如何在任何字节码库中使用最终静态字段创建 class？

Question

我正在尝试使用任何字节代码库创建一个包含静态最终对象字段的简单 class。我尝试过 BCEL 和 Byte Buddy，但没有成功。我要构建的 class 看起来像这样。谢谢

public class ConstructedClass{

   public static final MyClass a = new MyClass();

   public static final MyClass b = new MyClass(); 
}

我对 BCEL 的尝试：

ClassGen classGen=new ClassGen("org.test.lib.core", "java.lang.Object","core.java", Const.ACC_PUBLIC, null);
classGen.addEmptyConstructor(Const.ACC_PUBLIC); 

ConstantPoolGen constantPoolGen=classGen.getConstantPool();
int access_flags = Const.ACC_PUBLIC | Const.ACC_STATIC | Const.ACC_FINAL; 
final FieldGen FieldGen=new FieldGen( access_flags,Type.getType(Property.class), "test", constantPoolGen);
//FieldGen.setInitValue(new MyClass());

我第二次尝试也使用 BCEL：

private static final Type[] arg =  {Type.getType(MyClass.class)};
InstructionList init = new InstructionList();
InstructionFactory factory=new InstructionFactory(classGen);
//init.append(new PUSH(constantPoolGen, new MyClass())); 
init.append(factory.createInvoke(MyClass.class.getName(), "valueOf", 
             Type.getType(MyClass.class), arg, Const.INVOKESTATIC)); 
init.append(factory.createPutStatic("org.test.lib.core", "test", Type.getType(Property.class)));

注释行是推动我的对象不起作用的地方。

Answer 1

使用 ByteBuddy 您可以使用 ByteCodeAppender 生成静态初始化块。这将导致 class 与您想要的略有不同，但我认为足够接近了：

public class ConstructedClass {

   public static final MyClass a;

   static {
        a = new MyClass();
   } 
}

生成代码：

public class ByteBuddyTest {

    public static void main(String[] args) throws Exception {
        DynamicType.Loaded loaded =
            new ByteBuddy()
            .subclass(Object.class)
                .initializer( new ByteCodeAppender() {
                    @Override public Size apply(MethodVisitor methodVisitor, Implementation.Context implementationContext,
                                                MethodDescription instrumentedMethod) {
                        StackManipulation.Size size = new StackManipulation.Compound(
                            TypeCreation.of(new TypeDescription.ForLoadedType(MyClass.class)),
                            Duplication.SINGLE,
                            MethodInvocation.invoke(new TypeDescription.ForLoadedType(MyClass.class).getDeclaredMethods().filter(ElementMatchers.isDefaultConstructor()).getOnly()),
                            FieldAccess.forField(implementationContext.getInstrumentedType().getDeclaredFields().filter(ElementMatchers.named("a")).getOnly()).write()
                        ).apply(methodVisitor, implementationContext);

                        return new Size(size.getMaximalSize(), instrumentedMethod.getStackSize());
                    }
                })
            .name("org.test.lib.core.ConstructedClass")
            .modifiers(Opcodes.ACC_PUBLIC)
            .defineField("a", MyClass.class, Opcodes.ACC_PUBLIC | Opcodes.ACC_STATIC | Opcodes.ACC_FINAL)
            .make()
            .load(ByteBuddyTest.class.getClassLoader(), ClassLoadingStrategy.Default.INJECTION);

        Object obj = loaded.getLoaded().getConstructor().newInstance();
        System.out.println(obj.getClass().getField("a").get(obj));
    }

    public static class MyClass {
        public MyClass(String arg) {}

        public static MyClass createMyClass(String arg) {
            return new MyClass(arg);
        }
    }
}

评论更新

要调用静态工厂方法而不是构造函数你只需要替换构造函数调用：

StackManipulation.Size size = new StackManipulation.Compound(
    new TextConstant("test"),
    MethodInvocation.invoke(new TypeDescription.ForLoadedType(MyClass.class).getDeclaredMethods().filter(ElementMatchers.named("createMyClass")).getOnly()),
         FieldAccess.forField(implementationContext.getInstrumentedType().getDeclaredFields().filter(ElementMatchers.named("a")).getOnly()).write()
).apply(methodVisitor, implementationContext);

多个字段

    DynamicType.Builder builder = new ByteBuddy().subclass(Object.class);
    List<String> fields = Lists.newArrayList("a", "b", "c");

    for (String str : fields) {
        builder = builder.defineField(str, MyClass.class, Opcodes.ACC_PUBLIC | Opcodes.ACC_STATIC | Opcodes.ACC_FINAL);
    }

    DynamicType.Loaded loaded = builder.make().load(ByteBuddyTest.class.getClassLoader(), ClassLoadingStrategy.Default.INJECTION);

    Object obj = loaded.getLoaded().getConstructor().newInstance();
    System.out.println(obj.getClass().getField("a"));
    System.out.println(obj.getClass().getField("c"));

如何在任何字节码库中使用最终静态字段创建 class？

How to create class with final static fields in any byte code library?

java

bytecode

bcel

byte-buddy