Java and Client Server Models
The Role of Client Servers on the Web
Client server models provide the essential mechanisms for working with the Internet. In fact, most of the World Wide Web is built according to this paradigm. In client server models the web browsers run by millions of users are the clients. On the other side of the equation, are the web hosting systems that run at host sites and provide access to processes and data requested by the client. In this case, these hosting systems are the server.
This definition is based on software programs, where the client is a program running on a remote machine that communicates with a server, a program running at a single site and providing responses to client requests, such as web pages or data.
Java is a programming language that has been developed specifically for the distributed environment of the Internet. It resembles C++ language, but is easier to use. C++ is a high level programming language that performs low level functions. In computing, low level functions are those that focus on individual components and the interaction between them as opposed to abstracted and systemic features (high level). Java, like C++, is a language that is multi paradigm and supports object oriented programming (OOP), procedural programming, and data abstraction.
Object oriented programming is increasingly being used in client server technology. It refers to a programming language model that is organized by objects rather than actions, data rather than logic. OOP identifies objects (sets of data) and defines their relationship to each other. These objects are then generalized into a class. Methods or sequences of logic are applied to classes of objects. Methods provide computational instructions and class object features provide the data that is acted upon. Users communicate with objects and objects with each other through specifically defined interfaces called messages.
Java can create applications that are distributed between clients and servers in a network. Java source code (signaled by .java extension) is compiled (source code transformed into object code) into bytecode (signaled by .class extension). A Java interpreter then executes this bytecode format. Java interpreters and runtime environment run on Java Virtual Machines (JVMs). The portability and usability that characterizes Java stems from the fact that JVMs exist for most operating systems, from UNIX, to Windows, to Mac OS.
Java is one of the most well suited languages for working on the World Wide Web and the client server model is the primary models for working on distributed networks, of which the World Wide Web is just one. There is natural affinity between the two and this article will discuss some major characteristics of Java and how it can be utilized in building client server systems.
Major Characteristics of Java
This section will list and discuss some of Java’s major characteristic to support later discussions on how Java is used in client server systems. Perhaps the first and most obvious characteristics are that, given its syntactical similarity to C and C++ languages, Java is simple, simpler, in fact, than the languages it emulates. Java is also a robust programming language, which means it creates software that will identify and correct errors and handle abnormal conditions intelligently.
Another major characteristic of Java is that it is object oriented programming, which was described above. OOP is characterized by three properties also present in Java programming: inheritance, encapsulation, and polymorphism. Inheritance is a major component in OOP which defines a general class and then specializes these classes by adding additional details in the already written class. Programmers only have to write the new features since the specialized class inherits all the features of the generalized class.
In OOP, encapsulation is the inclusion of all methods and data required for an object to function. Objects publish their interfaces and other objects communicate with these object interfaces to use them without having to understand how the encapsulated object performs its functions. It is the separation of interface and implementation. Polymorphism in OOP in general and Java specifically, is the ability to assign a different set of behaviors to an object in a subclass from the methods describe in the more general class. Therefore, subclasses can behave differently from the parent class without the parent class having to understand why or change itself.
Multi threading is also an important characteristic of Java that increases interactive responsiveness and real time performance. Threading is the way a program splits or forks itself into two or more tasks running simultaneously. This allows for thread based multi tasking. Multi threading creates the effect of multiple threads running in parallel on different machines simultaneously.
Socket based Client Server Systems in Java
Java builds client server systems with sockets. Sockets are the endpoints of two way communication between programs running in a network. They are software objects that connect applications to network protocols, so they become intelligible. For example, in UNIX a program opens a socket that enables it to send and receive messages from the socket. This simplifies software development because programmers only have to change or specify the socket, while the operating system is left in tact.
In client server models the server contains sockets bound to specific port numbers. Port numbers identify specific processes that are to be forwarded over a network, like the Internet, in the form of messages to the server. The server only has to monitor the socket and respond when a client requests a connection. Once connections have been made and bound to port numbers, the two endpoints, client and server, can communicate through the socket. Java sockets are client side sockets, known simply as sockets and server side sockets known as server sockets. Each belong to their own class within a Java package. The client initiates the socket, which contains a host name and port number, by requesting connections.
The server socket class in Java allows for servers to monitor requests for connections. As the socket communicates over the network, Java’s server socket class assigns a port number to each client in the server and creates a socket object or instance. This server socket instance creates a connection and produces a thread or string of processes through which the server can communicate with the client through the socket. Java web servers are built according to this model.
TCP (Transmission Control Protocol) works in conjunction with IP (Internet Protocol) to send messages between computers over the Internet. When communicating over the Internet, the client program and the server program each attach a socket to their end of the connection. Then the client and server can read from and write to the socket. Java provides operating system sockets that allow two or more processes to send and receive data, regardless of computer location.
Java’s RMI System
The other method for using Java to build client server systems is RMI. RMI stands for Remote Method Invocation. By using Java language and functions, programmers write object oriented programming, so that objects that are distributed over a network can interact with each other. RMI allows for client objects in JVMs to request services through a network from another computer (host or server). Client objects included with the request may call upon methods in the remote computer that change the results. Methods are programmed procedures attributed to a class that are contained in each of its objects (or instances). It is a characteristic of object oriented programming.
Classes and, therefore, objects can have more than one method and methods can be used for more than one object. Responses then run as if they were local (on the same computer.) This passing back and forth of objects and methods attached to objects is called ‘object serialization’. Simply put, RMI requests call upon the method of a remote object. As previously stated, it uses the same syntax it would locally. To make this intelligible to the servers or sites being called upon requires three layers: a client side stub program, a remote reference layer, and a transport connection layer. Each request travels down the layers of the client computer and up the layers of the server.
The client side stub program initiates the request. Stub programs are small sections of programs containing routines from larger programs. It is a substitute for programs that may take too long to load or are located remotely on a network. They are provided by the server at the client’s request. Stubs accept client requests and communicate requested procedures (through another program) to remote applications. They also return the results to the client or requesting program. These stubs mimic the program being called for service.
In Java, stub programs are also referred to as ‘proxy’. Remote reference layers manage reference variables for remote objects, using the transport layer’s TCP (Transmission Control Protocol) connection to the server. Reference variables contain class data and therefore include methods. The transport layer protects and maintains end to end communication through a network. The most used transport layer is TCP. After the client requests pass through the transport layer, they pass through another remote reference layer before requesting implementation of the request by the server from a skeleton. These skeletons are written in high level IDL (Interface Definition Language). The server receives the request and sends the response along the same channels, but in the other direction.
