Java 如何使泛型类型生效

Question

Below is code sample which I working on.

interface KafkaRecord {}

class Book implements KafkaRecord { 
    //some properties here with getter setter
}

class BookDeserializer implements Deserializer<Book> {
    //Deserialiazing bytes to Book 
}


// props is Properties which having key value pair for configuration. 
Consumer<String, KafkaRecord> consumer = new KafkaConsumer<>(props);
ConsumerRecords<String, KafkaRecord> records = 
        consumer.poll(Duration.ofMillis(getTimeout()));

records.foreach(bean -> SomeComponent.process((Book) bean));

This work as expected. I created another bean Pencil and forgot to implement KafkaRecord, and below code is still working.

class Pencil {
  //some getters and setters
}

class PencilDeserializer implements Deserializer<Pencil> {
  //Deserialiazing bytes to Pencil
}

// props is Properties which having key value pair for configuration.
Consumer<String, KafkaRecord> consumer = new KafkaConsumer<>(props); 

// Pencil does not implement KafkaRecord
ConsumerRecords<String, KafkaRecord> records =
        consumer.poll(Duration.ofMillis(getTimeout()));

records.foreach(bean -> SomeOtherComponent.process((Pencil) bean));

ConsumerRecords definition(kafka-clients-2.4.0.jar):

public class ConsumerRecords<K, V> implements Iterable<ConsumerRecord<K, V>> {
    //Behavior and getter/setters
}

Seems like below issue is happening because of generic erasure. How can I fix this issue?

After some more digging -

Internal working from AbstractConfig.java- For setting deserilizer -

1. Its calling getClass(key) //key is coming from configuration
2. It then create instance which it got from step 1.
3. Then it cast to Class t(where t is Deserializer, in our case T should be KafkaRecord)

What I believe is that as run time type is not present, it allowing to set Deserializer<Pencil> to be set in first place. So its able to deserialize it and able to assign it in ConsumerRecords<String, KafkaRecord> records , as type is not there.

Sample GIT - https://github.com/jitendraVishnoi/kafka

Edit - I added antoher method in CustomExecutor.java -

private void playWithRecord(KafkaRecord record) {
    System.out.println("Record class: " + record.getClass()); // Record class: class kafka.beans.Pencil
    System.out.println("record instanceof KafkaRecord: " + (record instanceof KafkaRecord)); // record instanceof KafkaRecord: false
    System.out.println("record instanceof Pencil: " + (record instanceof Pencil)); // record instanceof Pencil: true
    System.out.println("toString() :" + record); // toString() :Pencil{color='Red'}
  }

and which being called from-

ConsumerRecords<String, KafkaRecord> pencilRecords = pencilConsumer.poll(Duration.ofMillis(1000));
pencilRecords.forEach(bean -> playWithRecord(bean.value()));

first line works but second line break with ClassCastException.

Exception in thread "Thread-0" java.lang.ClassCastException: kafka.beans.Pencil cannot be cast to kafka.beans.KafkaRecord
        at CustomExecutor.lambda$run$0(CustomExecutor.java:28)
        at java.lang.Iterable.forEach(Unknown Source)
        at CustomExecutor.run(CustomExecutor.java:28)
        at java.lang.Thread.run(Unknown Source)

If we update playWithRecord() to accept argument of type Object instead of KafkaRecord it works.

Looks to me its happening as Type has been erased for -

ConsumerRecords<String, KafkaRecord> pencilRecords = pencilConsumer.poll(Duration.ofMillis(1000));

Answer 1

Actually, this is not a generic type / erasure problem at all.

Consider this:

SomeOtherComponent.process((Pencil) bean))

At compile time, the question is: can bean ... which we know implements KafakaRecord ... possibly be Pencil or some subclass of Pencil ?

Now we know that it can't be an instance of Pencil , because Pencil does not implement KafakaRecord . But suppose I declare the following:

class ColoredPencil extends Pencil implements KafkaRecord {
    ...
}

If our records collection at runtime consisted of KafkaRecord instances that were ColouredPencil objects, then they would be instances of Pencil , so the type cast should not be a compile time error.

In general, a cast from an interface type to a class type is NOT a compile time error, unless the class type is final and the class does not directly or indirectly implement the interface.

---

Is there any way I can report this issue while writing code. That Pencil must implement KafkaRecord .

Not with the code as written.

You could try this approach:

interface <T extends KafkaRecord> KafkaDeserializer extends Deserializer<T> {}

class PencilDeserializer implements KafkaDeserializer<Pencil> {
     // Deserialiazing bytes to Pencil
}

The latter will give a compilation error if Pencil hasn't been declared as a subtype of KafkaRecord . That might be sufficient for your needs, especially if you can ensure that all of your deserializers implement KafkaDeserializer rather than Deserializer . (I don't know enough about Kafka to know if this actually makes sense.)

And another approach would be to simply declare Pencil as final .

Answer 2

It seems like this is happening because of generic erasure. https://coderanch.com/t/739304/java/Java-Generic-run-time

I had to discuss this in another forum as I was not getting much response here. @Stephen has already explained how to force java to report it compile time.