ASM MethodVisitor::visitMethodInsn gets descriptor when reading enum - bug or wrong usage?

1 answer

Abstract

I am reading class files using ASM, and my MethodVisitor gets a strange argument when visiting an enum: The owner argument to visitMethodInsn is supposed to be an internal name (e.g., mre/DoStuff), but for an enum, I get an owner in the form of an array descriptor, e.g., [Lmre/Stuff;.

Explanation with Example

A condensed Groovy version of how I am using the ClassReader, ClassVisitor, and MethodVisitor is the following:

package mre  import org.objectweb.asm.* import java.nio.file.Paths  class ClassTracer extends ClassVisitor {     ClassTracer() { super(Opcodes.ASM8) }      @Override     void visit(int version, int access, String name, String signature, String superName, String[] interfaces) {         println "C:visit($version, $access, $name, $signature, $superName, $interfaces)"     }      @Override     MethodVisitor visitMethod(int access, String name, String descriptor, String signature, String[] exceptions) {         println "C:visitMethod($access, $name, $descriptor, $signature, $exceptions)"         new MethodTracer(super.visitMethod(access, name, descriptor, signature, exceptions))     } }  class MethodTracer extends MethodVisitor {     MethodTracer(MethodVisitor parent) { super(Opcodes.ASM8, parent) }      @Override     void visitMethodInsn(int opcode, String owner, String name, String descriptor, boolean isInterface) {         println "M:visitMethodInsn($opcode, $owner, $name, $descriptor, $isInterface)"         super.visitMethodInsn(opcode, owner, name, descriptor, isInterface)     } }  static void main(String[] args) {     if (!args) throw new IllegalArgumentException(("Need class file path argument"))     new ClassReader(Paths.get(args[0]).toFile().bytes).accept(new ClassTracer(), ClassReader.SKIP_FRAMES) } 

When using this with, e.g., the mre/OneClass.class from this example:

class OtherClass { void run() {} }  class OneClass {     void runOther() {         new OtherClass().run();     } } 

... then I get the expected internal name argument mre/OtherClass for the run method call:

M:visitMethodInsn(182, mre/OtherClass, run, ()V, false) 

However, when run on the mre/OneEnum.class of this enum:

enum OneEnum {a, b} 

... then I unexpectedly get a descriptor argument [Lmre/OneEnum; to a clone method visitation:

M:visitMethodInsn(182, [Lmre/OneEnum;, clone, ()Ljava/lang/Object;, false) 

While this inconsistency seems like a bug to me, I am wondering whether I am missing something. I have tried toggling the generated byte code version between 7,8, and 11, but it seems to make no difference.

Question

So, in a nutshell: Am I using the visitors correctly, and is my confusion about the descriptor argument for the enum justified?

All answers to this question, which has the identifier 60998619

The best answer:

The receiver of a method invocation can be an array type.

To demonstrate it without using the ASM libary:

public class ArrayMethodCall {     enum SomeEnum { ;         public static String[] example(String[] array) {             return array.clone();         }     }     public static void main(String[] args) throws IOException, InterruptedException {         Path javap = Paths.get(System.getProperty("java.home"), "bin", "javap");         new ProcessBuilder(                 javap.toString(), "-c",// "-v",                 "-cp", System.getProperty("java.class.path"),                 "ArrayMethodCall$SomeEnum"         ).inheritIO().start().waitFor();     } } 

prints

Compiled from "ArrayMethodCall.java" final class ArrayMethodCall$SomeEnum extends java.lang.Enum<ArrayMethodCall$SomeEnum> {   public static ArrayMethodCall$SomeEnum[] values();     Code:        0: getstatic     #1                  // Field $VALUES:[LArrayMethodCall$SomeEnum;        3: invokevirtual #2                  // Method "[LArrayMethodCall$SomeEnum;".clone:()Ljava/lang/Object;        6: checkcast     #3                  // class "[LArrayMethodCall$SomeEnum;"        9: areturn    public static ArrayMethodCall$SomeEnum valueOf(java.lang.String);     Code:        0: ldc           #4                  // class ArrayMethodCall$SomeEnum        2: aload_0        3: invokestatic  #5                  // Method java/lang/Enum.valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;        6: checkcast     #4                  // class ArrayMethodCall$SomeEnum        9: areturn    public static java.lang.String[] example(java.lang.String[]);     Code:        0: aload_0        1: invokevirtual #7                  // Method "[Ljava/lang/String;".clone:()Ljava/lang/Object;        4: checkcast     #8                  // class "[Ljava/lang/String;"        7: areturn    static {};     Code:        0: iconst_0        1: anewarray     #4                  // class ArrayMethodCall$SomeEnum        4: putstatic     #1                  // Field $VALUES:[LArrayMethodCall$SomeEnum;        7: return } 

which shows that both clone() calls, the one in example on a string array and the one in the compiler-generated values() method on an enum array, use the array type as method receiver.

Note that array types also may appear in class literals (String[].class), type casts, and as the second argument to the instanceof operator. A type cast to an array type does already happen in the shown code, right after the clone() invocations. In all these cases, the instruction will refer to a CONSTANT_Class_info pool entry whose internal name will be an array signature.

Consider §5.1 of the JVM specification:

A symbolic reference to a class or interface is derived from a CONSTANT_Class_info structure (§4.4.1). Such a reference gives the name of the class or interface in the following form:

  • For a nonarray class or an interface, the name is the binary name (§4.2.1) of the class or interface.

  • For an array class of n dimensions, the name begins with n occurrences of the ASCII [ character followed by a representation of the element type:

    • If the element type is a primitive type, it is represented by the corresponding field descriptor (§4.3.2).

    • Otherwise, if the element type is a reference type, it is represented by the ASCII L character followed by the binary name of the element type followed by the ASCII ; character.

Whenever this chapter refers to the name of a class or interface, the name should be understood to be in the form above. (This is also the form returned by the Class.getName method.)

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