Getting T.class despite Java's type-erasure

user_103842 0
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I'm trying to bind an interface to its implementation as read from a configuration file so that I can feed it to my IoC container. Here's roughly what I'm trying to do:

public class PropertyImplementationBinder<T> {
    // ...
    public Class getInterfaceClass() {
        return T.class; // OR Class<T>, note T is not newable
    }
    public Class getImplementationClass() {
        return /* read config file to get implementation class */;
    }
}

Is it somehow possible to get T.class?

type-erasure generics java
5 Answers

You need to explicitly pass the class into the constructor (and store it yourself).

private final Class<T> clazz;

PropertyImplementationBinder(Class<T> clazz){
    this.clazz = clazz;
}

public Class<T> getInterfaceClass() {
    return clazz;
}
user_149550

You can get the actual type arguments for a generic superclass of a class. This blog post explores the possibilities presented by this, including a nice little trick using trivial anonymous inner classes. To quote directly:

It turns out that while the JVM will not track the actual type arguments for instances of a generic class, it does track the actual type arguments for subclasses of generic classes. In other words, while a new ArrayList<String>() is really just a new ArrayList() at runtime, if a class extends ArrayList<String>, then the JVM knows that String is the actual type argument for List's type parameter.

user_1915840

Contrary to what is widely accepted and rarely known type erasure can be avoided, which means that the callee do have the ability to know which generic parameters were employed during the call.

Please have a look at: Using TypeTokens to retrieve generic parameters

The article also talks about the experiences of our users with the technique. In a nutshell, we ended up falling back to the...

Conventional and widely used technique: "Pass class types in constructors"

user_621310

No it's not possible.

The only exception to Java's type erasure is that via reflection you can find out the parameterized type via reflection on a class's fields.

cletus_183930
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