October 26, 2016

In our post today, we're talking about Spring Framework Annotations. We've gathered up all the commonly-used and useful annotations developers will use and packed them into a one-page Spring annotations cheat sheet. It covers everything from the basic annotations you need to get your project started, to service discovery annotations.

You can grab a copy of the cheat sheet at the bottom of the page, or you can skip to a relevant section to grab an annotation to go.

Important Spring Annotations

Spring uses dependency injection to configure and to bring your application together. It means that you just declare the components of your system and how they interact. Then Spring wires it all together at runtime. Here are the most important annotations you should know of.

@Configuration - annotation is used to mark a class as a source of the bean definitions. Beans are the components of the system that you want to wire together. A method marked with the @Bean annotation is a bean producer. Spring will handle the life cycle of the beans for you, and it will use these methods to create the beans.

- annotation is used to mark a class as a source of the bean definitions. Beans are the components of the system that you want to wire together. A method marked with the @Bean annotation is a bean producer. Spring will handle the life cycle of the beans for you, and it will use these methods to create the beans. @ComponentScan - use to make sure that Spring knows about your configuration classes and can initialize the beans correctly. It makes Spring scan the packages configured with it for the @Configuration classes.

use to make sure that Spring knows about your configuration classes and can initialize the beans correctly. It makes Spring scan the packages configured with it for the @Configuration classes. @Import - If you need even more precise control of the configuration classes, you can always use @import to load additional configuration. This one works even when you specify the beans in an XML file like it's 1999.

If you need even more precise control of the configuration classes, you can always use @import to load additional configuration. This one works even when you specify the beans in an XML file like it's 1999. @Component - Another way to declare a bean is to mark a class with a @Component annotation. Doing this turns the class into a Spring bean at the auto-scan time.

- Another way to declare a bean is to mark a class with a @Component annotation. Doing this turns the class into a Spring bean at the auto-scan time. @Service - Mark a specialization of a @Component. It tells Spring that it's safe to manage them with more freedom than regular components. Remember, services have no encapsulated state.

Mark a specialization of a @Component. It tells Spring that it's safe to manage them with more freedom than regular components. Remember, services have no encapsulated state. @Autowired - To wire the application parts together, use the @Autowired on the fields, constructors, or methods in a component. Spring's dependency injection mechanism wires appropriate beans into the class members marked with @Autowired.

More Complex Spring Annotations

Now it's time to go a bit deeper. The bean initialization is eager by default. Spring will try to initialize all the beans and wire them all together. You can mark a @Bean or @Component with @Lazy to have them be initialized on demand. It can save some startup time!

If you have multiple beans that can be wired into the field marked with @Autowired, use @Qualifier to filter which beans should be used there.

Another useful annotation is @Value. This one indicates a default value expression for the field or parameter to initialize the property with. Typically something like the following expression, referencing a configuration property will be used there: "#{systemProperties.myProp}" .

And to make the system more robust, you can enable the verification to make Spring fail when a bean to autowire is not found. Use @Required to fail the wiring if the dependency cannot be injected.

@Lazy - Initialize bean on demand

Initialize bean on demand @Value - indicate default value expression for field or parameter

indicate default value expression for field or parameter @Required - Fail wiring if dependency can't be injected

Armed with these annotations you can make the application come together with a very little effort. Naturally, there are more Spring annotations that you might want to use, but these here are the core of the framework that enables the flexibility Spring is known for!

Important Spring Boot Annotations

Let's look at some of the most frequently used annotations in the context of web apps. Most of our readers are either backend engineers or are doing full stack developer jobs. So it makes sense to popularize the Spring Framework annotations that make web development easier.

@SpringBootApplicaiton

First one of the most basic, super helpful annotations, our all time favorite @SpringBootApplication. There's nothing magical about it. It's syntactic sugar for combining other annotations that we'll look at in just a moment. @SpringBootApplication is @Configuration, @EnableAutoConfiguration and @ComponentScan annotations combined, configured with their default attributes.

@Configuration and @ComponentScan

The @Configuration and @ComponentScan annotations that we described above make Spring create and configure the beans and components of your application. It's a great way to decouple the actual business logic code from wiring the app together.

@EnableAutoConfiguration

Now the @EnableAutoConfiguration annotation is even better. It makes Spring guess the configuration based on the JAR files available on the classpath. It can figure out what libraries you use and preconfigure their components without you lifting a finger. It is how all the spring-boot-starter libraries work. Meaning it's a major lifesaver both when you're just starting to work with a library as well as when you know and trust the default config to be reasonable.

Important Spring Boot Web Annotations

The following annotations make Spring configure your app to be a web application, capable of serving the HTTP response.

@Controller

@Controller marks the class as a web controller, capable of handling the HTTP requests. Spring will look at the methods of the class marked with the @Controller annotation and establish the routing table to know which methods serve which endpoints.

@ResponseBody

The @ResponseBody is a utility annotation that makes Spring bind a method's return value to the HTTP response body. When building a JSON endpoint, this is an amazing way to magically convert your objects into JSON for easier consumption.

@RestController

Then there's the @RestController annotation -- a convenience syntax for @Controller and @ResponseBody together. This means that all the action methods in the marked class will return the JSON response.

@RequestMapping(method = RequestMethod.GET, value = "/path")

The @RequestMapping(method = RequestMethod.GET, value = "/path") annotation specifies a method in the controller that should be responsible for serving the HTTP request to the given path. Spring will work the implementation details of how it's done. You simply specify the path value on the annotation and Spring will route the requests into the correct action methods.

@RequestParam(value="name", defaultValue="World")

Naturally, the methods handling the requests might take parameters. To help you with binding the HTTP parameters into the action method arguments, you can use the @RequestParam(value="name", defaultValue="World") annotation. Spring will parse the request parameters and put the appropriate ones into your method arguments.

@PathVariable("placeholderName")

Another common way to provide information to the backend is to encode it in the URL. Then you can use the @PathVariable("placeholderName") annotation to bring the values from the URL to the method arguments.

Most Useful Spring Cloud Annotations

Now you have a fully functioning web app that is both neat on the code level as well as separated into beans and service classes. The app is declaratively configuring background jobs and gets constructed and configured at runtime without extra uproar.

If you're in the mood to build a larger system of such web apps, you'll need to work more on the organizational problems. These include issues like how to configure these multiple apps, how to make them communicate with each other, and how to make sure that network failures will not crash the whole system.

The Spring Cloud project tries to precisely answer these questions and help you take your apps to the cloud.

@EnableConfigServer

@EnableConfigServer turns your application into a server other apps can get their configuration from. You need a configuration server to have a central repository of all the appropriate configurations. The good thing is that creating one is just an annotation away.

Now when you have the central configuration server available, you can use the spring.application.cloud.config.uri property in the client Spring apps to point them to the configuration server. When these apps start, they will reach out to the configuration server and obtain the necessary properties. You don't have to package your configuration together with the app code. This separation of concerns works wonders to simplify your deployments.

Spring Discovery Service Annotations

The next bit we want to cover is even cooler. Imagine a situation when you have many small services that are context bounded, excel at one thing and put together, form a complex system that brings your real business value. How do you wire the services together? You cannot go with hardcoding the URLs in code or even the configuration -- it just won't scale. What you need is a discovery service.

You probably know where I'm going. You can get yourself a fully functioning Eureka service discovery server just by using a single annotation.

@EnableEurekaServer

Just by adding the @EnableEurekaServer annotation to your brand new Spring Boot app, you make it an Eureka discovery service. Now other apps can locate services through it. To take advantage of this wonderful topology, you need to instruct the apps that they should become clients for this Eureka server.

@EnableDiscoveryClient

Luckily for you, there is a @EnableDiscoveryClient annotation. This makes your app register in the service discovery server and then consult with it to discover the other services you need. When all the apps use the discovery client, you don't need to hardcode any IP addresses or URLs. You can always say: please Eureka, give me a location of the service by a given name.

@EnableCircuitBreaker

Now with all this fancy distributed system going around, you cannot expect things never to break down. Network can and will fail on you. You need a way to mitigate the damage, and still serve something meaningful back to your clients. So, how can developers ensure resiliency in applications? Resilience patterns.

In this instance, you need a resilience pattern called the circuit breaker pattern. When things go south, you'll have a way to specify the fallback methods for retrieving data from another service or to respond with reasonable defaults. To make your app aware of the circuit breaker patterns, add the @EnableCircuitBreaker annotation. This will configure Hystrix circuit breaker protocols for your application.

And then mark the important methods with the @HystrixCommand(fallbackMethod = "fallbackMethodName") annotations. This says that in the case of an emergency when the method throws exceptions or times out, your app should make use of the fallback method. When things are going well, and everything works, this approach will help keep your code neat and maintainable.

Final Thoughts

In this post, we've looked at many annotations that a Java developer should know if they want to use the Spring Framework. We've covered the most frequently used and perhaps the most important annotations -- those that enable dependency injection for your components, the ways to bind your code to respond to HTTP requests, and how to enable the correct patterns for cloud-based applications and microservices architectures.

Download the Spring Annotations Cheat Sheet

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Additional Resources

If you're looking for additional Java cheat sheets, be sure to check out our Java cheat sheet collection. It's packed full of cheat sheets and shortcuts for popular Java technologies.

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