How to Implement a Class Mediator for WSO2 ESB

A Mediator is simply a processing unit in the ESB. A mediator can do any modification to the messages.

In WSO2 ESB there are so many built-in mediators available. Also we can write custom mediators and add more to the available capabilities of the ESB.

Among those built in mediators Class Mediator is capable of creating an instance of a custom-specified class and set it as a mediator. There are two approaches to create custom mediators.

• Extend the org.apache.synapse.mediators.AbstractMediator class
• Implement the org.apache.synapse.api.Mediator interface 

In this post I'm going to show you how to implement a simple Class Mediator and using it in a proxy service.

1. Create a maven project with the name OrderServiceMediator.
2. You need to add the following dependency to your pom.xml

        
            org.apache.synapse
            synapse-core
            2.1.2-wso2v4
        

3. Also you will have to configure the required maven repository as follows. If this is not configured once you try mvn clean install it will give the following error.
   [ERROR] Failed to execute goal on project OrderServiceMediator: Could not resolve dependencies for project

        
            wso2-nexus
            WSO2 internal Repository
            http://maven.wso2.org/nexus/content/groups/wso2-public/
            
                true
                daily
                ignore
            
        

4. Here is my pom.xml after configuring everything.

    4.0.0

    org.wso2.training.manorama
    OrderServiceMediator
    1.0

    
    
    
        
            wso2-nexus
            WSO2 internal Repository
            http://maven.wso2.org/nexus/content/groups/wso2-public/
            
                true
                daily
                ignore
            
        
    

    
        
            org.apache.synapse
            synapse-core
            2.1.2-wso2v4
        

    
    

Next step is to write the Class Mediator Java code. 

1. Create a new package called mediator (or any other package name you prefer).  If you skip this step once you try to deploy the mediator in ESB Carbon will not make a bundle from it. So you will get the following error.
   wso2 esb org.eclipse.osgi.internal.loader.BundleLoader.findClassInternal(BundleLoader.java:501)
2. Add a new Java class CustomMediator which extends the AbstractMediator.
3. Now you will have override the public boolean mediate(MessageContext messageContext) method.
4. Implement the logic inside this method to do whatever modifications you need to do to the messages .
5. Here in my simple class mediator I am going to give a discount for the orders submit to the OrderProcessingService which is deployed in the Axis2 server. (This is the same OrderProcessingService described in my previous post).

package mediator;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.synapse.MessageContext;
import org.apache.synapse.mediators.AbstractMediator;

import javax.xml.namespace.QName;

/**
 * Created with IntelliJ IDEA.
 * User: manorama
 * Date: 1/05/15
 * Time: 10:25 AM
 * To change this template use File | Settings | File Templates.
 */
public class CustomMediator extends AbstractMediator {

    private static final Log log = LogFactory.getLog(CustomMediator.class);

    private String discountFactor = "10";
    private String bonusFor = "10";
    private int bonusCount = 0;

    @Override
    public boolean mediate(MessageContext messageContext) {

        // xs:schema targetNamespace given in the WSDL is "http://ws.apache.org/axis2"

        String price = messageContext.getEnvelope().getBody().getFirstElement().
                getFirstChildWithName(new QName("http://ws.apache.org/axis2","amount")).getText();

        int discount = Integer.parseInt(discountFactor);
        int bonusNo = Integer.parseInt(bonusFor);
        double currentPrice = Double.parseDouble(price);

        // Discounting factor is deducted from current price form every response
        Double lastPrice = new Double(currentPrice - currentPrice * discount / 100);

        // Special discount of 5% offers for the first responses as set in the bonusFor property
        if (bonusCount <= bonusNo) {
            lastPrice = new Double(lastPrice.doubleValue() - lastPrice.doubleValue() * 0.05);
            bonusCount++;
        }

        String discountedPrice = lastPrice.toString();

        messageContext.getEnvelope().getBody().getFirstElement().
                getFirstChildWithName(new QName("http://ws.apache.org/axis2","amount")).setText(discountedPrice);

        log.info("Order price discounted");
        log.info("Original price: " + price);
        log.info("Discounted price: " + discountedPrice);

        return true;
    }

    public String getType() {
        return null;
    }

    public void setTraceState(int traceState) {
        traceState = 0;
    }

    public int getTraceState() {
        return 0;
    }

    public void setDiscountFactor(String discount) {
        discountFactor = discount;
    }

    public String getDiscountFactor() {
        return discountFactor;
    }

    public void setBonusFor(String bonus) {
        bonusFor = bonus;
    }

    public String getBonusFor() {
        return bonusFor;
    }
}

6. Note that the setter methods are implemented, to set the required property values through the mediator.
7. Next we will create the proxy service to the back-end service deployed in the Axis2 server. You can follow the steps mentioned in the WSO2 ESB user guide to create the proxy service. Here is my proxy service configuration.

   
       
           
               
                   
                   
               
           
           
               
               
           
           
               
   
           
   
       
       scenario1
       
       
       
   
   

8. Now try to send a message to this proxy service and see the discounted price is assigned for to the amount. You can see the resulted value after going through the mediator using this url.
   
   http://localhost:8080/axis2/services/OrderProcessingService/getAmount?orderID=25

References :
[1] https://docs.wso2.com/display/ESB481/Sample+380:+Writing+your+own+Custom+Mediation+in+Java
[2] https://docs.wso2.com/display/ESB481/Class+Mediator
[3] http://stackoverflow.com/questions/24260091/wso2-esb-jar-file-with-callback-handler-not-loading