Mega Code Archive
A simple demonstration of how the event handling features of the Velocity Servlet Engine are used
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/*
* 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 $
*/
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
*/
public void throwException()
throws Exception
{
throw new Exception("Hello from throwException()");
}
/**
* Event handler for when a reference is inserted into the output stream.
*/
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 $floobie - anything
* else we let go
*/
if ( reference.equals("floobie") )
{
s = "<no floobie value>";
}
}
return s;
}
/**
* Event handler for when the right hand side of
* a #set() directive is null, which results in
* a log message. This method gives the application
* a chance to 'vote' on msg generation
*/
public boolean shouldLogOnNullSet( String lhs, String rhs )
{
if (lhs.equals("$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
*/
public void init( RuntimeServices rs )
{
return;
}
/**
* This is the key method needed to implement a logging interface
* for Velocity.
*/
public void logVelocityMessage(int level, String message)
{
if (logOutput)
{
System.out.print("Velocity Log : level : " + level + " msg : " + message);
}
}
}
velocity-Event-Handle.zip( 1,876 k)
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