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Members of Spring Components Should Be Injected

Medium
maintainabilitybest practices

What is it?​

This practice is triggered when a Spring @Component, @Controller, @RestController, @Service, or @Repository singleton contains non-static members that are not managed by Spring through specific annotations.

Why apply it?​

Ensures that all non-static members in singleton Spring beans are properly managed by Spring, increasing maintainability of the code and preventing unexpected behavior by ensuring correct dependency injection.

How to fix it?​

Add annotations like @Resource, @Inject, @Autowired or @Value to all non-static members of Spring singleton components to ensure they are managed by the Spring framework.

Examples​

Example 1:​

Negative​

The negative example fails to use dependency injection, which could lead to maintainability issues or unexpected behavior.

import org.springframework.stereotype.Controller;
@Controller
public class GreetingController {

  private GreetingService greetingService = new GreetingService();  /* Noncompliant */

  @RequestMapping("/hello", method = RequestMethod.GET)
  public String greet(String name) {
    return greetingService.sayHello(name);
  }
}

Example 2:​

Positive​

The positive example uses @Autowired to inject a dependency, ensuring the Spring framework manages the non-static member.

import org.springframework.stereotype.Controller;
@Controller
public class GreetingController {

  @Autowired
  private GreetingService greetingService;  /* Compliant */

  @RequestMapping("/hello", method = RequestMethod.GET)
  public String greet(String name) {
    return greetingService.sayHello(name);
  }
}

Negative​

The negative example does not utilize annotations like @Value, leading to less maintainable code where the dependency is not defined clearly.

import org.springframework.stereotype.Service;
@Service
public class WelcomeService {

  private String welcomeMessage = "Welcome!";  /* Noncompliant */

  public String getWelcomeMessage() {
    return welcomeMessage;
  }
}

Example 3:​

Positive​

The positive example uses @Value for injecting configuration properties, maintaining clear management of the property by Spring.

import org.springframework.stereotype.Service;
@Service
public class WelcomeService {

  @Value("${welcome.message}")
  private String welcomeMessage;  /* Compliant */

  public String getWelcomeMessage() {
    return welcomeMessage;
  }
}