mcculls · Sep 28, 2026 · Sep 28, 2026
@@ -25,8 +25,8 @@
 /**
  * Generates glue bytecode for a {@link BiFunction} around {@code ClassLoader.defineClass} that
  * accepts a map of class-names to bytecode and a class-loader (or protection domain) and returns
- * the defined classes. The glue bytecode is encoded as a UTF-16BE string and written as a constant
- * inside a Java class at build-time.
+ * the defined classes. The glue bytecode is encoded as an ISO-8859-1 string and written as a
+ * constant inside a Java class at build-time.
  *
  * <p>At runtime the glue bytecode is decoded from the string and defined as a hidden/anonymous
  * class using instrumentation. This class provides access to {@code ClassLoader.defineClass}
@@ -414,11 +414,6 @@ static byte[] generateBytecode(String unsafeNamespace) {
     mv.visitMaxs(-1, -1);
     mv.visitEnd();
 
-    // pad bytecode to even number of bytes, to make string encoding/decoding easier
-    if ((unsafeNamespace.length() & 0x01) == 0) {
-      cw.newConst(0);
-    }
-
     cw.visitEnd();
 
     return cw.toByteArray();

@@ -290,9 +290,12 @@ public void visitCode() {
         mv.visitLdcInsn(DefineClassGlue.V8);
       }
 
-      // unpack the UTF-16BE encoded string back into bytecode
+      // unpack the ISO-8859-1 encoded glue string back into bytecode
       mv.visitFieldInsn(
-          GETSTATIC, "java/nio/charset/StandardCharsets", "UTF_16BE", "Ljava/nio/charset/Charset;");
+          GETSTATIC,
+          "java/nio/charset/StandardCharsets",
+          "ISO_8859_1",
+          "Ljava/nio/charset/Charset;");
       mv.visitMethodInsn(
           INVOKEVIRTUAL, "java/lang/String", "getBytes", "(Ljava/nio/charset/Charset;)[B", false);
 

@@ -1,5 +1,6 @@
 package datadog.instrument.glue;
 
+import static java.nio.charset.StandardCharsets.ISO_8859_1;
 import static org.assertj.core.api.Assertions.assertThat;
 
 import java.io.PrintWriter;
@@ -22,4 +23,18 @@ void generatedBytecodePassesVerification(String unsafeNamespace) {
 
     assertThat(errors.toString()).isEmpty();
   }
+
+  /** ClassInjector inlines the glue unpacking, so check the packed glue decodes as expected. */
+  @ParameterizedTest
+  @ValueSource(strings = {"sun/misc", "jdk/internal/misc"})
+  void packedGlueUnpacksToGeneratedBytecode(String unsafeNamespace) {
+    byte[] originalBytecode = DefineClassGlueGenerator.generateBytecode(unsafeNamespace);
+    String packedBytecode =
+        "sun/misc".equals(unsafeNamespace) ? DefineClassGlue.V8 : DefineClassGlue.V9;
+
+    // mirrors exactly what ClassInjector's generated bytecode does at runtime
+    byte[] decodedBytecode = packedBytecode.getBytes(ISO_8859_1);
+
+    assertThat(decodedBytecode).isEqualTo(originalBytecode);
+  }
 }
@@ -6,7 +6,7 @@
 
 package datadog.instrument.glue;
 
-import static java.nio.charset.StandardCharsets.UTF_16BE;
+import static java.nio.charset.StandardCharsets.ISO_8859_1;
 import static java.util.Objects.requireNonNull;
 
 import datadog.instrument.utils.JVM;
@@ -26,7 +26,7 @@ private Glue() {}
    * @return the unpacked bytecode
    */
   public static byte[] unpackBytecode(String bytecode) {
-    return bytecode.getBytes(UTF_16BE);
+    return bytecode.getBytes(ISO_8859_1);
   }
 
   /**

@@ -51,31 +51,20 @@ public static void classHeader(List<String> lines, String className) {
   /**
    * Packs the given glue bytecode as a string literal and writes it out in source code form.
    *
-   * <p>The bytecode must be padded to have even length, as this simplifies the encoding/decoding.
-   *
    * @param lines the list collecting the source code
    * @param bytecode the bytecode to pack
    */
   public static void packBytecode(List<String> lines, byte[] bytecode) {
-    if ((bytecode.length & 0x01) == 1) {
-      throw new IllegalStateException("Bytecode length is not even; requires padding");
-    }
     StringBuilder buf = new StringBuilder("      \"");
-    // encode bytes using lenient form of UTF_16BE: no BOM, allow unpaired surrogates
-    // this is so we can unpack it later without requiring any special/complex code
-    for (int i = 0; i < bytecode.length; i += 2) {
+    // encode bytes using ISO_8859_1, where each char maps directly to the byte with the same value
+    // this is so we can unpack it later with a single String.getBytes(ISO_8859_1) call
+    for (byte b : bytecode) {
       if (buf.length() > 120) {
         lines.add(buf + "\"");
         buf.setLength(0);
         buf.append("          + \"");
       }
-      // pack 2 bytes into one char using big-endian ordering
-      char c = (char) ((bytecode[i] << 8) | (0x00FF & bytecode[i + 1]));
-      if (c <= 0x00FF) {
-        buf.append(String.format("\\%03o", (int) c));
-      } else {
-        buf.append(String.format("\\u%04x", (int) c));
-      }
+      buf.append(String.format("\\%03o", 0x00FF & b));
     }
     lines.add(buf + "\";");
   }

@@ -11,7 +11,6 @@
 import java.net.URL;
 import java.net.URLClassLoader;
 import java.nio.file.Files;
-import java.util.ArrayList;
 import java.util.List;
 import javax.tools.JavaCompiler;
 import javax.tools.SimpleJavaFileObject;
@@ -51,9 +50,11 @@ void glueGenerator(@TempDir File resourceDir, @TempDir File javaDir, @TempDir Fi
     assertTrue(glueCompilationTask.call());
 
     String packedBytecode;
+    String packedAllByteValues;
     try (URLClassLoader cl = URLClassLoader.newInstance(new URL[] {classesDir.toURI().toURL()})) {
       Class<?> testGlueClass = cl.loadClass("datadog.instrument.glue.TestGlue");
       packedBytecode = (String) testGlueClass.getField("BYTECODE").get(null);
+      packedAllByteValues = (String) testGlueClass.getField("ALL_BYTE_VALUES").get(null);
     }
 
     byte[] expectedBytecode = {
@@ -66,23 +67,14 @@ void glueGenerator(@TempDir File resourceDir, @TempDir File javaDir, @TempDir Fi
 
     assertArrayEquals(expectedBytecode, Files.readAllBytes(testGlueResourceFile.toPath()));
     assertArrayEquals(expectedBytecode, Glue.unpackBytecode(packedBytecode));
-  }
-
-  @Test
-  void paddingRequired() {
 
-    byte[] exampleBytecodeNeedsPadding = {
-      -54, -2, -70, -66, 0, 0, 0, 52, 0, 10, 10, 0, 2, 0, 3, 7, 0, 4, 12, 0, 5, 0, 6, 1, 0, 16, 106,
-      97, 118, 97, 47, 108, 97, 110, 103, 47, 79, 98, 106, 101, 99, 116, 1, 0, 6, 60, 105, 110, 105,
-      116, 62, 1, 0, 3, 40, 41, 86, 7, 0, 8, 1, 0, 2, 69, 103, 1, 0, 4, 67, 111, 100, 101, 0, 32, 0,
-      7, 0, 2, 0, 0, 0, 0, 0, 1, 0, 0, 0, 5, 0, 6, 0, 1, 0, 9, 0, 0, 0, 17, 0, 1, 0, 1, 0, 0, 0, 5,
-      42, -73, 0, 1, -79, 0, 0, 0, 0, 0, 0
-    };
+    // every possible byte value, with an odd length to check no padding is required
+    byte[] expectedAllByteValues = new byte[257];
+    for (int i = 0; i < expectedAllByteValues.length; i++) {
+      expectedAllByteValues[i] = (byte) i;
+    }
 
-    assertThrows(
-        IllegalStateException.class,
-        () -> GlueGenerator.packBytecode(new ArrayList<>(), exampleBytecodeNeedsPadding),
-        "Bytecode length is not even; requires padding");
+    assertArrayEquals(expectedAllByteValues, Glue.unpackBytecode(packedAllByteValues));
   }
 
   @Test

@@ -20,6 +20,15 @@ public class TestGlueGenerator {
     0, 1, 0, 1, 0, 0, 0, 5, 42, -73, 0, 1, -79, 0, 0, 0, 0, 0, 0
   };
 
+  // every possible byte value, plus one more so the length is odd
+  private static final byte[] allByteValues = new byte[257];
+
+  static {
+    for (int i = 0; i < allByteValues.length; i++) {
+      allByteValues[i] = (byte) i;
+    }
+  }
+
   public static void generateGlue(Path resourcePath, Path javaPath) throws IOException {
     Files.write(resourcePath.resolve("test.glue"), exampleBytecode);
 
@@ -28,6 +37,8 @@ public static void generateGlue(Path resourcePath, Path javaPath) throws IOExcep
     classHeader(lines, "TestGlue");
     lines.add("  String BYTECODE =");
     packBytecode(lines, exampleBytecode);
+    lines.add("  String ALL_BYTE_VALUES =");
+    packBytecode(lines, allByteValues);
     lines.add("}");
     Files.write(testGlue, lines, StandardCharsets.UTF_8);
   }