Socket Basics
A socket is a connection between two hosts. It can perform seven basic operations:
- Connect to a remote machine
- Send data
- Receive data
- Close a connection
- Bind to a port
- Listen for incoming data
- Accept connections from remote machines on the bound port
Java's Socket class, which is used by both clients and servers, has methods that correspond to the first four of these operations. The last three operations are needed only by servers, which wait for clients to connect to them. They are implemented by the ServerSocket class, which is discussed in the next chapter. Java programs normally use client sockets in the following fashion:
- The program creates a new socket with a constructor.
- The socket attempts to connect to the remote host.
- Once the connection is established, the local and remote hosts get input and output streams from the socket and use those streams to send data to each other. This connection is full-duplex; both hosts can send and receive data simultaneously. What the data means depends on the protocol; different commands are sent to an FTP server than to an HTTP server. There will normally be some agreed-upon hand-shaking followed by the transmission of data from one to the other.
- When the transmission of data is complete, one or both sides close the connection. Some protocols, such as HTTP 1.0, require the connection to be closed after each request is serviced. Others, such as FTP, allow multiple requests to be processed in a single connection.