Reflection Use Cases

While reflection should be avoided in basic business logic, it is essential for writing frameworks that must parse and execute arbitrary user code dynamically.

Real-World Applications

• Dependency Injection (Spring): Resolves constructor parameters and autowires matching bean instances using reflection lookup.
• JSON Serialization (Jackson): Uses reflection to scan fields, match them to JSON keys, and instantiate classes dynamically during deserialization.
• Unit Testing (JUnit): Scans class structures for methods annotated with @Test and executes them.
• Dynamic Proxies: Java's Proxy class generates runtime implementations of interfaces, enabling Aspect-Oriented Programming (AOP) for transaction management.

Code Walkthrough

This program demonstrates how a basic test runner locates and runs test methods using reflection.

import java.lang.annotation.*;
import java.lang.reflect.Method;
@Retention(RetentionPolicy.RUNTIME)
@interface MyTest {}
class SampleTest {
    @MyTest
    public void testOne() { System.out.println("Test one passed!"); }
    public void normalMethod() { System.out.println("Ignored."); }
}
public class TestRunner {
    public static void main(String[] args) throws Exception {
        Class clazz = SampleTest.class;
        Object instance = clazz.getConstructor().newInstance();
for (Method method : clazz.getDeclaredMethods()) {
            if (method.isAnnotationPresent(MyTest.class)) {
                method.invoke(instance); // Executes testOne() dynamically
            }
        }
    }
}

Interview-Relevant Information