.springframework.webflow.execution.FlowExecutionException;
 
 /**
  * A strategy for handling an exception that occurs at runtime during the execution of a flow definition.
  * 
+ * Note: special care should be taken when implementing custom flow execution exception handlers. Exception handlers are
+ * like Transitions in that they direct flow control when exceptions occur. They are more complex than Actions, which
+ * can only execute behaviors and return results that drive flow control. For this reason, if implemented incorrectly, a
+ * FlowExecutionHandler can leave a flow execution in an invalid state, which can render the flow execution unusable or
+ * its future use undefined. For example, if an exception thrown at flow startup gets routed to an exception handler,
+ * the handler must take responsibility for ensuring the flow execution returns control to the caller in a consistent
+ * state. Concretely, this means the exception handler must transition the flow to its start state. The handler should
+ * not simply return leaving the flow with no current state set.
+ * 
+ * Note: Because flow execution handlers are more difficult to implement correctly, consider catching exceptions in your
+ * web flow action code and returning result events that drive standard transitions. Alternatively, consider use of the
+ * existing {@link TransitionExecutingFlowExecutionExceptionHandler} which illustrates the proper way to implement an
+ * exception handler.
+ * 
  * @author Keith Donald
  */
 public interface FlowExecutionExceptionHandler {
@@ -32,8 +47,8 @@ public interface FlowExecutionExceptionHandler {
 	public boolean canHandle(FlowExecutionException exception);
 
 	/**
-	 * Handle the exception in the context of the current request, optionally making an error view selection that should
-	 * be rendered.
+	 * Handle the exception in the context of the current request. An implementation is expected to transition the flow
+	 * to a state using {@link RequestControlContext#execute(Transition)}.
 	 * @param exception the exception that occurred
 	 * @param context the execution control context for this request
 	 */