Java Reflection, extract generic type from method

Question

I have a java method with the following signature:

static <ContentType> Map<Object,ContentType> foo();

I want to use reflection to dynamically change the behavior of the method according to ContentType. To achieve this, I must be able to handle ContentType as an object (maybe an instance of java.lang.reflect.Type). Does anyone know how to do this? Is that event possible?

it will not be possible before java 9 because of type erasure. — Arnaud Denoyelle, Jun 02 '15 at 08:08
The only way to do this is by adding an argument: static Map — Maurice Perry, Jun 02 '15 at 08:17

score 2 · Accepted Answer · edited May 23 '17 at 11:58

2

It's not possible. Generics in Java are "syntactic sugar". They are only used at compile-time but are then removed and never make it into the class file.

This question has some realy good information on this.

edited May 23 '17 at 11:58

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answered Jun 02 '15 at 08:08

Phil Anderson

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Note : It will probably be possible from Java 9. – Arnaud Denoyelle Jun 02 '15 at 08:08
@ArnaudDenoyelle Why do you say it will be possible in Java 9? If you're talking about the generic changes included as part of the [value types proposal](http://openjdk.java.net/jeps/169), that is currently targeted for Java 10 (and is very far from completion, so this might not end up being possible). – Jun 02 '15 at 08:11
@PhilAnderson It is called `reification` and as says Toby, it is not clear whether it will be ready for Java 9 or Java 10. https://blogs.oracle.com/java/entry/the_javaone_2013_technical_keynote – Arnaud Denoyelle Jun 02 '15 at 08:13
@ArnaudDenoyelle From my understanding full reification is no longer a goal ([JEP 218: Generics over Primitive Types](https://bugs.openjdk.java.net/browse/JDK-8046267)), although this could of course change. – Jun 02 '15 at 08:16

score 1 · Answer 2 · answered Jun 02 '15 at 08:38

At runtime inspecting a parameterizable type itself, like java.util.List, there is no way of knowing what type is has been parameterized to. But, when you inspect the method that declares the use of a parameterized type, you can see at runtime what type the parameterizable type was parameterized to

Method method = MyClass.class.getMethod("getStringList", null);

Type returnType = method.getGenericReturnType();
if(returnType instanceof ParameterizedType){
  ParameterizedType type = (ParameterizedType) returnType;
  Type[] typeArguments = type.getActualTypeArguments();
  for(Type typeArgument : typeArguments){
      Class typeArgClass = (Class) typeArgument;
      System.out.println("typeArgClass = " + typeArgClass);
  }
}

Java Reflection, extract generic type from method

2 Answers2