Design Patterns Interview: 5 Ways to Write Singleton and Factory Pattern in Spring

The role of design patterns in interviews: proving you're not just a coder who only knows CRUD.

5 Ways to Write the Singleton Pattern

1. Eager Initialization (Simplest)

public class Singleton {
    private static final Singleton INSTANCE = new Singleton();
    private Singleton() {}
    public static Singleton getInstance() { return INSTANCE; }
}

Disadvantage: It's created when the class is loaded. If it's never used, memory is wasted.

2. Lazy Initialization (Has Thread Safety Issues) If two threads check instance == null simultaneously, two instances will be created.

3. Double-Checked Locking (DCL, Most Common)

public class Singleton {
    private static volatile Singleton instance;  // volatile is very important!
    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

volatile prevents instruction reordering — otherwise, you might get an object that hasn't been fully initialized.

4. Static Inner Class (Recommended) Leverages the class-loading mechanism to ensure thread safety and lazy loading.

5. Enum (Safest)

public enum Singleton {
    INSTANCE;
}

Prevents reflection attacks and serialization from breaking the singleton. This is the approach recommended by Joshua Bloch (author of Effective Java).

Factory Pattern in Spring

• BeanFactory: The root interface of the Spring IoC container, the purest form of the factory pattern
• FactoryBean: Implement this interface when you need complex Bean creation logic (e.g., MyBatis's SqlSessionFactoryBean)
• Difference: BeanFactory manages all Beans, while FactoryBean is a factory that produces a specific Bean

How to Answer in an Interview

Write the code first, then explain why it's written that way. Design patterns detached from code are just hollow theory.