Mastering Spring 5.0: Master reactive programming, microservices, Cloud Native applications, and more

Spring Framework was released in March 2004. When the first version of Spring Framework was released, the popular way of developing an enterprise application was using Enterprise Java Beans (EJB) 2.1.

Developing and deploying EJBs was a cumbersome process. While EJBs made the distribution of components easier, developing, unit testing, and deploying them was not easy. The initial versions of EJBs (1.0, 2.0, 2.1) had a complex Application Programmer Interface (API), leading to a perception (and truth in most applications) that the complexity introduced far outweighed the benefits:

Spring Framework was introduced as a lightweight framework aimed at making developing Java EE applications simpler.

Earlier versions of EJBs were very difficult to unit test. In fact, it was difficult to run EJBs outside the container (as of version 2.1). The only way to test them was to deploy them in a container.

Spring Framework brought in the concept of Dependency Injection (DI). We will discuss dependency injection in complete detail in Chapter 2, Dependency Injection.

The dependency injection enables unit testing by making it easy to replace the dependencies with their mocks. We do not need to deploy the entire application to unit test it.

Simplifying unit testing has multiple benefits:

Before Spring Framework, typical J2EE (or Java EE, as it is called now) applications contained a lot of plumbing code. For example: getting a database connection, exception handling code, transaction management code, logging code, and a lot more.

Let's take a look at a simple example of executing a query using prepared statement:

    PreparedStatement st = null;
    try {
          st = conn.prepareStatement(INSERT_TODO_QUERY);
          st.setString(1, bean.getDescription());
          st.setBoolean(2, bean.isDone());
          st.execute();
        } 
    catch (SQLException e) {
          logger.error("Failed : " + INSERT_TODO_QUERY, e);
     } finally {
                if (st != null) {
           try {
           st.close();
          } catch (SQLException e) {
           // Ignore - nothing to do..
          }
       }
     }

In the preceding example, there are four lines of business logic and more than 10 lines of plumbing code.

With Spring Framework, the same logic can be applied in a couple of lines:

    jdbcTemplate.update(INSERT_TODO_QUERY, 
    bean.getDescription(), bean.isDone());

Spring Framework is modular. It is built as a set of independent modules built on top of the core Spring modules. Most of the Spring modules are independent--you can use one of them without having to use others.

Let's look at a few examples:

Spring Framework does not want to be the jack-of-all-trades. While focusing on its core job of reducing coupling between different parts of the application and making them testable, Spring provides great integration with frameworks of your choice. This means you have flexibility in your architecture--if you do not want to use a specific framework, you can easily replace it with another.

The first version of Spring Framework focused on making applications testable. However, as time moved on, there were new challenges. Spring Framework managed to evolve and stay ahead of the curve with the flexibility and modules that are offered. A couple of examples are listed as follows:

Another important thing Spring brings in is the umbrella of Spring Projects. Spring Framework is just one of the many projects under Spring Projects. We will discuss the different Spring Projects in a separate section. The following examples illustrate how Spring managed to stay ahead of times with new Spring Projects:

Spring Core Container provides the core features of Spring Framework--dependency injection, IoC (Inversion of Control) container, and the application context. We will learn more about DI and IoC Container in Chapter 2, Dependency Injection.

Important core Spring modules are listed in the following table:

Cross-cutting concerns are applicable to all application layers--logging and security, among others. AOP is typically used to implement cross-cutting concerns.

Unit tests and integration tests fit this category since they are applicable to all layers.

Important Spring modules related to cross-cutting concerns are listed as follows:

Spring provides its own MVC framework, Spring MVC, other than providing great integration with popular web frameworks such as Struts.

Important artifacts/modules are listed as follows:

We will cover Spring MVC and develop web applicaitions and rest services with it in Chapter 3, Building Web Application with Spring MVC and Chapter 5, Building Microservices with Spring Boot.

The business layer is focused on executing the business logic of the applications. With Spring, business logic is typically implemented in Plain Old Java Object (POJO).

Spring Transactions (spring-tx) provides declarative transaction management for POJO and other classes.

The data layer in applications typically talks to the database and/or the external interfaces. Some of the important Spring modules related to the data layer are listed in the following table:

Some of the challenges while developing microservices and web applications are as follows:

Spring Boot solves all these problems out of the box by taking an opinionated view of how applications have to be developed.

We will look at Spring Boot in depth in two chapters--Chapter 5, Building Microservices with Spring Boot and Chapter 7, Advanced Spring Boot Features.

It is not an exaggeration to say The world is moving to the Cloud.

Cloud Native microservices and applications are the order of the day. We will discuss this in detail in Chapter 4, Evolution toward Microservices and Cloud-Native Applications.

Spring is taking rapid strides toward making application development for the Cloud simpler with Spring Cloud.

Spring Cloud provides solutions for common patterns in distributed systems. Spring Cloud enables developers to quickly create applications that implement common patterns. Some of the common patterns implemented in Spring Cloud are listed as follows:

We will discuss Spring Cloud and its varied range features in more detail in Chapter 9, Spring Cloud.

There are multiple sources of data in today's world--SQL (relational) and a variety of NOSQL databases. Spring Data tries to provide a consistent data-access approach to all these different kinds of databases.

Spring Data provides integration with a varied range of specifications and/or data stores:

Some of the important features are listed as follows:

We will discuss Spring Data in more detail in Chapter 8, Spring Data.

Enterprise applications today process large volumes of data using batch programs. The needs of these applications are very similar. Spring Batch provides solutions for high- volume batch programs with high performance requirements.

Important features in Spring Batch are as follows:

Authentication is the process of identifying the user. Authorization is the process of ensuring that a user has access to perform the identified action on the resource.

Authentication and authorization are critical parts of Enterprise applications, both web applications and web services. Spring Security provides declarative authentication and authorization for Java based applications.

Important features in Spring Security are as follows:

We will discuss how to secure web applications with Spring Security in Chapter 3, Building Web Application with Spring MVC.

We will discuss how to secure REST Services with Basic and OAuth authentication mechanisms using Spring Security in Chapter 6, Extending Microservices.

HATEOAS stands for Hypermedia as The Engine of Application State. Though it sounds complex, it is quite a simple concept. Its main aim is to decouple the server (the provider of the service) from the client (the consumer of the service).

The service provider provides the service consumer with information about what other actions can be performed on the resource.

Spring HATEOAS provides a HATEOAS implementation--especially for the REST services implemented with Spring MVC.

Important features in Spring HATEOAS are as follows:

We will discuss how to use HATEOAS in Chapter 6, Extending Microservices.

Spring Framework 5.0 has JDK 8 and Java EE 7 baseline. Basically, it means that previous JDK and Java EE versions are not supported anymore.

Some of the important baseline Java EE 7 specifications for Spring Framework 5.0 are listed as follows:

There are many changes to the minimum supported versions of several Java frameworks. The following list contains some of the minimum supported versions of prominent frameworks:

The following list shows the supported server versions:

Applications using earlier versions of any of the preceding specifications/frameworks need to be upgraded at least to the previously listed versions before they can use Spring Framework 5.0.

JDK 9 is expected to be released mid-2017. Spring Framework 5.0 is expected to have runtime compatibility with JDK 9.

The Spring Framework 4.x baseline version is Java SE 6. This means that it supports Java 6, 7, and 8. Having to support Java SE 6 and 7 puts constraints on the Spring Framework code. The framework code cannot use any of the new features in Java 8. So, while the rest of the world upgraded to Java 8, the code in Spring Framework (at least the major parts) was restricted to using earlier versions of Java.

With Spring Framework 5.0, the baseline version is Java 8. Spring Framework code is now upgraded to use the new features in Java 8. This will result in more readable and performant framework code. Some of the Java 8 features used are as follows:

Reactive programming is one of the most important features of Spring Framework 5.0.

Microservices architectures are typically built around event-based communication. Applications are built to react to events (or messages).

Reactive programming provides an alternate style of programming focused on building applications that react to events.

While Java 8 does not have built-in suppport for reactive programming, there are a number of frameworks that provide support for reactive programming:

We will discuss Reactive Programming and how you can implement it with Spring Web Flux in Chapter 11, Reactive Programming.

Building on top of the reactive features, Spring 5 also provides a functional web framework.

A functional web framework provides features to define endpoints using functional programming style. A simple hello world example is shown here:

    RouterFunction<String> route =
    route(GET("/hello-world"),
    request -> Response.ok().body(fromObject("Hello World")));

A functional web framework can also be used to define more complex routes, as shown in the following example:

    RouterFunction<?> route = route(GET("/todos/{id}"),
    request -> {
       Mono<Todo> todo = Mono.justOrEmpty(request.pathVariable("id"))
       .map(Integer::valueOf)
       .then(repository::getTodo);
       return Response.ok().body(fromPublisher(todo, Todo.class));
      })
     .and(route(GET("/todos"),
     request -> {
       Flux<Todo> people = repository.allTodos();
       return Response.ok().body(fromPublisher(people, Todo.class));
     }))
    .and(route(POST("/todos"),
    request -> {
      Mono<Todo> todo = request.body(toMono(Todo.class));
      return Response.ok().build(repository.saveTodo(todo));
    }));

A couple of important things to note are as follows:

We will discuss Mono and Flux in more detail in Chapter 11, Reactive Programming.

Until Java 8, the Java platform was not modular. A couple of important problems resulted out of this:

Open System Gateway initiative (OSGi) is one of the initiatives, started way back in 1999, to bring modularity into Java applications.

Each module (referred to as bundle) defines the following:

Each module can have its own life cycle. It can be installed, started, and stopped on its own.

Jigsaw is an initiative under Java Community Process (JCP), started with Java 7, to bring modularity into Java. It has two main aims:

Jigsaw is expected to be part of Java 9 and Spring Framework 5.0 is expected to include basic support for Jigsaw modules.

Kotlin is a statically typed JVM language that enables code that is expressive, short, and readable. Spring framework 5.0 has good support for Kotlin.

Consider a simple Kotlin program illustrating a data class, as shown here:

    import java.util.*
    data class Todo(var description: String, var name: String, var  
    targetDate : Date)
    fun main(args: Array<String>) {
      var todo = Todo("Learn Spring Boot", "Jack", Date())
      println(todo)
        //Todo(description=Learn Spring Boot, name=Jack, 
        //targetDate=Mon May 22 04:26:22 UTC 2017)
      var todo2 = todo.copy(name = "Jill")
      println(todo2)
         //Todo(description=Learn Spring Boot, name=Jill, 
         //targetDate=Mon May 22 04:26:22 UTC 2017)
      var todo3 = todo.copy()
      println(todo3.equals(todo)) //true
    }  

In fewer than 10 lines of code, we created and tested a data bean with three properties and the following functions:

Kotlin is strongly typed. But there is no need to specify the type of each variable explicitly:

    val arrayList = arrayListOf("Item1", "Item2", "Item3") 
    // Type is ArrayList

Named arguments allow you to specify the names of arguments when calling methods, resulting in more readable code:

    var todo = Todo(description = "Learn Spring Boot", 
    name = "Jack", targetDate = Date())

Kotlin makes functional programming simpler by providing default variables (it) and methods such as take, drop, and so on:

    var first3TodosOfJack = students.filter { it.name == "Jack"   
     }.take(3)

You can also specify default values for arguments in Kotlin:

    import java.util.*
    data class Todo(var description: String, var name: String, var
    targetDate : Date = Date())
    fun main(args: Array<String>) {
      var todo = Todo(description = "Learn Spring Boot", name = "Jack")
    }

With all its features making the code concise and expressive, we expect Kotlin to be a language to be learned for the .

We will discuss more about Kotlin in Chapter 13, Working with Kotlin in Spring.

Spring Framework 5 is a major Spring release with substantial increase in the baselines. Along with the increase in baseline versions for Java, Java EE and a few other frameworks, Spring Framework 5 removed support for a few frameworks:

If you are using any of the preceding frameworks, it is recommended that you plan a migration and stay with Spring Framework 4.3--which has support until 2019.