Chapter 16. The java.net Package

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This interface is implemented by: Subclasses of these should override the methods of this interface in order to support their own options. The methods and constants which specify options in this interface are for implementation only. Disable Nagle's algorithm for this connection. Written data to the network is not buffered java so_bindaddr acknowledgement of previously written data. Fetch the local address binding of a socket this option cannot be "set" only "gotten", since sockets are bound at creation time, and so the locally bound address cannot be changed.

A multi-homed host can use this option to accept connections to only one of its addresses in the case of a ServerSocket or DatagramSocket java so_bindaddr, or to specify its return address to the peer for a Socket or DatagramSocket. The parameter of this option is an InetAddress. This is used only for MulticastSockets in java, and it is set by default for MulticastSockets. This java so_bindaddr enables and disables the ability of the process to send broadcast messages.

It is supported java so_bindaddr only datagram sockets and only on networks that support the java so_bindaddr of a broadcast message e. Ethernet, token ring, etc. Set which outgoing interface on which to send multicast packets. Useful on hosts with multiple network interfaces, where applications want to use other than the system default.

This option enables or disables local loopback of multicast datagrams. This option is enabled by default for Multicast Sockets. Specify a linger-on-close timeout. Enabling this option with a non-zero Integer timeout means that a close will block pending the transmission and acknowledgement of all data written to the peer, at which point the socket is closed gracefully.

Enabling java so_bindaddr option with a timeout of zero does a forceful close immediately. If the specified timeout value exceeds 65, it will be reduced java so_bindaddr 65, The option must be set prior to entering a blocking operation to take effect. If the timeout java so_bindaddr and the operation would continue to block, java. The Socket is not closed in this case. When used in set, this is a suggestion to the kernel java so_bindaddr the application about the size of buffers to use for the data to be sent over the socket.

When used in get, this must return the size java so_bindaddr the buffer actually used by the platform when sending out data on this socket. Valid for all sockets: When used in set, this is a suggestion to the java so_bindaddr from the application about the size of buffers to use for the data to be received over the socket. When used in get, this must return the size of the buffer actually used by the platform java so_bindaddr receiving in data on this socket. When the keepalive option is set for a TCP java so_bindaddr and no data has been exchanged across the socket in either direction for 2 hours NOTE: This probe is a TCP segment to which the java so_bindaddr must respond.

One of three responses is expected: The peer responds java so_bindaddr the expected ACK. The application is not notified since everything is OK. TCP will send another probe following another 2 hours of inactivity.

The socket is closed. There is no response from the peer. The purpose of this option is to detect if the peer host crashes. Valid only for TCP socket: When the option is disabled which is the default urgent data is silently discarded. If the option is to be enabled, and it takes an option-specific "value", this java so_bindaddr passed in value. The actual type of value is option-specific, and it is an error to pass something that isn't of the expected type: Fetch the value of an option.

Binary options will return java. Boolean true if enabled, java. Boolean false if disabled, e. For options that take a particular type as a parameter, getOption int will return the parameter's value, else it will return java. Valid for TCP only: This option must be specified in the constructor. Valid only for TCP: Set a timeout on blocking Socket operations:

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The last three operations are needed only by servers, which wait for clients to connect to them. They are implemented by the ServerSocket class. Java programs normally use client sockets in the following fashion:. 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. There will normally be some agreed-upon handshaking 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. This protocol is defined in RFC Reading that, you see that the daytime server listens on port 13, and that the server sends the time in a human-readable format and closes the connection. You can test the daytime server with Telnet like this:.

The other lines are produced either by the Unix shell or by the Telnet program. RFC does not specify any particular format for the output other than that it be human readable.

In this case, you can see this connection was made on March 24, , at 1: These details are all NIST specific. They are not part of the daytime standard. First, open a socket to time. It actually makes the connection across the network. In Java 7, Socket implements Autocloseable so you can use try-with-resources:. The next step is optional but highly recommended. Set a timeout on the connection using the setSoTimeout method. Timeouts are measured in milliseconds, so this statement sets the socket to time out after 15 seconds of nonresponsiveness:.

Setting a timeout on the socket means that each read from or write to the socket will take at most a certain number of milliseconds. Exactly how long a timeout to set depends on the needs of your application and how responsive you expect the server to be. In general, a server can send any bytes at all; but in this specific case, the protocol specifies that those bytes must be ASCII:. You can, of course, use any data structure that fits your problem to hold the data that comes off the network.

Example 1 puts this all together in a program that also allows you to choose a different daytime server. In most network programs like this, the real effort is in speaking the protocol and comprehending the data formats.

For instance, rather than simply printing out the text the server sends you, you might want to parse it into a java. Example 2 shows you how to do this. It just added a bunch of code to turn strings into dates. For example, the time protocol specified in RFC specifies that the time be sent as the number of seconds since midnight, January 1, , Greenwich Mean Time.

Rather, it is sent as a bit, unsigned, big-endian binary number. A Java program that connects to time servers must read the raw bytes and interpret them appropriately. When speaking other protocols, you may encounter data formats even more alien to Java.

For instance, a few network protocols use bit fixed-point numbers. You simply have to grit your teeth and code the math you need to handle the data in whatever format the server sends. Writing to a server is not noticeably harder than reading from one.

You simply ask the socket for an output stream as well as an input stream. In the most common pattern, the client sends a request. Then the server responds.

The client may send another request, and the server responds again. This continues until one side or the other is done, and closes the connection. This tells the server to send a definition of the word gold using its English-to-Latin dictionary. Different servers have different dictionaries installed. After the first definition is received, the client can ask for another.

You can explore dict with Telnet like this:. You can see that control response lines begin with a three-digit code. The actual definition is plain text, terminated with a period on a line by itself.

Of course, you could also find this out, and a lot more, by reading the RFC. First, open a socket to a dict server— dict. In the dict protocol, the client speaks first, so ask for the output stream using getOutputStream:.

The getOutputStream method returns a raw OutputStream for writing data from your application to the other end of the socket. The server should now respond with a definition. When you see a period on a line by itself, you know the definition is complete. You can then send the quit over the output stream:. Example 4 shows a complete dict client.

It connects to dict. It filters out all the metadata lines that begin with response codes such as or Example 4 is line oriented. It reads a line of input from the console, sends it to the server, and waits to read a line of output it gets back. The close method shuts down both input and output from the socket. On occasion, you may want to shut down only half of the connection, either input or output.

The shutdownInput and shutdownOutput methods close only half the connection:. Neither actually closes the socket. Further reads from the input stream after shutting down input return —1. Further writes to the socket after shutting down output throw an IOException. Many protocols, such as finger, whois, and HTTP, begin with the client sending a request to the server, then reading the response.

It would be possible to shut down the output after the client has sent the request. The isInputShutdown and isOutputShutdown methods tell you whether the input and output streams are open or closed, respectively.

You can use these rather than isConnected and isClosed to more specifically ascertain whether you can read from or write to a socket:. This class itself uses native code to communicate with the local TCP stack of the host operating system.

Each Socket constructor specifies the host and the port to connect to. Hosts may be specified as an InetAddress or a String. Remote ports are specified as int values from 1 to These constructors connect the socket i. In this constructor, the host argument is just a hostname expressed as a String. If the domain name server cannot resolve the hostname or is not functioning, the constructor throws an UnknownHostException.

If the socket cannot be opened for some other reason, the constructor throws an IOException. There are many reasons a connection attempt might fail: The first step to securing a system is understanding it. This program helps you understand what your system is doing so you can find and close possible entrance points for attackers. You may also find rogue servers: Three constructors create unconnected sockets.

These provide more control over exactly how the underlying socket behaves, for instance by choosing a different proxy server or an encryption scheme:. Two constructors specify both the host and port to connect to and the interface and port to connect from:.

This socket connects to the host and port specified in the first two arguments. It connects from the local network interface and port specified by the last two arguments.

The network interface may be either physical e. If 0 is passed for the localPort argument, Java chooses a random available port between and Selecting a particular network interface from which to send data is uncommon, but a need does come up occasionally.

Incoming connections would be accepted on one interface, processed, and forwarded to the local network from the other interface. Suppose you were writing a program to periodically dump error logs to a printer or send them over an internal mail server. This constructor can throw an IOException or an UnknownHostException for the same reasons as the previous constructors.

For instance, a program running on a. You could take deliberate advantage of this to restrict a compiled program to run on only a predetermined host.

It would require customizing distributions for each computer and is certainly overkill for cheap products. Furthermore, Java programs are so easy to disassemble, decompile, and reverse engineer that this scheme is far from foolproof.