Velocity 事件处理 (Event Handler) 演示，velocityhandler,EventExample

Velocity 事件处理 (Event Handler) 演示，velocityhandler,EventExample

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 $ */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);        }    }}