Support parsing class-file data at a given offset

class-match/src/main/java/datadog/instrument/classmatch/ClassFile.java

@@ -52,7 +52,18 @@ private ClassFile() {}
    * @return class header containing class-name, super-name, interfaces
    */
   public static ClassHeader header(byte[] bytecode) {
-    return parse(bytecode, true);
+    return parse(bytecode, 0, true);
+  }
+
+  /**
+   * Extracts a {@link ClassHeader} from the given class-file content.
+   *
+   * @param bytecode the class-file content to parse
+   * @param offset the offset of the class-file content
+   * @return class header containing class-name, super-name, interfaces
+   */
+  public static ClassHeader header(byte[] bytecode, int offset) {
+    return parse(bytecode, offset, true);
   }
 
   /**
@@ -62,7 +73,18 @@ public static ClassHeader header(byte[] bytecode) {
    * @return class outline containing header, fields, methods, annotations
    */
   public static ClassOutline outline(byte[] bytecode) {
-    return (ClassOutline) parse(bytecode, false);
+    return (ClassOutline) parse(bytecode, 0, false);
+  }
+
+  /**
+   * Extracts a {@link ClassOutline} from the given class-file content.
+   *
+   * @param bytecode the class-file content to parse
+   * @param offset the offset of the class-file content
+   * @return class outline containing header, fields, methods, annotations
+   */
+  public static ClassOutline outline(byte[] bytecode, int offset) {
+    return (ClassOutline) parse(bytecode, offset, false);
   }
 
   /**
@@ -115,9 +137,9 @@ private static UtfKey annotationKey(String internalName) {
   }
 
   /** Parse class-file content, skipping over uninteresting sections, */
-  private static ClassHeader parse(byte[] bytecode, boolean onlyHeader) {
+  private static ClassHeader parse(byte[] bytecode, int offset, boolean onlyHeader) {
     // skip preamble
-    int cursor = 8;
+    int cursor = offset + 8;
 
     int cpLen = u2(bytecode, cursor);
     cursor += 2;

class-match/src/test/java/datadog/instrument/classmatch/ClassFileTest.java

@@ -19,8 +19,11 @@
 
 class ClassFileTest {
 
+  static int SAMPLE_OFFSET = 1357;
+
   static final byte[] sampleUnicodeClass;
   static final byte[] sampleParametersClass;
+  static final byte[] sampleClassAtOffset;
 
   static {
     try {
@@ -35,6 +38,9 @@ class ClassFileTest {
     } catch (IOException e) {
       throw new UncheckedIOException(e);
     }
+    sampleClassAtOffset = new byte[sampleUnicodeClass.length + SAMPLE_OFFSET];
+    System.arraycopy(
+        sampleUnicodeClass, 0, sampleClassAtOffset, SAMPLE_OFFSET, sampleUnicodeClass.length);
   }
 
   @Test
@@ -90,6 +96,21 @@ void parameterParsing() {
         outline.methods[1].descriptorBoundaries());
   }
 
+  @Test
+  void offsetParsing() {
+    ClassHeader header = ClassFile.header(sampleClassAtOffset, SAMPLE_OFFSET);
+    assertEquals("sample/My例クラス", header.className);
+    assertEquals("java/util/AbstractCollection", header.superName);
+    assertArrayEquals(new String[] {"java/io/Serializable"}, header.interfaces);
+
+    ClassOutline outline = ClassFile.outline(sampleClassAtOffset, SAMPLE_OFFSET);
+    assertEquals("sample/My例クラス", outline.className);
+    assertEquals("java/util/AbstractCollection", outline.superName);
+    assertArrayEquals(new String[] {"java/io/Serializable"}, outline.interfaces);
+    assertEquals(1, outline.fields.length);
+    assertEquals(2, outline.methods.length);
+  }
+
   @SuppressWarnings("SameParameterValue")
   private static void testParsing(String sampleJar, Consumer<byte[]> parser) {
     byte[] buf = new byte[16384];