fixes Issue 462 (In Java21, the ConstPool API throws NPE for MethodParameters attributes without parameter names)

[wuwen5]

fix #462

package test4;
public class MethodParamInnerClassTest {
    class InnerClass implements Runnable {
        public void run() {
        }
    }
}

When compiling with Java21 (without the -parameters), the information is as follows

Classfile /Users/wuwen/opensource/javassist/temp/jdk21/MethodParamInnerClassTest$InnerClass.class
  Last modified 2023年10月8日; size 481 bytes
  SHA-256 checksum a40905df13d2760afd54f301c8ef0e06dd4f3a620b2e5946f363ed771172fc38
  Compiled from "MethodParamInnerClassTest.java"
class test4.MethodParamInnerClassTest$InnerClass implements java.lang.Runnable
  minor version: 0
  major version: 65
  flags: (0x0020) ACC_SUPER
  this_class: #7                          // test4/MethodParamInnerClassTest$InnerClass
  super_class: #2                         // java/lang/Object
  interfaces: 1, fields: 0, methods: 2, attributes: 3
Constant pool:
   #1 = Methodref          #2.#3          // java/lang/Object."<init>":()V
   #2 = Class              #4             // java/lang/Object
   #3 = NameAndType        #5:#6          // "<init>":()V
   #4 = Utf8               java/lang/Object
   #5 = Utf8               <init>
   #6 = Utf8               ()V
   #7 = Class              #8             // test4/MethodParamInnerClassTest$InnerClass
   #8 = Utf8               test4/MethodParamInnerClassTest$InnerClass
   #9 = Class              #10            // java/lang/Runnable
  #10 = Utf8               java/lang/Runnable
  #11 = Utf8               (Ltest4/MethodParamInnerClassTest;)V
  #12 = Utf8               Code
  #13 = Utf8               LineNumberTable
  #14 = Utf8               MethodParameters
  #15 = Utf8               run
  #16 = Utf8               SourceFile
  #17 = Utf8               MethodParamInnerClassTest.java
  #18 = Utf8               NestHost
  #19 = Class              #20            // test4/MethodParamInnerClassTest
  #20 = Utf8               test4/MethodParamInnerClassTest
  #21 = Utf8               InnerClasses
  #22 = Utf8               InnerClass
{
  test4.MethodParamInnerClassTest$InnerClass(test4.MethodParamInnerClassTest);
    descriptor: (Ltest4/MethodParamInnerClassTest;)V
    flags: (0x0000)
    Code:
      stack=1, locals=2, args_size=2
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 5: 0
    MethodParameters:
      Name                           Flags
      <no name>                      final mandated


  public void run();
    descriptor: ()V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=0, locals=1, args_size=1
         0: return
      LineNumberTable:
        line 7: 0
}
SourceFile: "MethodParamInnerClassTest.java"
NestHost: class test4/MethodParamInnerClassTest
InnerClasses:
  #22= #7 of #19;                         // InnerClass=class test4/MethodParamInnerClassTest$InnerClass of class test4/MethodParamInnerClassTest

It still contains MethodParameters, but there is no parameter names

MethodParameters:
      Name                           Flags
      <no name>                      final mandated

[wuwen5]

@chibash

[liach]

This fix is not correct: you should check for 0-index at

javassist/src/main/javassist/bytecode/MethodParametersAttribute.java

Line 64 in 1223557

return getConstPool().getUtf8Info(name(i));

to become:

int name = name(i);
return name == 0 ? null : getConstPool().getUtf8Info(name); 

and

javassist/src/main/javassist/bytecode/MethodParametersAttribute.java

Line 35 in 1223557

ByteArray.write16bit(cp.addUtf8Info(names[i]), data, i * 4 + 1);

to become

String name = names[i];
ByteArray.write16bit(name == null ? 0 : cp.addUtf8Info(name), data, i * 4 + 1); 

instead.

[scottmarlow]

@liach could you please approve the pull request if you think the change you requested was made.

@chibash are there any other committers on this project besides you and me? I propose that you pick from those that actively review others pull requests like @liach has done here.

Thanks,
Scott

[liach]

Looks good, though I'm not a project member.