org.bitbucket.cpointe.mash.velocity.VelocityMediator Maven / Gradle / Ivy
package org.bitbucket.cpointe.mash.velocity;
import java.io.StringWriter;
import java.util.Properties;
import org.apache.commons.io.IOUtils;
import org.apache.commons.lang3.StringUtils;
import org.apache.velocity.Template;
import org.apache.velocity.VelocityContext;
import org.apache.velocity.app.Velocity;
import org.apache.velocity.app.VelocityEngine;
import org.apache.velocity.runtime.resource.loader.ClasspathResourceLoader;
import org.bitbucket.cpointe.mash.MediationException;
import org.bitbucket.cpointe.mash.Mediator;
/**
* A mediator that uses Velocity to transform some input object to a String. The
* velocity template to use should be specified in your mediation definition's
* property with the name "velocity-template".
*
* Optionally, you can include a velocity-input-class parameter that will validate
* the input type to this mediation, throwing a {@link MediationException} if the
* expected type does not match the passed type.
*
* For example:
* [{
* "inputType": "string",
* "outputType": "string",
* "className": "org.bitbucket.cpointe.mash.velocity.VelocityMediator",
* "properties": [{
* "key": "velocity-template",
* "value": "/somepath/my-transformation-template.vm"
* },
* {
* "key": "velocity-input-class",
* "value":"org.bitbucket.cpointe.mash.some.package.ExpectedInputClass"}
* ]
* }]
*
* You can extend this class to add additional values to the
* {@link VelocityContext} by overriding getContext().
*/
public class VelocityMediator extends Mediator {
/**
* The file path to the jolt-specification to be used in this mediator.
*/
public static final String VELOCITY_MEDIATION_TEMPLATE = "velocity-template";
/**
* Provides the fully qualified class name that a class must be an instance of to be processed.
*/
public static final String VELOCITY_INPUT_CLASS = "velocity-input-class";
/**
* The variable in the context that contains the input to the mediation routine.
*/
public static final String INPUT = "input";
private String templatePath;
public VelocityMediator() {
Properties props = new Properties();
String className = ClasspathResourceLoader.class.getName();
props.setProperty("classpath." + VelocityEngine.RESOURCE_LOADER + ".class", className);
props.setProperty(VelocityEngine.RESOURCE_LOADER, "classpath");
Velocity.init(props);
}
@Override
protected Object performMediation(Object input, Properties properties) {
Object output = null;
if (input != null) {
StringWriter writer = null;
try {
validateInputClass(input, properties);
templatePath = properties.getProperty(VELOCITY_MEDIATION_TEMPLATE);
Template template = getTemplate();
VelocityContext context = getContext();
context.put(INPUT, input);
writer = new StringWriter();
template.merge(context, writer);
output = writer.toString();
} finally {
IOUtils.closeQuietly(writer);
}
}
return output;
}
protected Template getTemplate() {
if (StringUtils.isBlank(templatePath)) {
throw new MediationException(VELOCITY_MEDIATION_TEMPLATE + " was not specified!");
}
Template template = Velocity.getTemplate(templatePath);
if (template == null) {
throw new MediationException("Template '" + templatePath +"' could not be found!");
}
return template;
}
protected void validateInputClass(Object input, Properties properties) {
String className = properties.getProperty(VELOCITY_INPUT_CLASS, Object.class.getName());
Class expectedInputClass;
try {
expectedInputClass = Class.forName(className);
} catch (ClassNotFoundException e) {
throw new MediationException("Could not find mediator input class type!", e);
}
if (!expectedInputClass.isInstance(input)) {
throw new MediationException("Input (" + input.getClass().getName() + ") is not an instance of "
+ expectedInputClass.getName() + "!");
}
}
/**
* Extension point opportunity to add more information to the context for use in
* your velocity template.
*
* @return {@link VelocityContext}
*/
protected VelocityContext getContext() {
return new VelocityContext();
}
}