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Chapter 2: TroubleShooting Jini

Contents

• The Chapter that Shouldn't Exist
• Java Packages
• Jini Versions
• Jini Packages
• Lookup Service
• RMI Stubs
• Debugging

This chapter looks at some of the problems that can arise in a Jini system. Most of them are configuration problems of some kind. This chapter is woefully incomplete :-((( Contributions welcome!!!

Jini is advertised as ``network plug and play''. This carries the idea of ``zero administration'' where you buy a device, switch it on and voila it is there and available. Well, this may happen in the future, but right now there are a number of back-room games that you have to succeed at. Once you have won at these, ``network plug and play'' does work, but if you lose at any stage then it can be all uphill!

What is hard is issues of getting the right files in the right places with the right permissions. About 50% of the messages in the Jini mailing list are all about these configuration problems. These shouldn't occur, and that is why this is ``The Chapter that Shouldn't Exist''.

This is the second chapter in this book, so right now you shouldn't have managed to fail at anything! Each of the early sections contains instructions on what to do to get the example programs working. So skip on to the next chapters, but come back here when things go wrong. Your luck may vary: I got quite a reasonable way on my first attempts without problems, and some are even luckier. Some aren't...

Typical error:

Exception in thread "main" java.lang.NoClassDefFoundError: 
         basic/InvalidLookupLocator

Most of the code in this tutorial is organised into packages. To run the examples the classes must be accessible from your class path. For example, one of the programs in the basic directory is InvalidLookupLocator.java. This defines the class InvalidLookupLocator in the package basic. The program must be run using the fully qualified path name, as in

java basic.InvalidLookupLocator

In order to find this class, the classpath must be set correctly for the Java runtime. If you have copied the zip file classes.zip then the class files for this tutorial are in there. You only need to reference this

CLASSPATH=classes.zip:...

If you have downloaded the source files, then they are all in subdirectories such as basic, complex, etc. After compilation the class files should also be in these subdirectories, such as basic/InvalidLookupLocator.class. An alternative to using classes.zip is to set the classpath to include the directory containing those subdirectories. For example, if the full path is say /home/jan/classes/basic/InvalidLookupLocator.class, then set classpath to

CLASSPATH=/home/jan/classes:...

An alternative to setting the CLASSPATH environment variable is to use the -classpath option to the Java runtime engine

java -classpath /home/jan/classes basic.InvalidLookupLocator

There are currently two versions of Jini: 1.0 and a version of 1.1 in alpha. The core classes are all the same. The only changes in version 1.1 for the programmer are that some classes from Jini 1.0 have been specified better and are in different packages, and that some classes are new.

The main ones that have changed are

1. JoinManager
2. LeaseRenewalManager
3. ServiceIDListener

The main new ones are

1. LookupLocatorDiscovery
2. LookupDiscoveryManager
3. ClientLookupManager

If you get syntax errors or runtime errors relating to these, then it is possible that you are using Jini 1.0 instead of Jini 1.1. If you get ``deprecated'' warnings, then it is likely that you are using the Jini 1.0 classes in a Jini 1.1 environment. The old classes are supported for now, but are not approved.

Typical error

Exception in thread "main" java.lang.NoClassDefFoundError: 
             net/jini/discovery/DiscoveryListener

The Jini class files are all in jar files. The Jini distribution has them in a subdirectory lib when it is unpacked. There are a whole bunch of these jar files:

jini-core.jar         mahalo-dl.jar         sun-util.jar
jini-examples-dl.jar  mahalo.jar            tools.jar
jini-examples.jar     reggie-dl.jar
jini-ext.jar          reggie.jar

The jar file jini-core.jar contains the major packages of Jini

net.jini.core
net.jini.core.discovery
net.jini.core.entry
net.jini.core.event
net.jini.core.lease
net.jini.core.lookup
net.jini.core.transaction

The jar file jini-ext.jar contains a set of packages which are not in the core, but are still heavily used

net.jini.admin
net.jini.discovery
net.jini.entry
net.jini.lease
net.jini.lookup
net.jini.lookup.entry
net.jini.space

The jar file sun-util.jar contains the packages from the com.sun.jini hierarchy. These contain a number of ``convenience'' classes that are not essential but can be useful. These are less frequently used.

A compile or run of a Jini application will typically have an environment set something like

JINI_HOME=wherever_Jini_home_is
CLASSPATH=.:$JINI_HOME/lib/jini-core.jar:$JINI_HOME/lib/jini-ext.jar

Typical error

java.rmi.activation.ActivationException: ActivationSystem not running;
nested exception is: 
        java.rmi.NotBoundException: java.rmi.activation.ActivationSystem
java.rmi.NotBoundException: java.rmi.activation.ActivationSystem

Typical error

java.rmi.StubNotFoundException: 
         Stub class not found: rmi.FileClassifierImpl_Stub; 
nested exception is: 
        java.lang.ClassNotFoundException: rmi.FileClassifierImpl_Stub

Many of the examples export services as remote RMI objects. These objects are subclasses of UnicastRemoteObject. What gets exported is not the object itself, but a stub that will act as proxy for the object (which continues to run back in the server). The stub has to be created using the rmic compiler as in

rmic -v1.2 -d . rmi.FileClassifierImpl

Another typical error is

java.rmi.ServerException: RemoteException occurred in server thread; nested exception is: 
        java.rmi.UnmarshalException: error unmarshalling arguments; nested exception is: 
        java.lang.ClassNotFoundException: rmi.FileClassifierImpl_Stub

Debugging a Jini application is difficult because there are so many bits to it, and these bits are all running separately: the server for a service, the client, lookup services, remote activation daemons and HTTP servers. There are a few (not many) errors within the Jini objects themselves, but more importantly many of these objects are implemented using multiple threads and the flow of execution is not always clear. There are no magic ``debug'' flags that can be turned on to show what is happening.

On either the client or service side, a debugger such as jdb can be used to step through or trace execution of each one. Lots of print statements help too. There are also three flags that can be turned on to help:

java -Djava.security.debug=access \
     -Dnet.jini.discovery.debug=1 \
     -Djava.rmi.server.logCalls=true ...

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