The Reflection API

Class<?> type = Class.forName("com.example.Service");
Object instance = type.getDeclaredConstructor().newInstance();

Method m = type.getDeclaredMethod("execute", String.class);
m.setAccessible(true);
Object result = m.invoke(instance, "payload");

getX vs getDeclaredX

getMethods() returns public members including inherited ones. getDeclaredMethods() returns everything declared on that exact class, including private, but nothing inherited. Confusing the two is the most common reflection bug.

Costs

• Nothing is checked until runtime; a rename breaks it silently.
• invoke boxes arguments into an Object[] and wraps any thrown exception in InvocationTargetException — always unwrap with getCause().
• Reflective calls resist inlining, though the modern JIT narrows the gap.
• Since Java 9, setAccessible on another module’s internals is refused unless that module opens the package. This is why older libraries broke on 9+.

Reading generics back

Erasure removes type arguments from values but keeps signatures in the class file, so declared types remain readable:

Type t = field.getGenericType();
if (t instanceof ParameterizedType pt) {
    Type arg = pt.getActualTypeArguments()[0];   // e.g. String
}