错误而重新定义与ByteBuddy的方法：“重新定义类失败：尝试添加一个方法”

Question

我学习字节好友和我试图做到以下几点：

• 从给定创建一个子类类或接口
• 然后在子类中替换的方法

注意，子类是在ClassLoader之前其方法的一个（sayHello）被重新定义“加载”。它失败，出现以下错误信息：

java.lang.UnsupportedOperationException: class redefinition failed: attempted to add a method 
at sun.instrument.InstrumentationImpl.redefineClasses0(Native Method) 
at sun.instrument.InstrumentationImpl.redefineClasses(InstrumentationImpl.java:170) 
at net.bytebuddy.dynamic.loading.ClassReloadingStrategy$Strategy$1.apply(ClassReloadingStrategy.java:293) 
at net.bytebuddy.dynamic.loading.ClassReloadingStrategy.load(ClassReloadingStrategy.java:173) 
... 

下面是一组JUnit测试的代码。第一个测试shouldReplaceMethodFromClass通过，因为Bar类在重新定义其方法之前未被分类。当给定的Bar类或Foo接口被分类时，另外两个测试失败。

我读到应该将新方法委托给一个单独的类，这就是我使用CustomInterceptor类所做的工作，并且我还在测试启动时安装了ByteBuddy代理并用于加载子类，但即使使用该类，我还是失去了一些东西，我什么也看不见:(

任何人有一个想法

public class ByteBuddyReplaceMethodInClassTest { 

    private File classDir; 

    private ByteBuddy bytebuddy; 

    @BeforeClass 
    public static void setupByteBuddyAgent() { 
    ByteBuddyAgent.install(); 
    } 

    @Before 
    public void setupTest() throws IOException { 
    this.classDir = Files.createTempDirectory("test").toFile(); 
    this.bytebuddy = new ByteBuddy().with(Implementation.Context.Disabled.Factory.INSTANCE); 
    } 

    @Test 
    public void shouldReplaceMethodFromClass() 
     throws InstantiationException, IllegalAccessException, Exception { 
    // given 
    final Class<? extends Bar> modifiedClass = replaceMethodInClass(Bar.class, 
     ClassFileLocator.ForClassLoader.of(Bar.class.getClassLoader())); 
    // when 
    final String hello = modifiedClass.newInstance().sayHello(); 
    // then 
    assertThat(hello).isEqualTo("Hello!"); 
    } 

    @Test 
    public void shouldReplaceMethodFromSubclass() 
     throws InstantiationException, IllegalAccessException, Exception { 
    // given 
    final Class<? extends Bar> modifiedClass = replaceMethodInClass(createSubclass(Bar.class), 
     new ClassFileLocator.ForFolder(this.classDir)); 
    // when 
    final String hello = modifiedClass.newInstance().sayHello(); 
    // then 
    assertThat(hello).isEqualTo("Hello!"); 
    } 

    @Test 
    public void shouldReplaceMethodFromInterface() 
     throws InstantiationException, IllegalAccessException, Exception { 
    // given 
    final Class<? extends Foo> modifiedClass = replaceMethodInClass(createSubclass(Foo.class), 
     new ClassFileLocator.ForFolder(this.classDir)); 
    // when 
    final String hello = modifiedClass.newInstance().sayHello(); 
    // then 
    assertThat(hello).isEqualTo("Hello!"); 
    } 


    @SuppressWarnings("unchecked") 
    private <T> Class<T> createSubclass(final Class<T> baseClass) { 
    final Builder<T> subclass = 
     this.bytebuddy.subclass(baseClass); 
    final Loaded<T> loaded = 
     subclass.make().load(ByteBuddyReplaceMethodInClassTest.class.getClassLoader(), 
      ClassReloadingStrategy.fromInstalledAgent()); 
    try { 
     loaded.saveIn(this.classDir); 
     return (Class<T>) loaded.getLoaded(); 
    } catch (IOException e) { 
     throw new RuntimeException("Failed to save subclass in a temporary directory", e); 
    } 
    } 

    private <T> Class<? extends T> replaceMethodInClass(final Class<T> subclass, 
     final ClassFileLocator classFileLocator) throws IOException { 
    final Builder<? extends T> rebasedClassBuilder = 
     this.bytebuddy.redefine(subclass, classFileLocator); 
    return rebasedClassBuilder.method(ElementMatchers.named("sayHello")) 
     .intercept(MethodDelegation.to(CustomInterceptor.class)).make() 
     .load(ByteBuddyReplaceMethodInClassTest.class.getClassLoader(), 
      ClassReloadingStrategy.fromInstalledAgent()) 
     .getLoaded(); 
    } 

    static class CustomInterceptor { 
    public static String intercept() { 
     return "Hello!"; 
    } 
    } 


} 

的Foo接口和Bar类有：？

public interface Foo { 

    public String sayHello(); 

} 

和

public class Bar { 

    public String sayHello() throws Exception { 
     return null; 
    } 

} 

Answer 1

的问题是，你首先创建的Bar一个子类，然后加载它，但后来重新定义它来添加方法sayHello。类演变如下：

1. 子类创建

   class Bar$ByteBuddy extends Bar { 
       Bar$ByteBuddy() { ... } 
   } 
   

2. 重新定义子类的

   class Bar$ByteBuddy extends Bar { 
       Bar$ByteBuddy() { ... } 
       String sayHello() { ... } 
   } 
   

的HotSpot虚拟机和大多数其他虚拟机不允许后添加方法类加载。您可以通过添加方法来子类中定义它首次之前解决这个问题，即设置：

DynamicType.Loaded<T> loaded = bytebuddy.subclass(baseClass) 
    .method(ElementMatchers.named("sayHello")) 
    .intercept(SuperMethodCall.INSTANCE) // or StubMethod.INSTANCE 
    .make() 

这种方式，该方法重新定义的时候和字节好友只需更换它的字节代码，而不是需要已经存在添加该方法。请注意，Byte Buddy尝试重新定义，因为少数虚拟机实际上支持它（如有兴趣的话可以在某些时候合并到HotSpot中，参见JEP 159）。