1
CMSC 433 – Programming Language
Technologies and Paradigms
Fall 2008
Java RMI
2
Distributed Computing
• Programs that cooperate and communicate
over a network
– E-mail
– Web server and web client
– SETI @Home
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Java Remote Method Invocation (RMI) in Distributed Computing, Study Guides, Projects, Research of Programming Languages
An overview of java rmi in the context of distributed computing. It covers key features of distributed computation, different approaches, and the implementation of remote method invocation. The document also includes a simple example of a chat server and client using rmi, as well as information on remote objects, stubs, and compiling stubs.
Typology: Study Guides, Projects, Research
Pre 2010
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CMSC 433 – Programming Language
Technologies and Paradigms
Fall 2008
Java RMI
2
Distributed Computing
• Programs that cooperate and communicate
over a network
- Web server and web client
- SETI @Home
3
Key Features of Distrib. Comp.
• Machines are not all the same
- But all adhere to same communication protocol
• Network is “slow”
- Sending a message takes a lot of time
• Network is unreliable
- Machines may join and leave with no warning
- Part of the network may fail 4
Different Approaches to
Distributed Computation
- Connecting via sockets
- E.g., project 1
- Custom protocols for each application
- RPC/DCOM/CORBA/RMI
- Make what looks like a normal function call
- Function actually invoked on another machine
- Arguments are marshalled for transport
- Value is unmarshalled on return
7
Remote Objects
• Object should
- Extend java.rmi.server.UnicastRemoteObject
- Constructor declared to throw RemoteException
- Implement a remote interface
- A remote interface extends java.rmi.Remote
- All methods in a remote interface throw RemoteException - “Something bad happened on the network”
- Side note: actually, don’t need to extend
UnicastRemoteObject, but it’s much easier
8
Remote Interfaces
9
Stubs
• Client only sees the RemoteInterface
- ConcreteObject can have other methods
• Remote objects represented using stub
- Stub sends arguments over network
- Stub receives result back from network 10
Compiling Stubs with rmic*
- Generates stub code for a class
- For 1.1, also generates skeleton class
- Stub on client side communicates with skeleton on remote side
- Skeleton not needed for 1.2+
- And 1.2+ generates position-independent code
- Use -v1.2 if you want
- For 1.1, also generates skeleton class
- Generates stubs for all methods declared in the
class’ Remote interface
- Other methods don’t get a stub
- *Don’t need to use rmic anymore
13
Stub Code
• Objects contain both data and code
- When you receive a remote object, you need
the stub for that remote object
• Solution #1: All clients have stub code on
their classpath
- Or stub code for another class with same
remote interface
14
Downloading Code
• Solution #2: Provide a code base where
stub code for objects can be downloaded
java -Djava.rmi.server.codebase= ...
- Specifies location of classes originating from
this jvm
- URL can be, e.g., http:// or file:/
15
Getting the First Remote Object
• Can make objects available in RMI registry
- Each object has a name (that you specify)
- Registry listens on a port (1099 default)
• Naming.lookup(url) gets object from reg.
- E.g., Naming.lookup(“rmi://localhost/Chat”);
- Use to get first reference to remote object
- Don’t need to lookup objects returned by
remote methods
16
Starting an RMI Registry
• Method 1: Separate RMI registry process
- Command rmiregistry
- Run with stubs in classpath, or specify codebase
- Listens on port 1099 by default
• Method 2: Start in same JVM
- LocateRegistry.createRegistry(int port)
- Advantage: dies when your program dies
- No registries lying around on machine
19
Server
interface Server extends Remote { Connection logon(String name, Client c) throws RemoteException; } 20
Connection
interface Connection extends Remote { / Say to everyone / void say(String msg) throws RemoteException; / ** Say to one person / void say(String who, String msg) throws RemoteException; String [] who() throws RemoteException; void logoff() throws RemoteException; }
21
Client
interface Client extends Remote { void said(String who, String msg) throws RemoteException; void whoChanged(String [] who) throws RemoteException; } 22
Server’s Remote Object creation
ServerImpl Server s = new ServerImpl(); Hosted Remote Objects s
Server
Object added to table because it implements extension of Remote interface
25
Client looks up Server
Server s = (Server) Naming.lookup (“//host/ChatServer”); s ServerImpl Stub Hosted Remote Objects ServerImpl
Client RMI Registry Server
ChatServer ServerImpl Stub lookup returns stub 26
After lookup finished
ClientImpl Hosted Remote Objects c s ServerImpl Stub Hosted Remote Objects ServerImpl
Client Server
27
Invokes remote Server method
Connection conn = s.logon(“Bill”, c); s ServerImpl Stub
Client
Stub code for remote logon call String “Bill” Stub for c Method: logon … to server process logon c ClientImpl 28
Receives remote call
Server
(Skeleton) code for remote logon call String “Bill” Stub for c Method: logon … from client process Hosted Remote Objects ServerImpl “Bill” ClientImpl Stub c unmarshalled arguments
31
Receives the result
s ServerImpl Stub
Client
Stub code for remote logon call Stub for conn Return value: … from server process logon Conn Stub conn unmarshalled return value 32
Security Manager
- When using a code base, we must download stub
code from a remote site. This is potentially risky
- Need to limit what downloaded code could do
- Must install a Security Manager before you download any code from RMI code bases
- Can use System.setSecurityManager( new RMISecurityManager());
33
Policy Files
• In addition to security manager, need to
specify a security policy
grant { permission java.net.SocketPermission “: 1024-65535”, “connect,accept”; permission java.net.SocketPermission “:80”, “connect”; };**
• Set security policy when JVM started
- java -Djava.security.policy=