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EventExample.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 <a href="mailto:geirm@optonline.net">Geir Magnusson Jr.</a>
 * @version $Id: EventExample.java,v 1.3.10.1 2004/03/04 00:18:29 geirm Exp $
 */

00045 public class EventExample implements ReferenceInsertionEventHandler, 
                                     NullSetEventHandler, MethodExceptionEventHandler,
                                     LogSystem
{

    private boolean logOutput = false;
    private boolean exceptionSwitch = false;

    public static void main( String args[] )
    {
        EventExample ee = new EventExample();
    }

    public EventExample()
    {
        try
        {
            /*
             *  this class implements the LogSystem interface, so we
             *  can use it as a logger for Velocity
             */
            
            Velocity.setProperty(Velocity.RUNTIME_LOG_LOGSYSTEM, this );
            Velocity.init();
        }
        catch(Exception e)
        {
            System.out.println("Problem initializing Velocity : " + e );
            return;
        }

        /* 
         *  lets make a Context and add some data
         */

        VelocityContext context = new VelocityContext();

        context.put("name", "Velocity");

        /*
         *  Now make an event cartridge, register all the 
         *  event handlers (at once) and attach it to the
         *  Context
         */

        EventCartridge ec = new EventCartridge();
        ec.addEventHandler(this);
        ec.attachToContext( context );
  
        try
        {
            /* 
             *  lets test each type of event handler individually
             *  using 'dynamic' templates
             *
             *  First, the reference insertion handler
             */

            System.out.println("");
            System.out.println("Velocity Event Handling Demo");
            System.out.println("============================");
            System.out.println("");

            String s = "The word 'Velocity' should be bounded by emoticons :  $name.";
            
            StringWriter w = new StringWriter();
            Velocity.evaluate( context, w, "mystring", s );

            System.out.println("Reference Insertion Test : ");
            System.out.println("   " +  w.toString());
            System.out.println("");

            /*
             *  using the same handler, we can deal with 
             *  null references as well
             */

            s = "There is no reference $floobie, $nullvalue or anything in the brackets : >$!silentnull<";

            w = new StringWriter();
            Velocity.evaluate( context, w, "mystring", s );

            System.out.println("Reference Insertion Test with null references : ");
            System.out.println("   " + w.toString());
            System.out.println("");

            /*
             *  now lets test setting a null value - this test
             *  should result in *no* log output.
             *  Turn on the logger output.
             */

            logOutput = true;

            s = "#set($settest = $NotAReference)";
            w = new StringWriter();

            System.out.println("NullSetEventHandler test : " );
            System.out.print("      There should be nothing between >");
            Velocity.evaluate( context, w, "mystring", s );
            System.out.println("< the brackets.");
            System.out.println("");

            /*
             *  now lets test setting a null value - this test
             *  should result in log output.
             */

            s = "#set($logthis = $NotAReference)";
            w = new StringWriter();

            System.out.println("NullSetEventHandler test : " );
            System.out.print("     There should be a log message between >");
            Velocity.evaluate( context, w, "mystring", s );
            System.out.println("< the brackets.");
            System.out.println("");

            logOutput = false;

            /*
             *  finally, we test a method exception event - we do this 
             *  by putting this class in the context, and calling 
             *  a method that does nothing but throw an exception.
             *  we use a little switch to turn the event handling
             *  on and off
             *
             *  Note also how the reference insertion process
             *  happens as well
             */
            
            exceptionSwitch = true;

            context.put("this", this );

            s = " $this.throwException()";
            w = new StringWriter();

            System.out.println("MethodExceptionEventHandler test : " );
            System.out.print("    This exception will be controlled and converted into a string : ");
            Velocity.evaluate( context, w, "mystring", s );
            System.out.println("   " + w.toString());
            System.out.println("");

            /*
             *  now, we turn the switch off, and we can see that the 
             *  exception will propgate all the way up here, and 
             *  wil be caught by the catch() block below
             */

            exceptionSwitch = false;

            s = " $this.throwException()";
            w = new StringWriter();

            System.out.println("MethodExceptionEventHandler test : " );
            System.out.println("    This exception will NOT be controlled. "
                             + " The next thing you should see is the catch() output ");
            Velocity.evaluate( context, w, "mystring", s );
            System.out.println("If you see this, it didn't work!");

        }
        catch( ParseErrorException pee )
        {
            /*
             * thrown if something is wrong with the
             * syntax of our template string
             */
            System.out.println("ParseErrorException : " + pee );
        }
        catch( MethodInvocationException mee )
        {
            /*
             *  thrown if a method of a reference
             *  called by the template
             *  throws an exception. That won't happen here
             *  as we aren't calling any methods in this
             *  example, but we have to catch them anyway
             */
            System.out.println("   Catch Block : MethodInvocationException : " + mee );
        }
        catch( Exception e )
        {
            System.out.println("Exception : " + e );
        }
    }

    /**
     *  silly method to throw an exception to demonstrate
     *  the method invocation exception event handling
     */
00235     public void throwException()
        throws Exception
    {
        throw new Exception("Hello from throwException()");
    }

    /**
     *  Event handler for when a reference is inserted into the output stream.
     */
00244     public Object referenceInsert( String reference, Object value  )
    {
        /*
         *  if we have a value
         *  lets decorate the reference with emoticons
         */

        String s = null;

        if( value != null )
        {
            s = " ;) " + value.toString() + " :-)";
        }
        else
        {
            /*
             * we only want to deal with $floobie - anything
             *  else we let go
             */
            if ( reference.equals("floobie") )
            {
                s = "<no floobie value>";
            }
        }
        return s;
    }

    /**
     *  Event handler for when the right hand side of
     *  a #set() directive is null, which results in 
     *  a log message.  This method gives the application
     *  a chance to 'vote' on msg generation
     */
00277     public boolean shouldLogOnNullSet( String lhs, String rhs )
    {
        if (lhs.equals("$settest"))
            return false;
        
        return true;
    }

    public Object methodException( Class claz, String method, Exception e )
         throws Exception
    {
        /*
         *  only do processing if the switch is on
         */

        if( exceptionSwitch && method.equals("throwException"))
        {
            return "Hello from the methodException() event handler method.";
        }

        throw e;
    } 

      /**
       *  Required init method for LogSystem
       *  to get access to RuntimeServices
       */ 
00304        public void init( RuntimeServices rs )
       {
            return;
       }

    /**
     *  This is the key method needed to implement a logging interface
     *  for Velocity.
     */ 
00313     public void logVelocityMessage(int level, String message)
    {
        if (logOutput)
        {
            System.out.print("Velocity Log : level : " + level + " msg : " + message);
        }
    }

}

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