io.dropwizard.util.Generics Maven / Gradle / Ivy
package io.dropwizard.util;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.lang.reflect.TypeVariable;
import static java.util.Objects.requireNonNull;
/**
* Helper methods for class type parameters.
* @see Super Type Tokens
*/
public class Generics {
private Generics() { /* singleton */ }
/**
* Finds the type parameter for the given class.
*
* @param klass a parameterized class
* @return the class's type parameter
*/
public static Class getTypeParameter(Class klass) {
return getTypeParameter(klass, Object.class);
}
/**
* Finds the type parameter for the given class which is assignable to the bound class.
*
* @param klass a parameterized class
* @param bound the type bound
* @param the type bound
* @return the class's type parameter
*/
@SuppressWarnings("unchecked")
public static Class getTypeParameter(Class klass, Class bound) {
Type t = requireNonNull(klass);
while (t instanceof Class) {
t = ((Class) t).getGenericSuperclass();
}
/* This is not guaranteed to work for all cases with convoluted piping
* of type parameters: but it can at least resolve straight-forward
* extension with single type parameter (as per [Issue-89]).
* And when it fails to do that, will indicate with specific exception.
*/
if (t instanceof ParameterizedType) {
// should typically have one of type parameters (first one) that matches:
for (Type param : ((ParameterizedType) t).getActualTypeArguments()) {
if (param instanceof Class) {
final Class cls = determineClass(bound, param);
if (cls != null) {
return cls;
}
} else if (param instanceof TypeVariable) {
for (Type paramBound : ((TypeVariable) param).getBounds()) {
if (paramBound instanceof Class) {
final Class cls = determineClass(bound, paramBound);
if (cls != null) {
return cls;
}
}
}
}
}
}
throw new IllegalStateException("Cannot figure out type parameterization for " + klass.getName());
}
@SuppressWarnings("unchecked")
private static Class determineClass(Class bound, Type candidate) {
if (candidate instanceof Class) {
final Class cls = (Class) candidate;
if (bound.isAssignableFrom(cls)) {
return (Class) cls;
}
}
return null;
}
}