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Avoid Injecting Data into Static Fields in Spring

High
correctnessmaintainabilityreliability

What is it?​

Injecting data into static fields in Spring using annotations like @Value, @Inject, or @Autowired is ineffective as these annotations are ignored, leaving fields uninitialized.

Why apply it?​

Static fields with these annotations will not be initialized by Spring, leading to NullPointerExceptions. It also makes code misleading, as developers might expect the fields to be populated automatically by Spring when they aren't.

How to fix it?​

Instead of using static fields, apply the injection annotations to instance fields to ensure they are initialized by Spring. Alternatively, remove the annotations and manually initialize the static fields.

Examples​

Example 1:​

Negative​

This negative example incorrectly uses the @Value annotation on a static field, which will not be initialized.

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Component;

@Component
public class MyService {

    @Value("${my.app.name}")
    private static String appName;  // Noncompliant: static field will not be injected

    public static void printAppName() {
        System.out.println("Application Name: " + appName);
    }
}

Example 2:​

Positive​

In this positive example, the @Value annotation is used properly on an instance field, ensuring it gets initialized by Spring.

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Component;

@Component
public class MyService {

    @Value("${my.app.name}")
    private final String appName;  // Compliant: instance field used

    public MyService(@Value("${my.app.name}") String appName) {
        this.appName = appName;
    }

    public void printAppName() {
        System.out.println("Application Name: " + appName);
    }
}

Negative​

This negative example incorrectly applies @Autowired to a static field.

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component
public class GreetingService {

    @Autowired
    private static MessageService messageService;  // Noncompliant: static fields are not injected

    public static void greet() {
        System.out.println(messageService.getMessage());
    }
}

Example 3:​

Positive​

Another positive example where @Autowired is used correctly on an instance field within a service class.

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component
public class GreetingService {

    private final MessageService messageService;

    @Autowired
    public GreetingService(MessageService messageService) {  // Compliant: using constructor injection
        this.messageService = messageService;
    }

    public void greet() {
        System.out.println(messageService.getMessage());
    }
}