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
– Arnaud Denoyelle, Commented Jun 2, 2015 at 8:08
The only way to do this is by adding an argument: static <ContentType> Map<Object,ContentType> foo(Class<ContentType> type); — Maurice Perry
– Maurice Perry, Commented Jun 2, 2015 at 8:17

Community · Accepted Answer · 2017-05-23 11:58:13Z

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, 2017 at 11:58

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answered Jun 2, 2015 at 8:08

Phil Anderson

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4 Comments

Arnaud Denoyelle Over a year ago

Note : It will probably be possible from Java 9.

user1038550 Over a year ago

@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, that is currently targeted for Java 10 (and is very far from completion, so this might not end up being possible).

Arnaud Denoyelle Over a year ago

@PhilAnderson It is called reification and as says Toby, it is not clear whether it will be ready for Java 9 or Java 10. blogs.oracle.com/java/entry/the_javaone_2013_technical_keynote

user1038550 Over a year ago

@ArnaudDenoyelle From my understanding full reification is no longer a goal (JEP 218: Generics over Primitive Types), although this could of course change.

Arjit · Accepted Answer · 2015-06-02 08:38:18Z

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);
  }
}

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