ASM动态子类创建 - NoClassDefFoundError BeanInfo

时间:2016-10-16 23:28:42

标签: java class dynamic java-bytecode-asm beaninfo

我正在尝试使用ASM Framework动态创建子类。 我能够创建类并实例化它。但是当我尝试做的时候

org.apache.commons.beanutils.BeanUtils.copyProperties(processedEntity, entity); 

它引发了这个异常:

java.lang.NoClassDefFoundError: com/wheelsup/app/benefits/service/XpOWErhNBiBeanInfo (wrong name: com/wheelsup/app/benefits/service/XpOWErhNBi)

这是我用来创建子类的代码:

Class<? extends T> get() throws Exception {
    String superClassInternalName = getInternalName(superClass);

    String subClassSimpleName = RandomStringUtils.random(10, true, false);
    String subClassInternalName = getClass().getPackage().getName().replaceAll("\\.", "/").concat("/").concat(subClassSimpleName);
    String subClassName = getClass().getPackage().getName().concat(".").concat(subClassSimpleName);

    ClassWriter classWriter = new ClassWriter(ClassWriter.COMPUTE_FRAMES);
    classWriter.visit(Opcodes.V1_6,
            ACC_PUBLIC,
            subClassInternalName,
            null,
            superClassInternalName,
            null);

    visitDefaultConstructor(classWriter, superClassInternalName);

    classWriter.visitEnd();

    return SubClassLoader.<T>init().load(classWriter.toByteArray(), subClassName);
}

private void visitDefaultConstructor(ClassWriter classWriter, String superClassInternalName) {
    MethodVisitor methodVisitor = classWriter.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null);
    methodVisitor.visitCode();
    methodVisitor.visitVarInsn(ALOAD, 0);
    methodVisitor.visitMethodInsn(INVOKESPECIAL, superClassInternalName, "<init>", "()V");
    methodVisitor.visitInsn(RETURN);
    methodVisitor.visitMaxs(0, 0);
    methodVisitor.visitEnd();
}

private static class SubClassLoader<T> {
    private final ClassLoader contextClassLoader;

    private SubClassLoader(ClassLoader contextClassLoader) {
        this.contextClassLoader = contextClassLoader;
    }

    static <U> SubClassLoader<U> init() {
        return new SubClassLoader<>(Thread.currentThread().getContextClassLoader());
    }

    @SuppressWarnings("unchecked")
    Class<? extends T> load(byte[] classBytes, String className) throws Exception {
        return (Class<? extends T>) new SubClassLoader.DynamicClassLoader(contextClassLoader, classBytes).loadClass(className);
    }

    private static class DynamicClassLoader extends ClassLoader {
        private byte[] rawClassBytes;

        private DynamicClassLoader(ClassLoader contextClassLoader, byte[] classBytes) {
            super(contextClassLoader);
            this.rawClassBytes = classBytes;
        }

        @Override
        public Class findClass(String name) {
            return defineClass(name, this.rawClassBytes, 0, this.rawClassBytes.length);
        }
    }
}

我不明白 BeanInfo 的事情;它是什么?以及如何解决我的问题?

感谢。

2 个答案:

答案 0 :(得分:1)

所以问题出在ClassLoader

private static class SubClassLoader<T> {
    private final ClassLoader contextClassLoader;

    private SubClassLoader(ClassLoader contextClassLoader) {
        this.contextClassLoader = contextClassLoader;
    }

    static <U> SubClassLoader<U> init() {
        return new SubClassLoader<>(Thread.currentThread().getContextClassLoader());
    }

    @SuppressWarnings("unchecked")
    Class<? extends T> load(byte[] classBytes, String className) throws Exception {
        return (Class<? extends T>) new DynamicClassLoader(contextClassLoader, classBytes, className).loadClass(className);
    }

    private static class DynamicClassLoader extends ClassLoader {
        private byte[] classBytes;
        private final String className;

        private DynamicClassLoader(ClassLoader contextClassLoader, byte[] classBytes, String className) {
            super(contextClassLoader);
            this.classBytes = classBytes;
            this.className = className;
        }

        @Override
        public Class findClass(String className) throws ClassNotFoundException {
            if (StringUtils.equals(this.className, className)) {
                return defineClass(className, this.classBytes, 0, this.classBytes.length);
            }

            throw new ClassNotFoundException(className);
        }
    }
}

答案 1 :(得分:1)

问题在于,无论调用者要求哪个类,您的findClass实现都会尝试返回一个生成的类。在某些情况下,无法加载类是正确的操作。

BeanUtils类依赖于Introspector,它允许为被检查类提供可选的显式beaninfo实现,因此如果被要求BeanInfo Foo,它将会尝试首先加载一个类FooBeanInfo,如果失败,它将为Foo构建一个通用的bean信息。

但是,由于您的findClass实施尝试(重新)以错误的名称XpOWErhNBi构建XpOWErhNBiBeanInfo类,而不是报告缺少XpOWErhNBiBeanInfo,因此出现问题。

您必须更改SubClassLoader以接收生成的类的预期名称。然后,您可以将findClass实施更改为

@Override
public Class findClass(String name) throws ClassNotFoundException {
    if(!name.equals(expectedName))
        throw new ClassNotFoundException(name);
    return defineClass(name, this.rawClassBytes, 0, this.rawClassBytes.length);
}

一个更简单但是hacky的解决方案是在第一个类构造之后null rawClassBytes并为每个后续类加载请求抛出ClassNotFoundException作为继承标准{{ 1}}实现保证仅为尚未加载的类调用loadClass,因此只要立即加载生成的类的程序逻辑没有改变,所有后续请求都是关于不同的,不支持的类。

然而,由于关键点是程序逻辑不能改变,我不建议使用hacky,脆弱的解决方案。将生成的类的名称传递给自定义加载器并验证它,首先是更多的代码,但更清晰。