Friday, August 26, 2011

Discover runtime type of Generic class

A common misconception about generics in Java 5 is that you can't access them at runtime.

What you can't find out at runtime is which generic type is associated with an instance of an object. However you can use reflection to look at which types have been staticly associated with a member of a class.

public Map perGlobalSettings = new HashMap();
public void test() {
  Field field = Test.class.getField("perGlobalSettings");
  ParameterizedType type0=(ParameterizedType) field.getGenericType();
  System.out.println(type0);  //java.util.Map
  Class clazz=(Class)type0.getActualTypeArguments()[1]; //GlobalSettings
  ParameterizedType t=(ParameterizedType)perGlobalSettings.getClass().getGenericSuperclass();
  System.out.println(t);	//java.util.AbstractMap
Accessing generic type information at runtime
Some notes on discovering your type parameter using the Reflection API

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