Is Kotlin null safety implemented in compiller?
Multi tool use
Clash Royale CLAN TAG#URR8PPPIs Kotlin null safety implemented in compiller?
I wonder if Kotlin somehow need to wrap objects or to carry some flags to support null safety.
Is this information only available during compilation (about null safety) or it is leaked to .class file or bytecode?
.class
Can I check Kotlin null contract from Java code (for method arguments and return value)?
2 Answers
2
The Kotlin compiler tries really hard to protect from NullPointerExceptions and forces you to handle possible issues appropriately. But even with these very sophisticated means to avoid NPEs there are some situations when it is possible. The best example is when we want to use some java methods in Kotlin. The compiler cannot say if the type (also known as platform type) coming from Java can be null or not. When a value of a platform type is assigned to a variable the compiler allows it but it can lead to the NullPointerException later. That is why Kotlin heavily relies on nullability annotations (@Nullable and @NotNull). If these attributes are defined in the Java code which we want to call than the compiler can ensure that no NPE will occur. When the Kotlin compiler generates bytecode it also provides the nullability annotations:
@Nullable
@NotNull
@Nullable
Integer myFunction(@NotNull MyClass first_value, @Nullable String second_value)
This is the way how you can find out about the Kotlin null contract from Java code and as far as I know Kotlin doesn't wrap objects with some additional flag because it would make Kotlin-Java interoperability complicated.
What package do
@Nullable and @NotNull actually belong to? There are several candidates: kotlinlang.org/docs/reference/…– gavenkoa
yesterday
@Nullable
@NotNull
These annotations belong to the
org.jetbrains.annotations package.– Sergii Zhevzhyk
yesterday
org.jetbrains.annotations
I wonder if Kotlin somehow need to wrap objects or to carry some flags to support null safety.
Just like type safety, null safety pertains to compile time. Not only doesn't Kotlin need to wrap objects, it is the very essence of this feature that it proves at compile time that no runtime checks will be needed.
Is this information only available during compilation (about null safety) or it is leaked to .class file or bytecode?
.class
The nullability information is available during compilation precisely because it is present in .class files. Kotlin only compiles your own code, it doesn't compile the entire Standard Library and all your dependencies.
.class
Can I check Kotlin null contract from Java code (for method arguments and return value)?
You can, through reflection (by looking for the nullability annotations). However, as explained above, it would be quite pointless to do so.
See my comment on Question. Unless you explicitly tell the compiler, there ARE runtime checks added by the Kotlin compiler.
– Mikezx6r
yesterday
I know there are still some kinks and ways to get
null in a non-nullable type, but conceptually speaking, null-safety is about eliminating the need for those checks. Many devs coming from Java seem to miss that point and keep thinking that it's about automatic runtime checks.– Marko Topolnik
9 hours ago
null
Very true. If you have 100% Kotlin, it should be impossible to have an NPE. If you have a mix, it's very hard. And as you say, although the compiler is playing it extra safe by injecting runtime checks, the majority of the time, the checks are unnecessary.
– Mikezx6r
6 hours ago
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I don't see any of the answers (yet) mentioning this. The Kotlin compiler explicitly adds calls to Intrinsics.checkNotNull in the generated .class file. There are currently advanced compiler options so that these are not generated, but there is no conclusive evidence that they do/don't appreciably impact performance. But ultimately, this means it will throw a KotlinNullPointerException if some code didn't get caught during compilation time. Others have described how that can happen.
– Mikezx6r
yesterday