问题
Is it possible to load a class in Java and 'fake' the package name/canonical name of a class? I tried doing this, the obvious way, but I get a "class name doesn't match" message in a ClassDefNotFoundException.
The reason I'm doing this is I'm trying to load an API that was written in the default package so that I can use it directly without using reflection. The code will compile against the class in a folder structure representing the package and a package name import. ie:
./com/DefaultPackageClass.class
// ...
import com.DefaultPackageClass;
import java.util.Vector;
// ...
My current code is as follows:
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);
}
}
回答1:
As Pete mentioned, this can be done using the ASM bytecode library. In fact, that library actually ships with a class specifically for handling these class name re-mappings (RemappingClassAdapter). Here is an example of a class loader using this class:
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;
}
}
}
}
To illustrate, I created two classes, both of which belong to the default package:
public class Customer {
}
and
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;
}
}
This is the listing of Order before any re-mapping:
> 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
}
This is the listing of Order after remapping (using com.mycompany as the default package):
> 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
}
As you can see, the remapping has changed all Order references to com.mycompany.Order and all Customer references to com.mycompany.Customer.
This class loader would have to load all classes that either:
- belong to the default package, or
- use other classes that belong to the default package.
回答2:
You should be able to knock something up with ASM, though it would be easier to do the package rename once at build time rather than at load time.
回答3:
Maybe it would be easier to move the API from the default package into a more reasonable spot? It sounds like you don't have access to the source code. I am not sure if the package is encoded into class files, so simply moving the API class might be worth a try. Otherwise, Java decompilers like JAD usually do a good job, so you could change the package name in the decompiled source and compile it again.
来源：https://stackoverflow.com/questions/2897685/dynamic-loading-a-class-in-java-with-a-different-package-name