在java中动态加载一个具有不同包名的类

Question

是否可以在 Java 中加载类并“伪造”类的包名称/规范名称？我尝试这样做，这是显而易见的方法，但我在 ClassDefNotFoundException 中收到“类名不匹配”消息。

我这样做的原因是我试图加载一个在默认包中编写的 API，以便我可以直接使用它而不使用反射。该代码将针对表示包和包名称导入的文件夹结构中的类进行编译。即：

./com/DefaultPackageClass.class

// ...
import com.DefaultPackageClass;
import java.util.Vector;
// ...

我当前的代码如下：

public Class loadClass(String name) throws ClassNotFoundException {
    if(!CLASS_NAME.equals(name))
            return super.loadClass(name);

    try {
        URL myUrl = new URL(fileUrl);
        URLConnection connection = myUrl.openConnection();
        InputStream input = connection.getInputStream();
        ByteArrayOutputStream buffer = new ByteArrayOutputStream();
        int data = input.read();

        while(data != -1){
            buffer.write(data);
            data = input.read();
        }

        input.close();

        byte[] classData = buffer.toByteArray();

        return defineClass(CLASS_NAME,
                classData, 0, classData.length);

    } catch (MalformedURLException e) {
        throw new UndeclaredThrowableException(e);
    } catch (IOException e) {
        throw new UndeclaredThrowableException(e); 
    }

}

Answer 1

正如皮特提到的，这可以使用 ASM 字节码库来完成。事实上，该库实际上附带了一个专门用于处理这些类名重新映射的类（RemappingClassAdapter）。下面是使用此类的类加载器的示例：

public class MagicClassLoader extends ClassLoader {

    private final String defaultPackageName;

    public MagicClassLoader(String defaultPackageName) {
        super();
        this.defaultPackageName = defaultPackageName;
    }

    public MagicClassLoader(String defaultPackageName, ClassLoader parent) {
        super(parent);
        this.defaultPackageName = defaultPackageName;
    }

    @Override
    public Class<?> loadClass(String name) throws ClassNotFoundException {
        byte[] bytecode = ...; // I will leave this part up to you
        byte[] remappedBytecode;

        try {
            remappedBytecode = rewriteDefaultPackageClassNames(bytecode);
        } catch (IOException e) {
            throw new RuntimeException("Could not rewrite class " + name);
        }

        return defineClass(name, remappedBytecode, 0, remappedBytecode.length);
    }

    public byte[] rewriteDefaultPackageClassNames(byte[] bytecode) throws IOException {
        ClassReader classReader = new ClassReader(bytecode);
        ClassWriter classWriter = new ClassWriter(classReader, 0);

        Remapper remapper = new DefaultPackageClassNameRemapper();
        classReader.accept(
                new RemappingClassAdapter(classWriter, remapper),
                0
            );

        return classWriter.toByteArray();
    }

    class DefaultPackageClassNameRemapper extends Remapper {

        @Override
        public String map(String typeName) {
            boolean hasPackageName = typeName.indexOf('.') != -1;
            if (hasPackageName) {
                return typeName;
            } else {
                return defaultPackageName + "." + typeName;
            }
        }

    }

}

为了说明这一点，我创建了两个类，它们都属于默认包：

public class Customer {

}

这

public class Order {

    private Customer customer;

    public Order(Customer customer) {
        this.customer = customer;
    }

    public Customer getCustomer() {
        return customer;
    }

    public void setCustomer(Customer customer) {
        this.customer = customer;
    }

}

是 Order before 任何重新映射：

> javap -private -c Order
Compiled from "Order.java"
public class Order extends java.lang.Object{
private Customer customer;

public Order(Customer);
  Code:
   0:   aload_0
   1:   invokespecial   #10; //Method java/lang/Object."":()V
   4:   aload_0
   5:   aload_1
   6:   putfield    #13; //Field customer:LCustomer;
   9:   return

public Customer getCustomer();
  Code:
   0:   aload_0
   1:   getfield    #13; //Field customer:LCustomer;
   4:   areturn

public void setCustomer(Customer);
  Code:
   0:   aload_0
   1:   aload_1
   2:   putfield    #13; //Field customer:LCustomer;
   5:   return

}

这是Orderafter重新映射的列表（使用com.mycompany作为默认包）：

> javap -private -c Order
Compiled from "Order.java"
public class com.mycompany.Order extends com.mycompany.java.lang.Object{
private com.mycompany.Customer customer;

public com.mycompany.Order(com.mycompany.Customer);
  Code:
   0:   aload_0
   1:   invokespecial   #30; //Method "com.mycompany.java/lang/Object"."":()V
   4:   aload_0
   5:   aload_1
   6:   putfield    #32; //Field customer:Lcom.mycompany.Customer;
   9:   return

public com.mycompany.Customer getCustomer();
  Code:
   0:   aload_0
   1:   getfield    #32; //Field customer:Lcom.mycompany.Customer;
   4:   areturn

public void setCustomer(com.mycompany.Customer);
  Code:
   0:   aload_0
   1:   aload_1
   2:   putfield    #32; //Field customer:Lcom.mycompany.Customer;
   5:   return

}

如您所见，重新映射已更改对 com.mycompany.Order 的所有 Order 引用以及对 com.mycompany.Customer 的所有 Customer 引用代码>.

该类加载器必须加载以下所有类：

• 属于默认包，或
• 使用属于默认包的其他类。

Answer 2

您应该能够使用 ASM 进行一些修改，尽管在以下位置重命名一次包会更容易构建时而不是加载时。

Answer 3

也许将 API 从默认包移动到更合理的位置会更容易？听起来您无权访问源代码。我不确定包是否被编码到类文件中，因此简单地移动 API 类可能值得一试。否则，像 JAD 这样的 Java 反编译器通常会做得很好，因此您可以更改反编译源中的包名称并重新编译。