Session Beans

🏢 Jakarta EJB Session Beans: Your Building’s Smart Workers

Imagine a big office building where different workers help you get things done. Some workers remember you, some don’t, and one special worker is always there. That’s what EJB Session Beans are!

🎯 What is Jakarta EJB?

EJB = Enterprise JavaBeans

Think of a HUGE toy factory. You can’t run it alone! You need:

• Workers who build toys 🧸
• Someone to remember orders 📝
• A manager who’s always available 👔

Jakarta EJB gives your Java app these smart workers called “Session Beans”.

graph TD
    A["Your App"] --> B["EJB Container"]
    B --> C["Session Beans"]
    C --> D["Does the Work!"]

Why Use EJB?

Simple Example:

@Stateless
public class CalculatorBean {
    public int add(int a, int b) {
        return a + b;
    }
}

This bean adds numbers. The container handles everything else!

🎭 Session Beans: The Three Helpers

Session Beans are like three different types of helpers at a restaurant:

graph TD
    A["Session Beans"] --> B["🔄 Stateless"]
    A --> C["🧠 Stateful"]
    A --> D["👑 Singleton"]
    B --> E["Forgets you instantly"]
    C --> F["Remembers your order"]
    D --> G["One manager for all"]

🚪 No-Interface View: Keep It Simple!

The Old Way vs The New Way

Old way: You needed an extra “interface” file.

New way (No-Interface View): Just write one class!

// That's ALL you need! No extra files!
@Stateless
public class GreeterBean {
    public String sayHello(String name) {
        return "Hello, " + name + "!";
    }
}

🎁 Why is this great?

Think of it like ordering pizza:

• Old way: Fill form A, then form B, then form C 📋📋📋
• New way: Just say what you want! 🍕

graph LR
    A["Your Code"] --> B["Bean"]
    B --> C["Done!"]

No middleman. No extra files. Just works!

🔄 Stateless Session Beans: The Goldfish Workers

What Does “Stateless” Mean?

Imagine a worker with goldfish memory 🐠

• You ask: “Add 2 + 2”
• Worker says: “4!”
• You ask: “What did I just ask?”
• Worker says: “…I don’t know! 🤷”

They do one task, then forget everything.

Real Example

@Stateless
public class MathBean {

    public int multiply(int a, int b) {
        return a * b;
    }

    public double divide(int a, int b) {
        return (double) a / b;
    }
}

Why Use Stateless?

🌟 The Pool Magic

The container keeps a pool of stateless beans:

graph TD
    A["User 1"] --> P["Bean Pool"]
    B["User 2"] --> P
    C["User 3"] --> P
    P --> X["Bean A"]
    P --> Y["Bean B"]
    P --> Z["Bean C"]

Any user can use any bean. Super efficient!

🧠 Stateful Session Beans: The Waiter Who Remembers

What Does “Stateful” Mean?

Imagine a perfect waiter at a fancy restaurant 🍽️

• You order appetizer → They remember ✓
• You order main course → They remember ✓
• You order dessert → They remember ✓
• At the end: “Here’s your complete bill!”

They remember YOUR conversation until you leave.

Real Example: Shopping Cart

@Stateful
public class CartBean {

    private List<String> items =
        new ArrayList<>();

    public void addItem(String item) {
        items.add(item);
    }

    public List<String> getItems() {
        return items;
    }

    public void checkout() {
        // Process order
        items.clear();
    }
}

How It Works

graph TD
    A["User A"] --> B["Cart Bean #1"]
    C["User B"] --> D["Cart Bean #2"]
    E["User C"] --> F["Cart Bean #3"]
    B --> G["Apples, Milk"]
    D --> H["Bread, Butter"]
    F --> I["Cookies"]

Each user gets their OWN bean!

When to Use?

👑 Singleton Beans: The One and Only Boss

What is a Singleton?

Imagine your school has ONE principal 🏫

• Not one per class
• Not one per student
• Just ONE for the whole school!

Singleton = One bean for the ENTIRE application.

Real Example: Visitor Counter

@Singleton
@Startup
public class CounterBean {

    private int visitors = 0;

    public int addVisitor() {
        visitors++;
        return visitors;
    }

    public int getCount() {
        return visitors;
    }
}

The Magic of @Startup

graph TD
    A["App Starts"] --> B["@Startup"]
    B --> C["Singleton Created"]
    C --> D["Ready Before Anyone Asks!"]

@Startup = “Create me immediately when app starts!”

When to Use Singleton?

🔄 EJB Lifecycle: Birth, Life, and Goodbye

Every bean has a life story, just like a butterfly! 🦋

Stateless Bean Lifecycle

graph TD
    A["Does Not Exist"] --> B["Container Creates"]
    B --> C["Ready Pool"]
    C --> D["Doing Work"]
    D --> C
    C --> E["Container Destroys"]

Lifecycle Methods:

@Stateless
public class WorkerBean {

    @PostConstruct
    public void init() {
        // "I'm born! Setting up..."
    }

    @PreDestroy
    public void cleanup() {
        // "Goodbye! Cleaning up..."
    }
}

Stateful Bean Lifecycle

graph TD
    A["Does Not Exist"] --> B["Created for User"]
    B --> C["Ready"]
    C --> D["Working"]
    D --> C
    C --> E["Passivated - Sleeping"]
    E --> F["Activated - Awake!"]
    F --> C
    C --> G["Removed"]

Extra Methods for Stateful:

@Stateful
public class CartBean {

    @PostConstruct
    void init() { /* Born! */ }

    @PrePassivate
    void sleep() { /* Going to sleep... */ }

    @PostActivate
    void wake() { /* Waking up! */ }

    @PreDestroy
    void bye() { /* Goodbye! */ }
}

Singleton Bean Lifecycle

graph TD
    A["App Starts"] --> B["Created Once"]
    B --> C["Lives Forever"]
    C --> D["App Stops"]
    D --> E["Destroyed"]

Simple and eternal!

🎯 Quick Comparison Chart

🎪 The Restaurant Analogy (Complete Picture)

graph TD
    R["Restaurant = Your App"]
    R --> K["Kitchen = EJB Container"]
    K --> C1["👨‍🍳 Line Cooks = Stateless"]
    K --> C2["🧑‍🍳 Personal Waiter = Stateful"]
    K --> C3["👨‍💼 Manager = Singleton"]
    C1 --> T1["Cook any order, forget immediately"]
    C2 --> T2["Remember YOUR entire meal"]
    C3 --> T3["One boss, knows everything"]

💡 Key Takeaways

1. EJB Container = Your smart helper that manages everything
2. Stateless = Fast workers who forget (use for quick tasks)
3. Stateful = Personal assistants who remember (use for user sessions)
4. Singleton = One boss for everyone (use for shared data)
5. No-Interface = Less code, same power!
6. Lifecycle = Beans are born, work, and say goodbye

🚀 You Did It!

Now you understand how enterprise Java apps use Session Beans to organize work. Think of them as your smart workers who handle different jobs:

• 🔄 Stateless for quick, forgettable tasks
• 🧠 Stateful for remembering user conversations
• 👑 Singleton for shared, app-wide resources

Next time you see @Stateless or @Singleton, you’ll know exactly what’s happening! 🎉