代码之家 › 专栏 › 技术社区 › C. Ross trotttrotttrott

用不同的包名称动态加载Java中的类

dynamic-class-loaders class java

C. Ross trotttrotttrott · 技术社区 · 15 年前

是否可以在Java中加载类和“假”类的包名/规范名?我试过这样做,很明显,但是我得到了一个“类名不匹配”的消息 ClassDefNotFoundException .

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

./com/DefaultPackageClass.class

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

我的当前代码如下:

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); 
    }

}

3 回复 | 直到 15 年前

Community CDub 8 年前

As Pete mentioned ,这可以使用ASM字节码库来完成。实际上,该库实际上附带了一个类,专门用于处理这些类名重新映射( RemappingClassAdapter )下面是一个使用此类的类加载器示例:

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 {

}

和

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;
    }

}

这是 Order 之前任何重新映射:

> 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 秩序 之后重新映射(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

}

如你所见,重新映射已经改变了所有 秩序 参考文献 com.mycompany.Order 以及所有 Customer 参考文献 com.mycompany.Customer .

这个类加载器必须加载所有的类:

属于默认包,或
使用属于默认包的其他类。

Pete Kirkham 15 年前

你应该能用 ASM 尽管在构建时而不是在加载时对包进行一次重命名会更容易。

FelixM 15 年前

也许将API从默认包移到更合理的位置会更容易?听起来您没有访问源代码的权限。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反编译程序通常会做得很好,所以您可以更改反编译源中的包名并重新编译它。