feat: Refactor LoadClassPatch for readability

Introduce dedicated defineClass method per Java version.
Extract bytecode unpack to dedicated method for reuse.

Author: PerfectSlayer

class-inject/src/main/java/datadog/instrument/classinject/ClassInjector.java

@@ -169,74 +169,13 @@ public void visitCode() {
       mv.visitJumpInsn(IFEQ, notDatadogGlueRequest);
 
       if (JVM.atLeastJava(15)) {
-        // on Java 15+ prepare MethodHandles.lookup()
-        mv.visitMethodInsn(
-            INVOKESTATIC,
-            "java/lang/invoke/MethodHandles",
-            "lookup",
-            "()Ljava/lang/invoke/MethodHandles$Lookup;",
-            false);
-        mv.visitLdcInsn(DefineClassGlue.V9);
+        defineClassUsingMethodHandles();
       } else if (JVM.atLeastJava(9)) {
         // on Java 9+ prepare jdk.internal.misc.Unsafe
-        mv.visitMethodInsn(
-            INVOKESTATIC,
-            "jdk/internal/misc/Unsafe",
-            "getUnsafe",
-            "()Ljdk/internal/misc/Unsafe;",
-            false);
-        mv.visitLdcInsn(Type.getType(ClassLoader.class));
-        mv.visitLdcInsn(DefineClassGlue.V9);
+        defineClassUsingUnsafe("jdk/internal/misc/Unsafe", DefineClassGlue.V9);
       } else {
         // on Java 8 prepare sun.misc.Unsafe
-        mv.visitMethodInsn(
-            INVOKESTATIC, "sun/misc/Unsafe", "getUnsafe", "()Lsun/misc/Unsafe;", false);
-        mv.visitLdcInsn(Type.getType(ClassLoader.class));
-        mv.visitLdcInsn(DefineClassGlue.V8);
-      }
-
-      // unpack the UTF-16BE encoded string back into bytecode
-      mv.visitFieldInsn(
-          GETSTATIC, "java/nio/charset/StandardCharsets", "UTF_16BE", "Ljava/nio/charset/Charset;");
-      mv.visitMethodInsn(
-          INVOKEVIRTUAL, "java/lang/String", "getBytes", "(Ljava/nio/charset/Charset;)[B", false);
-
-      if (JVM.atLeastJava(15)) {
-        // on Java 15+ use MethodHandles.lookup().defineHiddenClass(...)
-        mv.visitInsn(ICONST_0);
-        mv.visitInsn(ICONST_0);
-        mv.visitTypeInsn(ANEWARRAY, "java/lang/invoke/MethodHandles$Lookup$ClassOption");
-        mv.visitMethodInsn(
-            INVOKEVIRTUAL,
-            "java/lang/invoke/MethodHandles$Lookup",
-            "defineHiddenClass",
-            "([BZ[Ljava/lang/invoke/MethodHandles$Lookup$ClassOption;)Ljava/lang/invoke/MethodHandles$Lookup;",
-            false);
-        // use lookupClass() to retrieve the hidden class we just defined
-        mv.visitMethodInsn(
-            INVOKEVIRTUAL,
-            "java/lang/invoke/MethodHandles$Lookup",
-            "lookupClass",
-            "()Ljava/lang/Class;",
-            false);
-      } else if (JVM.atLeastJava(9)) {
-        // on Java 9+ use jdk.internal.misc.Unsafe.defineAnonymousClass(...)
-        mv.visitInsn(ACONST_NULL);
-        mv.visitMethodInsn(
-            INVOKEVIRTUAL,
-            "jdk/internal/misc/Unsafe",
-            "defineAnonymousClass",
-            "(Ljava/lang/Class;[B[Ljava/lang/Object;)Ljava/lang/Class;",
-            false);
-      } else {
-        // on Java 8 use sun.misc.Unsafe.defineAnonymousClass(...)
-        mv.visitInsn(ACONST_NULL);
-        mv.visitMethodInsn(
-            INVOKEVIRTUAL,
-            "sun/misc/Unsafe",
-            "defineAnonymousClass",
-            "(Ljava/lang/Class;[B[Ljava/lang/Object;)Ljava/lang/Class;",
-            false);
+        defineClassUsingUnsafe("sun/misc/Unsafe", DefineClassGlue.V8);
       }
 
       mv.visitInsn(ARETURN);
@@ -246,6 +185,65 @@ public void visitCode() {
       mv.visitFrame(F_SAME, 0, null, 0, null);
     }
 
+    private void defineClassUsingMethodHandles() {
+      // on Java 15+ prepare MethodHandles.lookup()
+      mv.visitMethodInsn(
+          INVOKESTATIC,
+          "java/lang/invoke/MethodHandles",
+          "lookup",
+          "()Ljava/lang/invoke/MethodHandles$Lookup;",
+          false);
+      mv.visitLdcInsn(DefineClassGlue.V9);
+      unpackByteCode();
+      // on Java 15+ use MethodHandles.lookup().defineHiddenClass(...)
+      mv.visitInsn(ICONST_0);
+      mv.visitInsn(ICONST_0);
+      mv.visitTypeInsn(ANEWARRAY, "java/lang/invoke/MethodHandles$Lookup$ClassOption");
+      mv.visitMethodInsn(
+          INVOKEVIRTUAL,
+          "java/lang/invoke/MethodHandles$Lookup",
+          "defineHiddenClass",
+          "([BZ[Ljava/lang/invoke/MethodHandles$Lookup$ClassOption;)Ljava/lang/invoke/MethodHandles$Lookup;",
+          false);
+      // use lookupClass() to retrieve the hidden class we just defined
+      mv.visitMethodInsn(
+          INVOKEVIRTUAL,
+          "java/lang/invoke/MethodHandles$Lookup",
+          "lookupClass",
+          "()Ljava/lang/Class;",
+          false);
+    }
+
+    private void defineClassUsingUnsafe(String owner, String glue) {
+      String descriptor = "()L"+owner+";";
+      // on Java 9+ prepare jdk.internal.misc.Unsafe
+      mv.visitMethodInsn(
+          INVOKESTATIC,
+          owner,
+          "getUnsafe",
+          descriptor,
+          false);
+      mv.visitLdcInsn(Type.getType(ClassLoader.class));
+      mv.visitLdcInsn(glue);
+      unpackByteCode();
+      // on Java 9+ use jdk.internal.misc.Unsafe.defineAnonymousClass(...)
+      mv.visitInsn(ACONST_NULL);
+      mv.visitMethodInsn(
+          INVOKEVIRTUAL,
+          owner,
+          "defineAnonymousClass",
+          "(Ljava/lang/Class;[B[Ljava/lang/Object;)Ljava/lang/Class;",
+          false);
+    }
+
+    private void unpackByteCode() {
+      // unpack the UTF-16BE encoded string back into bytecode
+      mv.visitFieldInsn(
+          GETSTATIC, "java/nio/charset/StandardCharsets", "UTF_16BE", "Ljava/nio/charset/Charset;");
+      mv.visitMethodInsn(
+          INVOKEVIRTUAL, "java/lang/String", "getBytes", "(Ljava/nio/charset/Charset;)[B", false);
+    }
+
     @Override
     public void visitMaxs(int maxStack, int maxLocals) {
       // ensure we have enough stack allocated for our code