[kotlin debugger] IDEA-357870 Introduce MetadataDebugHelper

Kotlin compiler stores meta information about every class in the `kotlin.Metadata` annotation. Unfortunately, there is no way to fetch this annotation using plain JDI interface, because unlike method information, class annotations are not exposed to the debugger. This commit adds a helper class that allows fetching `kotlin.Metadata` of a specific class as a JSON string.

GitOrigin-RevId: 4d463abcd21f75acf90a2358d5782ee1b8b524d2

java/java-runtime/src/com/intellij/rt/debugger/MetadataDebugHelper.java

@@ -0,0 +1,132 @@
+// Copyright 2000-2025 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
+package com.intellij.rt.debugger;
+
+import java.lang.annotation.Annotation;
+import java.lang.reflect.Method;
+import java.nio.charset.StandardCharsets;
+import java.util.Arrays;
+
+public class MetadataDebugHelper {
+  private static final String METADATA_CLASS_NAME = "kotlin.Metadata";
+
+  private static final String KIND = "kind";
+  private static final String METADATA_VERSION = "metadataVersion";
+  private static final String DATA_1 = "data1";
+  private static final String DATA_2 = "data2";
+  private static final String EXTRA_STRING = "extraString";
+  private static final String PACKAGE_NAME = "packageName";
+  private static final String EXTRA_INT = "extraInt";
+
+  private static final String KIND_METHOD_NAME = "k";
+  private static final String METADATA_VERSION_METHOD_NAME = "mv";
+  private static final String DATA_1_METHOD_NAME = "d1";
+  private static final String DATA_2_METHOD_NAME = "d2";
+  private static final String EXTRA_STRING_METHOD_NAME = "xs";
+  private static final String PACKAGE_NAME_METHOD_NAME = "pn";
+  private static final String EXTRA_INT_METHOD_NAME = "xi";
+
+  /*
+   * Kotlin debugger uses this function to fetch @kotlin.Metadata for a given class.
+   * Originally, reading annotations from the debugger is not supported by JVM.
+   *
+   * The metadata is returned as a string in JSON format, where the `data1` field is
+   * base64 encoded to keep the format valid.
+   *
+   * Returning metadata as a single string allows us to minimize the number of JDI calls from
+   * the debugger side. Any JDWP communication can be very expensive when debugging remotely,
+   * especially when debugging Android applications on a phone.
+   */
+  @SuppressWarnings("unchecked")
+  public static String getDebugMetadataAsJson(Class<?> cls) {
+    try {
+      Class<?> metadataClass = Class.forName(METADATA_CLASS_NAME, false, cls.getClassLoader());
+      Method kindMethod = metadataClass.getDeclaredMethod(KIND_METHOD_NAME);
+      Method metadataVersionMethod = metadataClass.getDeclaredMethod(METADATA_VERSION_METHOD_NAME);
+      Method data1Method = metadataClass.getDeclaredMethod(DATA_1_METHOD_NAME);
+      Method data2Method = metadataClass.getDeclaredMethod(DATA_2_METHOD_NAME);
+      Method extraStringMethod = metadataClass.getDeclaredMethod(EXTRA_STRING_METHOD_NAME);
+      Method packageNameMethod = metadataClass.getDeclaredMethod(PACKAGE_NAME_METHOD_NAME);
+      Method extraIntMethod = metadataClass.getDeclaredMethod(EXTRA_INT_METHOD_NAME);
+
+      Object metadata = cls.getAnnotation((Class<Annotation>)metadataClass);
+
+      StringBuilder sb = new StringBuilder();
+      sb.append("{");
+
+      int kind = (int)kindMethod.invoke(metadata);
+      appendAsJsonValue(sb, KIND, kind);
+
+      int[] metadataVersion = (int[]) metadataVersionMethod.invoke(metadata);
+      appendAsJsonValue(sb, METADATA_VERSION, Arrays.toString(metadataVersion));
+
+      String[] data1 = (String[])data1Method.invoke(metadata);
+      appendAsJsonValue(sb, DATA_1, Arrays.toString(toJsonBase64Encoded(data1)));
+
+      String[] data2 = (String[])data2Method.invoke(metadata);
+      appendAsJsonValue(sb, DATA_2, Arrays.toString(toJson(data2)));
+
+      String extraString = (String) extraStringMethod.invoke(metadata);
+      appendAsJsonValue(sb, EXTRA_STRING, toJson(extraString));
+
+      String packageName = (String) packageNameMethod.invoke(metadata);
+      appendAsJsonValue(sb, PACKAGE_NAME, toJson(packageName));
+
+      int extraInt = (int)extraIntMethod.invoke(metadata);
+      appendAsJsonValueNoComma(sb, EXTRA_INT, extraInt);
+
+      sb.append("}");
+
+      return sb.toString();
+    } catch (Exception ex) {
+      return null;
+    }
+  }
+
+  private static void appendAsJsonValue(StringBuilder sb, String name, Object value) {
+    appendAsJsonValueNoComma(sb, name, value).append(',');
+  }
+
+  private static StringBuilder appendAsJsonValueNoComma(StringBuilder sb, String name, Object value) {
+    return sb.append(toJson(name)).append(':').append(value);
+  }
+
+  private static String toJson(String str) {
+    StringBuilder sb = new StringBuilder();
+    sb.append('"');
+    for (int i = 0; i < str.length(); i++) {
+      char c = str.charAt(i);
+      if (c == '"') {
+        sb.append("\\\"");
+      } else {
+        sb.append(c);
+      }
+    }
+    sb.append('"');
+    return sb.toString();
+  }
+
+  private static String[] toJson(String[] array) {
+    String[] result = new String[array.length];
+    for (int i = 0; i < array.length; i++) {
+      result[i] = toJson(array[i]);
+    }
+    return result;
+  }
+
+  @SuppressWarnings("PrimitiveArrayArgumentToVarargsMethod")
+  private static String[] toJsonBase64Encoded(String[] array) throws Exception {
+    Class<?> base64Class = Class.forName("java.util.Base64");
+    Method getEncoding = base64Class.getDeclaredMethod("getEncoder");
+    Object encoder = getEncoding.invoke(base64Class);
+    Class<?> encoderClass = Class.forName("java.util.Base64$Encoder");
+    Method encodeToString = encoderClass.getDeclaredMethod("encodeToString", byte[].class);
+
+    String[] result = new String[array.length];
+    for (int i = 0; i < array.length; i++) {
+      byte[] bytes = array[i].getBytes(StandardCharsets.UTF_8);
+      String encoded = (String)encodeToString.invoke(encoder, bytes);
+      result[i] = toJson(encoded);
+    }
+    return result;
+  }
+}