Basics

The Reflection API allows one to check the class structure of the code at runtime and invoke code dynamically. This is very powerful, but it is also dangerous since the compiler is not able to statically determine whether dynamic invocations are valid.

A simple example would be to get the public constructors and methods of a given class:

import java.lang.reflect.Constructor;
import java.lang.reflect.Method;

// This is a object representing the String class (not an instance of String!)
Class<String> clazz = String.class;

Constructor<?>[] constructors = clazz.getConstructors(); // returns all public constructors of String
Method[] methods = clazz.getMethods(); // returns all public methods from String and parents

With this information it is possible to instance the object and call different methods dynamically.

Reflection and Generic Types

Generic type information is available for:

• method parameters, using getGenericParameterTypes().
• method return types, using getGenericReturnType().
• public fields, using getGenericType.

The following example shows how to extract the generic type information in all three cases:

import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.List;
import java.util.Map;

public class GenericTest {

    public static void main(final String[] args) throws Exception {
        final Method method = GenericTest.class.getMethod("testMethod", Map.class);
        final Field field = GenericTest.class.getField("testField");

        System.out.println("Method parameter:");
        final Type parameterType = method.getGenericParameterTypes()[0];
        displayGenericType(parameterType, "\\t");

        System.out.println("Method return type:");
        final Type returnType = method.getGenericReturnType();
        displayGenericType(returnType, "\\t");

        System.out.println("Field type:");
        final Type fieldType = field.getGenericType();
        displayGenericType(fieldType, "\\t");

    }

    private static void displayGenericType(final Type type, final String prefix) {
        System.out.println(prefix + type.getTypeName());
        if (type instanceof ParameterizedType) {
            for (final Type subtype : ((ParameterizedType) type).getActualTypeArguments()) {
                displayGenericType(subtype, prefix + "\\t");
            }
        }

    }

    public Map<String, Map<Integer, List<String>>> testField;

    public List<Number> testMethod(final Map<String, Double> arg) {
        return null;
    }

}

This results in the following output:

Method parameter:

java.util.Map<java.lang.String, java.lang.Double> java.lang.String java.lang.Double

Method return type:

java.util.List<java.lang.Number> java.lang.Number

Field type:

java.util.Map<java.lang.String, java.util.Map<java.lang.Integer, java.util.List<java.lang.String>>> java.lang.String java.util.Map<java.lang.Integer, java.util.List<java.lang.String>> java.lang.Integer java.util.List<java.lang.String> java.lang.String