UseNullable

Use @Nullable

To eliminate NullPointerExceptions in your codebase, you must be disciplined about null references. We've been successful at this by following and enforcing a simple rule:

Every injected parameter is non-null unless explicitly specified.

Guava: Google Core Libraries for Java has simple APIs to get a nulls under control. Preconditions.checkNotNull can be used to fast-fail if a null reference is found. Guice takes this a step further by requiring all injected values to be non-null unless annotated as @Nullable.

import static com.google.common.base.Preconditions.checkNotNull;

public class Person {
  ...

  @Inject
  public Person(
    @FirstName String firstName,
    @LastName String lastName,
    @Nullable Phone phone) {
    // The checkNotNull calls are not required if Person is always
    // instantiated via Guice. Guice guarantees they'll be non-null.
    this.firstName = checkNotNull(firstName, "firstName");
    this.lastName = checkNotNull(lastName, "lastName");
    this.phone = phone;
  }

Guice forbids null by default

Guice checks for nulls during injection - not during the creation of the injector. So if something tries to supply null for an object, Guice will refuse to inject it and throw a NULL_INJECTED_INTO_NON_NULLABLE ProvisionException error instead. If null is permissible by your class, you can annotate the field or parameter with @Nullable.

Note on null injected into @Provides method

Due to an oversight, Guice allowed null to be injected into @Provides methods in the past. When this oversight was fixed, an option to control the enforcement level was added to avoid breaking existing code:

• IGNORE: null is allowed to be injected into @Provides methods
• WARN: a warning is logged when null is injected into a @Provides method
• ERROR: an error is thrown when when null is injected into a @Provides method

ERROR level enforcement is recommended so that:

• Guice consistently rejects null unless @Nullable is used
• NullPointerExceptions are caught early by Guice in all types of injections

Guice by default uses ERROR so you don't need to do anything to configure this. However, if for some reason that you need to relax this enforcement level, you can do so by setting the JVM property "-Dguice_check_nullable_provides_params" to either WARN or IGNORE.

Supported @Nullable annotations

Guice recognizes any @Nullable annotation that targets ElementType.PARAMETER (for parameters), ElementType.FIELD (for fields), or TYPE_USE like edu.umd.cs.findbugs.annotations.Nullable, jakarta.annotation.Nullable, javax.annotation.Nullable or org.checkerframework.checker.nullness.qual.Nullable. (Note: If an annotation does not explicitly list a target, it implicitly supports all relevant targets.)

If you've already annotated the injection site with @Nullable and still getting error then it's likely that you are using a type of @Nullable annotation that is not supported by Guice. Consider switching to one of the supported types.