Java Reflection API: Class<T>, Methods, Fields, and Constructors0%

Invoking Methods and Accessing Private Fields via Reflection

Java Reflection API: Class<T>, Methods, Fields, and Constructors

Advanced15 min read•Updated: 2025-01-15

🪞 Java Reflection API: Class<T>, Methods, Fields, and Constructors

Introduction

Reflection is Java's meta-programming mechanism that allows an executing program to inspect, examine, and modify the internal structure and behavior of classes, interfaces, fields, methods, and constructors at runtime—even when their names are unknown at compile time.

Frameworks like Spring, Hibernate, and JUnit rely fundamentally on Reflection for dependency injection, object-relational mapping, and test discovery.

🔍 1. Obtaining a `Class<?>` Object

Every loaded type has a unique Class instance maintained by the JVM ClassLoader. There are 3 ways to obtain it:

1// 1. Via class literal (Compile-time known)
2Class<Student> clazz1 = Student.class;
3 
4// 2. Via instance method getClass()
5Student s = new Student();
6Class<? extends Student> clazz2 = s.getClass();
7 
8// 3. Dynamic loading via Class.forName() (Dynamic string at runtime)
9Class<?> clazz3 = Class.forName("com.mskinstitute.models.Student");

🔬 2. Inspecting Class Modifiers and Metadata

1Class<?> clazz = Class.forName("com.mskinstitute.models.Student");
2 
3System.out.println("Full Name:    " + clazz.getName());
4System.out.println("Simple Name:  " + clazz.getSimpleName());
5System.out.println("Superclass:   " + clazz.getSuperclass().getName());
6System.out.println("Is Interface: " + clazz.isInterface());
7System.out.println("Modifiers:    " + Modifier.toString(clazz.getModifiers()));

📋 3. Inspecting Fields, Methods, and Constructors

Java provides two sets of reflection queries:

getFields() / getMethods(): Returns public members, including those inherited from superclasses.
getDeclaredFields() / getDeclaredMethods(): Returns all members (private, protected, default, public) declared directly within this class.

1// Inspecting all declared fields
2Field[] fields = clazz.getDeclaredFields();
3for (Field f : fields) {
4    System.out.printf("Field: %s %s (Type: %s)%n", 
5        Modifier.toString(f.getModifiers()), f.getName(), f.getType().getSimpleName());
6}
7 
8// Inspecting all declared methods
9Method[] methods = clazz.getDeclaredMethods();
10for (Method m : methods) {
11    System.out.printf("Method: %s %s (Return: %s, Params: %d)%n",
12        Modifier.toString(m.getModifiers()), m.getName(), 
13        m.getReturnType().getSimpleName(), m.getParameterCount());
14}

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Invoking Methods and Accessing Private Fields via Reflection

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Introduction

Frameworks like Spring, Hibernate, and JUnit rely fundamentally on Reflection for dependency injection, object-relational mapping, and test discovery.

🔍 1. Obtaining a Class<?> Object

Every loaded type has a unique Class instance maintained by the JVM ClassLoader. There are 3 ways to obtain it:

1// 1. Via class literal (Compile-time known)
2Class<Student> clazz1 = Student.class;
3 
4// 2. Via instance method getClass()
5Student s = new Student();
6Class<? extends Student> clazz2 = s.getClass();
7 
8// 3. Dynamic loading via Class.forName() (Dynamic string at runtime)
9Class<?> clazz3 = Class.forName("com.mskinstitute.models.Student");

🔬 2. Inspecting Class Modifiers and Metadata

1Class<?> clazz = Class.forName("com.mskinstitute.models.Student");
2 
3System.out.println("Full Name:    " + clazz.getName());
4System.out.println("Simple Name:  " + clazz.getSimpleName());
5System.out.println("Superclass:   " + clazz.getSuperclass().getName());
6System.out.println("Is Interface: " + clazz.isInterface());
7System.out.println("Modifiers:    " + Modifier.toString(clazz.getModifiers()));

📋 3. Inspecting Fields, Methods, and Constructors

Java provides two sets of reflection queries:

getFields() / getMethods(): Returns public members, including those inherited from superclasses.

getDeclaredFields() / getDeclaredMethods(): Returns all members (private, protected, default, public) declared directly within this class.

1// Inspecting all declared fields
2Field[] fields = clazz.getDeclaredFields();
3for (Field f : fields) {
4    System.out.printf("Field: %s %s (Type: %s)%n", 
5        Modifier.toString(f.getModifiers()), f.getName(), f.getType().getSimpleName());
6}
7 
8// Inspecting all declared methods
9Method[] methods = clazz.getDeclaredMethods();
10for (Method m : methods) {
11    System.out.printf("Method: %s %s (Return: %s, Params: %d)%n",
12        Modifier.toString(m.getModifiers()), m.getName(), 
13        m.getReturnType().getSimpleName(), m.getParameterCount());
14}

Previous Lesson

Next Lesson

Connection Pooling Concepts with HikariCP

Invoking Methods and Accessing Private Fields via Reflection

🪞 Java Reflection API: Class<T>, Methods, Fields, and Constructors

Introduction

🔍 1. Obtaining a Class<?> Object

🔬 2. Inspecting Class Modifiers and Metadata

📋 3. Inspecting Fields, Methods, and Constructors

Invoking Methods and Accessing Private Fields via Reflection

Related Lessons

Practice Quiz

Loading Page Content...

🪞 Java Reflection API: Class<T>, Methods, Fields, and Constructors

Introduction

🔍 1. Obtaining a Class<?> Object

🔬 2. Inspecting Class Modifiers and Metadata

📋 3. Inspecting Fields, Methods, and Constructors

Invoking Methods and Accessing Private Fields via Reflection

Related Lessons

Practice Quiz

🔍 1. Obtaining a `Class<?>` Object

🔍 1. Obtaining a `Class<?>` Object