fixes GitHub Issue 462 (Internal class issues in the Java 21)

src/main/javassist/bytecode/MethodParametersAttribute.java

@@ -32,7 +32,8 @@ public class MethodParametersAttribute extends AttributeInfo {
         byte[] data = new byte[names.length * 4 + 1];
         data[0] = (byte)names.length;
         for (int i = 0; i < names.length; i++) {
-            ByteArray.write16bit(cp.addUtf8Info(names[i]), data, i * 4 + 1);
+            String name = names[i];
+            ByteArray.write16bit(name == null ? 0 : cp.addUtf8Info(name), data, i * 4 + 1);
             ByteArray.write16bit(flags[i], data, i * 4 + 3);
         }
 
@@ -61,7 +62,8 @@ public class MethodParametersAttribute extends AttributeInfo {
      * @param i         the position of the parameter.
      */
     public String parameterName(int i) {
-        return getConstPool().getUtf8Info(name(i));
+        int index = name(i);
+        return index == 0 ? null : getConstPool().getUtf8Info(index);
     }
 
     /**
@@ -87,7 +89,8 @@ public class MethodParametersAttribute extends AttributeInfo {
         String[] names = new String[s];
         int[] flags = new int[s];
         for (int i = 0; i < s; i++) {
-            names[i] = cp.getUtf8Info(name(i));
+            int index = name(i);
+            names[i] = index == 0 ? null : cp.getUtf8Info(index);
             flags[i] = accessFlags(i);
         }
 

src/test/javassist/JvstTest5.java

@@ -1,5 +1,8 @@
 package javassist;
 
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.IOException;
 import java.lang.annotation.Annotation;
 import java.lang.reflect.TypeVariable;
 
@@ -9,6 +12,8 @@ import javassist.bytecode.AttributeInfo;
 import javassist.bytecode.ClassFile;
 import javassist.bytecode.ConstPool;
 import javassist.bytecode.InnerClassesAttribute;
+import javassist.bytecode.MethodInfo;
+import javassist.bytecode.MethodParametersAttribute;
 import javassist.bytecode.NestHostAttribute;
 import javassist.bytecode.NestMembersAttribute;
 import javassist.expr.ExprEditor;
@@ -593,4 +598,31 @@ public class JvstTest5 extends JvstTestRoot {
         }
         catch (CannotCompileException e) {}
     }
+
+    public void testGithubIssue462Java21WithoutParameters() throws IOException {
+
+        //This is a class file compiled by Java-21
+        //javac Java21InnerClassWithoutParameters.java
+        //public class Java21InnerClassWithoutParameters {
+        //    class InnerClass implements Runnable {
+        //        public void run() {
+        //        }
+        //    }
+        //}
+        String classFileName = "./Java21InnerClassWithoutParameters$InnerClass.class";
+        ClassLoader classLoader = getClass().getClassLoader();
+        File classFile = new File(classLoader.getResource(classFileName).getFile());
+        
+        CtClass cc = sloader.makeClass(new FileInputStream(classFile));
+        cc.getClassFile().compact();
+
+        ClassFile cf = cc.getClassFile2();
+        ConstPool cp = cf.getConstPool();
+        
+        MethodInfo minfo = cf.getMethod("<init>");
+        MethodParametersAttribute attr
+                = (MethodParametersAttribute)minfo.getAttribute(MethodParametersAttribute.tag);
+        assertEquals(1, attr.size());
+        assertNull(attr.parameterName(0));
+    }
 }