Why does using a method reference generate an inner class in byte code?

Question

I wrote simple example of using method reference:

public class Main {
private static String identity(String param) {
    return param;
}

public static void main(String... args) {
    Function<String, String> fun = Main::identity;
    System.out.println(fun.apply("Hello"));
}}

And in generated byte code is InnerClass:

InnerClasses:
 public static final #68= #67 of #71; //Lookup=class java/lang/invoke/MethodHandles$Lookup of class java/lang/invoke/MethodHandles
BootstrapMethods:
0: #35 invokestatic java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
Method arguments:
  #36 (Ljava/lang/Object;)Ljava/lang/Object;....

I supposed this innerClass is used in lambda bootstrap method, but I do not know when jvm create object of this class and what information will be store in object of this class. Could anyone explain it to me?

No object is created because your method is `static`. I'm not sure I understand your question. — Boris the Spider, Feb 03 '18 at 19:57
@BoristheSpider I think OP is talking about the `fun` object? — Jorn Vernee, Feb 03 '18 at 19:58
I wondered what is this inner class and why compiler generated it. If I see "InnerClasses" in byte code I supposed that jvm eventually create object of this class. — lolcio, Feb 03 '18 at 20:03
This might help: https://stackoverflow.com/questions/30002380/why-are-java-8-lambdas-invoked-using-invokedynamic. Lambdas can possibly be converted into anonymous classes that are a type of inner class: https://docs.oracle.com/javase/tutorial/java/javaOO/innerclasses.html. — tsolakp, Feb 03 '18 at 22:53

apangin · Accepted Answer · 2018-02-04T01:33:19.640

This is not a generated inner class, but rather InnerClasses attribute specified in JVMS §4.7.6. The specification requires that this attribute lists every class (referenced in the constant pool) that is not a member of a package.

Basically, javac generates InnerClasses attribute for all inner classes it sees in the code. E.g.

public class Test {
    public static void main(String[] args) {
        for (Map.Entry e : Collections.emptyMap().entrySet()) {

        }
    }
}

The bytecode for above class will refer to Map.Entry as an inner class of Map:

InnerClasses:
   public static #31= #7 of #23; //Entry=class java/util/Map$Entry of class java/util/Map

In JDK lambdas are implemented with invokedynamic instruction. According to the specification, resolution of invokedynamic involves calling a bootstrap method with 4 arguments of the following types:

MethodHandle
MethodHandles.Lookup
String
MethodType

Java Runtime creates these objects internally, but since the bytecode refers to MethodHandles.Lookup class, javac generates InnerClasses attribute for it.

That part of §4.7.6 annoys me for years now… Why does *every* class have to redundantly repeat the information about inner class relations of referenced classes. I can’t imagine any actual use case for that information, as no-one would consider consulting the byte code of `Test` to find out whether `Map.Entry` is a nested type. — Holger, Feb 05 '18 at 16:52

Why does using a method reference generate an inner class in byte code?

1 Answers1