Loading Xlet via .jar

Question

Hi everybody,
i've got an Xlet containing about 100 classes and some resources. Since the startup-time is beyond the pale when files are loaded separetely by the object carousel, I want to load them with some .jar-files.
Therefore I wrote a custom ClassLoader that overrides the loadClass(String,boolean)-method as required in JDK1.1.8, reads the .jar as dsmccObject and defines/resolves the class. And everything's working so far.
part of sample code for loading from repository:
<code>
DSMCCObject dsmccZipFile = new DSMCCObject("test1.jar");
dsmccZipFile.asynchronousLoad(this);
FileInputStream fi = new FileInputStream(dsmccZipFile);
ZipInputStream zis = new ZipInputStream(fi);
ZipEntry entry = null;
ByteArrayOutputStream baos = new ByteArrayOutputStream();
while((entry = zis.getNextEntry()) != null){
     String entryName = entry.getName();
     if(entryName.equals(className+".class")){
          buffer = new byte[1024];
          int len=0;
          while((len=zis.read(buffer)) != -1){
               baos.write(buffer, 0, len);
          zis.closeEntry();
          zis.close();
          byte[]classBytes = baos.toByteArray();
          baos.close();
          return classBytes;
</code>
Cause I'm a newbie to classLoaders, I have the following problem:
The classes from the jar files are not known at build-time, so that I'm not able to do typecasting and calling the classes' methods.
The only way I handled the problem was the ReflectionAPI.
<code>
Class c = mcl.loadClass("test", true);
Object o = c.newInstance();
Method method = c.getMethod("aMethod", new Class[0]);
method.invoke(o, new Object[0]);
</code>
It works, but i don't like this solution, since it will result in complex code and the reflection's performance is not so high in comparison to direct method calls.
Another solution would probably be a common known interface, which calls methods, but that's worse i think and i had to implement the interface in all classes.
=> my questions:
1) Is there a common solution for jar-loading in MHP (i'm surely not the only one with a bad startup-time if loading files seperately)?
2) If not: is there a better solution than mine (using the reflection api) (the source will be the same: initial ClassLoader.class and .jar's send from same source over the oc)?
3) Should I extend org.dvb.lang.DVBClassLoader (I simply extended java.lang.ClassLoader) and which parentClassLoader should I use as second parameter? I tested it once with the Xlet's ClassLoader (getClass().getClassLoader()) but the problem is of course still the same.
4) Is there a reason that there is no jar support in the mhpAPI / environments (resp. those i know)? I simply used ZipInputStream instead of JarInputStream. Are jar classes available for download anywhere?
THX
regards,
Ben

Answer 1

Thanks to desperado for pointing out that you couldn't load classes from JAR files. I thought that was the case, but couldn't find where in the spec said it, so I assumed I was wrong. Ho hum.
To answer your questions:
- caching can be done on either the module level of the file level. MHP is pretty flexible about this, and doesn't define how much cache will be present (having no cache at all is acceptable, but most boxes have some)
- module arrangement will not always help, but it does make things easier. What you can do in DSM-CC that you can't do in HTTP is change the repetition rate of modules, so that commonly-used modules are transmitted more often (and thus have less latency) than less commonly used modules. Of course, this does affect the time needed to transmit the entire carousel.
- jars work pretty well in an internet app, but they're much less use in a DTV environment because you're transmitting your apps in a carousel. See below for why this is the case.
- you are right that you have to load files separately and not from JAR files. Loading from JAR files is not interoperable.
Loading classes from a JAR file would be slower because you have to wait for the entire file to load before you get any classes out of it. Depending on the size of your file, this can have a number of problems:
1) you need to load the entire JAR file. This means that you have to load all of the files, not just the ones you need, which takes longer. You're still only transmitting your object carousel at the same speed.
2) using modules that group together your files in a logical way may make better use of the DSM-CC cache in the receiver. If the receiver caches modules, then grouping files that are used together in the same module means that you will have less latency when loading files froma cached module. This gives you some of the benefits if using a JAR file in a more flexible way.
3) DSM-CC can compress the data in your files before transmitting it, so using a JAR file won't actually help you much in terms of reducing space.
The only tutorials that I know of that discuss DSM-CC are mine and the ETSI documentation. If you find any more, please let me know.
Steve.