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Java RMI, JAX-RPC and
JWSDP
B. Ramamurthy
1
Inside RMI
• http://java.sun.com/j2se/1.5.0/docs/index.html
• 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
• 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
• 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
• Action of remote reference module: See
RemoteRef.java interface
– 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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•
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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.
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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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•
•
•
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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
• 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.
• Skeleton has been deprecated since JDk1.4.x
• Read more about reflection model of computing.
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A Little bit of Reflection
• 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
• 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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Putting it all together
Server side:
 Write a an interface and implement it.
Implements Remote,
 Inside code publishes the object by (exporting
to runtime) and by registering it.
Client Side:
 Code: look up server object name from the
registry {host,port};
 Invoke opertions.
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Lifecycle of a remote call
1. First time, an operation is invoked, remote
object reference is obtained from remote
registry, addRef() is sent to remote server, an
entry made in the local ref table and proxy is
created.
2. Proxy has message ids while the client’s ref
table has remote object reference.
3. Remote ref, method id and arguments are
marshaled into a message and sent across via
the communication module.
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Lifecycle of a remote call (contd.)
• On the server side RMI runtime maps the
remote reference to a local object.
• Unmarshalls the operation and parameters and
uses reflection to “invoke” the method on the
object reference.
• The result is marshaled back into the response
and sent back to the caller.
• “Skeleton” that includes the “dispatch” is
subsumed into the RMI runtime in the latest
versions of Java.
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Critique of RMI (Sun Java’s) /RPC
(Microsoft’s)
• Performs very well for single-platform limited distributed
system.
• Platform dependent
• Tightly coupled
• Inherently synchronous (No chance for eventing or
notification)
• Object-oriented: Objects not deployable units
• Non-standard
• Not scalable, location dependent, no global
registry/discovery
• Object reference passed as parameter and/or returned
as result.
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Web Services
• Web Services is a technology that allows for applications to
communicate with each other in a standard format.
• A Web Service exposes an interface that can be accessed through
messaging.
• Deployable unit.
• A Web service uses protocol to describe an operation and the data
exchange with another web service. Ex: SOAP
• Platform independent, say, through WSDL.
• Publishable, discoverable, searchable, queryable
• Scalability issues: A group of web services collaborating accomplish
the tasks of a large-scale application. The architecture of such an
application is called Service-Oriented Architecture (SOA).
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A Little bit of History: XML to SOAP
• Simple xml can facilitate sending message
to receive information.
• The message could be operations to be
performed on objects.
• Standardize the tags for object access.
• Simple Object Access Protocol (SOAP).
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SOAP Request (Not WS request)
<soap:Envelope
xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
<soap:Body>
<getProductDetails xmlns="http://warehouse.example.com/ws">
<productId>827635</productId>
</getProductDetails>
</soap:Body>
</soap:Envelope>
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SOAP Reply
<soap:Envelope
xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
<soap:Body>
<getProductDetailsResponse
xmlns="http://warehouse.example.com/ws">
<getProductDetailsResult>
<productName>Toptimate 3-Piece Set</productName>
<productId>827635</productId>
<description>3-Piece luggage set. Black
Polyester.</description>
<price>96.50</price>
<inStock>true</inStock>
</getProductDetailsResult>
</getProductDetailsResponse>
</soap:Body>
</soap:Envelope>
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SOAPWeb Services (WS)
Take a look at
1. http://www.w3.org/DesignIssues/WebServices.html
2. O’Reilly book on Web Services: Kim Topley’s
Webservices in a Nutshell:
http://www.oreilly.com/catalog/javawsian/index.html
This link has a sample chapter (SAAJ) and zip of all
examples in the book.
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Web Services (Colouris)
• A web service provides a service interface
enabling clients to interact with servers in a
more general way than web browsers do.
• Clients access operations in the interface
usually by XML messages over http.
• However other architectural models such as
REST and CORBA could access WS.
• WSDL provides additional details than for
standard operation: for encoding, security,
communication and location.
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Web services infrastructure and
components
Applications
Directory
service
Web Services
Security
Choreography
Service descriptions (in WSDL)
SOAP
URIs (URLs or URNs)
XML
HTTP, SMTP or other transport
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SOAP message in an envelope
envelope
header
header element
header element
body element
body element
body
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Example of a simple request
without headers
env:envelope
xmlns:env =namespace URI for SOAP envelopes
env:body
m:exchange
xmlns:m = namespace URI of the service description
m:arg1
Hello
m:arg2
World
In this figure and the next, each XML element is represented by a
shaded box with its name in italic followed by any attributes and its
content
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Example of a reply
corresponding to the request
xmlns:env = in
namespace URI for SOAP envelope
env:envelope
env:body
m:exchangeResponse
xmlns:m = namespace URI for the service description
m:res1
World
m:res2
Hello
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Host: www.cdk4.net
Content-Type: application/soap+xml
Action: http://www.cdk4.net/examples/stringer#exchange
action
HTTP
header
Use of HTTP POST Request in
SOAP client-server
communication
POST /examples/stringer
endpoint address
<env:envelope xmlns:env=namespace URI for SOAP
> envelope
<env:header> </env:header>
<env:body> </env:body>
</env:Envelope>
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Services, ports and bindings
• 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.
– 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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WS Interoperability Infrastructure
WSDL
Service Description
SOAP
XML Messaging
HTTP
Network
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JAX-RPC
• 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
•
•
•
•
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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JAX-RPC Runtime
Client Appl.
Server Impl.
Stub
Ties
JAX-RPC
Runtime
JAX-RPC
Runtime
SOAP over HTTP
Read the semantics of exchanges in your text book pages 798, 799:
You will observe very similar semantics for stub and ties
as proxy and skeleton in RMI. Exception: no object ref is passed but
request, response go through XML format and are extracted and dispatched.
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Stubs and Ties
•
Client Side: Stub object has the same methods as the service
implementation class.
– 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:
– 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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Web Service Clients and
Servers
• 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
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software is provided by JAX-RPC runtime.
Java web service interface
ShapeList
import java.rmi.*;
public interface ShapeList extends Remote {
int newShape(GraphicalObject g) throws
RemoteException;
int numberOfShapes()throws RemoteException;
int getVersion() throws RemoteException;
int getGOVersion(int i)throws RemoteException;
GraphicalObject getAllState(int i) throws
RemoteException;
}
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Java implementation of the
ShapeList server
import java.util.Vector;
public class ShapeListImpl implements ShapeList {
private Vector theList = new Vector();
private int version = 0;
private Vector theVersions = new Vector();
public int newShape(GraphicalObject g) throws RemoteException{
version++;
theList.addElement(g);
theVersions.addElement(new Integer(version));
return theList.size();
}
public int numberOfShapes(){}
public int getVersion() {}
public int getGOVersion(int i){ }
public GraphicalObject getAllState(int i) {}
}
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Java implementation of the
ShapeList client
package staticstub;
import javax.xml.rpc.Stub;
public class ShapeListClient {
public static void main(String[] args) { /* pass URL of service */
try {
Stub proxy = createProxy();
proxy._setProperty
(javax.xml.rpc.Stub.ENDPOINT_ADDRESS_PROPERTY, args[0]);
ShapeList aShapeList = (ShapeList)proxy;
GraphicalObject g = aShapeList.getAllState(0);
} catch (Exception ex) { ex.printStackTrace(); }
}
private static Stub createProxy() {
return
(Stub) (new MyShapeListService_Impl().getShapeListPort());
}
}
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SEI Invocation Code
• Service End Point (SEI) invocation code:
Stub stub = (Stub)(new MyHelloService_Impl().getHelloIFPort());
stub._setProperty(javax.xml.rpc.Stub.ENDPOINT_ADDRESS
_PROPERTY,
"http://localhost:8080/hello-jaxrpc/hello");
HelloIF hello = (HelloIF)stub;
resp = hello.sayHello(request.getParameter("username"));
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SOAP binding and service
definitions
binding
name "=ShapeListBinding
"
type ="tns:ShapeList
"
soap:binding
transport = URI
for schemas for soap/http
style="rpc"
operation
name=
" newShape
"
input
soap:body
encoding, namespace
service
name "=MyShapeListService
"
endpoint
name =
" ShapeListPort
"
binding "=tns:ShapeListBinding
"
soap:address
location = service URI
output
soap:body
encoding, namespace
soap:operation
soapAction
the service URI is:
“http://localhost:8080/ShapeListjaxrpc/ShapeList”
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The main elements in a WSDL
description
definitions
types
target namespace
message
interface/
(porttype)
bindings
document style request-reply style how
abstract
services
where
concrete
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Message exchange patterns for
WSDL operations
Name
Messages sent by
In-Out
In-Only
Client
Server
Request
Reply
Delivery
Fault message
may replaceReply
Request
no fault message
Robust In-Only Request
Reply
Out-In
guaranteed
Request
may replaceReply
Out-Only
Request
no fault message
Robust Out-Only
Request
guaranteed
may be sent
may send fault
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