Apache NiFi custom processor expression language

[#1]

I'm trying to make a custom processor in Apache NiFi that can add an attribute/string to the JSON object in the flowfile content. At the moment it works when I just use a string but it's not working when I use NiFi's expression language although I have it supported in my code.

The expression language is 100% correct as it works in another processor and I've also tried different attributes to make sure it's not the attribute. The property:

public static final PropertyDescriptor ADD_ATTRIBUTE = new PropertyDescriptor
        .Builder().name("Add Attribute")
        .description("Example Property")
        .required(true)
        .addValidator(StandardValidators.NON_EMPTY_VALIDATOR)
        .expressionLanguageSupported(true)
        .build();

Later in my code when I want to get the value and put in the JSON object I use:

jsonObject.put("hostname", context.getProperty(ADD_ATTRIBUTE).evaluateAttributeExpressions().getValue());

I also made a Unit Test and it works when I assign a text value to the testrunner.setProperty. However I don't know how I can assign an attribute to the testrunner or how I can use expression language in my test. Thanks in advance for any suggestions or a solution!

[#2 · Accepted solution]

If the expression refers to attributes on a flow file, you will need to pass a reference to the flow file into evaluateAttributeExpressions:

FlowFile flowFile = session.get();
jsonObject.put("hostname", context.getProperty(ADD_ATTRIBUTE).evaluateAttributeExpressions(flowFile).getValue());

If the property contains an attribute name (rather than an Expression containing an attribute name) and you want the value from the flow file:

jsonObject.put("hostname", flowFile.getAttribute(context.getProperty(ADD_ATTRIBUTE).getValue()));

If the attribute value itself contains Expression Language and you want to evaluate it, take a look at the following class:

org.apache.nifi.attribute.expression.language.Query

[#3]

Thank you very much for this answer!

[#4]

Matt, how would the properties set in custom properties file be accessed within the custom controller service's onConfigured method? Thanks.

[#5]

That sounds like a different issue altogether, perhaps you'd get more/better responses if you create a new question in HCC to deal with it. The short answer is: your ControllerService implementation can have a method annotated with @OnEnabled, looks like you're calling yours onConfigured(). That method can take a ConfigurationContext:

@OnEnabled
public void onConfigured(final ConfigurationContext context) throws InitializationException {
// Do stuff here 
}

And you can call getProperty(), getProperties(), etc. on it

[#6]

Matt,

Appreciate if you provide an example on how to use the org.apache.nifi.attribute.expression.language.Query using Groovy. Thanks.