2. Distributed Objects
and Remote Invocation
Chapter 2.5 and 2.6
Design of Distributed Software
Distributed Software Technologies

SUN Remote Procedure Call (RPC)
 JAVA Remote Method Invocation (RMI)
 Common Object Request Broker Architecture
(CORBA)
 Web Centric Architecture



Servlets
Java Server Pages (JSP)
Enterprise Java Beans (EJB)

Web Services
 .Net Remoting
Design of Distributed Software
2
Schema ODS
ODS - Ontwerp van geDistribueerde Software
Week 1 - 23/09
VM Intro (Bart)
Week 2 - 30/09
VM Vervolg intro + RMI Hfdst2 1.-4. (Bart)
Week 3 - 07/10
VM Hfstk 3 - part 1 (Filip)
Week 4 - 14/10
VM Hfstk 3 - part 2 (Filip)
Lab Sessie 1 CORBA/RMI *
Week 5 - 21/10
VM Hfstk 4 (Bart)
Lab Sessie 2 Web Centric *
Week 6 - 28/10
VM Hfstk 5 (Bart)
Lab Sessie 3 Web Services *
Week 7 - 04/11
VM Hfstk 6+9 (Filip)
Project Sessie 1 *
Week 8 - 11/11 = verlof
////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////
Week 9 - 18/11
VM Hfstk 7+8 (Bart)
Project Sessie 2 *
Week 10 - 25/11
Project Sessie 3 (groep A) *
Project Sessie 3 (groep B) *
Week 11 - 02/12
VM Hfdstk 10 (Filip)
,
Week 12 - 09/12
,
Feedback
Design of Distributed Software
NM Vervolg Hfdst 2 (Filip) (Aud. Zuiderpoort)
3
2.5 CORBA : Common Object
Request Broker Architecture
Design of Distributed Software
2.5 Outline
Introduction + Motivation
Corba Framework
Basic Architecture
Corba Object Model
Corba Object Refs
Basic Corba Example
Corba Implementations
Advanced
Architecture
Portable Object Adapter
Extended Corba Example
Design of Distributed Software
5
Situating CORBA

Socket Interface:


no concept of methods, objects
no services:



Binding service, Activation Service, Persistent object storage
service, Object location service, Transaction service, Event service
support for multiple programming languages
JAVA RMI/ C RPC


concept of methods, objects
services:


Binding service, Activation Service, “light” Persistent object storage
service
single programming language
Design of Distributed Software
6
CORBA: Basic Idea
Need for middleware that allows applications
to communicate irrespective of their programming
language and that offers all required services

Recognized by OMG in 1989



OMG = Object Management Group
Industry Group of over 800 participants
Their goal : design middleware that allows applications
to communicate irrespective of (i) their programming
language, (ii) their hardware and software platforms
and (iii) the networks they communicate over.
Design of Distributed Software
7
CORBA ORB

OMG introduced ORB



ORB = Object Request Broker
Software component which helps a client to invoke a
method on an object
Timeline :
Design of Distributed Software
8
CORBA framework components

IDL = Interface Definition Language
 CORBA Architecture


GIOP = General Inter-Orb protocol


CDR = Common Data Representation
Object Reference Definition


ORB Core, Object Adapter, Skeletons, Client
stubs/proxies, Implementation repository, Interface
repository, Dynamic Invocation Interface, Dynamic
Skeleton Interface
IOR = Interoperable Object Reference
CORBA Services

Naming, Event, Notification, Security, Transaction,
Concurrency, Trading
Design of Distributed Software
9
RMI : architecture
a
proxy_b
Remote Ref. Mod.
communication
client
• proxy
• 1 proxy per remote object
• local representative of remote object
• implements all methods of remote interface
• handles (un)marshalling
Remote Ref Module :
•remote object reference table
Design
Distributed
Software
• tableof of
local proxies
dispatcher
skeleton
b
Remote Ref. Mod.
communication
server
• dispatcher
• typically 1 per class
• identify message to invoke in skeleton
• skeleton
• typically 1 per class
• (un)marshalling
• invokes method on remote object
• sends reply
10
CORBA : architecture
a
proxy_b
dispatcher
Object
Adapter
ORB Core
client
• ORB Core
• cfr Communication Module
• Provides interface:
• start/stop ORB
• convert between remote
objects and strings
• provide argument lists for
requests using dynamic invocation
Design of Distributed Software
skeleton
b
ORB Core
server
• Object Adapter
• Remote Ref + Dispatcher
• creates remote object references
for CORBA objects
• dispatches each invocation to
the appropriate servant
• activates objects
• Portable Object Adapters = POAs11
CORBA architecture (2)

Skeletons





generated by IDL compiler
remote method invocations are dispatched via
appropriate skeleton to a particular servant
unmarshals the arguments
marshals exceptions and results
Client Stubs/proxies



generated by IDL compiler
marshals the arguments
unmarhals exceptions and results
Design of Distributed Software
12
CORBA RMI : Object Model

CORBA’s object model :
A

B
C
D
E
Differences with Java RMI :


Clients are not necessarily objects
CORBA object:

implements an IDL interface
 has a remote object reference
 able to respond to invocations of methods on its IDL interface


No class objects in CORBA
Data structures of various types and complexity can be passed as
arguments
Design of Distributed Software
13
CORBA Objects
data
A
Client
Method 1
Method 2
Method 3
CORBA
Object
Design of Distributed Software
14
CORBA IDL interface example
#ifndef __ECHO_IDL__
#define __ECHO_IDL__
echo.idl :
/**
* The Echo interface which is basically an interface that contains an echo
operation and an accompanying operation.
* @author AT&T omniORB, adapted by Filip De Turck
* @version $Revision: 1.6 $, $Date: 2001/06/26 00:26:46 $
**/
interface echo {
/**
* Echoes the string received
* @param mesg The received string which has to be echoed
* @return The echoed string, which has to be <U>freed by the caller</U>
**/
string echoString(in string mesg);
/**
* @return The number of times the <code> echoString </code> operation is called
**/
long
};
times_called();
Design of Distributed Software
#endif
15
CORBA RMI : IDL

CORBA IDL






IDL interface specifies a name and a set of methods
that clients can request
Parameters and results: in, out, inout keywords
Passing CORBA objects, primitive and constructed
types
Type Object: common supertype of all IDL interface
types
Exceptions: defined in interfaces and thrown by their
methods
Invocation semantics: at-most-once

maybe semantics by oneway keyword
Design of Distributed Software
16
CORBA RMI : pseudo objects

CORBA implementations provide some interfaces
to the functionality of the ORB

pseudo objects:




Cannot be passed as arguments
IDL interface
Implemented as libraries
Example: ORB interface

represents the functionality that programmers need to
access:




init method : to initialize the ORB
connect method : to register CORBA objects with the ORB
shutdown method : to stop CORBA objects
conversion between remote object references and strings
Design of Distributed Software
17
CORBA client and server example

IDL compiler :
*.idl
files
IDL compiler
(idlj)
echo.idl
$ idlj -fall –oldImplBase echo.idl
Design of Distributed Software
Java Interface files
echo.java
Server Skeletons
_echoImplBase.java
Proxy Classes or
Client Stubs
_echoStub.java
Java classes
for structs
Helper and
Holder classes
echoHelper.java
18
Java interface file
/**
* echo.java .
* Generated by the IDL-to-Java compiler
* from echo.idl
*/
public interface echo extends org.omg.CORBA.Object
{
public String echoString(String mesg);
public int times_called();
} // interface Echo
Design of Distributed Software
19
Servant Class Implementation
import org.omg.CORBA.*;
class echoServant extends _echoImplBase {
private ORB theORB;
private int counter;
public echoServant(ORB orb) {
theOrb = orb;
counter = 0;
}
public String echoString(String mesg) {
counter++;
String p = mesg;
return p;
}
public int times_called() {
return counter;
}
}
Design of Distributed Software
20
Servant Classes

Extends the corresponding skeleton class
 Implements the interface methods
 Uses the method signatures defined in the
equivalent Java interface
 ORB private attribute: to connect new CORBA
objects to the ORB (connect method) or
shutdown objects (shutdown method)
Design of Distributed Software
21
Server Class Implementation
import org.omg.CORBA.*;
public class echoServer
{
public static void main(String args[]) throws
org.omg.CORBA.UserException {
try{
java.util.Properties props = System.getProperties();
ORB orb = ORB.init(args, props);
echoServant echoRef = new echoServant(orb);
orb.connect(echoRef);
System.out.println(“echoServer ready and waiting ...");
orb.run();
} catch (Exception e) {
System.err.println("ERROR: " + e);
e.printStackTrace(System.out);
}
System.out.println(“Echo Server Exiting...");
} //main
} // class
Design of Distributed Software
22
CORBA server





Java class with Main method
Creates and initializes the ORB
Creates an instance of Servant class
Registers it to the ORB (through the connect
method)
Waits for incoming client requests
Design of Distributed Software
23
Client Implementation
import org.omg.CORBA.*;
public class echoClient
{
public static void main(String args[])
java.util.Properties props = System.getProperties();
org.omg.CORBA.ORB orb = org.omg.CORBA.ORB.init(args, props);
if (args.length == 0) {
System.err.println("usage: java echoClient <object reference>\n");
System.exit(1);}
try {
org.omg.CORBA.Object obj = orb.string_to_object(args[0]);
if (obj==null) {
System.err.println("Got a nil reference.\n"); System.exit(1);}
Echo echoRef = echoHelper.narrow(obj);
String src = "Hello!";
String answer = echoRef.echoString(src);
System.out.println("I said:\""+src+"\"."+
" The Object said:\""+answer+"\“.");
System.out.println("The function echoString on the object is called already "+
echoRef.times_called()+" time(s)");
} catch(Exception ex) {
System.err.println("Caught an exception!!.\n");
}
orb.destroy();
System.exit(0);
} //mainDesign of Distributed Software
} // class
24
Client Program

Creates and initializes the ORB
 Narrow method to cast Object to particular
required type
 Invokes the methods

Catch CORBA System exceptions

Objects cannot be passed by value in CORBA
Design of Distributed Software
25
Running the server and client

Start orbd :



Start the Echo server :




UNIX command shell : $ orbd
MS-DOS system prompt : start orbd
UNIX command shell :
$ java echoServer
MS-DOS system prompt :
start java echoServer
Result: echoServer ready and waiting ...
Run the client application :


$ java echoClient ior_server
ResuIt: I said: “Hello!“. The Object said: “Hello!“.
Design of Distributed Software
26
Chapter Outline
Introduction + Motivation
Corba Framework
Basic Architecture
Corba Object Model
Corba Object Refs
Basic Corba Example
Corba Implementations
Advanced
Architecture
Portable Object Adapter
Extended Corba Example
Design of Distributed Software
27
CORBA implementations

Interesting URLs :

http://cmeerw.org/freeorbwatch

http://dmoz.org/Computers/Programming/
Component_Frameworks/CORBA/
Object_Request_Brokers/

http://java.sun.com/developer/
onlineTraining/corba/corba.html

http://java.sun.com/j2se/1.5.0/docs/
guide/idl/index.html
Design of Distributed Software
28
Portable Object Adapter
Object
dispatcher
Adapter
skeleton
b
ORB Core
object adapter : connects a
request using an object
reference with the proper
code to service that request
server
Portable Object Adapter (POA):
 Allow programmers to construct object implementations
that are portable between different ORB products.
 Allow a single servant to support multiple object identities
simultaneously.
Design of Distributed Software
29
Example: Echo Servant with POA
import org.omg.CORBA.*;
import org.omg.PortableServer.POA;
class echoServant extends echoPOA {
private ORB theOrb;
private int counter;
public echoServant(ORB orb) {
theOrb = orb;
counter = 0;
}
public String echoString(String mesg) {
counter++;
String p = mesg;
return p;
}
public int times_called() {
return counter;
}
}
Design of Distributed Software
30
Example: Echo Server with POA
import org.omg.CORBA.*;
Import org.omg.PortableServer.*;
public class echoServer
{
public static void main(String args[]) throws org.omg.CORBA.UserException {
try{
// create and initialize the ORB
ORB orb = ORB.init(args, null);
// get reference to rootpoa
POA rootpoa = POAHelper.narrow(orb.resolve_initial_references("RootPOA"));
// create servant and register it with the ORB
echoServant eS = new echoServant(orb);
//activate the server object
rootpoa.activate(echoServantRef);
// get and narrow object reference from the servant
org.omg.CORBA.Object ref = rootpoa.servant_to_reference(eS);
echo echoRef = echoHelper.narrow(ref);
// wait for invocations from clients
orb.run();
} catch (Exception e) {
System.err.println("ERROR: " + e);
e.printStackTrace(System.out);
}
System.out.println(“Echo Server Exiting...");
} //main
Design of Distributed Software
} // class
31
Server Implementation with POA
1. Get the root POA
org.omg.CORBA.Object obj = orb.resolve_initial_references("RootPOA");
POA rootpoa = POAHelper.narrow(obj);
2. Create the servant instance
echoServant echoServantRef = new echoServant(orb);
3. Activate the server object
rootpoa.activate(echoServantRef);
4. Create the object reference from the POA
org.omg.CORBA.Object ref = rootpoa.servant_to_reference(echoServantRef);
5. Narrow the object reference
echo echoRef = echoHelper.narrow(ref);
6. Wait for incoming requests from clients
orb.run()
Design of Distributed Software
32
Interface Definition Language


Additional keywords (compared to C++):

interface, any, attribute, in, out, inout, readonly, raises

module : group interfaces and idl types in logical units
C++ preprocessing capabilities
Design of Distributed Software
33
Interface Definition Language (2)





Attributes



IDL types: short, long, unsigned short, usigned long,
float, double, char, boolean, octet, any
const keyword
Object = common supertype
arrays or sequences in arguments : typedef !
cfr public class fields in Java
readonly : only getter method is provided
Inheritance:


multiple inheritance is allowed
scoped names to make distinction between types
Design of Distributed Software
34
More advanced IDL example:
Whiteboard.idl
struct Rectangle{
long width;
long height;
long x;
long y;
} ;
struct GraphicalObject {
string type;
Rectangle enclosing;
boolean isFilled;
};
interface Shape {
long getVersion() ;
GraphicalObject getAllState() ; // returns state of the GraphicalObject
};
typedef sequence <Shape, 100> All;
interface ShapeList {
exception FullException{ };
Shape newShape(in GraphicalObject g) raises (FullException);
All allShapes(); // returns sequence of remote object references
long getVersion() ;
};
Design of Distributed Software
•
35
IDL module

Modules allow interfaces and associated
definitions to be grouped.
 A module defines a naming scope.
module Whiteboard {
struct Rectangle{
...} ;
struct GraphicalObject {
...};
interface Shape {
...};
typedef sequence <Shape, 100> All;
interface ShapeList {
...};
};
Design of Distributed Software
36
2.6 Invocations in JEE Architecture
1.
2.
3.
4.
5.
6.
7.
Introduction
Servlets
Java Server Pages
Java Server Faces (JSF)
Apache Struts
Invocation of Enterprise Java Beans (EJB)
Ajax
Design of Distributed Software
Web centric architectures
1. Introduction
HTTP request
webclient
webserver
HTTP response
Thin client :
• web browser
• plug-ins
Advantages
• ubiquitous and uniform client
• no network blockages (firewalls) for HTTP (port 80)
• huge user base
Design of Distributed Software
38
Protocol messages : HTTP
1. Introduction
Application level message formats and behavior
Request
Response
• GET (data encoded in URL)
• POST (data encoded in message)
• header
• requested content (URL)
• HTTP version
• response types accepted (MIME, language)
• client browser
• connection type
• POST
• content of request
• content length
• REPLY
• header
• status (success/failure)
• response data + MIME type
(Multipurpose Internet Mail Extensions)
Stateless protocol
• 1 transaction for each request/response pair
• each request/response pair INDEPENDENT of others
Design of Distributed Software
39
Addressing resources : URL
1. Introduction
<protocol>://<hostname>:<port>/<fully qualified pathname>?<parameters>
<protocol> : http, https, ftp, telnet, …
<hostname> : IP-address or DNS-name
<port> : port for webserver (default : http : 80, https: 343)
<fully qualified pathname> : directory + file name (default index.html)
<parameters> : normally used to specify (name, value)-pairs
? : separates resource name from parameters
+ : represents space
& : separate (name,value) pairs
= : binds value to name
% : escape character
followed by 2 hex-symbols representing ASCII value
elections!
e.g. http://www.spy.cia.com:80/reports/interest?subject=elections%21
Design of Distributed Software
40
Standard interaction
1. Introduction
Retrieving a (HTML) document
HTTP GET
webclient
webserver
HTTP REPLY
Typical interactions
1. Connect : client opens connection with server
2. Request : client sends message to identify resource/service
3. Response : server sends message to respond to request
4. Disconnect : close client-server connection
Design of Distributed Software
41
1. Introduction
Dynamic content : CGI
• Common Gateway Interface
• URL
• specifies (small) program
• gives input through URL-parameters
• program generates html-output
(or output for plug in)
HTTP POST
webclient
HTTP REPLY
webserver
INVOKE
CGI-program
• Performance/resource issues
• new separate process for each request
• programs often in scripting languages (e.g. Perl)
• resources (e.g. database connections) created/destroyed per
request
Design of Distributed Software
42
Dynamic content : HTML for CGI
1. Introduction
HTML <FORM>-tag
• METHOD-field : specifies HTTP message type
• ACTION-field : specifies URL (without parameters)
• parameters specified in subsequent INPUT fields
<html>
<head>
<title> Personal ID </title>
</head>
<body>
<FORM METHOD="GET" ACTION="http://java.sun.com/prog/enter">
<P>Enter first name <INPUT NAME="first"></INPUT></P>
<P>Enter family name <INPUT NAME="family"></INPUT></P>
<INPUT TYPE="submit“ VALUE=“Submit Query">
</FORM>
</body>
http://java.sun.com/prog/enter?first=John+F.&family=Kennedy
</html>
Design of Distributed Software
43
1. Introduction
Servlets and JSP
• Plug-in object at server side
• Runs separate thread per request
• Shares resources with other servlets
• Servlet life cycle managed by container process
(single process, single JVM) :
Created/destroyed/activated/passivated
HTTP POST
webclient
HTTP REPLY
webserver
INVOKE
webclient
Servlet Container
Design of Distributed Software
44
1. Introduction
Servlets and JSP
• Servlet disadvantage :
• pages contain static + dynamic data
• static data created dynamically -> not optimal
• Solution
• mix static and dynamic code in single webpage
• compile webpage to extract servlets and static portions
• configure webserver to
• automatically invoke servlets to create dynamic content
• merge dynamic and static content to response to client
JSP : Java Server Page
passive
JSP
compiler
HTML
code
active
passive
Design of Distributed Software
Servlet class
Actually : JSP = Servlet
45
1. Introduction
EJB
• Server side component technology to code application logic
• model business logic
• model business objects (“database record”)
Web tier
webclient
webserver
Servlet Engine
Application
client
Design of Distributed Software
Persistent Storage tier
Entreprise Systems Tier
Application logic
Client tier
Business tier
Middle Tier
46
EJB
1. Introduction
• Session beans
• Transient, DO NOT survive server shutdown
• Associated with ONE client (at any given time)
Stateless session beans
• no state between successive invocations
Stateful session beans
• state info kept consistent between invocations
• Entity beans
• Persistent, DO survive server shutdown
• State persists method calls/server shutdown
• Can be shared between multiple clients
Container Managed Persistence (CMP)
Bean Managed Persistence (BMP)
Design of Distributed Software
47
Web Centric Architecture
Client
Webserver
Application Server
1. Introduction
Database system
Web container
webserver
applet
applet
browser
EJB container
Servlet
JSP
EJB
Web container services
• component life cycle management
• handle communication with
webserver HTTP <-> servlet
• session tracking
Design of Distributed Software
EJB container services
• component life cycle management
• transaction service
• persistence service
• security handling
• resource pooling
48
Servlet life cycle
2. Servlets
If webcontainer receives request for non-instantiated servlet
1. Load Servlet class
2. Instantiate object of Servlet class
3. Initialize Servlet
-> call init() method
4. Invoke service()-method to handle request
5. Handle any new requests by invoking service()
6. If idle-time > timeout or request by admin
Remove servlet from webcontainer
-> call destroy() method
(clean up)
Design of Distributed Software
49
2. Servlets
Servlet life cycle
Application Server
Request
webserver
Response
ServletContext
Servlet
Servlet
Servlet
Servlet
Web container
• Single Servlet instance handles all
requests for given Servlet class (exception : SingleThreadModel)
• Servlet invoked per request
• Servlet state does not survive requests
• “long lived” storage :
- through ServletContext object (e.g. Session persistence)
- in database (persistent storage)
GenericServlet : protocol neutral Servlet
HttpServlet : has access to HTTP specifics
Design of Distributed Software
50
Servlet types
2. Servlets
GenericServlet
abstract public void service(ServletRequest req,ServletResponse res)
throws ServletException, IOException
• method called for each request
• method not synchronized by default !
HttpServlet
Inherits from GenericServlet
Overrides service-method
- automatically dispatches to HTTP-message related methods
public void doGet(HttpServetRequest req,HttpServletResponse res)
throws ServletException, IOException
public void doPost(HttpServetRequest req,HttpServletResponse res)
throws ServletException, IOException
Design of Distributed Software
51
A GenericServlet
2. Servlets
import java.io.*;
import javax.servlet.*;
public class RandomGenericServlet extends GenericServlet {
public void service(ServletRequest req,ServletResponse res)
throws ServletException,IOException {
res.setContentType("text/html");
PrintWriter out=res.getWriter();
out.println("<HTML><HEAD><TITLE>RandomGenericServlet Output
</TITLE></HEAD>");
out.println("<BODY> Your random number is : "+
((int)(Math.random()*10))+"</BODY>");
out.println("</HTML>");
out.close();
}
}
• Override service()-method
• Set MIME-content type BEFORE writing to Response-object
Design of Distributed Software
52
An HttpServlet
2. Servlets
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class RandomHttpServlet extends HttpServlet {
public void doGet(HttpServletRequest req,HttpServletResponse res)
throws ServletException,IOException {
res.setContentType("text/html");
PrintWriter out=res.getWriter();
out.println("<HTML><HEAD><TITLE>RandomGenericServlet Output
</TITLE></HEAD>");
out.println("<BODY> Your random number is : "+
((int)(Math.random()*10))+"</BODY>");
out.println("</HTML>");
out.close();
}
public void doPost(HttpServletRequest req,HttpServletResponse res)
throws ServletException,IOException {
doGet(req,res);
}
• No service-method
}
• Override doXXX()-methods
XXX=Get,Post,Put,Trace,Delete,Options
Design of Distributed Software
53
Servlet framework methods
2. Servlets
Automatically called methods
public void init(ServletConfig config)
• called by container when Servlet is loaded
• if overriden : first statement should be super.init(config)
• calls init() method
(better to override init() !)
public void init()
• called by init(ServletConfig)
• code non-standard initialization here (e.g. Open database connection)
public void destroy()
• called by container when Servlet unloaded or timeout
• locate clean-up code here (close file handles, database connections, sockets, etc.)
• DANGER : make this method thread-safe !!!
public abstract void service(ServletRequest req, ServletResponse res)
• called when request received
• overriden in HttpServlet class to dispatch to HTTP-related methods
Convenience method
public void log(String log) : write (error)message to container specific log file
Design of Distributed Software
54
2. Servlets
Servlet environment
ServletContext I
ServletRequest
Servlet
ServletResponse
Servlet
Servlet
Web container
Servlet
XML
deployment
Description
Application I
XML
deployment
Description
Application II
ServletContext II
Servlet configuration
- static info used for deploying the Servlet
- initialisation parameters
Servlet context
- object allowing to interact with web application
- used to track sessions !
Servlet request/response
- request holds input parameters for Servlet
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ServletConfig
2. Servlets
Servlet config object passes servlet-specific info (NO request specific info)
public ServletConfig getServletConfig()
public String getServletInfo()
info on author, version, ... (override to supply info)
public String getServletName()
returns Servlet instance name (as known to container)
public String getInitParameter(String parName)
ALWAYS type String (Servlet should parse)
public Enumeration getInitParametersNames()
returns Enumeration of names for all known init parameters
init parameters
- parameter (name,value)-pairs stored in war-file
- initialized when servlet is loaded
- setting init-parameters is server dependent
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ServletConfig
2. Servlets
Intialize database connection
public void init() {
String dbHost=getInitParameter(“host”);
int port=Integer.parseInt(getInitParameter(“dbport”));
String user=getInitParameter(“user”);
String pass=getInitiParameter(“password”);
// initialize connection to database
}
Show all init-parameters
public String getServletInfo() {
String res=“”;
Enumeration e=getInitParameterNames();
while(e.hasMoreElements())
res+=(String)(enum.nextElement());
return res;
}
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2. Servlets
ServletContext
Servlet context object serves as interface to container
One context per web application
If distributed web application : one context per JVM per web application
public ServletContext getServletContext()
public ServletContext getServletContext(String uri)
returns context object for uri specified servlet
public String getInitParameter(String parName)
public Enumeration getInitParametersNames()
public RequestDispatcher getRequestDispatcher(String path)
returns RequestDispatcher for given path
public RequestDispatcher getNamedDispatcher(String name)
returns RequestDispatcher for referred servlet
public Object getAttribute(String name)
return Object with given name
public Enumeration getAttributeNames()
return names for all context attributes
public void removeAttribute(String name)
removes named attribute from context
public void setAttribute(String name,Object obj)
add attribute obj, name it “name”
Design of Distributed Software
Same as for config object,
But for CONTEXT wide init
Parameters (shared between servlets)
58
ServletRequest
2. Servlets
- request holds input parameters for Servlet
- created by container and handed to service()-method
Same as for context object,
public Object getAttribute(String name)
But for Request Scope
public Enumeration getAttributeNames()
Attributes
public void removeAttribute(String name)
public void setAttribute(String name,Object obj)
public String getServerName()
public int getServerPort()
public String getRemoteAddress()
public String getRemoteHoser()
public BufferedReader getReader() // buffered char reader
public ServletInputStream getInputStream() // binary reader
public String getProtocol()
public int getContentLength()
public String getParameter()
Request encoded parameters
-request body – POST
public String[] getParameterValues()
-URL encoded - GET
public Enumeration getParameterNames()
public Map getParameters() // name – value map
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2. Servlets
HttpServletRequest
public String getHeader(String name)
public Enumeration getHeaders(String name)
public Enumeration getHeaderNames()
public String getMethod()
public Cookie[] getCookies()
public HttpSession getSession()
public HttpSession getSession(boolean create)
Session
management
http://somehost:80/directory/servlet/pathinfo?query
public String getPathInfo()
public String getQueryString()
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HttpServletRequest : headers
User-Agent
Referer
Host
Accept
Accept-Charset
Accept-encoding
Accept-Language
Authorization
Connection
Pragma
2. Servlets
information on client side software
URL from which Servlet is accessed
Internet host and port of requested resource
comma-separated list of MIME-types accepted by the client
character set accepted by client
accepted content-encoding (mainly compression)
e.g. “compress”, “deflate”, “gzip”
preferred language by client software
(e.g. “en”, “de”,”en-us”,...)
used to authorize user (server specific)
options for this connection
(“close”, “keep-alive”)
destined for intermediary
(e.g. “no-cache” -> no proxy caching)
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HttpServletRequest : headers
2. Servlets
import java.util.*;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class HeaderServlet extends HttpServlet {
public void doGet(HttpServletRequest req,HttpServletResponse res)throws ServletException,IOException {
res.setContentType("text/html");
PrintWriter out=res.getWriter();
out.println("<HTML><HEAD><TITLE>HeaderServlet Output</TITLE></HEAD>");
Enumeration names=req.getHeaderNames();
out.println("<H1>Request Headers</H1>");
while(names.hasMoreElements()) {
String headerName=(String)names.nextElement();
String headerValue=req.getHeader(headerName);
out.println("<B>"+headerName+" : </B>"+headerValue+"<BR>");
}
out.println("</HTML>");
out.close();
}
public void doPost(HttpServletRequest req,HttpServletResponse res)throws ServletException,IOException {
doGet(req,res);
}
}
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HttpServletRequest : headers
2. Servlets
View Source
<HTML><HEAD><TITLE>HeaderServlet Output</TITLE></HEAD>
<H1>Request Headers</H1>
<B>accept : </B>image/gif, image/x-xbitmap, image/jpeg,
image/pjpeg, application/vnd.ms-excel, application/vnd.ms-powerpoint,
application/msword, application/x-shockwave-flash, */*<BR>
<B>accept-language : </B>nl-be<BR>
<B>accept-encoding : </B>gzip, deflate<BR>
<B>user-agent : </B>Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)<BR>
<B>host : </B>localhost:8080<BR>
<B>connection : </B>Keep-Alive<BR>
</HTML>
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HTML parameter encoding
2. Servlets
In FORM-tag
Field type
Value
Example
text
hidden
checkbox
String or null(empty)
String or null(empty)
checked : String
either “on”,”true”,”yes”
OR assigned tag value
unchecked : null
String
String or null
String or null (empty)
name=Kennedy
sessionID=12234
box=on
box=John
radio
select option
textarea
Design of Distributed Software
name=John
car=Fiat
text=this is some sample text
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Retrieving parameters
2. Servlets
<html><head><title> Personal ID </title></head>
<body>
<FORM METHOD="GET" ACTION="http://localhost:8080/process/input">
<P>Enter name <INPUT TYPE="text" NAME="name" SIZE=20></INPUT></P>
<P>Occupation :<BR>
<INPUT TYPE="checkbox" NAME="USpres">US president<BR>
<INPUT TYPE="checkbox" NAME="NMBS">NMBS manager<BR>
<INPUT TYPE="checkbox" NAME="prime">Belgian Prime Minister<BR>
</P>
<P>Sex : <BR>
<INPUT TYPE="radio" NAME="sex" VALUE="male" CHECKED>male<BR>
<INPUT TYPE="radio" NAME="sex" VALUE="female">female<BR>
<P>Select your favourite car : <BR>
<SELECT NAME="cars">
<OPTION VALUE="volvo">Volvo
<OPTION VALUE="saab">Saab
<OPTION VALUE="fiat">Fiat
• input to be processed on http://localhost:8080/process/input
<OPTION VALUE="audi">Audi
• using doGet()-method
</select>
• parameter names : name, USpres, NMBS, prime, ...
</P>
<P>Your comments :<BR>
<TEXTAREA NAME="comments" ROWS="5" COLS="30">
</TEXTAREA>
</P>
<INPUT TYPE="submit" VALUE="Submit ID card">
</FORM>
</body>
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</html>
Retrieving parameters
2. Servlets
Browser
output
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Retrieving parameters
2. Servlets
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class ProcessInput extends HttpServlet {
public void doGet(HttpServletRequest req,HttpServletResponse res)throws ServletException,IOException {
res.setContentType("text/html");
PrintWriter out=res.getWriter();
out.println("<HTML><HEAD><TITLE>Echo back to confirm.</TITLE></HEAD>");
out.println("<BODY>");
out.println("You have entered following data :<BR>");
out.println("Your name : "+req.getParameter("name")+"<BR>");
out.print("Your occupations : ");
if(req.getParameter("USpres")!=null) out.print("US president ");
if(req.getParameter("NMBS")!=null) out.print("NMBS manager ");
if(req.getParameter("prime")!=null) out.print("Belgian Prime Minister ");
out.println("<BR>");
out.println("Your sex : "+req.getParameter("sex")+"<BR>");
out.println("Your favourite car : "+req.getParameter("cars")+"<BR>");
out.println("Your comments : "+req.getParameter("comments")+"<BR>");
out.println("</BODY>");
out.println("</HTML>");
out.close();
}
}
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Retrieving parameters
2. Servlets
Browser
output
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ServletResponse
2. Servlets
Webcontainer
• retrieves ServletResponse object
• translates to HTTP and manages connection
public void setContentType(String type)
• sets MIME-type of response (e.g. “text/html”)
• MIME type MUST be set before generating output
public void setContentLength(int len)
public ServletOutputStream getServletOutputStream() // binary output
public PrintWriter getWriter() // formatted character output
public Enumeration getInitParameters()
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HttpServletResponse
2. Servlets
public void addHeader(String name,String value)
public void setHeader(String name,String value)
public boolean containsHeader()
public void addDateHeader(String name,long date) // ms since January 1, 1970
public void setDateHeader(String name,long date) // ms since January 1, 1970
public sendError(int http_statuscode)
public sendError(int http_statuscode,String message)
e.g. sendError(404,”Can not find this file here.”)
public sendRedirect(String url)
e.g. sendRedirect(http://newhost/index.html)
public setStatus(int http_statuscode)
e.g. setStatus(404)
public void addCookie(Cookie c) Session
management
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HttpServletResponse : headers
Age
Allow
Content-Encoding
Content-Language
Content-Length
Content-Type
Date
Expires
Last-Modified
Refresh
Server
WWW-Authenticate
2. Servlets
time elapsed since response generated (control caching)
list of supported http methods for requested document
type of encoding used for (e.g. compressed) response
natural language used for the response
number of bytes of the document’s body
MIME-type of response
Current date and time (GMT)
date and time (GMT) when content becomes stale
(can not be cached any longer)
date and time (GMT) since last update (used for cache control)
number of seconds before document should be reloaded
server info
for authorization, server specific
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Servlet collaboration
2. Servlets
Sharing data
Within same application : use ServletContext attributes
With another application/context ON SAME SERVER :
• use database
• use external object (on same or different server)
• use ServletContext of other application
ServletContext ownContext= getServletContext();
ServletContext otherContext=ownContext.getContext(“/otherapp/index.html”);
Object parameter=otherContext.getAttribute(“foreignAttribute”);
Sharing control
Use RequestDispatcher to forward/include to component contents
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Servlet chaining / forwarding
2. Servlets
• make sure all headers are set appropriately before forwarding requests
• through RequestDispatcher-object
Use getRequestDispatcher()-methods
forward(ServletRequest request, ServletResponse response)
-> forward request to other resource, hand off control
-> response NOT committed to client !
include(ServletRequest request, ServletResponse response)
-> include contents to response of this servlet,
original servlet stays in control
// …
String destination=“/servlet/NewServlet”;
RequestDispatcher dispatcher= req.getRequestDispatcher(destination);
dispatcher.forward(req,res);
// …
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Servlet forwarding : example
2. Servlets
Check credentials
Process input text
ValidServlet
forward
valid
invalid
Known to webserver
as “valid”
HTML-input form
Fields : name, password, text
ProcessServlet
Known to webserver
as “process”
Web application “forward” contains :
• HTML-input form “index.html”
• Servlet “valid” instance of class ValidServlet.class
• Servlet “process” instance of class ProcessServlet.class
forward.war
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Servlet forwarding : example
<html>
<head>
<title> Forward </title>
</head>
2. Servlets
index.html
<body>
<FORM METHOD="GET" ACTION="http://localhost:8080/forward/valid">
<P>Enter user name <INPUT TYPE="text" NAME="user" SIZE=20></INPUT></P>
<P>Password <INPUT TYPE="password" NAME="pass" SIZE=20></INPUT></P>
<P>Enter text to send to database :<BR>
<TEXTAREA NAME="text" ROWS="5" COLS="30">
</TEXTAREA>
</P>
<INPUT TYPE="submit" VALUE="Submit Text">
</FORM>
</body>
</html>
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Servlet forwarding : example
2. Servlets
ValidServlet.java
public class ValidServlet extends HttpServlet {
public void doGet(HttpServletRequest req,HttpServletResponse res)throws
ServletException,IOException {
String user=req.getParameter("user");
String pass=req.getParameter("pass");
if(user.equals("user")&&pass.equals("secret")) {
RequestDispatcher dispatcher=req.getRequestDispatcher("process");
dispatcher.forward(req,res);
} else {
res.setContentType("text/html");
PrintWriter out=res.getWriter();
out.println("<HTML><HEAD><TITLE>Answer from ValidServlet.</TITLE></HEAD>");
out.println("<BODY>");
out.println("You failed to supply valid credentials.<BR>");
out.println("</BODY>");
out.println("</HTML>");
out.close();
}
}
public void doPost(HttpServletRequest req,HttpServletResponse res) throws
ServletException,IOException {
doGet(req,res);
}
}
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Servlet forwarding : example
2. Servlets
ProcessServlet.java
public class ProcessServlet extends HttpServlet {
public void doGet(HttpServletRequest req,HttpServletResponse res)throws
ServletException,IOException {
String text=req.getParameter("text");
res.setContentType("text/html");
PrintWriter out=res.getWriter();
out.println("<HTML><HEAD><TITLE>Answer from ProcessServlet.</TITLE></HEAD>");
out.println("<BODY>");
out.println("Following text has been submitted :<BR>"+text);
out.println("</BODY>");
out.println("</HTML>");
out.close();
}
public void doPost(HttpServletRequest req,HttpServletResponse res)throws
ServletException,IOException {
doGet(req,res);
}
}
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3. JSP
Java Server Pages
• allows to use convenient syntax for static portions
• JAVA-scriptlets to code dynamic parts
• intuitive interaction with Java Beans components
• jsp-file is compiled in single Servlet
Application Server
request
ServletContext
Deployed
JSP/Servlet
reply
Servlet class
JAVA-compiler
Servlet
Web container
war-file
Servlet source
JSP-compiler
JSP-file
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A Simple Example
import java.io.*;
import javax.servlet.*;
3. JSP
Servlet version
public class RandomGenericServlet extends GenericServlet {
public void service(ServletRequest req,ServletResponse res)
throws ServletException,IOException {
res.setContentType("text/html");
PrintWriter out=res.getWriter();
out.println("<HTML><HEAD><TITLE>Servlet Output
</TITLE></HEAD>");
out.println("<BODY> Your random number is : "+
((int)(Math.random()*10))+"</BODY>");
out.println("</HTML>");
out.close();
}
<%@page contentType="text/html"%>
}
JSP-version
<html>
<head><title>JSP output</title></head>
<body>
Your random number is : <%=((int)(Math.random()*10))%>
</body>
</html>
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Object Scoping
3. JSP
Recap from Servlets :
Application scope
all JSP-pages and resources sharing same ServletContext
e.g.
String myString=“example variable”;
getServletContext.setAttribute(“example”,myString);
Session scope
all resources in current HttpSession
e.g.
HttpSession session=req.getSession(true);
sess.setAttribute(“example”,myString);
Request scope
this request only
e.g.
req.setAttribute(“example”,myString);
Page scope
this page only (no servlet equivalent) (default for JSP)
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3. JSP
Syntax
Declarations
application scope declarations
• variables and methods
• OUTSIDE servlet service()-method
• syntax :
<%!
the declarations
request scope declarations
• variables local to service()-method
• comes INSIDE servlet service()-method
• syntax : same as scriptlets
<%
the declarations %>
Scriptlet
Piece of java-code for service()-method
syntax :
<%
the java code
%>
public class TheServlet {
public void service(…) {
%>
}
}
Escaping
Literal sequence
<\%
%\>
\’
\”
Design of Distributed Software
meaning
<%
%>
‘
(for single quote in single quoted attribute)
“
(for double quote in double quoted attribute)
81
Syntax
3. JSP
Expressions
Dynamically generated output
• evaluate Java-expression
• apply toString() if necessary
• send to JSP output stream (end hence ServletOutputStream)
• syntax :
<%=
the java expression
%>
(no ; !)
Comments
HTML-comments
• visible in browser (“view source”)
• embedded scriptlets/expressions in HTML-comments are executed/evaluated
• syntax : <!-the comment
-->
Java-comments
• standard java-comment in scriptlets
• do not appear in HTML-output
• syntax : <%
// the comment
%> (or multi-line comment syntax)
JSP-comments
• ignored by the JSP-compiler (do not appear)
• syntax : <%-the comment
-->
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3. JSP
Syntax
Directives
Messages to the JSP engine
Syntax : <%@ directive attribute=“value” %>
page
Attributes
contentType
extends
import
info
errorPage
isErrorPage
session
meaning
sets MIME-type of response
<%@ page contentType=“text/html” %>
specify superclass of JSP’s associated Servlet
<%@ page extends=“com.SuperServlet” %>
specify packages to import
<%@ page import=“java.util.*” %>
sets ServletInfo variable
<%@ page info=“Hello Servlet echoes request” %>
specify jsp to redirect in case of uncaught exception
<%@ page errorPage=“ErrPage.jsp” %>
indicate if this page is error page for other page, true or false
<%@ page isErrorPage=“true” %>
true or false : participate in session ?
<%@ page session=“false” %>
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3. JSP
Syntax
include
Include other file (jsp, html, textfile, …, in current jsp)
• static include (at JSP-compile time)
• includes interpreted in “cut and paste” fashion
<%@ page include=“jspfiles/MyJSP.jsp” %>
taglib
Inclusion of Custom Tag Libraries
Extension mechanism for JSPs
Implicit objects
Objects made available to JSP-page
Name
application
config
exception
page
request
response
session
Design of Distributed Software
meaning
ServletContext
ServletConfig
Exception object (only for error pages)
this
HttpServletRequest
HttpServletResponse
HttpSession
84
3. JSP
Syntax
Actions
Statement
jsp:include
meaning
dynamic include of resource
<jsp:include page=“myjsps/included.jsp” flush=“true” />
second attribute MUST be true
forward request to other resource (specified as URL)
<jsp:forward page=“other.jsp” />
must be resource of same application
jsp:forward
forwarding WITH parameters :
<jsp:forward page=“other.jsp” >
<jsp:param name=“parname” value=“parval”/>
</jsp:forward>
jsp:plugin
include Applet using Java-plugin
<jsp:plugin type=”applet” code=“myApplet.class” with=“100” height=“200” >
<jsp:params>
<jsp:param name=“car” value=“Volvo” />
</jsp:params>
</jsp:plugin>
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Syntax
3. JSP
Actions : interaction with JavaBeans
Instantiating a Bean
<jsp:usebean id=“beanInstanceName” scope=“the scope” beandetails />
Scope attribute : valid values “page” (default), “request”, “session”, “application”
beandetails:
attributes :
class
Bean class file (usually required)
type
dynamic typing (cast)
beanName
class file, can be expression
If bean already exists with this name and scope, the bean is not re-instantiated
Bean class MUST have no-arg public constructor !
<jsp:usebean id=“myBean” scope=“session” class=“MyBean.class” type=“Object.class”/>
=
Object myBean=(Object) new MyBean();
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Syntax
3. JSP
Actions : interaction with JavaBeans
Bean properties
<jsp:getProperty name=“beanInstanceName” property=“propertyname”/>
Property value converted to String
<jsp:setProperty name=“beanInstanceName” property=“propertyname” value=“propValue” />
<jsp:setProperty name=“beanInstanceName” property=“propertyname”
param=“requestParameterName” />
How to transform requestParameter (always String) to BeanProperty ?
• if PropertyEditor available : setAsText() used
• primitives : valueOf(String) used
<jsp:setProperty name=“beanInstanceName” property=“propertyname” />
-> sets property according to matching request parameter
<jsp:setProperty name=“beanInstanceName” property=“*” />
-> sets all properties according to request parameters
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3. Application design
Technologies
Different technologies to execute code
JSP
Enterprise
Java Beans
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Servlet
Database
Stored procedures
88
Typical technology role
Business Logic
Database system
JDBC
Presentation
3. Application design
JSP : intuitive graphical representation
Servlet : execute business logic (e.g. data validation)
Enterprise Java Beans : local storage
Stored procedure (SQL) : optimize database access
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Java Server Faces (JSF)
4. Java Server Faces

Java-based Web application framework
 Goal: simplify development of user interfaces for Java EE
applications.
 The state of UI components is saved when the client requests
a new page and then is restored when the request is returned.
 JSF uses JavaServer Pages (JSP) for its display technology.
 There is a server side event model (e.g. ActionListeners and
ValueChangedListener).
 After compilation and deployment of the application, the clientserver interaction takes place the same way as with
Servlets/JSP.
 http://java.sun.com/javaee/javaserverfaces/
 http://java.sun.com/javaee/5/docs/tutorial/doc/bnaph.html
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Apache Struts








5. Apache Struts
Open-source web application framework for developing Java EE web
applications.
It uses and extends the Java Servlet API to encourage developers to adopt
a model-view-controller (MVC) architecture.
Goal: to separate the model (application logic that interacts with a
database) from the view (HTML pages presented to the client) and the
controller (instance that passes information between view and model).
Struts provides the controller (a servlet known as ActionServlet) and
facilitates the writing of templates for the view or presentation layer
(typically in JSP, but XML/XSLT is also supported).
The web application programmer is responsible for writing the model code,
and for creating a central configuration file struts-config.xml which binds
together model, view and controller.
After compilation and deployment of the application, the client-server
interaction takes again place the same way as with Servlets/JSP.
http://struts.apache.org/
http://struts.apache.org/2.0.11.2/docs/tutorials.html
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Invocation of EJBs
6. Invocation of EJBs

Enterprise Java Beans are Java objects, run inside a JEE
application server.
 The application server is responsible for the life cycle
management of the objects.
 Enterprise Java Beans run inside the same application server
can easily obtain a reference of each other and invoke
methods.
 Remote clients (i.e. run outside the application server, either
standalone or in another application server) can obtain a
reference to an Enterprise Java Bean (through a JNDI service,
as described in the next chapter) and invoke its methods.
 In terms of invocation, there is no distinction between session
beans or entities.
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Ajax






7. Ajax
AJAX = Asynchronous JavaScript and XML
based on xHTML and CSS, the Document Object Model, XML
and XSLT, the XMLHttpRequest object for asynchronous
communication with the back-end server and JavaScript to
bind everything together.
JavaScript code can communicate directly with the server,
through the JavaScript XMLHttpRequest object.
This object allows to send or retrieve information to or from a
server over HTTP.
At the client side, JavaScript takes care of creating the
XMLHttpRequest object, sending or retrieving information
through it and processing the results.
http://www.w3schools.com/ajax/
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