Lambda with anonymous or local class inside unnecessarily captures 'this'

[Remi Forax]

> From: "Chen Liang" <chen.l.liang at oracle.com>
> To: "Archie Cobbs" <archie.cobbs at gmail.com>, "Tagir Valeev" <amaembo at gmail.com>
> Cc: "compiler-dev" <compiler-dev at openjdk.org>
> Sent: Friday, November 14, 2025 4:15:47 PM
> Subject: Re: Lambda with anonymous or local class inside unnecessarily captures
> 'this'

> The situation in which non-member nested classes capture the enclosing instance
> unconditionally can be treated as an implementation artifact of javac; we
> discussed this within Oracle a while ago. The enclosing instance capture being
> the leading parameter and the null check are for linkage, but local and
> anonymous classes do not have such linkage links, and it should be
> JLS-compliant that unused enclosing instance capture is dropped altogether.
> There are local and anonymous classes in early construction context and static
> context that don't perform enclosing instance capture, yet reflectively they
> are not distinguished from the ones that do, so I believe the compatibility
> impact is low.

> Re Archie: I think Tagir means that for 23 and below, the lambdas with local or
> anonymous classes are still captured. It is interesting
> withLocalClassInsideNoInstance() is suddenly deduplicated.

> P.S. "Deduplication" is a confusing term - in javac I believe it refers to
> identical lambda expressions reusing the same InvokeDynamic constant (which
> still generates different call sites per JVMS rules), but in this thread it
> refers to the singleton lambda instances.

About deduplication, the term is usually used when you do not control the creation of the objects but you want later to have some sharing. 
In the case of javac, javac controls the objects representing the constant pool constants, so it's just sharing. 

> -Chen

regards, 
Rémi 

> From: compiler-dev <compiler-dev-retn at openjdk.org> on behalf of Archie Cobbs
> <archie.cobbs at gmail.com>
> Sent: Thursday, November 13, 2025 3:42 PM
> To: Tagir Valeev <amaembo at gmail.com>
> Cc: compiler-dev at openjdk.org <compiler-dev at openjdk.org>
> Subject: Re: Lambda with anonymous or local class inside unnecessarily captures
> 'this'
> Hi Tagir,

> This is speculation here...

> Possibly the new behavior is a side effect of [
> https://bugs.openjdk.org/browse/JDK-8164714 |
> JDK-8164714 ] ( [ https://github.com/openjdk/jdk/pull/23875/files | pr #23875 ]
> ) which ensures outer instances get provided as parameters, even if they are
> not otherwise used, so they can be checked non-null.

> I believe that parameter would later get detected by [
> https://github.com/openjdk/jdk/blob/6322aaba63b235cb6c73d23a932210af318404ec/src/java.base/share/classes/java/lang/invoke/InnerClassLambdaMetafactory.java#L226-L232
> |
> this logic ] in InnerClassLambdaMetafactory , which would cause it to not
> deduplicate the lambda instance.

> If the above is true, then to fix this, one could imagine that the metafactory
> logic might be made smarter, so it could filter out such "null check only"
> captured outer instances. I'm not sure exactly how that would be detectable...
> maybe by adding a new boolean BSM flag?

> -Archie

> On Thu, Nov 13, 2025 at 7:00 AM Tagir Valeev < [ mailto:amaembo at gmail.com |
> amaembo at gmail.com ] > wrote:

>> Hello!

>> I was playing with lambda instance deduplication and come up with the following
>> set of test cases:

>> import java.util.function.Supplier;

>> public final class CapturingLambda {
>> Runnable empty() {
>> return () -> {};
>> }

>> Runnable simple() {
>> return () -> {
>> System.out.println("simple");
>> };
>> }

>> Runnable withRunnableInside() {
>> return () -> {
>> Runnable r = () -> {};
>> r.run();
>> };
>> }

>> Runnable withAnonymousRunnableInside() {
>> return () -> {
>> Runnable r = new Runnable() {
>> @Override
>> public void run() {
>> }
>> };
>> r.run();
>> };
>> }

>> Runnable withLocalClassInside() {
>> return () -> {
>> class Local {}
>> new Local();
>> };
>> }

>> Runnable withLocalClassInsideNoInstance() {
>> return () -> {
>> class Local {}
>> };
>> }

>> void checkDeduplicated(String name, Supplier<Runnable> lambdaSupplier) {
>> Runnable lambda1 = lambdaSupplier.get();
>> Runnable lambda2 = lambdaSupplier.get();
>> if (lambda1 == lambda2) {
>> System.out.println("Deduplicated: "+name);
>> } else {
>> System.out.println("Not deduplicated: "+name);
>> }
>> }

>> private void test() {
>> checkDeduplicated("empty", this::empty);
>> checkDeduplicated("simple", this::simple);
>> checkDeduplicated("withRunnableInside", this::withRunnableInside);
>> checkDeduplicated("withAnonymousRunnableInside",
>> this::withAnonymousRunnableInside);
>> checkDeduplicated("withLocalClassInside", this::withLocalClassInside);
>> checkDeduplicated("withLocalClassInsideNoInstance",
>> this::withLocalClassInsideNoInstance);
>> }

>> public static void main(String[] args) {
>> new CapturingLambda().test();
>> }
>> }

>> Here, checkDeduplicated checks whether the particular lambda is deduplicated
>> when returned several times. We know that lambdas are deduplicated if they
>> don't capture anything. The output of this code in Java 24 and Java 25 is the
>> following:

>> Deduplicated: empty
>> Deduplicated: simple
>> Deduplicated: withRunnableInside
>> Not deduplicated: withAnonymousRunnableInside
>> Not deduplicated: withLocalClassInside
>> Deduplicated: withLocalClassInsideNoInstance

>> (side note: in Java 23 and below, withLocalClassInsideNoInstance is Not
>> deduplicated)

>> The problem is that lambdas returned from withAnonymousRunnableInside and
>> withLocalClassInside capture 'this', thus not deduplicated. For example, here's
>> the generated bytecode for withLocalClassInside:

>> private void lambda$withLocalClassInside$0();
>> descriptor: ()V
>> flags: (0x1002) ACC_PRIVATE, ACC_SYNTHETIC
>> Code:
>> stack=3, locals=1, args_size=1
>> 0: new #73 // class com/example/CapturingLambda$1Local
>> 3: dup
>> 4: aload_0
>> 5: invokespecial #75 // Method
>> com/example/CapturingLambda$1Local."<init>":(Lcom/example/CapturingLambda;)V
>> 8: pop
>> 9: return

>> So 'this' is passed to the generated constructor of the local class
>> CapturingLambda$1Local. However, inside the constructor, the only thing we do
>> with this parameter is check it against null:

>> com.example.CapturingLambda$1Local(com.example.CapturingLambda);
>> descriptor: (Lcom/example/CapturingLambda;)V
>> flags: (0x0000)
>> Code:
>> stack=2, locals=2, args_size=2
>> 0: aload_1
>> 1: dup
>> 2: invokestatic #1 // Method
>> java/util/Objects.requireNonNull:(Ljava/lang/Object;)Ljava/lang/Object;
>> 5: pop
>> 6: pop
>> 7: aload_0
>> 8: invokespecial #7 // Method java/lang/Object."<init>":()V
>> 11: return

>> Is it really necessary? I understand that the mandated parameter can be probably
>> helpful to maintain the (unspecified) binary compatibility with pre-Java-18
>> class files not to break the reflection code. However, can we probably omit the
>> null-check and pass null here? This would help to deduplicate lambda instances
>> more and remove the unnecessary strong reference to the outer object.
>> Currently, the lambda cannot outlive the enclosing object, and this looks sad.
>> What do you think?

>> With best regards,
>> Tagir Valeev

> --
> Archie L. Cobbs
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