Spring for beginners

This article assumes you have basic programming skills and an engineering/programming background.

Table of Contents

What is Spring?

Spring is an open-source framework for building enterprise-grade applications in Java. It provides a comprehensive programming and configuration model for modern Java-based enterprise applications, covering everything from basic infrastructure support to advanced integration patterns. Spring is widely used for building web applications, microservices, and other types of software systems.

The Spring framework is built around the core concept of dependency injection, which allows developers to write loosely coupled code that can be easily tested and maintained. Spring also provides support for a wide range of other features, including declarative transaction management, data access, security, and more.

One of the key advantages of Spring is its modular architecture, which allows developers to pick and choose the components they need for their specific use case. This makes Spring highly flexible and adaptable to a wide range of application requirements.

In addition to the core Spring framework, there are also many other Spring projects that provide additional functionality, such as Spring Boot, which provides a streamlined way to build and deploy Spring-based applications, and Spring Cloud, which provides support for building cloud-native applications.

What are some popular concepts within Spring

There are several popular concepts within the Spring framework, including:

1. Dependency Injection (DI): This is a technique for managing object dependencies by creating objects in one class and injecting them into another class where they are needed. Spring uses DI to create and manage objects, making it easier to write loosely coupled code that is more modular and easier to test.

2. Aspect-Oriented Programming (AOP): AOP is a programming paradigm that allows developers to separate cross-cutting concerns (such as logging, security, and caching) from business logic. Spring provides AOP support through its AspectJ integration, making it easy to apply cross-cutting concerns across multiple classes and modules.

3. Spring MVC: This is a web framework built on top of the Spring framework that provides support for building web applications. It uses a Model-View-Controller (MVC) architecture to separate the presentation layer (View) from the business logic (Model), and provides a DispatcherServlet to route requests to the appropriate controller.

4. Spring Data: This is a module within the Spring framework that provides support for working with data in a variety of databases and data stores. It provides a consistent API for working with different data sources, making it easier to switch between different databases and data stores.

5. Spring Security: This is a module within the Spring framework that provides security and authentication services for web applications. It provides a variety of authentication mechanisms (such as form-based authentication, basic authentication, and OAuth), as well as authorization mechanisms (such as role-based access control and expression-based access control).

What is a Spring bean?

In Spring, a "bean" is simply an object that is instantiated, assembled, and managed by the Spring IoC (Inversion of Control) container. In other words, a bean is an instance of a class that is defined and managed by the Spring framework.

Beans are the basic building blocks of Spring applications, and they are typically created and managed in a Spring configuration file using XML, Java annotations, or Java configuration classes. Once a bean is defined in the configuration file, the Spring container is responsible for creating an instance of the bean, setting its properties, and managing its lifecycle (such as initializing and destroying the bean).

Spring beans can be configured to have a wide range of behaviors and characteristics. For example, they can be scoped as singleton (one instance shared across the application), prototype (a new instance created for each request), or other scopes. Beans can also be configured to have dependencies injected, through constructor injection or setter injection, by the Spring IoC container.

In summary, a Spring bean is a Java object that is managed by the Spring framework and can be configured with a wide range of behaviors and characteristics to support the development of enterprise-grade applications.

What is inversion of control?

In software development, "control" refers to the flow of execution in a program. In traditional programming, you as the developer are in control of the flow of execution. You write code that instantiates objects, calls methods, and manages the flow of data between different parts of the program.

Inversion of Control (IoC) is a design pattern that flips this control around, so that instead of you controlling the flow of execution, control is given to a framework or container. In other words, the framework or container becomes responsible for managing the flow of execution, while you as the developer simply provide the building blocks (such as classes and configuration files) that the framework or container uses to assemble and run the application.

This has several benefits. First, it allows you to write more modular and reusable code, because you can write code that depends on abstractions (such as interfaces) rather than concrete implementations. Second, it makes your code more testable, because you can write tests that substitute mock implementations for the real implementations. Finally, it makes your code more flexible and extensible, because you can easily swap out different implementations or modules without having to modify the core of your application.

In the context of Spring, IoC is implemented through the use of a container called the Spring IoC container. The container is responsible for creating and managing objects (or "beans") that make up the application, as well as handling dependencies between beans. This allows you to write code that is more modular, testable, and flexible, while also reducing the amount of boilerplate code you have to write.

What are some examples of IoC?

Let's say you want to build a program that sends notifications to users when they complete a task. You might write a simple implementation like this:

public class NotificationService {
  private EmailService emailService = new EmailService();
  public void sendNotification(String email, String message) {
    emailService.sendEmail(email, message);
  }
}

In this implementation, the NotificationService class depends on the EmailService class, which is instantiated inside the NotificationService class itself. This means that NotificationService is tightly coupled to EmailService, and you can't easily swap out EmailService for a different implementation without modifying the code.

To implement IoC, you would invert the control so that the NotificationService class does not create the EmailService instance itself. Instead, you would use a container or framework (like Spring) to manage the dependencies between objects.

Here's how you might implement the same functionality using Spring:

public class NotificationService {
  private EmailService emailService;
  public NotificationService(EmailService emailService) {
    this.emailService = emailService;
  }
  public void sendNotification(String email, String message) {
    emailService.sendEmail(email, message);
  }
}

In this implementation, NotificationService no longer creates an instance of EmailService. Instead, the EmailService instance is injected into NotificationService through its constructor. The actual implementation of EmailService is managed by the Spring container, which creates an instance of EmailService and injects it into NotificationService when the application starts.

This implementation makes the code more modular and flexible, because you can easily swap out different implementations of EmailService without having to modify the NotificationService class. Additionally, it makes the code easier to test, because you can create a mock implementation of EmailService to use in your tests, rather than having to send actual emails.

What is the advantage of the code block with IoC compared to the one without?

In the first implementation, you could potentially swap out the EmailService implementation by modifying the code, but it's not as flexible and modular as the second implementation.

Let's say you want to swap out the EmailService implementation with a different implementation (e.g. a SmsService). In the first implementation, you would need to modify the code to instantiate the SmsService class and replace the EmailService instance:

public class NotificationService {
  private EmailService emailService = new SmsService(); // modify this line
  public void sendNotification(String email, String message) {
    emailService.sendEmail(email, message); // replace with emailService.sendSms(phone, message);
  }
}

This means that every time you want to change the implementation of the notification service, you would need to modify the code and recompile the application.

In the second implementation using Spring, you can swap out the implementation of the EmailService without modifying the NotificationService class. You can simply configure the Spring container to use a different implementation of the EmailService at runtime, without needing to modify the code:

@Configuration
public class AppConfig {
  @Bean
  public EmailService emailService() {
    return new SmsService();
  }
  @Bean
  public NotificationService notificationService(EmailService emailService) {
    return new NotificationService(emailService);
  }
}

In this configuration, the EmailService is defined as a bean in the Spring container, and the NotificationService is configured to use the EmailService bean. If you want to use a different implementation of the EmailService, you can simply modify the configuration file without needing to modify the code.

Overall, the second implementation using Spring is more modular, flexible, and easier to maintain than the first implementation. It separates the concerns of object instantiation and configuration from the application code, allowing you to change the implementation of the notification service without modifying the code.

Isn't the spring config also defined by code and thus non-separated from the code?

You are correct that the Spring configuration is also defined in code, but the key difference is that the configuration code is separate from the application code that defines the business logic. In the second implementation using Spring, the NotificationService class does not need to know anything about the implementation details of the EmailService.

This separation of concerns allows you to modify the implementation of the EmailService without changing the application code. For example, if you wanted to use a different implementation of the EmailService, you could simply modify the configuration code to use a different bean, without modifying the NotificationService class:

@Configuration
public class AppConfig {
  @Bean
  public EmailService emailService() {
    return new AnotherEmailService();
  }
  @Bean
  public NotificationService notificationService(EmailService emailService) {
    return new NotificationService(emailService);
  }
}

With this configuration change, the NotificationService class would automatically use the AnotherEmailService implementation instead of the previous EmailService implementation, without needing to modify the NotificationService class.

This makes the application more modular and easier to maintain, because changes to the implementation details can be made without affecting the application logic, and vice versa.

Does inversion of control give the control to the config file instead of the business logic?

Yes, that's correct. Inversion of Control (IoC) is a design pattern in which the control of object creation and dependency injection is inverted or moved from the application code to a container or framework. In the case of Spring, the Spring container takes over the responsibility of object creation and dependency injection, which is typically defined in the configuration files.

By doing this, the application code becomes decoupled from the object creation and configuration details, and the control is transferred to the configuration files. This allows for greater flexibility, modularity, and maintainability of the application.

In short, IoC is a way of designing applications such that the control of object creation and dependency injection is handled by a container or framework, rather than the application code. The Spring framework uses IoC to manage the creation and configuration of objects, which are defined in the configuration files.

What are some must-know concepts about IoC?

Here are some must-know concepts about Inversion of Control (IoC):

1. Dependency Injection: A key concept in IoC is Dependency Injection (DI), which is a design pattern that enables loose coupling between objects. DI is the process of supplying the dependencies of an object from outside rather than having the object create its dependencies. This is typically done through constructors or setters.

2. Inversion of Control Container: IoC containers are frameworks that manage object creation and dependency injection. They provide a way to configure and manage objects and their dependencies in a modular way. Spring is an example of an IoC container.

3. Bean: In Spring, a bean is an object that is managed by the IoC container. Beans are typically defined in configuration files and can be injected into other objects.

4. ApplicationContext: The ApplicationContext is the central interface for configuring a Spring application. It is responsible for managing the lifecycle of beans, handling dependencies, and providing other services such as internationalization, validation, and more.

5. Configuration: Configuration in Spring refers to the process of defining beans and their dependencies. This is typically done in XML files, Java configuration classes, or annotations.

6. Scopes: Scopes determine the lifecycle of a bean. Spring provides several scopes such as singleton, prototype, request, session, and others.

7. Autowiring: Autowiring is the process of automatically injecting dependencies into a bean. This can be done by type, by name, or by using custom annotations.

8. Profiles: Profiles allow you to define different configurations for different environments. This can be useful for configuring a development, testing, or production environment.

These are some of the key concepts related to Inversion of Control (IoC) that are important to understand in the context of Spring.

ApplicationContext examples

In Spring, ApplicationContext is an interface that represents the Spring IoC container and is responsible for managing the lifecycle of beans, handling dependencies, and providing other services. Here are some examples of how to use ApplicationContext in Spring:

1. Loading the ApplicationContext

You can load the ApplicationContext in a Spring application using the ClassPathXmlApplicationContext class, which loads the XML configuration file from the classpath:

ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");

This code loads the applicationContext.xml file from the classpath.

1. Accessing Beans

Once the ApplicationContext is loaded, you can access beans by their name using the getBean() method:

MyBean bean = (MyBean) context.getBean("myBean");

This code gets the bean named "myBean" from the ApplicationContext.

1. Injecting Dependencies

You can define dependencies between beans in the ApplicationContext configuration file using XML or Java annotations:

<bean id="customerService" class="com.example.CustomerServiceImpl">
  <property name="customerRepository" ref="customerRepository"/>
</bean>
<bean id="customerRepository" class="com.example.CustomerRepositoryImpl">
  <constructor-arg value="jdbc:mysql://localhost/mydb"/>
</bean>

This XML configuration defines two beans: "customerService" and "customerRepository". The "customerService" bean has a property named "customerRepository" which is set to the "customerRepository" bean.

1. Scopes

You can define the scope of a bean in the ApplicationContext configuration file. For example, you can define a bean as a singleton, which means that only one instance of the bean is created and shared between all clients:

<bean id="myBean" class="com.example.MyBean" scope="singleton">
  <property name="message" value="Hello World!"/>
</bean>

This code defines a singleton bean named "myBean" of type "MyBean".

1. Profiles

You can define different configurations for different environments using profiles. For example, you can define a "dev" profile for development and a "prod" profile for production:

<beans profile="dev">
  <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource">
    ...
  </bean>
</beans>
<beans profile="prod">
  <bean id="dataSource" class="org.apache.commons.dbcp.PoolingDataSource">
    ...
  </bean>
</beans>

This code defines two configurations for the "dataSource" bean: one for the "dev" profile and one for the "prod" profile.

In summary, the ApplicationContext is a central interface in Spring that represents the IoC container and provides services such as bean management, dependency injection, and profile configuration. It is loaded from a configuration file and can be used to access beans, define dependencies, and configure the application.

What is a 'scope' in the context of Spring?

In Spring, a scope defines the lifecycle of a bean and how many instances of the bean will be created. Here are some simple examples of the most common scopes in Spring:

1. Singleton

The singleton scope is the default scope in Spring. It means that only one instance of the bean is created and shared between all clients:

<bean id="myBean" class="com.example.MyBean" scope="singleton">
  <property name="message" value="Hello World!"/>
</bean>

In this example, the "myBean" bean is a singleton, so there is only one instance of the "MyBean" class created, and all clients that use the "myBean" bean will get the same instance.

1. Prototype

The prototype scope means that a new instance of the bean is created every time it is requested:

<bean id="myBean" class="com.example.MyBean" scope="prototype">
  <property name="message" value="Hello World!"/>
</bean>

In this example, the "myBean" bean is a prototype, so every time a client requests the "myBean" bean, a new instance of the "MyBean" class is created.

1. Request

The request scope is used in web applications and means that a new instance of the bean is created for every HTTP request:

<bean id="myBean" class="com.example.MyBean" scope="request">
  <property name="message" value="Hello World!"/>
</bean>

In this example, the "myBean" bean is a request scope, so every time a new HTTP request is made, a new instance of the "MyBean" class is created.

1. Session

The session scope is used in web applications and means that a new instance of the bean is created for every user session:

<bean id="myBean" class="com.example.MyBean" scope="session">
  <property name="message" value="Hello World!"/>
</bean>

In this example, the "myBean" bean is a session scope, so every time a new user session is created, a new instance of the "MyBean" class is created.

In summary, scopes in Spring define the lifecycle of a bean and how many instances of the bean will be created. The most common scopes are singleton and prototype, while request and session scopes are used in web applications.

Spring scope types

As we said before, Spring provides several scopes such as singleton, prototype, request, session, and others.

In Spring, a scope defines the lifecycle of a bean, which means how long a bean will live, and how many instances of the bean will be created. A scope defines the visibility and accessibility of a bean within the Spring application context.

Here is a brief overview of the most common scopes in Spring:

• Singleton: The default scope in Spring, it means only one instance of the bean is created and shared between all clients.

• Prototype: A new instance of the bean is created every time it is requested.

• Request: A new instance of the bean is created for every HTTP request in a web application.

• Session: A new instance of the bean is created for every user session in a web application.

Other scopes include global session, application, websocket, and others. Each scope has its unique characteristics, and it's up to the developer to choose the right scope based on the use case.

By defining the scope of a bean, Spring provides greater flexibility in managing the beans within the application context. It also helps in reducing the memory footprint of the application, as unnecessary objects can be disposed of as soon as they are no longer needed.

What is Autowiring?

Autowiring is a way to automatically inject dependencies into a bean without manually wiring them. Here are some simple examples:

1. By type

If there is only one bean of a certain type in the application context, Spring can automatically wire it into another bean that requires it:

@Component
public class MyService {
  private final MyRepository repository;
  public MyService(MyRepository repository) {
    this.repository = repository;
  }
  // ...
}
@Repository
public class MyRepositoryImpl implements MyRepository {
  // ...
}

In this example, the MyService class requires a dependency of type MyRepository. Since there is only one bean of type MyRepository in the application context (MyRepositoryImpl), Spring can automatically wire it into MyService.

1. By name

If there are multiple beans of the same type in the application context, we can use the @Qualifier annotation to specify which bean to wire:

@Component
public class MyService {
  private final MyRepository repository;
  public MyService(@Qualifier("myRepository2") MyRepository repository) {
    this.repository = repository;
  }
  // ...
}
@Repository("myRepository1")
public class MyRepositoryImpl1 implements MyRepository {
  // ...
}
@Repository("myRepository2")
public class MyRepositoryImpl2 implements MyRepository {
  // ...
}

In this example, the MyService class requires a dependency of type MyRepository, but there are two beans of that type in the application context. By using the @Qualifier annotation, we can specify which bean to wire into MyService.

1. Using custom annotations

We can also create custom annotations to specify how Spring should wire dependencies:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
@Autowired
public @interface MyDependency {
}
@Component
public class MyService {
  @MyDependency
  private final MyRepository repository;
  public MyService(MyRepository repository) {
    this.repository = repository;
  }
  // ...
}
@Repository
public class MyRepositoryImpl implements MyRepository {
  // ...
}

In this example, we have created a custom annotation @MyDependency that is used to mark a field that should be autowired by Spring. We can then use this annotation to wire the dependency in the MyService class.

By using autowiring, we can reduce the amount of boilerplate code required for wiring dependencies and make our code more concise and readable.

Resolving configurations at runtime seems dangerous. Is Autowired a good practice?

While autowiring can simplify the code by automatically injecting dependencies into beans, it can also lead to some issues if not used properly. Here are some of the potential downsides of autowiring:

Despite these potential issues, autowiring can still be a good practice if used judiciously. Here are some situations where autowiring can be beneficial:

Overall, it's up to the developer to decide whether or not to use autowiring in their Spring application. It's important to weigh the potential benefits and drawbacks and use it in a way that makes sense for the particular use case.

Annotations

What are @Component and @Repository. And how do they differ from @Autowired?

The @Component and @Repository annotations are used to indicate to Spring that the annotated classes are candidates for auto-detection as Spring-managed beans. Specifically:

• @Component: This annotation is a generic stereotype for any Spring-managed component. It indicates that the annotated class is a Spring-managed bean that can be injected into other Spring-managed beans. In this case, the MyService class is annotated with @Component, indicating that it is a Spring-managed bean.

• @Repository: This annotation is a specialization of @Component and is used to indicate that the annotated class is a Spring-managed repository. Repositories are typically used to manage data access operations and are a common use case in Spring applications. In this case, the MyRepositoryImpl class is annotated with @Repository, indicating that it is a Spring-managed repository.

In the example you provided, the MyService class has a constructor that takes a MyRepository parameter. This means that the MyRepository dependency must be provided to MyService when it is created. Since MyRepositoryImpl is annotated with @Repository, Spring will automatically create an instance of MyRepositoryImpl and make it available for injection into any Spring-managed bean that requires it.

While it's possible to use @Autowired to inject dependencies, in this case, the MyRepository dependency is provided through the constructor of MyService. This is known as constructor injection, and it is considered by many to be a best practice for dependency injection in Spring applications. By using constructor injection, you can ensure that all required dependencies are provided at object creation time and that the object is in a fully initialized state when it is returned from the constructor.

When would you use @Autowired instead of @Component and @Repository etc.?

You would typically use @Autowired when you have a large number of dependencies that need to be injected into a bean, or when you want to inject dependencies into properties or methods rather than the constructor.

Using @Autowired to inject dependencies can simplify your code and make it more concise, as you don't need to define constructors or setter methods for all of your dependencies. However, it can also make your code less explicit, as the dependencies are not clearly defined in the constructor or elsewhere in the class. This can make it more difficult to understand the dependencies of a class and to reason about its behavior.

In general, it's a good practice to use constructor injection when possible, as it makes the dependencies of a class explicit and makes it easier to reason about the behavior of the class. However, there may be cases where using @Autowired makes sense, particularly when you have many dependencies or when you need to inject dependencies into properties or methods.

Is this all there is to Spring?

We covered some important concepts related to Spring, including inversion of control, dependency injection, Spring beans, scopes, autowiring, and application context. However, Spring is a large and complex framework, and there are many other concepts and features that we didn't cover in this conversation.

Some additional concepts that are important to understand in the context of Spring include:

• AOP (aspect-oriented programming): This is a programming paradigm that allows you to modularize cross-cutting concerns, such as logging, caching, or security, into reusable components called aspects. Spring provides AOP support through the use of proxies and aspects.

• Transactions: Spring provides support for declarative transaction management, which allows you to define transaction boundaries declaratively using annotations or XML configuration.

• MVC (model-view-controller): Spring provides an MVC framework that can be used to build web applications. The MVC framework provides a structured approach to handling web requests and generating responses.

• Security: Spring provides support for building secure web applications by integrating with various authentication and authorization mechanisms.

• Testing: Spring provides a testing framework that can be used to write integration tests for Spring applications.

These are just a few examples of the many concepts and features that Spring provides. The key takeaway is that Spring is a powerful and flexible framework that can be used to build a wide variety of applications, and there are many different concepts and features that you can use to achieve your goals.

Conclusion

In this conversation, we discussed several important concepts related to the Spring framework, including inversion of control, dependency injection, Spring beans, scopes, and autowiring. We also talked about the role of the application context in managing the lifecycle of Spring beans.

We covered some basic examples to illustrate how these concepts work, and we also discussed some best practices and potential pitfalls to be aware of when using these features.

Overall, the conversation highlighted the power and flexibility of the Spring framework, and how it can be used to build a wide variety of applications. It also emphasized the importance of understanding the underlying principles and concepts of Spring, and using them effectively to build robust and maintainable software.

As an engineer, it's important to have a deep understanding of the technologies and frameworks that you work with, and to be able to explain those concepts clearly to others. This conversation demonstrated an ability to do just that, and would be a positive sign to a potential employer that the candidate has a strong understanding of Spring and its capabilities.

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