Camel :: Generic List<Enum> Converter

Question

Situation:

@Handler
public void handle(@Header("type") List<ListingType> listingType) {
    System.err.println(listingType);
}

...

public enum ListingType {
    TYPE_A,
    TYPE_B
}

If the value of listingType is some String that doesn't represent a value of the Enum, Camel resolves it to List<String>.

For example, if listingType = "foo"

System.err.println(listingType);
=> ["foo"]

I tryed it with a custom FallBackConverter

@FallbackConverter
public static <T> Object convertTo(Class<T> type, Exchange exchange, Object value, TypeConverterRegistry registry) {
    Class<?> concreteClass = ...
    if (List.class.isAssignableFrom(type) && concreteClass.isEnum()) {
        ...
    }
}

But how can I get the concrete generic type of the list?

Edited:

Or is there another solution to convert the String to the concrete list of enums?

Answer 1

You can't do it at Runtime. Generics in Java present only at compilation stage and don't affect on the program anyway.

Here you can read more about the Generics.

Answer 2

I don't know why you are keeping a list of strings in the first place instead of proper enum values.
I would try to explicitly convert the header before processing it.

<route>
    <from uri="direct:yourRoute">
    <to uri="bean:preprocessHeader" /> <!-- this converts the header to enum -->
    <to uri="bean:handle" /> <!-- this is your bean/method -->
</route>

Define a preprocessHeader bean with a method like

public process(Exchange exchange) {
    List<String> stringHeader = (List<String>)exchange.getIn().getHeader("type", List.class);

    List<ListingType> enumHeader = new ArrayList<>(stringHeader.size());

    for (String h : stringHeader) {
        try {
            ListingType t = ListingType.valueOf(h);
            enumHeader.add(t);
        } carch (IllegalArgumentException e) {
            // Handle the case where the string is not a proper enum value
            enumHeader.add(ListingType.UNKNOWN);
        }
    }

    exchange.getIn().setHeader("type", enumHeader);
}

Code not tested.