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Java RMI, JAX-RPC and
JWSDP
B. Ramamurthy
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Inside RMI
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http://java.sun.com/j2se/1.5.0/docs/index.html
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Basic RMI classes: /usr/java1.1/src/java/rmi
• java.rmi.registry.*
• java.rmi.Naming class (static/class methods)
• java.rmi.Remote interface (marker interface)
• java.rmi.server.*
• Default RMI port 1099
• Both lookup from local and remote are
acceptable.
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Implementation of RMI (5.2.5)
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The role of proxy and skeleton
in remote method invocation
server
client
object A proxy for B
Request
skeleton
& dispatcher
for B’s class
remote
object B
Reply
Remote Communication
reference module module
servant
CommunicationRemote reference
module
module
Object A invokes a remote object in Object B for which it holds a remote object
reference.
“System Model”
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RMI Internals: Communication
Module
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Carries out request-reply protocol;
On the client side {message type, message id,
remote reference to object} are gathered and
sent out. At most once invocation semantics;
On the server side, it gets local reference for
remote reference from remote reference
module, invokes a dispatcher with this
reference.
See UnicastRemote (implements
UnicastRemote)
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RMi Internals: Remote
Reference module
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Responsible for translating between
local and remote object references and
for creating remote object references.
A remote object table has a mapping
between local and remote references. A
table at server (entry for object ref for B)
and a table at client (entry for object ref
for proxy B).
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RMI Internals: Remote
References
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Action of remote reference module: See
RemoteRef.java interface
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When a remote object is to be passed as argument or
result for the first time, the remote ref is asked to
create a remote ref object which is added to the table.
When a remote object reference arrives in a request
or reply, the remote ref module is asked for
corresponding local object ref, which may either a
proxy or remote object. If it is not in the table RMI
runtime creates it and asks remote ref module to add it
to the table.
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RMI Internals: RMI software
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Layer of software between application level objects and communication and
remote reference modules: “Middleware”
Proxy: provides remote access transparency. One proxy for every remote
object in the client.
Dispatcher: A server has one dispatcher and skeleton for each class
representing a remote object.
• It receives request message from comm. Module
• It used MessageId to select appropriate method in skeleton.
• Proxy and dispatcher use same MessageId.
Skeleton: A class of remote object has a skeleton that implements of the remote
interface. All the access dependencies are hidden in this class. A remote object
has a servant that directly implements the methods. Java 5 creates this
dynamically.
Proxies, dispatcher and skeleton are automatically generated by interface
compiler.
Binder: binds textual names to remote object references. RMiRegistry is a
binder; Naming class; see fig.5.13
Server Threads: one thread per invocation
Distributed garbage collection: See Andrew Birell’s paper [1995].
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RMI Internals: Distributed
Garbage Collection
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Based on reference counts.
Local garbage collectors and a distributed support.
Each server holds the list of processes that hold remote object references: for
example, B.Holders
When a client C first receives a remote reference to a particular remote object,
say B, it makes a addRef(B) invocation to server of that remote object and then
creates proxy; server adds C to B.Holders.
When client C’s garbage collector finds that proxy is no longer reachable (ref
count), it makes a removeRef(B) invocation to server and then deletes proxy;
the server removes C from B.Holders.
When B.Holders is empty, server’s local garbage collector will reclaim the
space occupied B unless there are any local holders.
These extra calls for updates occur during proxy creation and deletion and do
not affect normal opertion.
Tolerates communication failures: addRef() and removeRef() are idempotent:
effects of N > 0 identical requests is the same as for a single request.
If addRef() fails with an exception, proxy is not created, removeRef() is
transmitted; removeRef() failures are dealt with by “leases” (Jini kind).
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RMI Internals: Use of Reflection
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What is reflection? See Reflection package
Reflection enables Java code to discover information
about the fields, methods and constructors of loaded
classes, and
To use reflected fields, methods, and constructors to
operate on their underlying counterparts on objects,
within security restrictions.
http://java.sun.com/docs/books/tutorial/reflect/class/index.html
 Reflection feature allowed for dynamic creation of skeleton and proxy
in Java 2 version onwards.
 Read more about reflection model of computing.
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A Little bit of Reflection
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Method class, invoke method
Invoke method requires two parameters:
first the object to receive invocation,
second an array of Object parameters.
Invoke executes the method on the
object and returns result as Object.
Method m;
Object result = M.invoke(String, Args);
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Using Reflection in RMI
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Proxy has to marshal info. about a method and its
arguments into a request message.
For a method it marshals an object of class Method into
the request. It then adds an array of objects for the
method’s arguments.
The dispatcher unmarshals the Method object and its
arguments from request message.
The remote object reference is obtained from remote ref.
table.
The dispatcher then calls the “invoke” method on the
object reference and array of arguments values.
After the method execution the dispatcher marshals the
result or any exceptions into the reply message.
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JAX-RPC
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JAX-RPC (The Java API for XML-based RPC) is
designed to provide a simple way for developers to
create Web services server and Web services client.
Based on remote procedure calls; so the
programming model is familiar to Java developers
who have used RMI or CORBA.
Major difference between RMI and JAX-RPC is that
messages exchanged are encoded in XML based
protocol and can be carried over a variety of transport
protocols such as HTTP, SMTP etc.
You can use JAX-RPC without having to be an expert
in XML, SOAP, or HTTP.
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The JAX-RPC Programming
Model
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Services, ports and bindings
JAX-RPC web service servers and clients
JAX-RPC service creation
JAX-RPC client and server programming
environments
Stubs and ties
Client invocation modes
Static and dynamic stubs and invocation
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Services, ports and bindings
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Service endpoint interface (SEI) or service
endpoint that defines one or more operations
that the web service offers.
Access to an endpoint is provided by binding it
to a protocol stack through a port.
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A port has an address that the client can use to
communicate with the service and invoke its
operations.
An endpoint can be bound to different ports
each offering a different suite of protocols for
interaction.
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Endpoint, Port and binding
Web service
endpoint
Port1
SOAP1.1
Over http
port2
SOAP 1.1 over
https
port3
Other. Ex:
ebXML over
SMTP
Web services Client
https 1.1 transport
soap1.1 messages
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Web Service Clients and Servers
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JAX-RPC maps a
• web service operation to a java method call.
• service endpoint to a Java Interface.
Thus one way to begin implementation of a web service
in JAX-RPC is to define a Java interface with a method
for each operation of the service along with a class that
implements the interface. Of course, following the rules of
remote invocation etc.
Now visualize client/server invocation in the same
address space and lets compare it with remote
invocation.
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Local Date Service
//server
public class DataService {
public Data getDate() {
return new Date();}
//client
Public class Appln {
public static void main (..) {
DateService instance = new DateService();
Date date = instance.getDate();
System.out.println (“ The date is” + date);
}
 In the case of the remote call a layer of software is used to
convey the method call from client to server. This layer of
software is provided by JAX-RPC runtime.
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JAX-RPC service creation
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A service definition describes the operations that it
provides and the data types that they require as
argument and provide as return values.
This definition can be made available as a document
written in WSDL.
From a WSDL document, JAX-RPC can generate the
Java code required to connect a client to a server leaving
one to write only the logic of the client application itself.
Since WSDL is language independent the server can be
in .net, Jax-rpc or any other compatible platform.
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JAX-RPC service creation
(contd.)
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Define the service a Java interface.
Generate WSDL using the tools provided
with JAX-RPC package.
Advertise it in a registry for the client to
lookup and import it.
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Client and Server Programming
Environment
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JAX-RPC API is distributed over a set of
packages:
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javax.xml.rpc
javax.xml.rpc.encoding
javax.xml.rpc.handler
javax.xml.rpc.handler.soap
javax.xml.rpc.holders
javax.xml.rpc.server
javac.xml.rpc.soap
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Stubs and Ties
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Client Side: Stub object has the same methods as the service
implementation class.
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Client application is linked with the stub.
When it invokes a method stub delegates the call to the JAX-RPC runtime so
that appropriate SOAP message can be sent to the server.
On completion the result return back in the reverse path as above.
Server side:
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Message received must be converted into a method call on actual service
implementation. This functionality is provided by another piece of glue called tie.
Tie extracts method name and parameter from SOAP message.
Tie also converts the result of the method call back into a response message to
be returned to client JAX-RPC runtime.
JAX-RPC comes with tools to generate these.
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Client Invocation Modes
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Synchronous request-response mode
(tightly coupled).
One-way RPC (loosely coupled): no
value returned, no exception thrown,
need to bypass stub layer, use Dynamic
Invocation Interface (DII).
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SEI Invocation Code
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Service End Point (SEI) invocation code:
Stub stub = (Stub)(new MyHelloService_Impl().getHelloIFPort());
stub._setProperty(javax.xml.rpc.Stub.ENDPOINT_ADDRESS_PRO
PERTY,
"http://localhost:8080/hello-jaxrpc/hello");
HelloIF hello = (HelloIF)stub;
resp = hello.sayHello(request.getParameter("username"));
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