Why Jersey / JAX-RS client can't handle generics?
I have a Jersery / JAX-RS client that hits a RESTful API (JSON) that should return a list of my POJOs:
// Hits: GET localhost:8080/myapp/fizz/widget/{widget_id}
@Override
public List<Widget> getWidgetsByUser(Long id) {
return webResource.path("fizz").path("widget").path(id.toString()).get(List.class);
}
And a driver to test the client with:
public class Driver {
public static void main(String[] args) {
Driver d = new Driver();
d.run();
}
public void run() {
MyAppService myService = getSomehow();
List<Widget> widgets = myService.getWidgetResource().getWidgetsByUser(2L);
for(Widget widget : widgets) {
System.out.println("\t...and it found Widget #" + widget.getCaseId());
}
}
}
When I run this I get:
Exception in thread "main" java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to com.me.myapp.Widget
<stack trace omitted for brevity>
Exception thrown from for loop:
for(Widget widget : widgets) {
Which tells me that the client is working, but I configured it incorrectly.
So either Jersey is trying to return pure JSON rather than trying to translate it back into the Widgets list, or I'm calling the method get(Class<?>)
incorrectly. Any ideas?
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Paul Choral's commentary is correct. Without a known type, Jackson (base deserilaizer) will map to LinkedHashMap
, so it will returnList<LinkedHashMap>
Fix:
For generic types, we have to use another method get()
that accepts GenericType
. Therefore, we have to do something like
...get(new GenericType<List<Widget>>(){});
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