Java RMI Essentials

1
Java RMI Essentials

• Based on Mastering RMI
• Rickard Oberg

2
Essentials of Remote Invocation

• What is RMI?
• The Principles of RMI
• How Does RMI Differ from Ordinary Java
• RMI/JRMP Architecture
• Stubs
• Marshalling
• RMI Threading Network Connection Management
• Distributed Garbage Collection
• Naming
• Summary

3
What is RMI?

• RMI is a specification (API) for accessing
  objects from a remote JVM.
• What is specified?
• How objects are to be coded.
• How objects can be located invoked.
• How parameters returned values are passed.
• Java Remote Method Protocol (JRMP) is Suns
  implementation of the RMI API.

4
The Principles of RMI

• Meta-principle
• Make RMI like MI as much as possible.
• Objects are invoked by calling methods.
• Expose interfaces not implementation.
• Exceptions report errors in the computation.
• GC determines the lifecycle of objects.
• Get classes that are not part of system classpath
  via classloading.

5
How Does RMI Differ from Local MI?

• Remote exceptions
• Remote methods throw java.rmi.RemoteExeption.
• Pass by value
• All arguments are pass-by-value.
• Serializing may hurt performance for large
  objects.
• Latency
• Invocations take much longer to complete.
• Security
• Arguments/returned value are sent over a network.
  Is privacy an issue?

6
RMI/JRMP Architecture

• Stubs
• Marshalling
• RMI Threading Network Connection Management
• Distributed Garbage Collection
• Naming

7
Stubs

• Stub
• The client has a proxy for the remote object the
  stub.
• The stub implements the remote objects
  interfaces.
• The RMI compiler (rmic) generates the stub.
• Remote methods invoked by the client are
  delegated to the JRMP engine.
• The JRMP forwards the call to the server.
• The server executes the method.
• The result is returned to the client.

8
CLIENT
MyServer
MyServer Stub
JRMP
RemoteRef hostmyhost, port1234, ObjID0
ltltcallgtgt
ltltsendgtgt
SERVER
JRMP
End-point
ltltcallgt
MyServer
ServerSocket port1234
ltltaccessgtgt
MyServerImpl
Object table
exported objects
MyServer instance maps to ObjID0
9
The Remote Interface

• It is a set of remotely invoked methods.
• It has the following characteristics
• _______________________________________
• public interface MyRemoteInterface
• extends java.rmi.Remote // possibly indirectly
• public ltreturn_typegt myMethod( lttypegt p1, )
• throws java.rmi.RemoteException
• // declare other remote methods
• _______________________________________
• All parameters return type are serializable.

10
The Hello Interface

• package masteringrmi.helloworld.interfaces
• import java.rmi.Remote
• import java.rmi.RemoteException
• public interface HelloWorld extends Remote
• public String helloWorld( String name )
• throws RemoteException

11
The Hello Interface

• Style create a contract package
• It contains the contract between client
  server
• Remote interfaces of server objects
• Application exceptions throwable by any remote
  interface
• Data container classes for data moved between
  client server.
• Client server get contract classes
  interfaces.
• Server also gets implementation classes.

12
Implementing the Remote Interface

• public class HelloWorldImpl extends
  UnicastRemoteObject
• implements HelloWorld
• public HelloWorldImpl() throws RemoteException
  
• public String helloWorld( String name )
• return Hello name !

13
Implementing the Remote Interface

• public class HelloWorldImpl implements HelloWorld
• public HelloWorldImpl() throws RemoteException
• UnicastRemoteObject.exportObject( this )
• public String helloWorld( String name )
• return Hello name !

14
Implementing the Remote Interface

• UnicastRemoteObject constructor exports the
  object
• Makes it available for incoming invocations.
• The exportObject method does this explicitly.
• Extending UnicastRemoteObject inherits
  distributed implementations of
• equals, hashCode, toString.

15
RMI/JRMP Architecture

• Stubs
• Marshalling
• RMI Threading Network Connection Management
• Distributed Garbage Collection
• Naming

16
Marshalling

• Marshalling creates a byte from an object.
• Unmarshalling does the reverse.
• To marshal, Java serializes the object.
• To unmarshal, Java deserializes the byte.
• public class foo implements Serializable

Copy of Foo
Foo
Bytes
Serialization
Deserialization
17
Dynamic Classloading

• We defer discussion of this until later.

18
Security

• To dynamically download stubs, the client
• Sets a security manager
• ________________________________________________
  ______
• import java.rmi.RMISecurityManager
• . . .
• if ( System.getSecurityManager() null )
  System.setSecurityManager( new
  RMISecurityManager() )
• ________________________________________________
• Has a policy file that permits downloading
• ________________________________________________
• grant
• permission java.security.AllPermission
• ________________________________________________

19
Class Versioning

• Server changes are propagated to clients that
  subsequently download the stub.
• What about running clients that already have the
  stub?
• Basically, this is a problem.
• serial version UID can be used to detect
  incompatibilities.
• Jini further ameliorates the version problem.

20
RMI/JRMP Architecture

• Stubs
• Marshalling
• RMI Threading Network Connection Management
• Distributed Garbage Collection
• Naming

21
RMI Threading Network Connection Management

• Since RMI on the same remote object may execute
  concurrently, a remote object implementation
  needs to make sure its implementation is
  thread-safe.
• The RMI specification, section 3.2

22
Network Connections

• RMI specifies socket factory interfaces to get
  sockets on the client server
• Java.rmi.server.RMIClientSocketFactory
• Java.rmi.server.RMIServerSocketFactory
• Override the default implementation
• (to provide encryption, authentication, )
• Selecting an features at run time is desirable
  Some jobs want encryption, some do not.
• Custom implementations are discussed later.

23
Threading Model

• The JRMP implementation for the server
• Instantiates a thread for each connection.
• It listens for its associated clients calls.
• It handles each call to completion.
• The JRMP implementation for the client
• Concurrent calls from a client cause concurrent
  threads connecting to the server.
• This may swamp a server.
• If your client makes concurrent calls, you may
  want a different implementation.

24
RMI/JRMP Architecture

• Stubs
• Marshalling
• RMI Threading Network Connection Management
• Distributed Garbage Collection
• Naming

25
Distributed Garbage Collection

• RMI has distributed garbage collection (DGC).
• Server tracks clients who have its stub.
• Server keeps a count of such clients.
• Count is decremented when client
• explicitly relinquishes reference OR
• doesnt renew its lease (default 10 min.), e.g.,
  client crashed
• Leasing is due to Miller Drexler, in
• Incentive Engineering for Computational Resource
  Management. K. E. Drexler M. S. Miller, B. A.
  Huberman (ed.), (Studies in Computer Science
  Artificial Intelligence), 1988.
• If local remote references 0, it is garbage.

26
The Unreferenced Interface

• A server can implement Unreferenced
• It has 1 method unreferenced.
• It is called when there are no remote references
  to the object.
• Being bound in the RMI registry counts as a
  remote reference.
• It must unbind itself before unreferenced can be
  invoked.

27
RMI/JRMP Architecture

• Stubs
• Marshalling
• RMI Threading Network Connection Management
• Distributed Garbage Collection
• Naming

28
Naming

• Rmiregistry allows
• A server to store its (serialized) stub on a web
  server
• A client to download deserialize the stub.
• The essential information the url of the
  serialized stub file.