How to write custom WSO2 synapse handler to validate the payload and response to the client?

In this article I'm going to cover below areas

1. How to write a Synapse handler
2. How to get the request payload
3. How to validate payload (In this example, I'm checking whether the payload is a valid JSON or not)
4. Respond to the customer without invoking the mediation flow.

Implement the handler class with extending the AbstractSynapseHandler class as below

package com.wso2.handler.sample;

import net.minidev.json.parser.JSONParser;
import org.apache.commons.io.IOUtils;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.synapse.AbstractSynapseHandler;
import org.apache.synapse.MessageContext;
import org.apache.synapse.core.axis2.Axis2MessageContext;
import org.apache.synapse.core.axis2.Axis2Sender;
import org.apache.synapse.transport.passthru.util.RelayUtils;

import java.io.IOException;
import java.io.InputStream;
import java.io.StringWriter;
import java.util.Map;

/**
 * Created by tharanga
 */
public class SampleMessageHandler extends AbstractSynapseHandler {

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

    public boolean handleRequestInFlow(MessageContext messageContext) {
        try {

            org.apache.axis2.context.MessageContext axis2MessageContext = ((Axis2MessageContext) messageContext)
                    .getAxis2MessageContext();
            Object headers = axis2MessageContext
                    .getProperty(org.apache.axis2.context.MessageContext.TRANSPORT_HEADERS);

            RelayUtils.buildMessage(((Axis2MessageContext) messageContext).getAxis2MessageContext());

            InputStream jsonPaylodStream = (InputStream) ((Axis2MessageContext) messageContext)
                    .getAxis2MessageContext().getProperty(
                            "org.apache.synapse.commons.json.JsonInputStream");
            StringWriter writer = new StringWriter();

            IOUtils.copy(jsonPaylodStream, writer);

            String payloadMessge = writer.toString();

            try {
                //Validate the json message.
                JSONParser jsonParser = new JSONParser();
                jsonParser.parse(payloadMessge);

                System.out.println(messageContext.getEnvelope().getBody().toString());

                
                return true;
            } catch (Exception ex) {
                Map headersMap = (Map) headers;
                headersMap.clear();
                axis2MessageContext.setProperty("HTTP_SC", "400");
                messageContext.setProperty("RESPONSE", "true");
                messageContext.setTo(null);

                Axis2Sender.sendBack(messageContext);

                return false;
            }

        } catch (Exception e) {
            e.printStackTrace();
        }

        return false;
    }

    public boolean handleRequestOutFlow(MessageContext messageContext) {
        return true;
    }

    public boolean handleResponseInFlow(MessageContext messageContext) {
        return true;
    }

    public boolean handleResponseOutFlow(MessageContext messageContext) {
        return true;
    }
}

In the above sample, I'm validating the JSON message and if it fails, send the 400 HTTP response code to the client.

Find the sample test result below

• Payload with valid JSON message

[2017-03-23 22:13:55,933] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "POST /services/Proxy1 HTTP/1.1[\r][\n]"
[2017-03-23 22:13:55,952] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Host: tharanga:8280[\r][\n]"
[2017-03-23 22:13:55,952] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Connection: keep-alive[\r][\n]"
[2017-03-23 22:13:55,952] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Content-Length: 19[\r][\n]"
[2017-03-23 22:13:55,952] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Postman-Token: 29dfad86-fa3e-5576-4bfd-25f503687baa[\r][\n]"
[2017-03-23 22:13:55,952] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Cache-Control: no-cache[\r][\n]"
[2017-03-23 22:13:55,952] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Origin: chrome-extension://fhbjgbiflinjbdggehcddcbncdddomop[\r][\n]"
[2017-03-23 22:13:55,956] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.87 Safari/537.36[\r][\n]"
[2017-03-23 22:13:55,956] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Content-Type: application/json[\r][\n]"
[2017-03-23 22:13:55,956] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Accept: */*[\r][\n]"
[2017-03-23 22:13:55,956] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Accept-Encoding: gzip, deflate[\r][\n]"
[2017-03-23 22:13:55,956] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Accept-Language: en-US,en;q=0.8[\r][\n]"
[2017-03-23 22:13:55,957] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "[\r][\n]"
[2017-03-23 22:13:55,957] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "{"name":"tharanga"}"
tharanga
[2017-03-23 22:13:56,239]  INFO - LogMediator status = 11111111111111111111111111111111111111111111
[2017-03-23 22:13:56,250] DEBUG - wire HTTP-Listener I/O dispatcher-1 << "HTTP/1.1 202 Accepted[\r][\n]"
[2017-03-23 22:13:56,251] DEBUG - wire HTTP-Listener I/O dispatcher-1 << "Date: Thu, 23 Mar 2017 16:43:56 GMT[\r][\n]"
[2017-03-23 22:13:56,251] DEBUG - wire HTTP-Listener I/O dispatcher-1 << "Transfer-Encoding: chunked[\r][\n]"
[2017-03-23 22:13:56,251] DEBUG - wire HTTP-Listener I/O dispatcher-1 << "Connection: keep-alive[\r][\n]"
[2017-03-23 22:13:56,251] DEBUG - wire HTTP-Listener I/O dispatcher-1 << "[\r][\n]"
[2017-03-23 22:13:56,252] DEBUG - wire HTTP-Listener I/O dispatcher-1 << "0[\r][\n]"
[2017-03-23 22:13:56,252] DEBUG - wire HTTP-Listener I/O dispatcher-1 << "[\r][\n]"

• Payload with invalid JSON message

[2017-03-23 22:14:10,283] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "POST /services/Proxy1 HTTP/1.1[\r][\n]"
[2017-03-23 22:14:10,283] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Host: tharanga:8280[\r][\n]"
[2017-03-23 22:14:10,283] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Connection: keep-alive[\r][\n]"
[2017-03-23 22:14:10,283] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Content-Length: 17[\r][\n]"
[2017-03-23 22:14:10,284] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Postman-Token: 194a9cd8-35ed-3998-08b7-fb94598797a3[\r][\n]"
[2017-03-23 22:14:10,284] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Cache-Control: no-cache[\r][\n]"
[2017-03-23 22:14:10,284] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Origin: chrome-extension://fhbjgbiflinjbdggehcddcbncdddomop[\r][\n]"
[2017-03-23 22:14:10,284] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.87 Safari/537.36[\r][\n]"
[2017-03-23 22:14:10,284] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Content-Type: application/json[\r][\n]"
[2017-03-23 22:14:10,284] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Accept: */*[\r][\n]"
[2017-03-23 22:14:10,284] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Accept-Encoding: gzip, deflate[\r][\n]"
[2017-03-23 22:14:10,284] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "Accept-Language: en-US,en;q=0.8[\r][\n]"
[2017-03-23 22:14:10,285] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "[\r][\n]"
[2017-03-23 22:14:10,285] DEBUG - wire HTTP-Listener I/O dispatcher-1 >> "{"name":"tharanga"
[2017-03-23 22:14:10,419] DEBUG - wire HTTP-Listener I/O dispatcher-1 << "HTTP/1.1 400 Bad Request[\r][\n]"
[2017-03-23 22:14:10,420] DEBUG - wire HTTP-Listener I/O dispatcher-1 << "Content-Type: application/json; charset=UTF-8[\r][\n]"
[2017-03-23 22:14:10,420] DEBUG - wire HTTP-Listener I/O dispatcher-1 << "Date: Thu, 23 Mar 2017 16:44:10 GMT[\r][\n]"
[2017-03-23 22:14:10,420] DEBUG - wire HTTP-Listener I/O dispatcher-1 << "Transfer-Encoding: chunked[\r][\n]"
[2017-03-23 22:14:10,420] DEBUG - wire HTTP-Listener I/O dispatcher-1 << "Connection: Close[\r][\n]"
[2017-03-23 22:14:10,420] DEBUG - wire HTTP-Listener I/O dispatcher-1 << "[\r][\n]"
[2017-03-23 22:14:10,421] DEBUG - wire HTTP-Listener I/O dispatcher-1 << "11[\r][\n]"
[2017-03-23 22:14:10,421] DEBUG - wire HTTP-Listener I/O dispatcher-1 << "{"name":"tharanga[\r][\n]"
[2017-03-23 22:14:10,421] DEBUG - wire HTTP-Listener I/O dispatcher-1 << "0[\r][\n]"
[2017-03-23 22:14:10,421] DEBUG - wire HTTP-Listener I/O dispatcher-1 << "[\r][\n]"