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March 03, 2025 00:51
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Jakarta EE Cheat Sheet

A quick reference guide to Jakarta EE, covering essential APIs and components for building enterprise Java applications.

Core Concepts

Containers

Definition: A container provides a runtime environment for Jakarta EE components, managing their lifecycle, dependencies, and security.

Types:

  • Web Container: Manages web components like Servlets, JSPs, and Faces.
  • EJB Container: Manages Enterprise JavaBeans (EJBs).

Responsibilities:

  • Lifecycle management (creation, destruction).
  • Dependency injection.
  • Security.
  • Transaction management.

Components

Servlets:

Java classes that handle HTTP requests and responses. Used for dynamic web content generation.

JSPs (JavaServer Pages):

Text-based documents that contain static content and dynamic Java code. Compiled into Servlets.

EJBs (Enterprise JavaBeans):

Server-side components that encapsulate business logic. Support transactions, security, and concurrency.

CDI Beans (Contexts and Dependency Injection):

Beans managed by the CDI container. Provide dependency injection and contextual lifecycle management.

JAX-RS Resources:

Java classes that expose RESTful web services. Annotated with @Path, @GET, @POST, etc.

Annotations

@WebServlet: Defines a Servlet.

@EJB: Injects an EJB.

@Inject: Injects a CDI bean.

@Path: Defines a resource path in JAX-RS.

@GET, @POST, @PUT, @DELETE: HTTP method annotations in JAX-RS.

@Produces, @Consumes: Defines MIME types in JAX-RS.

@ApplicationScoped, @RequestScoped, @SessionScoped: CDI scopes.

Web Tier

Servlets

Servlets handle client requests and generate responses. They are configured using annotations or deployment descriptors.

Example:

@WebServlet("/hello")
public class HelloServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.getWriter().println("Hello, world!");
    }
}

Key methods: init(), service(), doGet(), doPost(), destroy()

JavaServer Pages (JSPs)

JSPs allow embedding Java code within HTML. They are compiled into Servlets.

Example:

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<title>Hello</title>
</head>
<body>
    <h1>Hello, <%= request.getParameter("name") %>!</h1>
</body>
</html>

Implicit Objects: request, response, session, application, out, pageContext, config, page, exception

Jakarta Faces

A component-based UI framework for building web applications. Uses Facelets (XML-based view technology).

Key Components:

  • UI Components: Reusable UI elements like buttons, input fields, and data tables.
  • Managed Beans: Java classes that back the UI components.
  • Navigation: Defining page transitions.

Example Facelets:

<h:inputText value="#{myBean.name}" />
<h:commandButton value="Submit" action="#{myBean.submit}" />

Business Tier

Enterprise JavaBeans (EJBs)

EJBs are server-side components for encapsulating business logic. They are managed by the EJB container.

Types:

  • Session Beans: Represent business tasks.
  • Message-Driven Beans (MDBs): Handle asynchronous messages.

Example Session Bean:

@Stateless
public class MyBean {
    public String doSomething() {
        return "Something";
    }
}

Annotations: @Stateless, @Stateful, @Singleton, @MessageDriven, @EJB

Contexts and Dependency Injection (CDI)

CDI provides dependency injection and contextual lifecycle management for Java EE applications.

Key Concepts:

  • Beans: Managed objects.
  • Scopes: Define the lifecycle of beans (e.g., @ApplicationScoped, @RequestScoped).
  • Qualifiers: Used to differentiate between beans of the same type.
  • Events: Enable loosely coupled communication between components.

Example:

@ApplicationScoped
public class MyService {
    @Inject
    private MyRepository repository;
}

Transactions

Jakarta EE provides transaction management capabilities through JTA (Java Transaction API).

Annotations:

  • @Transactional: Marks a method or class as transactional.
  • @TransactionAttribute: Specifies the transaction behavior.

Example:

@Transactional
public void transfer(Account from, Account to, double amount) {
    //...
}

Transaction Attributes: REQUIRED, REQUIRES_NEW, MANDATORY, SUPPORTS, NOT_SUPPORTED, NEVER

Data Tier & APIs

Jakarta Persistence (JPA)

JPA provides an API for managing relational data in Java applications. It supports object-relational mapping (ORM).

Key Concepts:

  • Entities: Java classes that represent database tables.
  • EntityManager: Used to persist, retrieve, update, and delete entities.
  • JPQL (Java Persistence Query Language): Used to query entities.

Example Entity:

@Entity
public class Customer {
    @Id
    private Long id;
    private String name;
}

Annotations: @Entity, @Id, @GeneratedValue, @Column, @Table, @ManyToOne, @OneToMany

Jakarta RESTful Web Services (JAX-RS)

JAX-RS is an API for building RESTful web services.

Key Annotations:

  • @Path: Defines the resource path.
  • @GET, @POST, @PUT, @DELETE: HTTP method annotations.
  • @Produces, @Consumes: Define MIME types.
  • @PathParam, @QueryParam: Inject request parameters.

Example:

@Path("/customers")
public class CustomerResource {
    @GET
    @Produces("application/json")
    public List<Customer> getAll() {
        //...
    }
}

Jakarta Messaging (JMS)

JMS provides an API for asynchronous messaging between applications.

Key Concepts:

  • Message: The unit of communication.
  • Destination: The target of a message (Queue or Topic).
  • ConnectionFactory: Used to create connections to the JMS provider.
  • Session: Used to create messages, producers, and consumers.

Example MDB:

@MessageDriven(activationConfig = {
    @ActivationConfigProperty(propertyName = "destinationType", propertyValue = "javax.jms.Queue")
})
public class MyMDB implements MessageListener {
    public void onMessage(Message message) {
        //...
    }
}