Which @NotNull Java annotation should I use?

Since JSR 305 (whose goal was to standardize @NonNull and @Nullable) has been dormant for several years, I'm afraid there is no good answer. All we can do is to find a pragmatic solution and mine is as follows:

Syntax

From a purely stylistic standpoint I would like to avoid any reference to IDE, framework or any toolkit except Java itself.

This rules out:

• android.support.annotation
• edu.umd.cs.findbugs.annotations
• org.eclipse.jdt.annotation
• org.jetbrains.annotations
• org.checkerframework.checker.nullness.qual
• lombok.NonNull

Which leaves us with either javax.validation.constraints or javax.annotation.The former comes with JEE. If this is better than javax.annotation, which might come eventually with JSE or never at all, is a matter of debate.I personally prefer javax.annotation because I wouldn't like the JEE dependency.

This leaves us with

javax.annotation

which is also the shortest one.

There is only one syntax which would even be better: java.annotation.Nullable. As other packages graduatedfrom javax to java in the past, the javax.annotation wouldbe a step in the right direction.

Implementation

I was hoping that they all have basically the same trivial implementation,but a detailed analysis showed that this is not true.

First for the similarities:

The @NonNull annotations all have the line

public @interface NonNull {}

except for

• org.jetbrains.annotations which calls it @NotNull and has a trivial implementation
• javax.annotation which has a longer implementation
• javax.validation.constraints which also calls it @NotNull and has an implementation

The @Nullableannotations all have the line

public @interface Nullable {}

except for (again) the org.jetbrains.annotations with their trivial implementation.

For the differences:

A striking one is that

• javax.annotation
• javax.validation.constraints
• org.checkerframework.checker.nullness.qual

all have runtime annotations (@Retention(RUNTIME)), while

• android.support.annotation
• edu.umd.cs.findbugs.annotations
• org.eclipse.jdt.annotation
• org.jetbrains.annotations

are only compile time (@Retention(CLASS)).

As described in this SO answer the impact of runtime annotationsis smaller than one might think, but they have the benefitof enabling tools to do runtime checks in addition to thecompile time ones.

Another important difference is where in the code the annotations can be used.There are two different approaches. Some packages use JLS 9.6.4.1 style contexts. The following table gives an overview:

                                FIELD   METHOD  PARAMETER LOCAL_VARIABLE android.support.annotation      X       X       X   edu.umd.cs.findbugs.annotations X       X       X         Xorg.jetbrains.annotation        X       X       X         Xlombok                          X       X       X         Xjavax.validation.constraints    X       X       X   

org.eclipse.jdt.annotation, javax.annotation and org.checkerframework.checker.nullness.qual use the contexts defined in JLS 4.11, which is in my opinion the right way to do it.

This leaves us with

• javax.annotation
• org.checkerframework.checker.nullness.qual

in this round.

Code

To help you to compare further details yourself I list the code of every annotation below.To make comparison easier I removed comments, imports and the @Documented annotation.(they all had @Documented except for the classes from the Android package).I reordered the lines and @Target fields and normalized the qualifications.

package android.support.annotation;@Retention(CLASS)@Target({FIELD, METHOD, PARAMETER})public @interface NonNull {}

package edu.umd.cs.findbugs.annotations;@Retention(CLASS)@Target({FIELD, METHOD, PARAMETER, LOCAL_VARIABLE})public @interface NonNull {}

package org.eclipse.jdt.annotation;@Retention(CLASS)@Target({ TYPE_USE })public @interface NonNull {}

package org.jetbrains.annotations;@Retention(CLASS)@Target({FIELD, METHOD, PARAMETER, LOCAL_VARIABLE})public @interface NotNull {String value() default "";}

package javax.annotation;@TypeQualifier@Retention(RUNTIME)public @interface Nonnull {    When when() default When.ALWAYS;    static class Checker implements TypeQualifierValidator<Nonnull> {        public When forConstantValue(Nonnull qualifierqualifierArgument,                Object value) {            if (value == null)                return When.NEVER;            return When.ALWAYS;        }    }}

package org.checkerframework.checker.nullness.qual;@Retention(RUNTIME)@Target({TYPE_USE, TYPE_PARAMETER})@SubtypeOf(MonotonicNonNull.class)@ImplicitFor(    types = {        TypeKind.PACKAGE,        TypeKind.INT,        TypeKind.BOOLEAN,        TypeKind.CHAR,        TypeKind.DOUBLE,        TypeKind.FLOAT,        TypeKind.LONG,        TypeKind.SHORT,        TypeKind.BYTE    },    literals = {LiteralKind.STRING})@DefaultQualifierInHierarchy@DefaultFor({TypeUseLocation.EXCEPTION_PARAMETER})@DefaultInUncheckedCodeFor({TypeUseLocation.PARAMETER, TypeUseLocation.LOWER_BOUND})public @interface NonNull {}

For completeness, here are the @Nullable implementations:

package android.support.annotation;@Retention(CLASS)@Target({METHOD, PARAMETER, FIELD})public @interface Nullable {}

package edu.umd.cs.findbugs.annotations;@Target({FIELD, METHOD, PARAMETER, LOCAL_VARIABLE})@Retention(CLASS)public @interface Nullable {}

package org.eclipse.jdt.annotation;@Retention(CLASS)@Target({ TYPE_USE })public @interface Nullable {}

package org.jetbrains.annotations;@Retention(CLASS)@Target({FIELD, METHOD, PARAMETER, LOCAL_VARIABLE})public @interface Nullable {String value() default "";}

package javax.annotation;@TypeQualifierNickname@Nonnull(when = When.UNKNOWN)@Retention(RUNTIME)public @interface Nullable {}

package org.checkerframework.checker.nullness.qual;@Retention(RUNTIME)@Target({TYPE_USE, TYPE_PARAMETER})@SubtypeOf({})@ImplicitFor(    literals = {LiteralKind.NULL},    typeNames = {java.lang.Void.class})@DefaultInUncheckedCodeFor({TypeUseLocation.RETURN, TypeUseLocation.UPPER_BOUND})public @interface Nullable {}

The following two packages have no @Nullable, so I list them separately; Lombok has a pretty boring @NonNull.In javax.validation.constraints the @NonNull is actually a @NotNulland it has a longish implementation.

package lombok;@Retention(CLASS)@Target({FIELD, METHOD, PARAMETER, LOCAL_VARIABLE})public @interface NonNull {}

package javax.validation.constraints;@Retention(RUNTIME)@Target({ FIELD, METHOD, ANNOTATION_TYPE, CONSTRUCTOR, PARAMETER })@Constraint(validatedBy = {})public @interface NotNull {    String message() default "{javax.validation.constraints.NotNull.message}";    Class<?>[] groups() default { };    Class<? extends Payload>[] payload() default {};    @Target({ METHOD, FIELD, ANNOTATION_TYPE, CONSTRUCTOR, PARAMETER })    @Retention(RUNTIME)    @Documented    @interface List {        NotNull[] value();    }}

Support

From my experience, javax.annotation is at least supported by Eclipse and the Checker Framework out of the box.

Summary

My ideal annotation would be the java.annotation syntax with the Checker Framework implementation.

If you don't intend to use the Checker Framework the javax.annotation (JSR-305) is still your best bet for the time being.

If you are willing to buy into the Checker Framework just usetheir org.checkerframework.checker.nullness.qual.

Sources

• android.support.annotation from android-5.1.1_r1.jar
• edu.umd.cs.findbugs.annotations from findbugs-annotations-1.0.0.jar
• org.eclipse.jdt.annotation from org.eclipse.jdt.annotation_2.1.0.v20160418-1457.jar
• org.jetbrains.annotations from jetbrains-annotations-13.0.jar
• javax.annotation from gwt-dev-2.5.1-sources.jar
• org.checkerframework.checker.nullness.qual from checker-framework-2.1.9.zip
• lombok from lombok commit f6da35e4c4f3305ecd1b415e2ab1b9ef8a9120b4
• javax.validation.constraints from validation-api-1.0.0.GA-sources.jar

I very much like the Checker Framework, which is an implementation of type annotations (JSR-308) which is used to implement defect checkers like a nullness checker. I haven't really tried any others to offer any comparison, but I've been happy with this implementation.

I'm not affiliated with the group that offers the software, but I am a fan.

Four things I like about this system:

1. It has a defect checkers for nullness (@Nullable), but also has ones for immutability and interning (and others). I use the first one (nullness) and I'm trying to get into using the second one (immutability/IGJ). I'm trying out the third one, but I'm not certain about using it long term yet. I'm not convinced of the general usefulness of the other checkers yet, but its nice to know that the framework itself is a system for implementing a variety of additional annotations and checkers.

2. The default setting for nullness checking works well: Non-null except locals (NNEL). Basically this means that by default the checker treats everyhing (instance variables, method parameters, generic types, etc) except local variables as if they have a @NonNull type by default. Per the documentation:

   The NNEL default leads to the smallest number of explicit annotations in your code.

   You can set a different default for a class or for a method if NNEL doesn't work for you.

3. This framework allows you to use with without creating a dependency on the framework by enclosing your annotations in a comment: e.g. /*@Nullable*/. This is nice because you can annotate and check a library or shared code, but still be able to use that library/shared coded in another project that doesn't use the framework. This is a nice feature. I've grown accustom to using it, even though I tend to enable the Checker Framework on all my projects now.

4. The framework has a way to annotate APIs you use that aren't already annotated for nullness by using stub files.

I use the IntelliJ one, because I'm mostly concerned with IntelliJ flagging things that might produce a NPE. I agree that it's frustrating not having a standard annotation in the JDK. There's talk of adding it, it might make it into Java 7. In which case there will be one more to choose from!