[英]Dynamic loading a class in java with a different package name
是否可以在Java中加載一個類並“偽造”類的包名稱/規范名稱? 我嘗試這樣做,顯而易見的方法,但我在ClassDefNotFoundException
得到一個“類名不匹配”的消息。
我這樣做的原因是我正在嘗試加載一個在默認包中編寫的API,以便我可以直接使用它而不使用反射。 代碼將在表示包和包名稱導入的文件夾結構中針對類進行編譯。 即:
./com/DefaultPackageClass.class
// ...
import com.DefaultPackageClass;
import java.util.Vector;
// ...
我目前的代碼如下:
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);
}
}
正如Pete所說 ,這可以使用ASM字節碼庫來完成。 實際上,該庫實際上附帶了一個專門用於處理這些類名重新映射的類( RemappingClassAdapter
)。 以下是使用此類的類加載器的示例:
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;
}
}
}
}
為了說明,我創建了兩個類,這兩個類都屬於默認包:
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;
}
}
這是任何重新映射之前的Order
列表:
> 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 }
這是重新映射后的Order
列表(使用com.mycompany
作為默認包):
> 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 }
正如你所看到的,重新映射改變了這一切Order
到引用com.mycompany.Order
和所有的Customer
引用com.mycompany.Customer
。
這個類加載器必須加載以下所有類:
您應該能夠使用ASM來解決問題 ,但是在構建時而不是在加載時更容易重命名一次。
也許將API從默認包移動到更合理的位置會更容易? 聽起來你無法訪問源代碼。 我不確定包是否被編碼到類文件中,所以簡單地移動API類可能值得一試。 否則,像JAD這樣的Java反編譯器通常做得很好,因此您可以在反編譯源中更改包名稱並再次編譯。
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