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

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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

Sun’s implementation of the RMI API.

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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.

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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.

• LatencyInvocations take much longer to complete.

• SecurityArguments/returned value are sent over a network. Is

privacy an issue?

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RMI/JRMP Architecture

• Stubs

• Marshalling

• RMI Threading & Network Connection

Management

• Distributed Garbage Collection

• Naming

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Stubs

• Stub– The client has a proxy for the remote object: the stub.

– The stub implements the remote object’s interface[s].

– 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.

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MyServer

MyServer Stub

RemoteRef[host=myhost, port=1234, ObjID=0]

JRMP<<call>>

CLIENT

MyServerImpl

MyServer ServerSocket[port=1234]

End-point

<<send>>

Object table

<<access>>

JRMP

exported objects

SERVER

<<call>

MyServer instance maps to ObjID=0

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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 <return_type> myMethod( <type> p1, … )throws java.rmi.RemoteException

// declare other remote methods …}

_______________________________________• All parameters & return type are serializable.

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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;

}

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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.

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Implementing the Remote Interface

public class HelloWorldImpl extends UnicastRemoteObject

implements HelloWorld

{

public HelloWorldImpl() throws RemoteException {}

public String helloWorld( String name )

{

return “Hello “ + name + “!”;

}

}

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Implementing the Remote Interface …

public class HelloWorldImpl implements HelloWorld

{

public HelloWorldImpl() throws RemoteException

{

UnicastRemoteObject.exportObject( this );

}

public String helloWorld( String name )

{

return “Hello “ + name + “!”;

}

}

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Implementing the Remote Interface …

• UnicastRemoteObject constructor exports the objectMakes it available for incoming invocations.

• The exportObject method does this explicitly.

• Extending UnicastRemoteObject inherits distributed implementations of: – equals, hashCode, & toString.

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RMI/JRMP Architecture

• Stubs

• Marshalling

• RMI Threading & Network Connection

Management

• Distributed Garbage Collection

• Naming

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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

Foo Copy of FooBytesSerialization Deserialization

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Dynamic Classloading

• We defer discussion of this until later.

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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;}________________________________________________

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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

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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

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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.

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Threading Model

• The JRMP implementation for the server– Instantiates a thread for each connection.

– It listens for its associated client’s 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

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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– doesn’t 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.

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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

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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.