錯誤而重新定義與ByteBuddy的方法：「重新定義類失敗：嘗試添加一個方法」

Question

我學習字節好友和我試圖做到以下幾點：

• 從給定創建一個子類類或接口
• 然後在子類中替換的方法

注意，子類是在ClassLoader之前其方法的一個（sayHello）被重新定義「加載」。它失敗，出現以下錯誤信息：

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) 
... 

下面是一組JUnit測試的代碼。第一個測試shouldReplaceMethodFromClass通過，因爲Bar類在重新定義其方法之前未被分類。當給定的Bar類或Foo接口被分類時，另外兩個測試失敗。

我讀到應該將新方法委託給一個單獨的類，這就是我使用CustomInterceptor類所做的工作，並且我還在測試啓動時安裝了ByteBuddy代理並用於加載子類，但即使使用該類，我還是失去了一些東西，我什麼也看不見:(

任何人有一個想法

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類有：？

public interface Foo { 

    public String sayHello(); 

} 

和

public class Bar { 

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

} 

Answer 1

的問題是，你首先創建的Bar一個子類，然後加載它，但後來重新定義它來添加方法sayHello。類演變如下：

1. 子類創建

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

2. 重新定義子類的

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

的HotSpot虛擬機和大多數其他虛擬機不允許後添加方法類加載。您可以通過添加方法來子類中定義它首次之前解決這個問題，即設置：

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

這種方式，該方法重新定義的時候和字節好友只需更換它的字節代碼，而不是需要已經存在添加該方法。請注意，Byte Buddy嘗試重新定義，因爲少數虛擬機實際上支持它（如有興趣的話可以在某些時候合併到HotSpot中，參見JEP 159）。