交通大學資訊工程學系
Programming in Java
RMI
蔡文能交通大學資訊工程學系[email protected]
http://www.csie.nctu.edu.tw/~tsaiwn/java/
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Java RMI
Agenda
RMI IntroductionRPC vs. RMIRMI DeploymentRMI Example
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Java RMI
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Java RMI
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Java RMI
RMI Introduction RMI enables the programmer to create distributed Java
applications, in which the methods of remote Java objects can be invoked from other Java virtual machines, possibly on different hosts.
A Java program can make a call on a remote object once it obtains a reference to the remote object, either by looking up the remote object in the naming service provided by RMI or by receiving the reference as an argument or a return value. A client can call a remote object in a server, and that server can also be a client of other remote objects.
RMI uses object serialization to marshal and unmarshal parameters.
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Java RMI
Serialization
Action of encoding of an object into a stream of bytes
RMI performs marshalling/unmarshalling via Java Serialization Marshalling is the process of encoding arguments and results for
transmission
Thus, objects to be marshalled or unmarshalled must be serializable
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Java RMI
RMI Overview
Remote Method Invocation Java version of RPC
ha.nctu.edu.tw hehe.nctu.edu.tw
networknetworkObjectClient
RemoteObjectServer
method invocation
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Java RMI
blah, blah, blah
bar = foo(a,b);
blah, blah, blah
blah, blah, blah
bar = foo(a,b);
blah, blah, blah
int foo(int x, int y ) { if (x>100)
return(y-2); else if (x>10)
return(y-x); else
return(x+y);}
int foo(int x, int y ) { if (x>100)
return(y-2); else if (x>10)
return(y-x); else
return(x+y);}
ClientClient
ServerServer
protocol
Remote Procedure Call (RPC)
RPCRPC
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Java RMI
Middleware Layers
Applications
Middlewarelayers Request reply
protocoleXternal Data Representation(XDR)
Operating System
RPC, RMI, and events
分層負責 , 分工合作
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Java RMI
RPC – Remote Procedure CallThere are a number of popular RPC specifications.
Sun RPC is widely used.
NFS (Network File System) is RPC based.
RPC clients are processes that call remote procedures.
RPC servers are processes that include procedure(s) that can be called by clients.
Rich set of support tools.The RPC library is a collection of tools for automating the creation of RPC clients and servers.
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Java RMI
Sun RPC
RPCGEN There is a tool for automating the creation of RPC clients and
servers. The program rpcgen does most of the work for you. The input to rpcgen is a protocol definition in the form of a list of
remote procedures and parameter types.
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Java RMI
RPCGEN
Input File
rpcgen
Client Stubs XDR filters header file Server skeleton
C Source CodeC Source Code
ProtocolProtocolDescriptionDescription
foo_clnt.c foo_svc.c
foo.xx
% rpcgen –C foo.xx
foo_xdr.c foo.h
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Java RMI
RPC Programming
RPC library XDR routines RPC run time library
call rpc service
register with portmapper
dispatch incoming request to correct procedure
Program Generatorrpcgen
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Java RMI
RPC Run-time Library
High- and Low-level functions that can be used by clients and servers.
High-level functions provide simple access to RPC services. For client: int callrpc( . . . ) For server: int registerrpc( . . . ) svc_run() is a dispatcher on server
A dispatcher waits for incoming connections and invokes the appropriate function to handle each incoming request.
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Java RMI
Procedure Arguments
To reduce the complexity of the interface specification, Sun RPC includes support for a single argument to a remote procedure.*Typically the single argument is a structure that contains a number of values.
* Newer versions can handle multiple args.
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Java RMI
Procedure Identification
Each procedure is identified by: Hostname (IP Address) Program Identifier (32 bit integer) Procedure Identifier (32 bit integer)
Program Version identifierfor testing and migration.
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Java RMI
Program Identifiers
Each remote program has a unique ID.
Sun divided up the IDs:
0x00000000 - 0x1fffffff
0x20000000 - 0x3fffffff
0x40000000 - 0x5fffffff
0x60000000 - 0xffffffff
SunSun
SysAdmin SysAdmin
TransientTransient
ReservedReserved
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Java RMI
Procedure Identifiers &Program Version Numbers
Procedure Identifiers usually start at 1 and are numbered sequentially
Version Numbers typically start at 1 and are numbered sequentially.
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Java RMI
Iterative ServerSun RPC specifies that at most one remote procedure within a program can be invoked at any given time.
If a 2nd procedure is called, the call blocks until the 1st procedure has completed.
Having an iterative server is useful for applications that may share data among procedures.
Example: database - to avoid insert/delete/modify collisions.
We can provide concurrency when necessary...
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Java RMI
RPC example (1/7)This program shows you how to use a server program, a client program and an interface definition file to let the client program call the functions in the server program and get the results.Files: ① test_proc.c --- the server file ② test_client.c --- the client file ③ test.xx --- the Interface Definition file of RPC Files generated by rpcgen: rpcgen –C test.xx
test_clnt.c test_svc.c test.h
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Java RMI
RPC example (2/7)File test_proc.c (1/1) for server side
#include "test.h"int *addd_1_svc(int *argp, struct svc_req *rqstp){ static int result; result = *argp + 1; return (&result);}int *decc_1_svc(int *argp, struct svc_req *rqstp){ static int result; result = *argp - 1; return (&result);}
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Java RMI
RPC example (3/7)File test_client.c (1/3) for client side
#include "test.h" /* test_client.c , page1 */void test_prog_1(char *host){ CLIENT *clnt; int *result_1; int addd_1_arg; int *result_2; int decc_1_arg; #ifndef DEBUG clnt = clnt_create(host, TEST_PROG, TEST_VERS, "netpath"); if (clnt == (CLIENT *) NULL) /* NULL is 0 */ { clnt_pcreateerror(host); exit(1); } /* to be continued */
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Java RMI
RPC example (4/7)File test_client.c (2/3) for client side
#endif /* DEBUG */ /* test_client.c , page2 */ scanf ("%d",&addd_1_arg); scanf ("%d",&decc_1_arg); result_1 = addd_1(&addd_1_arg, clnt); if (result_1 == (int *) NULL) { clnt_perror(clnt, "call failed"); } result_2 = decc_1(&decc_1_arg, clnt); if (result_2 == (int *) NULL) { clnt_perror(clnt, "call failed"); } printf ("addd_1_result = %d\n",*result_1); printf ("decc_1_result = %d\n",*result_2); #ifndef DEBUG clnt_destroy(clnt); #endif/* DEBUG */} /* test_prog_1 *//* to be continued */
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Java RMI
RPC example (5/7)File test_client.c (3/3) for client side
/* test_client.c , page3 */main(int argc, char *argv[]){ char *host; if (argc < 2) /* no host name given */ { printf("usage: %s server_host\n", argv[0]); exit(1); } host = argv[1]; test_prog_1(host);}
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Java RMI
RPC example (6/7)File : test.xx
program TEST_PROG{ version TEST_VERS { int ADDD(int)=1; int DECC(int)=2; }=1;}=0x31234567;
rpcgen –C test.xx
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Java RMI
RPC example (7/7)Use rpcgen , C compiler, and Linking/loader By rpcgen test.xx , you can get test_clnt.c, test_svc.c and test.h. Compile/link on the client
gcc -o test test_client.c test_clnt.c -lnsl
Compile/link/run on the server (ccsun2.csie.nctu.edu.tw)
gcc -o test_svc test_svc.c test_proc.c -lrpcsvc -lnsl
./test_svc&
Run clien program on the client
./test ccsun2.csie.nctu.edu.tw
Demo on Sun machines
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Java RMI
rpcgen –C test.xx
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Java RMI
Test RPC -- Server Side
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Java RMI
Test RPC -- Client Side
把第一個數加 1,
把第二個數減一
輸入兩列 : 57 89
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Java RMI
Another example of rpcgen file
struct readargs {FileIdentifier f;FilePointer position;Length length;
};
program FILEREADWRITE { version VERSION {
void WRITE(writeargs)=1;Data READ(readargs)=2;2
}=2;} = 9999;
const MAX = 1000;typedef int FileIdentifier;typedef int FilePointer;typedef int Length;struct Data {
int length;char buffer[MAX];
};struct writeargs {
FileIdentifier f;FilePointer position;Data data;
};
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Java RMI
High-Level Library Limitation
The High-Level RPC library calls support UDP only (no TCP).
You must use lower-level RPC library functions to use TCP.
The High-Level library calls do not support any kind of authentication.
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Java RMI
Low-level RPC Library
Full control over all IPC options TCP & UDP Timeout values Asynchronous procedure calls
Multi-tasking Servers
Broadcasting
IPC : Inter-Process Communication
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Java RMI
The General RMI Architecture
The server must first bind its name to the registry
The client lookup the server name in the registry to establish remote references.
The Stub serializing the parameters to skeleton, the skeleton invoking the remote method and serializing the result back to the stub.
RMI Server
skeleton
stub
RMI Client
Registry
bind
lookupreturn call
Local Machine
Remote Machine
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Java RMI
The Stub and Skeleton
A client invokes a remote method, the call is first forwarded to stub.
The stub is responsible for sending the remote call over to the server-side skeleton
The stub opening a socket to the remote server, marshaling the object parameters and forwarding the data stream to the skeleton.
A skeleton contains a method that receives the remote calls, unmarshals the parameters, and invokes the actual remote object implementation.
Stu
b
RMI Client RMI Server
skeleton
return
call
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Java RMI
Stubs
Stub Client side Acts as an implementation of a remote interface Communicates with the real object over the network The stub class is generated from corresponding remote class
by the RMI compiler rmic
Note: As of the J2SE 5.0 release, stub classes for remote objects no longer need to be pregenerated using the rmic stub compiler, unless the remote object needs to support clients running in pre-5.0 VMs.
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Java RMI
skeletons
Skeleton Server side Carries on a conversation with the stub; Reads the parameters for the method call from the link Call the real remote service Writes the return value from the service back to the stub
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Java RMI
JVM Asociated
to ActivationGroup
RMI (in Java)
Client Code
network
NamingService
Server Object
ActivationDaemonClient
Stub
ServerSkeleton
Implement “Factories” using declarative descriptions of activatable objects
One JVM per ActivationGroup. Automatically launched by Activation daemon and contains (potentially) many small scale, semi-independent servers which can share resources (like connection pools) and which live under the same security restrictions
“Flat” directory structure which attempts to provide some notion of location transparency-- client code uses a single name to find a server
Copyright ?1997 Alex Chaffee
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Java RMI
RMI Layers
TCPRemote Reference Layer
Transport Layer
Java Virtual Machine
Client Object
Remote Reference Layer
Transport Layer
Java Virtual Machine
Stub
Remote Object
Skeleton
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Java RMI
Remote reference layer
Defines and supports the invocation semantics of the RMI connection
Provides a RemoteRef object that represents the link to the remote service implementation object
The stub objects use the invoke() method in RemoteRef to forward the method call
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Java RMI
Transport layer
Provides basic connectivity makes the connection between JVMs
Based on TCP/IP connections between machines in a network
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Java RMI
Finding Remote Objects
It would be awkward if we needed to include a hostname, port and protocol with every remote method invocation.
RMI provides a Naming Service through the RMI Registry that simplifies how programs specify the location of remote objects. This naming service is a JDK utility called rmiregistry that runs at a well known address (by default).
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Java RMI
Find Servers using Naming Service
Simple interface to registry
Implemented via 5 static methods on an object:
public static String[] list(String name) public static Remote lookup(String name) public static void bind(String name, Remote obj) public static void rebind(String name, Remote obj) public static void unbind(String name)
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Java RMI
java.rmi.Naming
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Java RMI
java.rmi.registry.LocateRegistry
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Java RMIjava.rmi.registry.Registry
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Java RMI
RMI Deployment
RMI Deployment( 佈署 ): Where does the server program finds it’s classes? Where does the client program find it’s classes? How are classes loaded from client to server and vice-versa?
Remember that Object Serialization does not send the classes, only the data
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Java RMI
Steps for Developing an RMI System
1. Define the Remote interface
2. Develop the remote object by implementing the remote interface.
3. Develop the client program.
4. Compile the Java source files.
5. Generate the client stubs and server skeletons.
6. Start the RMI registry.
7. Start the remote server objects.
8. Run the client
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Java RMI
RMI Class Loading It’s all in the codebase
Source from which to load classes into JVM Usual CLASSPATH variable “local codebase”
Set this while running both the server and the client but not rmiregistry RMI client needs stubs from server
Stubs downloaded from the server to the client using the java.rmi.server.codebase property
RMI server may need classes from the clientClasses downloaded from the client to the server using the java.rmi.server.codebase property setup by the client
RMI registry needs to find stub classesIf it finds them from CLASSPATH it will not convey the “true” code base associated with the stub class even if the server sets it.
Searches “local” code base initially, then searches the “java.rmi.server.codebase”
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Java RMI
RMIDownloading Stubs
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Java RMI
RMI
Downloading Classes
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Java RMI
Java Interfaces
Similar to Class
No implementation! All methods are abstract (virtual for C++ ).
Everything is public.
No constructor
an Interface is an API that can be implemented by a Class .
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Java RMI
Interfaces and Inheritence
In Java a class can only extend a single superclass (single inheritence).
A class can implement any number of interfaces. end result is very similar to multiple
inheritence.
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Java RMI
Creating Remote object
1. Define an interface which extends Remote interface
2. Create the server program which implements that interface
3. Compile the Java program
4. Create the Stub
Note: As of the J2SE 5.0 release, stub classes for remote objects no longer need to be pregenerated using the rmic stub compiler, unless the remote object needs to support clients running in pre-5.0 VMs.
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Java RMI
Create the Stub
A. In JBuilder use “JNI/RMI” tab in class properties to generate it automatically
B. By hand: use rmic tool:
rmic ComputeEngine
Note: As of the J2SE 5.0 release, stub classes for remote objects no longer need to be pregenerated using the rmic stub compiler, unless the remote object needs to support clients running in pre-5.0 VMs.
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Java RMI
Server Details – extending Remote
Create an interface the extends the java.rmi.Remote interface. This new interface includes all the public methods that will be
available as remote methods.
import java.rmi.*;
public interface MyRemote extends Remote {
public int foo(int x) throws RemoteException;
public String blah(int y) throws RemoteException;
. . .
}
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Java RMI
How the interface will be used
Class for your Remote Object
Your Interface UnicastRemoteObject
Remote Interface Class RemoteServer
extends
implementsextends
extendsprovides methods needed by
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Java RMI
Server Details – Implementation Class
Create a class that implements the interface. The class should also extend UnicastRemoteObject*
This class needs a constructor that throws RemoteException !
This class is now used by rmic to create the stub and skeleton code.
*It doesn’t have to extend UnicastRemoteObject, there is another way…
Note: As of the J2SE 5.0 release, stub classes for remote objects no longer need to be pregenerated using the rmic stub compiler, unless the remote object needs to support clients running in pre-5.0 VMs.
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Java RMI
Generating stubs and skeleton
Compile the remote interface and implementation:
> javac MyRemote.java MyRemoteImpl.java
Use rmic to generate MyRemoteImpl_stub.class, MyRemoteImpl_skel.class
> rmic MyRemoteImpl
Note: As of the J2SE 5.0 release, stub classes for remote objects no longer need to be pregenerated using the rmic stub compiler, unless the remote object needs to support clients running in pre-5.0 VMs.
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Java RMI
Server Detail – main()
The server main() needs to: create a remote object. register the object with the Naming service.
public static void main(String args[]) {
try {
MyRemoteImpl r = new MyRemoteImpl();
javaNaming.bind(“joe”,r);
} catch (RemoteException e) {
. . .
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Java RMI
Client Details
The client needs to ask the Naming service for a reference to a remote object. The client needs to know the hostname or IP address of the
machine running the server. The client needs to know the name of the remote object.
The Naming service uses URLs to identify remote objects.
rmi://hostname/objectname
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Java RMI
Using The Naming service
Naming.lookup() method takes a string parameter that holds a URL indicating the remote object to lookup.
rmi://hostname/objectname
Naming.lookup() returns an Object!
Naming.lookup() can throw RemoteException MalformedURLException
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Java RMI
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Java RMI
RMI registry (1/2)
A naming service Maps names to remote objects Provides clients with a mechanism to find remote services
running on RMI servers
Essential operations: bind/rebind, unbind, lookup Bind adds a service entry to the registry Unbind removes a service entry from the registry Lookup allows clients to find the service’s address using
service name
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Java RMI
RMI registry (2/2)
Names in the registry use unique names Recommend the name of the remote class that implements
the remote interface
For accessing a remote registry, use the following URL form rmi://host:port/serviceName host is the machine on which the registry is running The registry is listening on the port (default 1099)
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Java RMI
Start the Java RMI registry
By default, the registry runs on TCP port 1099. To start a registry on a different port, specify the port number from the command line. For example, to start the registry on port 2001 on a Windows platform:
start rmiregistry 2001
If the registry will be running on a port other than 1099, you'll need to specify the port number in the calls to LocateRegistry.getRegistry in the Server and Client classes. For example, if the registry is running on port 2001 in this example, the call to getRegistry in the server would be: Registry registry =
LocateRegistry.getRegistry(2001);
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Java RMI
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Java RMI
Java RMI classes
Java.rmi.Remote Interface supporting remote objects
java.rmi.server.UnicastRemoteObject Continuously running server
java.rmi.activation.Activatable Server started by rmid daemon
java.rmi.Naming Lookup: Returns stub given a name
Java.rmi.registry.Registry , LocateRegistryjava.rmi.RMISecurityManager Validates rights to access downloaded object
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Java RMI
Other Technologies
CORBA XML based
SOAP / Web Service
J2EE/EJB MicroSoft’s .NET
Has Common Language Runtime
Tight Integration of Web Services and XML
Multiple language integration However only one platform Windows
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Java RMI
Reference RMI docs
java.sun.com/products/jdk/rmi/ Java Tutorial on RMI:
http://java.sun.com/j2se/1.5.0/docs/guide/rmi
JavaIDLjava.sun.com/docs/books/tutorial/idl
java.sun.com/products/jdk/idl/
www.omg.org/news/begin.htm
Jiniwww.artima.com/jini/resources
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Programming Paradigms
Imperative Programming (FORTRAN, C, Pascal, …) The most common programming paradigm
Functional Programming (LISP, …)
Logic Programming (Prolog)
(Declarative programming language; 宣告式語言 )
Object-Oriented Programming
(Smalltalk, C++, Java, …)
Simply using C++/Java constructs does not automatically lead to well-organized Object-Oriented Programs.
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Java RMI
Why OO Programming?Better concepts and tools to model and represent the real world as closely as possible (including concurrency, e.g., in Windows GUI)=> model of reality=> behavior modelingBetter reusability & extensibility (inheritance)=> reduce the time/cost of development
Enhanced maintainability & improved reliability – “Encapsulation” and “Information Hiding” Object only accessible through the external interface Internal implementation details are not visible outside Localized changes to implementation of classes Completeness: all required operations are defined Independent testing and maintenance
Help writing good program more easily
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Java RMI
RPC / RMI
謝謝捧場http://www.csie.nctu.edu.tw/~tsaiwn/java/
蔡文能
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