Network Clients in Java

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622 Network Clients
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Network clients initiate connections and usually take charge of network transactions The server is there to fulfill the requests of the client a client does not fulfill the requests of a server Although the client is in control, some power still resides in the server, of course A client can tell a server to delete all files on the local file system, but the server isn't necessarily compelled to carry out that action (thankfully!) The network client speaks to the server using an agreed-upon standard for communication, the network protocol For example, an HTTP client uses a set of commands different from a mail client, and has a completely different purpose Connecting an HTTP client to a mail server, or a mail client to an HTTP server, will result not only in an error message but in an error message that the client will not understand For this reason, as part of the protocol specification, a port number is used so that the client can locate the server A Web server typically runs on port 80, and while some servers can run on nonstandard ports, the convention for a URL is not to list a port, as it is assumed that port 80 is used For more information on ports, see Section 612
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623 Network Servers
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The role of the network server is to bind to a specific port (which is used by the client to locate the server), and to listen for new connections While the client is temporary, and runs only when the user chooses, the server must run continually (even if no clients are actually connected) in the hope that someone, at some time, will want its services The server is often referred to as a daemon process, to use Unix parlance It runs indefinitely, and is normally automatically started when the host computer of the server is started So the server waits, and waits, and waits, until a client establishes a connection to the server port Some servers can handle only a single connection at a time, while others can handle many connections concurrently, through the use of threads Multi-threaded programming is discussed in depth in 7 When a connection is being processed, the server is submissive It waits for the client to send requests, and dutifully processes them (though the server is free to respond with an error message, particularly if the request violates some important precept of the protocol or presents a security risk) Some protocols, like HTTP/10, normally allow only one request per connection, whereas others, such as POP3, support a sequence of requests Servers will answer the client request by sending either a response or an error message the format of which varies from protocol to protocol Learning a network protocol (when writing either a client or a server) is a little like learning a new language, as the syntax changes Typically, though, the number of commands is much smaller, making things a little easier The behavior of the server is determined in part by the protocol and in part by the developer (Some commands may be optional, and are not always supported by server implementations)
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63 TCP Sockets and Java
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Java offers good support for TCP sockets, in the form of two socket classes, javanetSocket and javanetServerSocket When writing client software that connects to an existing service, the Socket class should be used When writing server software that binds to a local port in order to provide a service, the ServerSocket class should be employed This is different from the way a DatagramSocket works with UDP the function of connecting to servers, and the function of accepting data from clients, is split into a separate class under TCP
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