Why is this an issue?

Dependency injection frameworks such as Spring support dependency injection by using annotations such as @Inject and @Autowired. These annotations can be used to inject beans via constructor, setter, and field injection.

Generally speaking, field injection is discouraged. It allows the creation of objects in an invalid state and makes testing more difficult. The dependencies are not explicit when instantiating a class that uses field injection.

In addition, field injection is not compatible with final fields. Keeping dependencies immutable where possible makes the code easier to understand, easing development and maintenance.

Finally, because values are injected into fields after the object has been constructed, they cannot be used to initialize other non-injected fields inline.

This rule raises an issue when the @Autowired or @Inject annotations are used on a field.

How to fix it

Use constructor injection instead.

By using constructor injection, the dependencies are explicit and must be passed during an object’s construction. This avoids the possibility of instantiating an object in an invalid state and makes types more testable. Fields can be declared final, which makes the code easier to understand, as dependencies don’t change after instantiation.

Code examples

Noncompliant code example

public class SomeService {
    @Autowired
    private SomeDependency someDependency; // Noncompliant

    private String name = someDependency.getName(); // Will throw a NullPointerException
}

Compliant solution

public class SomeService {
    private final SomeDependency someDependency;
    private final String name;

    @Autowired
    public SomeService(SomeDependency someDependency) {
        this.someDependency = someDependency;
        name = someDependency.getName();
    }
}

Resources

Articles & blog posts