Java/Velocity/Velocity Event

A simple demonstration of how the event handling features of the Velocity Servlet Engine are used

   <source lang="java">

/*

* Copyright 2001,2004 The Apache Software Foundation.
* 
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
* 
*      http://www.apache.org/licenses/LICENSE-2.0
* 
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

import java.io.StringWriter; import java.util.Properties; import org.apache.velocity.app.Velocity; import org.apache.velocity.VelocityContext; import org.apache.velocity.exception.ParseErrorException; import org.apache.velocity.exception.MethodInvocationException; import org.apache.velocity.runtime.log.LogSystem; import org.apache.velocity.runtime.RuntimeServices; import org.apache.velocity.app.event.EventCartridge; import org.apache.velocity.app.event.ReferenceInsertionEventHandler; import org.apache.velocity.app.event.MethodExceptionEventHandler; import org.apache.velocity.app.event.NullSetEventHandler; import org.apache.velocity.context.Context; /**

*   This class is a simple demonstration of how the event handling
*   features of the Velocity Servlet Engine are used.  It uses a
*   custom logger as well to check the log message stream
*   when testing the NullSetEventHandler
*
* @author 

Event Handler: Exceptions

   <source lang="java">

import java.io.StringWriter; import java.io.Writer; import org.apache.velocity.Template; import org.apache.velocity.VelocityContext; import org.apache.velocity.app.Velocity; import org.apache.velocity.app.event.EventCartridge; import org.apache.velocity.app.event.MethodExceptionEventHandler; import org.apache.velocity.app.event.NullSetEventHandler; import org.apache.velocity.app.event.ReferenceInsertionEventHandler; import org.apache.velocity.context.Context; public class EventHandler implements ReferenceInsertionEventHandler,

   NullSetEventHandler, MethodExceptionEventHandler {
 public EventHandler(Context ctx) {
   EventCartridge ec = new EventCartridge();
   ec.addEventHandler(this);
   ec.attachToContext(ctx);
 }
 public Object referenceInsert(String reference, Object data) {
   System.out.println("referenceInsert: " + reference + " data: " + data);
   return data;
 }
 public boolean shouldLogOnNullSet(String lhs, String rhs) {
   System.out.println("shouldLogOnNullSet");
   System.out.println("lhs:" + lhs + " rhs:" + rhs);
     return true;
 }
 public Object methodException(Class cls, String method, Exception e)
     throws Exception {
   return "An " + e.getClass().getName() + " was thrown by the " + method
       + " method of the " + cls.getName() + " class [" + e.getMessage() + "]";
 }
 public static void main(String[] args) throws Exception {
   Velocity.init();
   Template t = Velocity.getTemplate("./src/eventHandler.vm");
   Context ctx = new VelocityContext();
   ctx.put("person", "Joe");
   ctx.put("exception", new ExceptionGenerator());
   EventHandler hdl = new EventHandler(ctx);
   Writer writer = new StringWriter();
   t.merge(ctx, writer);
   System.out.println(writer);
 }

}

import java.util.Random; public class ExceptionGenerator {

 public String generate() throws Exception {
   Random rnd = new Random();
   int x = rnd.nextInt(5);
   if (x == 2) {
     throw new Exception("Unlucky!");
   } else {
     return "No Exception";
   }
 }

}

---

1. set($myNull1 = $validNull)
2. set($myNull2 = $invalidNull)

This is $person $exception.generate() $exception.generate() $exception.generate() $exception.generate()

      </source>