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# Writing Interceptors

See the Interceptors page for an overview of how interceptors work.

## Interceptor interface

Interceptors must implement the `com.opensymphony.xwork2.interceptor.Interceptor` interface.

Interceptor.java

    public interface Interceptor extends Serializable {

        void destroy();

        void init();

        String intercept(ActionInvocation invocation) throws Exception;
    }

The `init` method is called the after interceptor is instantiated and before calling `intercept`. This is the place to allocate any resources used by the interceptor.

The `intercept` method is where the interceptor code is written. Just like an action method, `intercept` returns a result used by Struts to forward the request to another web resource. Calling `invoke` on the parameter of type `ActionInvocation` will execute the action (if this is the last interceptor on the stack) or another interceptor.

> Keep in mind that `invoke` will return after the result has been called (eg. after you JSP has been rendered), making it perfect for things like open-session-in-view patterns. If you want to do something before the result gets called, you should implement a `PreResultListener`.

Overwrite `destroy` to release resources on application shutdown.

## Thread Safety

Interceptors must be thread-safe!

> A Struts 2 Action instance is created for every request and do not need to be thread-safe. Conversely, Interceptors are shared between requests and must be thread-safe.

## AbstractInterceptor

The AbstractInterceptor class provides an empty implementation of `init` and `destroy`, and can be used if these methods are not going to be implemented.

## Mapping

Interceptors are declared using the `<interceptor/>` element, nested inside the `<interceptors/>` element. Example from `struts-default.xml`:

    <struts>
       ...

       <package name="struts-default">
          <interceptors>
             <interceptor name="alias" class="com.opensymphony.xwork2.interceptor.AliasInterceptor"/>
             <interceptor name="autowiring" class="com.opensymphony.xwork2.spring.interceptor.ActionAutowiringInterceptor"/>
             ...
          </interceptors>
       </package>

       ...
    </struts>

### Example

Assuming there is an action of type “MyAction”, with a setDate(Date) method, this simple interceptor will set the date of the action to the current date:

Interceptor Example

    import com.opensymphony.xwork2.ActionInvocation;
    import com.opensymphony.xwork2.interceptor.AbstractInterceptor;

    public class SimpleInterceptor extends AbstractInterceptor {

        public String intercept(ActionInvocation invocation) throws Exception {
           MyAction action = (MyAction)invocation.getAction();
           action.setDate(new Date());
           return invocation.invoke();
        }
    }

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