如何获取CGLIB代理类实例的字节码？

Question

我正在尝试使用BCEL获取cglib增强对象的字节码：

package app;

import cglib.MyInterceptor;
import net.sf.cglib.proxy.Enhancer;
import org.apache.bcel.Repository;
import org.apache.bcel.classfile.JavaClass;
import org.apache.bcel.classfile.Method;
import service.Tool;

public class CgLibApp {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException {
        // target object
        Tool tool = new Tool();

        // proxying
        Enhancer e = new Enhancer();
        e.setSuperclass(tool.getClass());
        e.setCallback(new MyInterceptor(tool));
        Tool proxifiedTool = (Tool) e.create();

        // trying to get proxy byte code
        JavaClass clazz = Repository.lookupClass(proxifiedTool.getClass());
        Method method = clazz.getMethod(Tool.class.getMethod("meth"));

        System.out.println(method.getCode().toString());
    }
}

但是我得到了：

Exception in thread "main" java.lang.ClassNotFoundException: SyntheticRepository could not load service.Tool$$EnhancerByCGLIB$$22a3afcc
at org.apache.bcel.util.SyntheticRepository.loadClass(SyntheticRepository.java:174)
at org.apache.bcel.util.SyntheticRepository.loadClass(SyntheticRepository.java:158)
at org.apache.bcel.Repository.lookupClass(Repository.java:74)
at app.CgLibApp.main(CgLibApp.java:21)

我该如何从增强对象中获取字节码？

- corvax

Answer 1

BCEL查询类加载器以获取表示.class文件的字节数组。对于动态生成的类，不存在这样的类文件。

为了获得类文件，必须在创建类文件时收集字节码。Cglib基于ASM构建，并允许您注册自己的ClassVisitor来收集类文件。

使用Enhancer，使用generateClass(ClassVisitor)方法并将后者方法交给ClassWriter。调用该方法后，可以从传递的类编写器对象中获取字节码。

- Rafael Winterhalter

Answer 2

我在研究如何在Spring Boot 3.0应用程序中保存CGLIB生成的类（例如处理带有@Transactional或@Configuration注释的类）时，发现了这个问题。这种简单的方法可能会有所帮助：

import org.springframework.cglib.core.ReflectUtils;
...
public class SpringCglibUtils {
    public static void initGeneratedClassHandler(String targetPath) {
        File dir = new File(targetPath);
        dir.mkdirs();
        ReflectUtils.setGeneratedClassHandler((String className, byte[] classContent) -> {
            try (FileOutputStream out = new FileOutputStream(new File(dir, className + ".class"))) {
                out.write(classContent);
            } catch (IOException e) {
                throw new UncheckedIOException("Error while storing " + className, e);
            }
        });
    }
}

然后在创建上下文之前，在您的主类中定义：

SpringCglibUtils.initGeneratedClassHandler("cglib");

Spring会将所有生成的类文件存储到targetPath目录中。

注意：很遗憾，在spring-boot 3之前不可用。

- seregamorph

Answer 3

这是打印生成的CGLIB类伪代码的示例代码。 visitEnd方法以文本格式打印生成的类。

package naga.cglib.demo;

import static org.objectweb.asm.Opcodes.ASM7;

import java.io.PrintWriter;
import java.lang.reflect.InvocationTargetException;

import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.FieldVisitor;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.util.TraceClassVisitor;

import net.sf.cglib.proxy.Enhancer;
import net.sf.cglib.proxy.FixedValue;

public class App {
public static void main(String[] args) throws Exception, IllegalArgumentException, InvocationTargetException {

    Enhancer enhancer = new Enhancer();
    enhancer.setSuperclass(SampleClass.class);
    enhancer.setCallback(new FixedValueImpl());
    SampleClass proxy = (SampleClass) enhancer.create();
    enhancer.generateClass(new CustomClassWriter());

    System.out.println("Hello cglib!" + proxy.test(null));

}
}

class SampleClass {
public String test(String input) {
    return "Hello world!";
}
}

class FixedValueImpl implements FixedValue {

@Override
public Object loadObject() throws Exception {
    // TODO Auto-generated method stub
    return "Hello cglib! from loadObject()";
}

}

class CustomClassWriter extends ClassVisitor {

TraceClassVisitor tracer;
PrintWriter pw = new PrintWriter(System.out);

public CustomClassWriter() {
    super(ASM7);
    tracer = new TraceClassVisitor(pw);
}

@Override
public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {
    System.out.println("method name is :" + name);
    return tracer.visitMethod(access, name, desc, signature, exceptions);

}

@Override
public FieldVisitor visitField(int access, String name, String desc, String signature, Object value) {

    System.out.println("field name is :" + name);
    return tracer.visitField(access, name, desc, signature, value);
}

public void visitEnd() {
    tracer.visitEnd();
    System.out.println(tracer.p.getText());

}
}

- NagararajaPALLA