Last Updated on October 22, 2024 by jt

The Java Programming language provided support for Annotations from Java 5.0. Leading Java frameworks were quick to adopt annotations and the Spring Framework started using annotations from the release 2.5. Due to the way they are defined, annotations provide a lot of context in their declaration.

Prior to annotations, the behavior of the Spring Framework was largely controlled through XML configuration. Today, the use of annotations provide us tremendous capabilities in how we configure the behaviors of the Spring Framework.

The Spring Framework provides a diverse range of annotations to streamline development and enhance functionality. Specifically, there are six main types of annotations available. Here’s a breakdown of each category:

1. Spring Stereotype Annotations
   These annotate classes which Spring component scanning will add and configure in the Spring context via component scans.
2. Spring Dependency Injection Annotations
   These annotations are essential for building web applications, managing requests, and handling responses effectively.
3. Spring Bean Annotations
   Annotations used to declare Spring Beans
4. Spring Boot Annotations
   Designed to simplify the bootstrapping and development of stand-alone Spring applications, these annotations offer streamlined configuration.
5. Spring MVC and REST Annotations
   These annotations are used to configure Spring Controllers.
6. Spring Cloud Annotations
   Configuration of Spring Cloud Components
7. Spring Framework Data Access Annotations
   These annotations facilitate data access and manipulation by integrating various data-handling technologies.
8. Cache Based Annotations
   Configure caching with Spring applications.
9. Task Execution and Scheduling Annotations
   Ideal for task scheduling, these enable the execution of tasks at specified intervals or times.
10. Spring Bean Annotations
    These help manage bean creation and lifecycle management within the Spring Container.

Understanding these categories empowers developers to leverage the full capabilities of the Spring Framework effectively.

Understanding Spring Bean Annotations and Their Role in Bean Configuration

Spring Bean Annotations are a powerful feature in the Spring Framework, providing a streamlined way to manage and configure beans within the application context. They are essential for defining the behavior and lifecycle of beans, and they simplify the configuration process by reducing the need for extensive XML setup.

Configuring Beans in Spring

In Spring, beans can be configured in several ways:

1. XML Configuration: Traditionally, XML files were used to declare beans. While still supported, it’s a more verbose method compared to annotations.
2. Java-based Configuration: By using the @Bean annotation within a configuration class, beans can be explicitly defined in a more concise, code-centric manner.
3. Component Scanning: This technique leverages annotations within the org.springframework.stereotype package, allowing the framework to automatically detect and manage beans during runtime.

Early version of Spring heavily used XML configuration. Today’s Spring Framework applications primarily use annotation based configuration.

Benefits of Using Annotations

• Simplified Configuration: Annotations reduce the boilerplate code needed for bean definitions, making the configuration quicker and more intuitive.
• Ease of Maintenance: By providing a clear indication of each class’s role within the application, annotations enhance readability and organization.
• Automatic Detection: Through component scanning, Spring can automatically detect and manage these annotated classes, streamlining the development process.

Spring Framework Key Stereotype Annotations

Understanding stereotype annotations is crucial for effectively managing Spring beans within an application context. These annotations not only streamline the process of bean creation but also categorize them logically across different application layers.

@Component

This annotation is used on classes to indicate a Spring component. The @Component annotation marks the Java class as a bean or say component so that the component-scanning mechanism of Spring can add it into the application context. Essentially, @Component is the primary stereotype annotation from which others are derived.

@Controller

The @Controller annotation is used to indicate the class is a Spring controller. This annotation can be used to identify controllers for Spring MVC or Spring WebFlux. It signifies classes acting as front controllers and is responsible for handling user requests and returning appropriate responses, which is especially prevalent in RESTful web services.

@Service

This annotation is used on a class. The @Service marks a Java class that performs some service, such as executing business logic, performing calculations, and calling external APIs. This annotation is a specialized form of the @Component annotation intended to be used in the service layer. It helps in clearly defining where the business logic resides within the application architecture.

@Repository

This annotation is used on Java classes which directly access the database. The @Repository annotation works as marker for any class that fulfills the role of repository or Data Access Object.

This annotation has a automatic translation feature. For example, when an exception occurs in the @Repository there is a handler for that exception and there is no need to add a try catch block.

In summary, the stereotype annotations—@Component, @Service, @Repository, and @Controller—serve as a powerful suite of tools for organizing and managing Spring beans. They provide a clear framework for dividing responsibilities across different parts of a Spring application, ensuring that each component is placed where it can perform its role most effectively.

Dependency Injection Annotations in Spring

Dependency Injection (DI) is a core feature of the Spring framework, designed to manage object dependencies within an application. It simplifies code by externally providing the necessary dependencies to objects, enhancing modularity and flexibility. Let’s delve into how annotations play a pivotal role in this process.

@Autowired

The @Autowired annotation is crucial for automatically injecting dependencies into your Spring-managed beans. When applied, it signals to the Spring container that a specific field, constructor, or setter method should have its dependencies wired at runtime. This removes hardcoded dependencies, promoting loose coupling within your application structure.

• Usage Areas:
  • Fields: Directly annotate private fields to have them injected by the Spring container.
  • Constructors: Annotate constructors that require dependency injection.
  • Setter Methods: Use for injecting dependencies through setters, allowing for more configurable elements.

Each approach serves a slightly different purpose:

Field Injection

With field injection, you directly annotate a field within a class. This is the simplest form and is often used for quick and straightforward dependency wiring. However, it can sometimes make unit testing more cumbersome since dependencies are not visible through constructor or setter methods.

public class Customer {
    @Autowired                               
    private Person person;                   
    private int type;
}

 Setter Injection

Setter injection is ideal for optional dependencies or situations where you want to allow configuration changes post-instantiation. This method provides flexibility as it enables modifying dependencies after object creation without altering its state significantly.

When you use @Autowired on setter methods, Spring tries to perform the by Type autowiring on the method. You are instructing Spring that it should initiate this property using setter method where you can add your custom code, like initializing any other property with this property.

public class Customer {                                                                                         
    private Person person;
    @Autowired                                                                                                      
    public void setPerson (Person person) {
     this.person=person;
    }
}

Consider a scenario where you need instance of class A, but you do not store A in the field of the class. You just use A to obtain instance of B, and you are storing B in this field. In this case setter method injection will better suite you. You will not have class level unused fields.

Constructor Injection

Constructor injection ensures that your class cannot be instantiated without its required dependencies, promoting immutability and robustness. By annotating a constructor, you clarify that the class requires specific dependencies to function.

When you use @Autowired on a constructor, constructor injection happens at the time of object creation. It indicates the constructor to autowire when used as a bean. One thing to note here is that only one constructor of any bean class can carry the @Autowired annotation.

@Component
public class Customer {
    private Person person;
    @Autowired
    public Customer (Person person) {					
      this.person=person;
    }
}

NOTE: As of Spring 4.3, @Autowired became optional on classes with a single constructor. In the above example, Spring would still inject an instance of the Person class if you omitted the @Autowired annotation.

Constructor Injection Best Practices

The majority of the Spring community considers using construction injection the best practice. Ideally, the injected property should be declared final. Following best practices, and using the automatic construction injection, the above Customer class could be re-written as follows:

@Component
public class Customer {
    private final Person person;
   
    public Customer (Person person) {					
      this.person=person;
    }
}

Since the Customer class only has one constructor, when Spring instantiates this bean, it will look for a Person bean to inject into the constructor at runtime.

@Required

This annotation is applied on bean setter methods. Consider a scenario where you need to enforce a required property. The @Required annotation indicates that the affected bean must be populated at configuration time with the required property. Otherwise an exception of type BeanInitializationException is thrown.

@Qualifier

This annotation is used along with @Autowired annotation. When you need more control of the dependency injection process, @Qualifier can be used. @Qualifier can be specified on individual constructor arguments or method parameters. This annotation is used to avoid confusion which occurs when you create more than one bean of the same type and want to wire only one of them with a property.

Consider an example where an interface BeanInterface is implemented by two beans BeanB1 and BeanB2.

@Component
public class BeanB1 implements BeanInterface {
  //
}
@Component
public class BeanB2 implements BeanInterface {
  //
}

Now if BeanA autowires this interface, Spring will not know which one of the two implementations to inject. One solution to this problem is the use of the @Qualifier annotation.

@Component
public class BeanA {
  @Autowired
  @Qualifier("beanB2")
  private BeanInterface dependency;
  ...
}

With the @Qualifier annotation added, Spring will now know which bean to autowire where beanB2 is the name of BeanB2.

@Lazy

This annotation is used on component classes. By default all autowired dependencies are created and configured at startup. But if you want to initialize a bean lazily, you can use @Lazy annotation over the class. This means that the bean will be created and initialized only when it is first requested for. You can also use this annotation on @Configuration classes. This indicates that all @Bean methods within that @Configuration should be lazily initialized.

@Value

The @Value annotation is used at the field, constructor parameter, and method parameter level. The @Value annotation indicates a default value expression for the field or parameter to initialize the property with. As the @Autowired annotation tells Spring to inject object into another when it loads your application context, you can also use @Value annotation to inject values from a property file into a bean’s attribute. It supports both #{...} and ${...} placeholders.

@Scope

The @Scope annotation is use to define the scope of the Spring Bean. By default, Spring uses a singleton scope, meaning one instance will be created.

Available Spring Bean Scopes:

• Singleton – Default, One instance created
• Prototype – A new instance is created each time it is requested.
• Request – One bean per HTTP Request
• Session – One bean per HTTP session.
• Global Session – One bean per global HTTP Session

Spring Bean Annotations

The Spring Bean Annotations help manage the creation and lifecycle of Spring Beans.

Spring Stereotype Annotations

We previously looked at the Spring Stereotype annotations.

• @Component
• @Service
• @Repository
• @Controller

@Configuration

This annotation is used on classes which define beans. @Configuration is an analog for XML configuration file – it is configuration using Java class. Java class annotated with @Configuration is a configuration by itself and will have methods to instantiate and configure the dependencies.

Here is an example:

@Configuration
public class DataConfig{ 
  @Bean
  public DataSource source(){
    DataSource source = new OracleDataSource();
    source.setURL();
    source.setUser();
    return source;
  }
  @Bean
  public PlatformTransactionManager manager(){
    PlatformTransactionManager manager = new BasicDataSourceTransactionManager();
    manager.setDataSource(source());
    return manager;
  }
}

@Bean

This annotation is used at the method level. @Bean annotation works with @Configuration to create Spring beans. As mentioned earlier, @Configuration will have methods to instantiate and configure dependencies. Such methods will be annotated with @Bean. The method annotated with this annotation works as bean ID and it creates and returns the actual bean.

Here is an example:

@Configuration
public class AppConfig{
  @Bean
  public Person person(){
    return new Person(address());
  }
  @Bean
  public Address address(){
    return new Address();
  }
}

@ComponentScan

This annotation is used with @Configuration annotation to allow Spring to know the packages to scan for annotated components. @ComponentScan is also used to specify base packages using basePackageClasses or basePackage attributes to scan. If specific packages are not defined, scanning will occur from the package of the class that declares this annotation.

Checkout this post for an in depth look at the Component Scan annotation.

Spring Boot Annotations

@EnableAutoConfiguration

This annotation is usually placed on the main application class. The @EnableAutoConfiguration annotation implicitly defines a base “search package”. This annotation tells Spring Boot to start adding beans based on classpath settings, other beans, and various property settings.

@SpringBootApplication

This annotation is used on the application class while setting up a Spring Boot project. The class that is annotated with the @SpringBootApplication must be kept in the base package. The one thing that the @SpringBootApplication does is a component scan. But it will scan only its sub-packages. As an example, if you put the class annotated with @SpringBootApplication in com.example then @SpringBootApplication will scan all its sub-packages, such as com.example.a, com.example.b, and com.example.a.x.

The @SpringBootApplication is a convenient annotation that adds all the following:

• @Configuration
• @EnableAutoConfiguration
• @ComponentScan

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Spring MVC and REST Annotations

@Controller

This annotation is used on Java classes that play the role of controller in your application. The @Controller annotation allows autodetection of component classes in the classpath and auto-registering bean definitions for them. To enable autodetection of such annotated controllers, you can add component scanning to your configuration. The Java class annotated with @Controller is capable of handling multiple request mappings.

This annotation can be used with Spring MVC and Spring WebFlux.

@RestController

This annotation is used at the class level. The @RestController annotation marks the class as a controller where every method returns a domain object instead of a view. By annotating a class with this annotation you no longer need to add @ResponseBody to all the RequestMapping method. It means that you no more use view-resolvers or send html in response. You just send the domain object as HTTP response in the format that is understood by the consumers like JSON.

@RestController is a convenience annotation which combines @Controller and @ResponseBody.

@RequestMapping

This annotation is used both at class and method level. The @RequestMapping annotation is used to map web requests onto specific handler classes and handler methods. When @RequestMapping is used on class level it creates a base URI for which the controller will be used. When this annotation is used on methods it will give you the URI on which the handler methods will be executed. From this you can infer that the class level request mapping will remain the same whereas each handler method will have their own request mapping.

Sometimes you may want to perform different operations based on the HTTP method used, even though the request URI may remain the same. In such situations, you can use the method attribute of @RequestMapping with an HTTP method value to narrow down the HTTP methods in order to invoke the methods of your class.

Here is a basic example on how a controller along with request mappings work:

@Controller
@RequestMapping("/welcome")
public class WelcomeController{
  @RequestMapping(method = RequestMethod.GET)
  public String welcomeAll(){
    return "welcome all";
  }	
}

In this example only GET requests to /welcome is handled by the welcomeAll() method.

This annotation also can be used with Spring MVC and Spring WebFlux.

The @RequestMapping annotation is very versatile. Please see my in depth post on Request Mapping bere.

@CookieValue

This annotation is used at method parameter level. @CookieValue is used as argument of request mapping method. The HTTP cookie is bound to the @CookieValue parameter for a given cookie name. This annotation is used in the method annotated with @RequestMapping.
Let us consider that the following cookie value is received with a http request:

JSESSIONID=418AB76CD83EF94U85YD34W

To get the value of the cookie, use @CookieValue like this:

@RequestMapping("/cookieValue")
  public void getCookieValue(@CookieValue "JSESSIONID" String cookie){
}

@CrossOrigin

This annotation is used both at class and method level to enable cross origin requests. In many cases the host that serves JavaScript will be different from the host that serves the data. In such a case Cross Origin Resource Sharing (CORS) enables cross-domain communication. To enable this communication you just need to add the @CrossOrigin annotation.

By default the @CrossOrigin annotation allows all origin, all headers, the HTTP methods specified in the @RequestMapping annotation and maxAge of 30 min. You can customize the behavior by specifying the corresponding attribute values.

An example to use @CrossOrigin at both controller and handler method levels is this.

@CrossOrigin(maxAge = 3600)
@RestController
@RequestMapping("/account")
public class AccountController {

@CrossOrigin(origins = "http://example.com")
@RequestMapping("/message")
  public Message getMessage() {
      // ...
    }
 
@RequestMapping("/note")
    public Note getNote() {
        // ...
    }
}

In this example, both getExample() and getNote() methods will have a maxAge of 3600 seconds. Also, getExample() will only allow cross-origin requests from http://example.com, while getNote() will allow cross-origin requests from all hosts.

Composed @RequestMapping Variants

Spring framework 4.3 introduced the following method-level variants of @RequestMapping annotation to better express the semantics of the annotated methods. Using these annotations have become the standard ways of defining the endpoints. They act as wrapper to @RequestMapping.

These annotations can be used with Spring MVC and Spring WebFlux.

@GetMapping

This annotation is used for mapping HTTP GET requests onto specific handler methods. @GetMapping is a composed annotation that acts as a shortcut for @RequestMapping(method = RequestMethod.GET)

@PostMapping

This annotation is used for mapping HTTP POST requests onto specific handler methods. @PostMapping is a composed annotation that acts as a shortcut for @RequestMapping(method = RequestMethod.POST)

@PutMapping

This annotation is used for mapping HTTP PUT requests onto specific handler methods. @PutMapping is a composed annotation that acts as a shortcut for @RequestMapping(method = RequestMethod.PUT)

@PatchMapping

This annotation is used for mapping HTTP PATCH requests onto specific handler methods. @PatchMapping is a composed annotation that acts as a shortcut for @RequestMapping(method = RequestMethod.PATCH)

@DeleteMapping

This annotation is used for mapping HTTP DELETE requests onto specific handler methods. @DeleteMapping is a composed annotation that acts as a shortcut for @RequestMapping(method = RequestMethod.DELETE)

@ExceptionHandler

This annotation is used at method levels to handle exception at the controller level. The @ExceptionHandler annotation is used to define the class of exception it will catch. You can use this annotation on methods that should be invoked to handle an exception. The @ExceptionHandler values can be set to an array of Exception types. If an exception is thrown that matches one of the types in the list, then the method annotated with matching @ExceptionHandler will be invoked.

@InitBinder

This annotation is a method level annotation that plays the role of identifying the methods which initialize the WebDataBinder – a DataBinder that binds the request parameter to JavaBean objects. To customise request parameter data binding , you can use @InitBinder annotated methods within our controller. The methods annotated with @InitBinder all argument types that handler methods support.
The @InitBinder annotated methods will get called for each HTTP request if you don’t specify the value element of this annotation. The value element can be a single or multiple form names or request parameters that the init binder method is applied to.

@Mappings and @Mapping

This annotation is used on fields. The @Mapping annotation is a meta annotation that indicates a web mapping annotation. When mapping different field names, you need to configure the source field to its target field and to do that you have to add the @Mappings annotation. This annotation accepts an array of @Mapping having the source and the target fields.

@MatrixVariable

This annotation is used to annotate request handler method arguments so that Spring can inject the relevant bits of matrix URI. Matrix variables can appear on any segment each separated by a semicolon. If a URL contains matrix variables, the request mapping pattern must represent them with a URI template. The @MatrixVariable annotation ensures that the request is matched with the correct matrix variables of the URI.

@PathVariable

This annotation is used to annotate request handler method arguments. The @RequestMapping annotation can be used to handle dynamic changes in the URI where certain URI value acts as a parameter. You can specify this parameter using a regular expression. The @PathVariable annotation can be used declare this parameter.

@RequestAttribute

This annotation is used to bind the request attribute to a handler method parameter. Spring retrieves the named attributes value to populate the parameter annotated with @RequestAttribute. While the @RequestParam annotation is used bind the parameter values from query string, the @RequestAttribute is used to access the objects which have been populated on the server side.

@RequestBody

This annotation is used to annotate request handler method arguments. The @RequestBody annotation indicates that a method parameter should be bound to the value of the HTTP request body. The HttpMessageConveter is responsible for converting from the HTTP request message to object.

@RequestHeader

This annotation is used to annotate request handler method arguments. The @RequestHeader annotation is used to map controller parameter to request header value. When Spring maps the request, @RequestHeader checks the header with the name specified within the annotation and binds its value to the handler method parameter. This annotation helps you to get the header details within the controller class.

@RequestParam

This annotation is used to annotate request handler method arguments. Sometimes you get the parameters in the request URL, mostly in GET requests. In that case, along with the @RequestMapping annotation you can use the @RequestParam annotation to retrieve the URL parameter and map it to the method argument. The @RequestParam annotation is used to bind request parameters to a method parameter in your controller.

@RequestPart

This annotation is used to annotate request handler method arguments. The @RequestPart annotation can be used instead of @RequestParam to get the content of a specific multipart and bind to the method argument annotated with @RequestPart. This annotation takes into consideration the “Content-Type” header in the multipart(request part).

@ResponseBody

This annotation is used to annotate request handler methods. The @ResponseBody annotation is similar to the @RequestBody annotation. The @ResponseBody annotation indicates that the result type should be written straight in the response body in whatever format you specify like JSON or XML. Spring converts the returned object into a response body by using the HttpMessageConveter.

@ResponseStatus

This annotation is used on methods and exception classes. @ResponseStatus marks a method or exception class with a status code and a reason that must be returned. When the handler method is invoked the status code is set to the HTTP response which overrides the status information provided by any other means. A controller class can also be annotated with @ResponseStatus which is then inherited by all @RequestMapping methods.

@ControllerAdvice

This annotation is applied at the class level. As explained earlier, for each controller you can use @ExceptionHandler on a method that will be called when a given exception occurs. But this handles only those exception that occur within the controller in which it is defined. To overcome this problem you can now use the @ControllerAdvice annotation. This annotation is used to define @ExceptionHandler, @InitBinder and @ModelAttribute methods that apply to all @RequestMapping methods. Thus if you define the @ExceptionHandler annotation on a method in @ControllerAdvice class, it will be applied to all the controllers.

@RestControllerAdvice

This annotation is applied on Java classes. @RestControllerAdvice is a convenience annotation which combines @ControllerAdvice and @ResponseBody. This annotation is used along with the @ExceptionHandler annotation to handle exceptions that occur within the controller.

@SessionAttribute

This annotation is used at method parameter level. The @SessionAttribute annotation is used to bind the method parameter to a session attribute. This annotation provides a convenient access to the existing or permanent session attributes.

@SessionAttributes

This annotation is applied at type level for a specific handler. The @SessionAtrributes annotation is used when you want to add a JavaBean object into a session. This is used when you want to keep the object in session for short lived. @SessionAttributes is used in conjunction with @ModelAttribute.
Consider this example.

@ModelAttribute("person")
public Person getPerson(){} 
// within the same controller as above snippet
@Controller
@SeesionAttributes(value="person", types={Person.class})
public class PersonController{}

The @ModelAttribute name is assigned to the @SessionAttributes as value. The @SessionAttributes has two elements. The value element is the name of the session in the model and the types element is the type of session attributes in the model.

Spring Cloud Annotations

@EnableConfigServer

This annotation is used at the class level. When developing a project with a number of services, you need to have a centralized and straightforward manner to configure and retrieve the configurations about all the services that you are going to develop. One advantage of using a centralized config server is that you don’t need to carry the burden of remembering where each configuration is distributed across multiple and distributed components.

You can use Spring cloud’s @EnableConfigServer annotation to start a config server that the other applications can talk to.

@EnableEurekaServer

This annotation is applied to Java classes. One problem that you may encounter while decomposing your application into microservices is that, it becomes difficult for every service to know the address of every other service it depends on. There comes the discovery service which is responsible for tracking the locations of all other microservices.
Netflix’s Eureka is an implementation of a discovery server and integration is provided by Spring Boot. Spring Boot has made it easy to design a Eureka Server by just annotating the entry class with @EnableEurekaServer.

@EnableDiscoveryClient

This annotation is applied to Java classes. In order to tell any application to register itself with Eureka you just need to add the @EnableDiscoveryClient annotation to the application entry point. The application that’s now registered with Eureka uses the Spring Cloud Discovery Client abstraction to interrogate the registry for its own host and port.

@EnableCircuitBreaker

This annotation is applied on Java classes that can act as the circuit breaker. The circuit breaker pattern can allow a micro service continue working when a related service fails, preventing the failure from cascading. This also gives the failed service a time to recover.

The class annotated with @EnableCircuitBreaker will monitor, open, and close the circuit breaker.

@HystrixCommand

This annotation is used at the method level. Netflix’s Hystrix library provides the implementation of Circuit Breaker pattern. When you apply the circuit breaker to a method, Hystrix watches for the failures of the method. Once failures build up to a threshold, Hystrix opens the circuit so that the subsequent calls also fail. Now Hystrix redirects calls to the method and they are passed to the specified fallback methods.
Hystrix looks for any method annotated with the @HystrixCommand annotation and wraps it into a proxy connected to a circuit breaker so that Hystrix can monitor it.

Consider the following example:

@Service
public class BookService{
    private final RestTemplate restTemplate; 
    public BookService(RestTemplate rest){
      this.restTemplate =   rest;
    }                                           
  @HystrixCommand(fallbackMethod = "newList")                                                                     public String bookList(){
    URI uri = URI.create("http://localhost:8081/recommended");                                                      return this.restTemplate.getForObject(uri, String.class);  
  }
  public String newList(){
    return "Cloud native Java";
  }
}

Here @HystrixCommand is applied to the original method bookList(). The @HystrixCommand annotation has newList as the fallback method. So for some reason if Hystrix opens the circuit on bookList(), you will have a placeholder book list ready for the users.

Spring Framework DataAccess Annotations

@Transactional

This annotation is placed before an interface definition, a method on an interface, a class definition, or a public method on a class. The mere presence of @Transactional is not enough to activate the transactional behaviour. The @Transactional is simply a metadata that can be consumed by some runtime infrastructure. This infrastructure uses the metadata to configure the appropriate beans with transactional behaviour.

The annotation further supports configuration like:

• The Propagation type of the transaction
• The Isolation level of the transaction
• A timeout for the operation wrapped by the transaction
• A read only flag – a hint for the persistence provider that the transaction must be read only
  The rollback rules for the transaction

Cache-Based Annotations

@Cacheable

This annotation is used on methods. The simplest way of enabling the cache behaviour for a method is to annotate it with @Cacheable and parameterize it with the name of the cache where the results would be stored.

@Cacheable("addresses")
public String getAddress(Book book){...}

In the snippet above , the method getAddress is associated with the cache named addresses. Each time the method is called, the cache is checked to see whether the invocation has been already executed and does not have to be repeated.

@CachePut

This annotation is used on methods. Whenever you need to update the cache without interfering the method execution, you can use the @CachePut annotation. That is, the method will always be executed and the result cached.

@CachePut("addresses")
public String getAddress(Book book){...}

Using @CachePut and @Cacheable on the same method is strongly discouraged as the former forces the execution in order to execute a cache update, the latter causes the method execution to be skipped by using the cache.

@CacheEvict

This annotation is used on methods. It is not that you always want to populate the cache with more and more data. Sometimes you may want remove some cache data so that you can populate the cache with some fresh values. In such a case use the @CacheEvict annotation.

@CacheEvict(value="addresses", allEntries="true")
public String getAddress(Book book){...}

Here an additional element allEntries is used along with the cache name to be emptied. It is set to true so that it clears all values and prepares to hold new data.

@CacheConfig

This annotation is a class level annotation. The @CacheConfig annotation helps to streamline some of the cache information at one place. Placing this annotation on a class does not turn on any caching operation. This allows you to store the cache configuration at the class level so that you don’t have declare things multiple times.

Task Execution and Scheduling Annotations

@EnableScheduling

Turns on Spring’s scheduling support. This annotation is necessary to schedule tasks using Spring’s built-in scheduling features.

@Scheduled

This annotation is a method level annotation. The @Scheduled annotation is used on methods along with the trigger metadata. A method with @Scheduled should have void return type and should not accept any parameters.

There are different ways of using the @Scheduled annotation:

@Scheduled(fixedDelay=5000)
public void doSomething() {
  // something that should execute periodically   
}

In this case, the duration between the end of last execution and the start of next execution is fixed. The tasks always wait until the previous one is finished.

@Scheduled(fixedRate=5000)
public void doSomething() { 
  // something that should execute periodically 
}

In this case, the beginning of the task execution does not wait for the completion of the previous execution.

@Scheduled(initialDelay=1000,fixedRate=5000)
public void doSomething() { 
 // something that should execute periodically after an initial delay  
}

The task gets executed initially with a delay and then continues with the specified fixed rate.

@Schedules

Allows multiple schedules to be defined on a single method.

@EnableAsync

By placing @EnableAsync on a Spring configuration class, you activate Spring’s asynchronous method execution capability. This tells Spring to look for methods annotated with @Async and run them in a separate thread.

When enabled, Spring can handle multiple requests concurrently by managing different threads. It utilizes a TaskExecutor to run these async methods, which you can configure to suit your application’s performance needs, such as defining thread pools.

@Async

This annotation is used on methods to execute each method in a separate thread. The @Async annotation is provided on a method so that the invocation of that method will occur asynchronously. Unlike methods annotated with @Scheduled, the methods annotated with @Async can take arguments. They will be invoked in the normal way by callers at runtime rather than by a scheduled task.

@Async can be used with both void return type methods and the methods that return a value. However methods with return value must have a Future typed return values.

Spring Framework Testing Annotations

@BootstrapWith

This annotation is a class level annotation. The @BootstrapWith annotation is used to configure how the Spring TestContext Framework is bootstrapped. This annotation is used as a metadata to create custom composed annotations and reduce the configuration duplication in a test suite.

@ContextConfiguration

This annotation is a class level annotation that defines a metadata used to determine which configuration files to use to the load the ApplicationContext for your test. More specifically @ContextConfiguration declares the annotated classes that will be used to load the context. You can also tell Spring where to locate for the file.
@ContextConfiguration(locations={"example/test-context.xml", loader = Custom ContextLoader.class})

@WebAppConfiguration

This annotation is a class level annotation. The @WebAppConfiguration is used to declare that the ApplicationContext loaded for an integration test should be a WebApplicationContext. This annotation is used to create the web version of the application context. It is important to note that this annotation must be used with the @ContextConfiguration annotation.The default path to the root of the web application is src/main/webapp. You can override it by passing a different path to the @WebAppConfiguration.

@Timed

This annotation is used on methods. The @Timed annotation indicates that the annotated test method must finish its execution at the specified time period(in milliseconds). If the execution exceeds the specified time in the annotation, the test fails.

@Timed(millis=10000)
public void testLongRunningProcess() {  ... }

In this example, the test will fail if it exceeds 10 seconds of execution.

@Repeat

This annotation is used on test methods. If you want to run a test method several times in a row automatically, you can use the @Repeat annotation. The number of times that test method is to be executed is specified in the annotation.

@Repeat(10)
@Test
public void testProcessRepeatedly() {  ... }

In this example, the test will be executed 10 times.

@Commit

This annotation can be used as both class-level or method-level annotation. After execution of a test method, the transaction of the transactional test method can be committed using the @Commit annotation. This annotation explicitly conveys the intent of the code. When used at the class level, this annotation defines the commit for all test methods within the class. When declared as a method level annotation @Commit specifies the commit for specific test methods overriding the class level commit.

@RollBack

This annotation can be used as both class-level and method-level annotation. The @RollBack annotation indicates whether the transaction of a transactional test method must be rolled back after the test completes its execution. If this true @Rollback(true), the transaction is rolled back. Otherwise, the transaction is committed. @Commit is used instead of @RollBack(false).

When used at the class level, this annotation defines the rollback for all test methods within the class.

When declared as a method level annotation @RollBack specifies the rollback for specific test methods overriding the class level rollback semantics.

@DirtiesContext

This annotation is used as both class-level and method-level annotation. @DirtiesContext indicates that the Spring ApplicationContext has been modified or corrupted in some manner and it should be closed. This will trigger the context reloading before execution of next test. The ApplicationContext is marked as dirty before or after any such annotated method as well as before or after current test class.

The @DirtiesContext annotation supports BEFORE_METHOD, BEFORE_CLASS, and BEFORE_EACH_TEST_METHOD modes for closing the ApplicationContext before a test.

NOTE: Avoid overusing this annotation. It is an expensive operation and if abused, it can really slow down your test suite.

@BeforeTransaction

This annotation is used to annotate void methods in the test class. @BeforeTransaction annotated methods indicate that they should be executed before any transaction starts executing. That means the method annotated with @BeforeTransaction must be executed before any method annotated with @Transactional.

@AfterTransaction

This annotation is used to annotate void methods in the test class. @AfterTransaction annotated methods indicate that they should be executed after a transaction ends for test methods. That means the method annotated with @AfterTransaction must be executed after the method annotated with @Transactional.

@Sql

This annotation can be declared on a test class or test method to run SQL scripts against a database. The @Sql annotation configures the resource path to SQL scripts that should be executed against a given database either before or after an integration test method. When @Sql is used at the method level it will override any @Sql defined in at class level.

@SqlConfig

This annotation is used along with the @Sql annotation. The @SqlConfig annotation defines the metadata that is used to determine how to parse and execute SQL scripts configured via the @Sql annotation. When used at the class-level, this annotation serves as global configuration for all SQL scripts within the test class. But when used directly with the config attribute of @Sql, @SqlConfig serves as a local configuration for SQL scripts declared.

@SqlGroup

This annotation is used on methods. The @SqlGroup annotation is a container annotation that can hold several @Sql annotations. This annotation can declare nested @Sql annotations.
In addition, @SqlGroup is used as a meta-annotation to create custom composed annotations. This annotation can also be used along with repeatable annotations, where @Sql can be declared several times on the same method or class.

@SpringBootTest

This annotation is used to start the Spring context for integration tests. This will bring up the full autoconfiguration context.

@DataJpaTest

The @DataJpaTest annotation will only provide the autoconfiguration required to test Spring Data JPA using an in-memory database such as H2.

This annotation is used instead of @SpringBootTest

@DataMongoTest

The @DataMongoTest will provide a minimal autoconfiguration and an embedded MongoDB for running integration tests with Spring Data MongoDB.

@WebMVCTest

The @WebMVCTest will bring up a mock servlet context for testing the MVC layer. Services and components are not loaded into the context. To provide these dependencies for testing, the @MockBean annotation is typically used.

@AutoConfigureMockMVC

The @AutoConfigureMockMVC annotation works very similar to the @WebMVCTest annotation, but the full Spring Boot context is started.

@MockBean

Creates and injects a Mockito Mock for the given dependency.

@JsonTest

Will limit the auto configuration of Spring Boot to components relevant to processing JSON.

This annotation will also autoconfigure an instance of JacksonTester or GsonTester.

@TestPropertySource

Class level annotation used to specify property sources for the test class.

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