An Introduction to the Java Reflection API

The Java Reflection API can be used to inspect and modify the runtime behavior of a program written in Java. In this Java programming tutorial, we will examine the Java Reflection API, its features and benefits, how it can be used, its drawbacks, and so forth.

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What is the Java Reflection API?

The Java Reflection API can be used to inspect and manipulate objects at runtime. With it, developers can access fields and methods (even private ones) as well as examine other object properties such as class name, hash code, and type at runtime.

Programmers can take advantage of the Java Reflection API to view and change the values of fields, invoke methods, and construct new objects. The Reflection API is not part of the core Java API, but it is included in the standard java.lang.reflect package.

Why use the Java Reflection API?

The Java Reflection API enables programmers to inspect and manipulate the structure and behavior of classes, objects, and interfaces at runtime. You can use it to create new classes at runtime, obtain class attributes such as private fields or methods without requiring them to be visible through an API.

You can also use it to implement dynamic proxies which are useful in distributed computing environments where one machine acts on behalf of another machine (such as remote procedure calls), discover what platform your application is running on (such as OS name/version), find all loaded classes in memory or get information about their members directly from the .class files.

Features and Benefits of the Java Reflection API

The Java Reflection API provides a way to examine the structure of a class and its methods at runtime. You can take advantage of this API to:

• Inspect fields, methods and constructors of a class
• Invoke methods of a class
• Examine the members (fields, methods and constructors) of a class, including their name, signature and modifiers

The benefits of Java reflection API are:

• You can use it to dynamically manipulate your code at runtime, which can be very useful for things like testing and debugging.
• It makes it easy to introspect your classes and objects, which can be helpful for understanding how your code works.
• It provides a way to access and modify the fields, methods and annotations of your classes and objects, which can be useful for things like creating custom adapters or decorators.
• You can use it with most Java frameworks and development tools.

Read: How to Use Java Reflection Effectively

What are the Drawbacks of Using the Java Reflection API

The Java Reflection API is a powerful tool for accessing information about classes at runtime. While it can be very useful, there are a few drawbacks to using the reflection API:

• It is slow compared to other methods of getting information from classes. Some JDK functions that use reflection may run slower than equivalent native code would run, especially if they need to do many reflective lookups or perform memory allocation.
• The reflection API is not thread-safe, which means that you must take care not to have multiple threads attempting to use the same class loader at once (this includes loading classes).
• The reflection API is not easy to use; meaning there will be some serious effort involved in learning how it works and getting familiar with best practices when using it. Not only does this make development time more expensive but also makes debugging harder when something goes wrong.

The Java.lang Package

Here is a list of Java classes in the Java.lang that can help you work with reflection in Java:

• Field: You can take advantage of this class to retrieve declarative information about a class. Such information may include access modifier, datatype, name, and value of variables.
• Method: You can take advantage of this class to collect declarative information about a method, such as its access modifier, argument types, return type, name, etc.
• Constructor: You can leverage this class to collect declarative information about a constructor’s name, access modifier, and argument types.
• Modifier: You can use this class to collect data about a specific access modifier.

The class java.lang.Class class primarily serves two functions:

• It offers methods for retrieving a class’s information at run time
• It provides methods for inspecting and changing a class’s run-time behavior.

Programming the Java Reflection API

Using the Reflection API is straightforward. All you need to do is obtain a reference to the object you want to inspect, then call one of the reflective methods on that object. For example, the following code snippet will display the name and type of all the fields in an instance:

Class obj = MyCustomClass.class;
for (Field field : obj.getDeclaredFields()) {
System.out.println(field.getName() + " " + field.getType());
}

Now consider the following two classes:

class Vehicle {
}
class Car extends Vehicle {
}

You can use the code snippet given below to display the name of the class:

Car car = new Car();
Class obj = car.getClass();
String name = obj.getName();
System.out.println("Name: " + name);

You can use the following code snippet to display the name of the base or super class:

Class superClass = obj.getSuperclass();
System.out.println("Superclass: " + superClass.getName());

Here’s the complete program for your reference:

import java.lang.Class;
import java.lang.reflect.*;
class Vehicle {
}
class Car extends Vehicle {
}
class Main {
  public static void main(String[] args) {
    try {
      Car car = new Car();
      Class obj = car.getClass();
      String name = obj.getName();
      System.out.println("Name: " + name);
      Class superClass = obj.getSuperclass();
      System.out.println("Superclass: " + superClass.getName());
    }
    catch (Exception e) {
      e.printStackTrace();
    }
  }
}

When you execute the preceding code, the following text will be displayed at the console window:

Name: Car
Superclass: Vehicle

Final Thoughts on Java Reflection API

The Java Reflection API is a library that allows you to inspect and modify the structure of objects at runtime. This can be useful for a variety of tasks, such as creating dynamic proxy objects, transforming objects between different formats, or retrieving metadata about classes and methods.

You can use this API to access fields and methods (even private ones) as well as examine other object properties such as class name, hash code, and type.

That said, one of the major downsides of this API is that it can be quite slow in real time. So, it is not really suitable for use in performance-sensitive areas such as real-time applications or high-traffic websites.

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