Writing a Custom Function

To write a custom function, create a class extending "org.wso2.siddhi.core.executor.function.FunctionExecutor", add the SiddhiExtionsion annotation, compile that class, and add the jar file to the class path <DAS_HOME>/repository/components/lib. Then add the fully-qualified class name for the implementation class in a new line, to the siddhi.extension file located at <DAS_HOME>/repository/conf/siddhi.

For example, if you have created the extension with namespace custom and function name plus, you can use that in the query as follows:

from InMediationStatsStream
select meta_host,timestamp,resource_id,direction,fault_count,custom:plus(fault_count,count) as totalCount
insert into OutMediationStatsStream;

import org.apache.log4j.Logger;
import org.wso2.siddhi.core.config.SiddhiContext;
import org.wso2.siddhi.core.exception.QueryCreationException;
import org.wso2.siddhi.core.executor.function.FunctionExecutor;
import org.wso2.siddhi.query.api.definition.Attribute;
import org.wso2.siddhi.query.api.extension.annotation.SiddhiExtension;

@SiddhiExtension(namespace = "custom", function = "plus")
public class CustomFunctionExtension extends FunctionExecutor {
    Logger log = Logger.getLogger(CustomFunctionExtension.class);
    Attribute.Type returnType;
    
	/**
     * Method will be called when initialising the custom function
     *
     * @param types
     * @param siddhiContext
     */
    @Override
    public void init(Attribute.Type[] types, SiddhiContext siddhiContext) {
        for (Attribute.Type attributeType : types) {
            if (attributeType == Attribute.Type.DOUBLE) {
                returnType = attributeType;
                break;
            } else if ((attributeType == Attribute.Type.STRING) || (attributeType == Attribute.Type.BOOL)) {
                throw new QueryCreationException("Plus cannot have parameters with types String or Bool");
            } else {
                returnType = Attribute.Type.LONG;
            }
        }
    }
    
	/**
     * Method called when sending events to process
     *
     * @param obj
     * @return
     */
    @Override
    protected Object process(Object obj) {

        if (returnType == Attribute.Type.DOUBLE) {
            double total = 0;
            if (obj instanceof Object[]) {
                for (Object aObj : (Object[]) obj) {
                    total += Double.parseDouble(String.valueOf(aObj));
                }
            }
            return total;
        } else {
            long total = 0;
            if (obj instanceof Object[]) {
                for (Object aObj : (Object[]) obj) {
                    total += Long.parseLong(String.valueOf(aObj));
                }
            }
            return total;
        }
    }
   
	@Override
    public void destroy() {
    }
    /**
     * Return type of the custom function mentioned
     *
     * @return
     */
    
	@Override
    public Attribute.Type getReturnType() {
        return returnType;
    }

}

Sample project file can be downloaded from here.