Interceptor Annotations

As well as supporting application-wide interceptors, also allows action bean-specific interceptors. Application-wide interceptors are executed before or after requests to each action bean defined in the Struts module concerned, while action bean-specific interceptors are only executed for the action declaring the interceptors.

Action bean-specific interceptors are declared using interceptor annotations. Two interceptor annotations are currently supported.

@BeforeInterceptors: used to identify BeforeInterceptor instances whose beforeExecute() method should be executed before any of the action bean interface methods are invoked.
@AfterInterceptors: used to identify AfterInterceptor instances whose afterExecute() method should be executed after all of the action bean interface methods are invoked.

An example is shown below:

@BeforeInterceptors(classes={CustomBeforeInterceptor1.class, CustomBeforeInterceptor2.class})
@AfterInterceptors(classes={CustomAfterInterceptor1.class, CustomAfterInterceptor2.class})
public class ExampleInterceptorAction implements BasicAction
{
	private String message;

	public void setMessage(String message)
	{
		this.message = message;
	}

	public String execute()
	{
		return "success";
	}

	public String getCurrentMessage()
	{
		return message;
	}
	
	public String getMessage()
	{
		return "Interceptor built message: " + message;
	}
}

CustomBeforeInterceptor1 and CustomBeforeInterceptor2 are executed before the action bean's execute() method is invoked. In the same way, CustomAfterInterceptor1 and CustomAfterInterceptor2 are executed afterwards.

One useful feature of action bean interceptors is that they can be parameterised using generics. Consider our implementation of CustomBeforeInterceptor1.

public class CustomBeforeInterceptor1 implements BeforeInterceptor<ExampleInterceptorAction>
{
	public void beforeExecute(ExampleInterceptorAction action, ActionContext context)
	{
		action.setMessage("CustomBeforeInterceptor1,");
	}
}

CustomBeforeInterceptor1 is parameterised using the action bean class type ExampleInterceptorAction. Without this capability, type casting would be necessary in the application code, as shown in the next example, which is not parameterized by the action bean type.

public class CustomBeforeInterceptor2 implements BeforeInterceptor
{
	public void beforeExecute(Object actionBean, ActionContext context)
	{
		if (actionBean instanceof ExampleInterceptorAction)
		{
			ExampleInterceptorAction action = (ExampleInterceptorAction) actionBean;
			action.setMessage(action.getCurrentMessage()+"CustomBeforeInterceptor2,");
		}
	}
}

Strecks will generate an intelligent runtime error message if you attempt to use an interceptor parameterised with a type incompatible with the action bean. Obviously it does not make sense to do this for application-wide interceptors.

@BeforeInterceptors

This annotation is used to identify the interceptor implementation classes. This annotation has a single required attribute, classes, which can either be a single Class instance or a Class array literal. Note that if application-wide BeforeInterceptors have been defined for the action, these will be executed before any of the interceptors declared using the @BeforeInterceptors annotation.

@AfterInterceptors

This annotation is used to identify the interceptor implementation classes. As with @BeforeInterceptors, this annotation has a single required attribute, classes, which can either be a single Class instance or a Class array literal. If application-wide AfterInterceptors have been defined for the action, these will be executed after any of the interceptors declared using the @AfterInterceptors annotation.