@Test
public void ModifyClassTest() throws Exception {
    // 读取的Person类的class文件
    ClassReader classReader = new ClassReader("org.javamaster.b2c.bytecode.model.Person");
    ClassWriter classWriter = new ClassWriter(ClassWriter.COMPUTE_MAXS);
    ClassAdapter classAdapter = new ClassAdapter(classWriter);
    classReader.accept(classAdapter, ClassReader.SKIP_DEBUG);

    // 插入新方法
    // Opcodes.ACC_PUBLIC:方法修饰符为public
    // sayHello:方法名为sayHello
    // ()V:没有入参,返回类型为void
    // new String[]{"javax.validation.ValidationException"}:声明抛出ValidationException异常
    MethodVisitor helloVisitor = classWriter
            .visitMethod(Opcodes.ACC_PUBLIC, "sayHello", "()V", null, new String[]{"javax.validation.ValidationException"});
    // 插入方法体内容,以下涉及到了虚拟机字节码指令
    helloVisitor.visitCode();
    // getstatic 指令:取静态字段
    helloVisitor.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
    // ldc 指令:取"hello world!"常量到操作数栈
    helloVisitor.visitLdcInsn("hello world!");
    // invokevirtual 指令: 调用实例方法println
    helloVisitor.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V");
    // return 指令: 从当前方法返回void
    helloVisitor.visitInsn(Opcodes.RETURN);
    helloVisitor.visitMaxs(1, 1);
    helloVisitor.visitEnd();

    byte[] bytes = classWriter.toByteArray();

    String path = ResourceUtils.getFile("classpath:").getAbsolutePath();
    File file = new File(path, "/org/javamaster/b2c/bytecode/PersonModify.class");
    FileOutputStream fileOutputStream = new FileOutputStream(file);
    fileOutputStream.write(bytes);
    fileOutputStream.close();
}
 

public byte[] patch(String className, byte[] classBytes) {

    if(wrappers.size() == 0) {
      return classBytes;
    }

    matchProps.put(PROP_CLASSNAME, className);

    if(patchesFilter != null) {
      boolean b = patchesFilter.match(matchProps);
      
      if(!b) {
        return classBytes;
      }
    }

    try {
      ClassReader  cr    = new ClassReader(classBytes);
      ClassWriter  cw    = new BundleClassWriter(ClassWriter.COMPUTE_MAXS,
                                                 patchedBundle.adapt(BundleWiring.class).getClassLoader());
      ClassAdapter trans = new ClassAdapterPatcher(cw,
                                                   className.replace('.', '/'),
                                                   patchedBundle.adapt(BundleWiring.class).getClassLoader(),
                                                   patchedBundle.getBundleId(),
                                                   this);

      cr.accept(trans, 0);

      byte[] newBytes = cw.toByteArray();

      if(bDumpClasses) {
        dumpClassBytes(className, newBytes);
      }
      classBytes = newBytes;
    } catch (Exception e) {
      throw new RuntimeException("Failed to patch " + className + "/"
                                 + patchedBundle.adapt(BundleWiring.class).getClassLoader() +": " +e);
    }
    return classBytes;
  }
 

byte[] nullAdaptClass(final InputStream is, final String name)
        throws Exception
{
    ClassReader cr = new ClassReader(is);
    ClassWriter cw = new ClassWriter(compute);
    ClassAdapter ca = new ClassAdapter(cw);
    cr.accept(ca, skipDebug);
    return cw.toByteArray();
}
 

byte[] counterAdaptClass(final InputStream is, final String name)
        throws Exception
{
    ClassReader cr = new ClassReader(is);
    ClassWriter cw = new ClassWriter(false);
    ClassAdapter ca = new CounterClassAdapter(cw);
    cr.accept(ca, false);
    return cw.toByteArray();
}