Which @NotNull Java annotation should I use?
I'm looking to make my code more readable as well as use tooling like IDE code inspection and/or static code analysis (FindBugs and Sonar) to avoid NullPointerExceptions. Many of the tools seem incompatible with each others' @NotNull / @NonNull / @Nonnull annotation and listing all of them in my code would be terrible to read. Any suggestions of which one is the 'best'? Here is the list of equivalent annotations I've found:
javax.validation.constraints.NotNull
Created for runtime validation, not static analysis.
documentation
edu.umd.cs.findbugs.annotations.NonNull
Used by Findbugs static analysis and therefore Sonar (now Sonarqube)
documentation
javax.annotation.Nonnull
This might work with Findbugs too, but JSR-305 is inactive. (See also: What is the status of JSR 305?) source
org.jetbrains.annotations.NotNull
Used by IntelliJ IDEA IDE for static analysis.
documentation
lombok.NonNull
Used to control code generation in Project Lombok.
Placeholder annotation since there is no standard.
source, documentation
android.support.annotation.NonNull
Marker annotation available in Android, provided by support-annotations package
documentation
org.eclipse.jdt.annotation.NonNull
Used by Eclipse for static code analysis
documentation
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:
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.
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.
This framework allows you to use with without creating a dependency on the framework by enclosing your annotations in a comment: eg /*@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.
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!
Since Oracle decided not to standardize @NonNull (and @Nullable) for the moment, 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:
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 graduated from javax to java in the past, the javax.annotation would be 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
The @Nullable annotations 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
all have runtime annotations (@Retention(RUNTIME), while
are only compile time (@Retention(CLASS)).
As described in this SO answer the impact of runtime annotations is smaller than one might think, but they have the benefit of enabling tools to do runtime checks in addition to the compile 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 X
org.jetbrains.annotation X X X X
lombok X X X X
javax.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
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 @NotNull and 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
Form 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 use their org.checkerframework.checker.nullness.qual.