Part 1. The Mirror Insight of Kotlin Reflection

Have you ever wondered how your Kotlin code can peek into its own structure, dynamically inspecting classes, methods, and properties at runtime? If so, you’ve stumbled upon the fascinating world of Kotlin reflection. In this blog post, we’ll embark on a journey behind the scenes, delving into the inner workings of Kotlin’s reflection capabilities.

What is Kotlin's Reflection?

In Kotlin, Reflection is a combination of language and library capabilities that allow you to introspect a program while it is running. Kotlin reflection is used at runtime to utilize a class and its members, such as properties, methods, and constructors. It is also used to create instances of classes, inspect annotations, lookup functions, and invoke them.

Kotlin features its own reflection API, built in a clean, functional style, in addition to the Java reflection API.

When you call a reflection API in Kotlin, several underlying processes occur to fulfill the reflective operation. Here’s an overview of what happens behind the scenes:

1. Class Loading: When you use reflection to work with a class, the JVM loads the class bytecode into memory if it hasn’t been loaded already. This involves finding the corresponding .class file and parsing its bytecode to create a representation of the class.

// If the class is not already loaded, the JVM loads it
val clazz = MyClass::class.java

2. Metadata Retrieval: The reflection API queries metadata about the class, method, property, or annotation involved in the operation. This metadata includes information such as the class’s constructors, methods, fields, annotations, parameter types, return types, etc.

// Querying metadata about the class
val constructors = clazz.constructors
val properties = clazz.declaredFields
val annotations = clazz.annotations

3. Runtime Checks: Depending on the nature of the reflective operation, the JVM may perform runtime checks to ensure the correctness and safety of the operation. For example, when invoking a method reflectively, the JVM may check if the method is accessible and if the arguments provided match the method’s signature.

// Performing runtime checks before invoking a method
val method = clazz.getDeclaredMethod("methodName")
if (Modifier.isPublic(method.modifiers)) {
    // Method is public, can be invoked
    method.invoke(instance)
} else {
    // Method is not accessible, handle accordingly
}

4. Method Resolution: In the case of a method invocation or property access, the JVM resolves the method or property to be invoked or accessed based on its name and signature. This involves searching through the class’s hierarchy and interfaces to find the appropriate method or property.

// Resolving a method to be invoked
val method = clazz.getDeclaredMethod("methodName")

5. Memory Operations: Once the method or property is resolved, the JVM performs the necessary memory operations to execute the method or access the property. For method invocation, this includes setting up the method’s arguments, invoking the method, and handling the return value.

// Invoking a method reflectively
method.invoke(instance)

6. Security Checks: If a security manager is present, the JVM may perform additional security checks to ensure that the reflective operation is authorized based on the security policy in place. This helps prevent malicious code from abusing reflection to access sensitive resources or compromise the integrity of the application.

// Performing security checks for reflective access
val securityManager = System.getSecurityManager()
if (securityManager != null) {
    securityManager.checkPermission(RuntimePermission("reflectPermission"))
}

7. Performance Considerations: Reflection operations can incur a performance overhead compared to direct method calls or property accesses, as they involve additional runtime checks, metadata lookups, and method resolution. Therefore, it’s essential to use reflection judiciously, especially in performance-critical code paths.

// Demonstrating performance overhead of reflection
val startTime = System.currentTimeMillis()
repeat(iterations) {
    method.invoke(instance)
}
val endTime = System.currentTimeMillis()
println("Elapsed time: ${endTime - startTime} ms")

Overall, when you call a reflection API in Kotlin, the JVM orchestrates a series of processes to fulfill the reflective operation, allowing your code to introspect and manipulate classes and objects dynamically at runtime.

Now we know what happens in the background whenever we use reflection APIs so in the next blog we will dive into deeper more practical usages of Kotlin’s reflection API.