从类文件实例化通用参数化类型

Question

So a client will create a class in the following fashion:

public class Tester implements Test<Type1, Type2, Type3> {
    @override
    public method1(...)
    ...
}

Then the client will pass the .class file (IE Tester.class) to us.

We would then like to instantiate the client's class and run their method1 based on the types that the generic uses. I have the following code so far but I don't think I'm going down the right path:

    //tempClass = <ClassName>.class
    Type[] genericTypes;
    Type[] genericInterfaces = tempClass.getGenericInterfaces();
    if(genericInterfaces[0] instanceof ParameterizedType){
        genericTypes = ((ParameterizedType)genericInterfaces[0])
                        .getActualTypeArguments();
    }
    Class<Tester<genericTypes[0], genericTypes[1], genericTypes[2]>> classTest
        = ( Class<Tester<genericTypes[0], genericTypes[1], genericTypes[2]>>) tempClass
    Constuctor<...> = ... //get the constructor
    //Create the new instance by calling .newInstance()

But eclipse is giving me a lot of errors in the Class> line, so I don't think this is the right path. What am I doing wrong?

Answer 1

Tere are a couple of issues here...

First, both Classname.class is inherently static construct - it is even called class literal . Hence, type name cannot be supplied at runtime, that's why Eclipse complains about your attempt to do so. To obtain the Class object given the class name as the string, you can use Class.forName :

Class<?> tempClass = Class.forName("Tester");

(fully qualified name and no .class suffix).

Second, so is Tester<Type1, Type2, Type3> . Type arguments must be known at the compile time. Now, I'm not sure why you needed this cast in this place, but I see a couple of possible reasons:

• To ensure the class represented by tempClass does implement the appropriate interface - then it is of no use even if it was a syntactically legal construct. Casting cannot dynamically check generic type arguments, since they are also entirely compile-time construct - they are not available in runtime, due to type erasure . To check if a given Class object A is a subclass/implementation of type represented by Class object B, we can use Class.isAssignableFrom :

   if (B.isAssignableFrom(A)) { // ... } 

• To avoid warnings - sorry, such casts really are unchecked, as there is no way to ensure its correctness at runtime - again, due to type erasuer. Use @SuppressWarnings in places where you know cast is valid, and Java type system is too dumb to be able to verify your claim.

Anyway, don't cast the class. Instead, cast the result of newInstance invocation. Now, as for running the right methods... Now you have all the information you need, in two pieces: object to invoke methods upon, and type arguments of implemented interface.