[jira] [Created] (KAFKA-8299) Add type-safe instantiation of generic classes to AbstractConfig

["Chris Egerton (JIRA)" <ji...@apache.org>]

Chris Egerton created KAFKA-8299:
------------------------------------

             Summary: Add type-safe instantiation of generic classes to AbstractConfig
                 Key: KAFKA-8299
                 URL: https://issues.apache.org/jira/browse/KAFKA-8299
             Project: Kafka
          Issue Type: Improvement
          Components: config
            Reporter: Chris Egerton
            Assignee: Chris Egerton


{{AbstractConfig.getConfiguredInstance(String key, Class<T> klass)}} and other similar methods isn't type-safe for generic types. For example, the following code compiles but generates a runtime exception when the created {{Consumer}} is invoked:

 
{code:java}
public class KafkaIssueSnippet {
    public static class PrintInt implements Consumer<Integer> {
        @Override
        public void accept(Integer i) {
            System.out.println(i);
        }
    }

    public static void main(String[] args) {
        final String stringConsumerProp = "string.consumer.class";

        AbstractConfig config = new AbstractConfig(
            new ConfigDef().define(
                stringConsumerProp,
                ConfigDef.Type.CLASS,
                ConfigDef.Importance.HIGH,
                "A class that implements Consumer<String>"
            ),
            Collections.singletonMap(
                stringConsumerProp,
                PrintInt.class.getName()
            )
        );

        Consumer<String> stringConsumer = config.getConfiguredInstance(
            stringConsumerProp,
            Consumer.class
        );

        stringConsumer.accept("Oops! ClassCastException");
    }
}{code}
The compiler (rightfully so) generates a warning about the unchecked cast from {{Consumer}} to {{Consumer<String>}} to indicate that exactly this sort of thing may happen, but it would be nice if we didn't have to worry about this in the first place and instead had the same guarantees for generic types that we do for non-generic types: that either the {{getConfiguredInstance(...)}} method returns an object to us that we know for sure is an instance of the requested type, or an exception is thrown.

Apache Commons contains a useful reflection library that could possibly be used to bridge this gap; specifically, its [TypeUtils|https://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/reflect/TypeUtils.html] and [TypeLiteral|https://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/reflect/TypeLiteral.html] classes could be used to add new {{getConfiguredInstance}} and {{getConfiguredInstances}} methods to the {{AbstractConfig}} class that accept instances of {{TypeLiteral}} instead of {{Class}} and then perform type checking to ensure that the requested class actually implements/extends from the requested type.

Since this affects public API it's possible a KIP will be required, but the changes are pretty lightweight (four new methods that heavily resemble existing ones). If a contributor or committer, especially one familiar with this section of the codebase, has an opinion on the necessity of a KIP their input would be appreciated.

 



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