Serialization and reflection

Question

I have a client/server application that talks to other instances of itself through ObjectStreams.
One instance of the application is host and another is client.
It is used to play multiplayer games.
Each game is contained in a jar file, from which the main-class is loaded in by the application at runtime with uRLClassLoader.loadClass("name of the main-class"). That main-class extends GameInterface.
The host tells the client which game it is running through a String.
The client then checks all the loaded GameInterfaces to see if it has the required game.
Once connnected, the server and client send Serializable objects to each other.
The problem is that some of those Serializable objects are classes from within a game's jar file (which are loaded in with reflection at runtime).
The host can get DiceSettings through the GameInterface of Dice for example. DiceSettings is in Dice's jar file.
But when the client receives DiceSettings in it's objectstream, it doesn't recognize it at the client:
java.lang.ClassNotFoundException: simplegamenet.games.dice.DiceSettings
1) How can I tell the ObjectInputStream that it also should use my URLClassCloader to create classes?
2) Which are the rules to be able to send an object through an objectStream?
Thanks for any and all help :)

Answer 1

Hi,
I worked myself on such an application. It works fine. Here is some code:
- for my class loader:
class JarClassLoader extends ClassLoader
private HashMap resMap=new HashMap(8000,0.9f);
JarClassLoader()
void addJar(byte[] jar)
try
JarInputStream jis=new JarInputStream(new ByteArrayInputStream(jar));
byte[] buffer=new byte[10240];
for(JarEntry je=jis.getNextJarEntry();je!=null;je=jis.getNextJarEntry())
String name=je.getName();
try
ArrayList data=new ArrayList();
int size=0;
for(int len=jis.read(buffer);len>0;len=jis.read(buffer))
byte[] real=new byte[len];
System.arraycopy(buffer,0,real,0,len);
data.add(real);
size+=len;
byte[] finalData=new byte[size];
int crt=0;
for(int f=0;f<data.size();f++)
byte[] chunk=(byte[])data.get(f);
System.arraycopy(chunk,0,finalData,crt,chunk.length);
crt+=chunk.length;
resMap.put(name,finalData);
catch(Exception e)
System.out.println(e);
jis.close();
catch(Throwable t)
t.printStackTrace(System.out);
public InputStream getResourceAsStream(String name)
byte[] bytes=(byte[])(resMap.get(name));
if(bytes==null)
return null;
return new ByteArrayInputStream(bytes);
public Class findClass(String name) throws ClassNotFoundException
String name1=name.replace('.','/').concat(".class");
byte[] b=(byte[])(resMap.get(name1));
if(b==null)
throw new ClassNotFoundException("Class:"+name+" not found");
Class c=defineClass(name,b,0,b.length);
return c;
- for the launcher:
ois=the ObjectInputStream
jcl=the JarClassLoader
byte[] jar=downloadFile("myjar.jar",ois);
String command=ois.readObject().toString();
jcl.addJar(jar);
runCommand(command,args);
private void runCommand(final String command,final String[] args)
try
Class c=Class.forName(command,true,jcl);
Method m=c.getMethod("main",new Class[] {args.getClass()});
m.invoke(null,new Object[] {args});
catch(Throwable t)
t.printStackTrace(System.out);
}