Class ServerSocket

All Implemented Interfaces:
Closeable, AutoCloseable
Direct Known Subclasses:
SSLServerSocket
public class ServerSocket
extends Object
implements Closeable

This class implements server sockets. A server socket waits for requests to come in over the network. It performs some operation based on that request, and then possibly returns a result to the requester.

The actual work of the server socket is performed by an instance of the SocketImpl class. An application can change the socket factory that creates the socket implementation to configure itself to create sockets appropriate to the local firewall.

Since:
1.0
See Also:
SocketImpl, setSocketFactory(java.net.SocketImplFactory), ServerSocketChannel

Constructors

Constructor Description
ServerSocket()

Creates an unbound server socket.
ServerSocket​(int port)

Creates a server socket, bound to the specified port.
ServerSocket​(int port, int backlog)

Creates a server socket and binds it to the specified local port number, with the specified backlog.
ServerSocket​(int port, int backlog, InetAddress bindAddr)

Create a server with the specified port, listen backlog, and local IP address to bind to.

Methods

Modifier and Type Method Description
Socket accept()

Listens for a connection to be made to this socket and accepts it.
void bind​(SocketAddress endpoint)

Binds the ServerSocket to a specific address (IP address and port number).
void bind​(SocketAddress endpoint, int backlog)

Binds the ServerSocket to a specific address (IP address and port number).
void close()

Closes this socket.
ServerSocketChannel getChannel()

Returns the unique ServerSocketChannel object associated with this socket, if any.
InetAddress getInetAddress()

Returns the local address of this server socket.
int getLocalPort()

Returns the port number on which this socket is listening.
SocketAddress getLocalSocketAddress()

Returns the address of the endpoint this socket is bound to.
<T> T getOption​(SocketOption<T> name)

Returns the value of a socket option.
int getReceiveBufferSize()

Gets the value of the SO_RCVBUF option for this ServerSocket, that is the proposed buffer size that will be used for Sockets accepted from this ServerSocket.
boolean getReuseAddress()

Tests if SO_REUSEADDR is enabled.
int getSoTimeout()

Retrieve setting for SO_TIMEOUT. 0 returns implies that the option is disabled (i.e., timeout of infinity).
protected void implAccept​(Socket s)

Subclasses of ServerSocket use this method to override accept() to return their own subclass of socket.
boolean isBound()

Returns the binding state of the ServerSocket.
boolean isClosed()

Returns the closed state of the ServerSocket.
<T> ServerSocket setOption​(SocketOption<T> name, T value)

Sets the value of a socket option.
void setPerformancePreferences​(int connectionTime, int latency, int bandwidth)

Sets performance preferences for this ServerSocket.
void setReceiveBufferSize​(int size)

Sets a default proposed value for the SO_RCVBUF option for sockets accepted from this ServerSocket.
void setReuseAddress​(boolean on)

Enable/disable the SO_REUSEADDR socket option.
static void setSocketFactory​(SocketImplFactory fac)

Sets the server socket implementation factory for the application.
void setSoTimeout​(int timeout)

Enable/disable SO_TIMEOUT with the specified timeout, in milliseconds.
Set<SocketOption<?>> supportedOptions()

Returns a set of the socket options supported by this server socket.
String toString()

Returns the implementation address and implementation port of this socket as a String.

Methods declared in class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructors

ServerSocket

public ServerSocket()
             throws IOException

Creates an unbound server socket.

Throws:
IOException - IO error when opening the socket.

ServerSocket

public ServerSocket(int port)
             throws IOException

Creates a server socket, bound to the specified port. A port number of 0 means that the port number is automatically allocated, typically from an ephemeral port range. This port number can then be retrieved by calling getLocalPort.

The maximum queue length for incoming connection indications (a request to connect) is set to 50. If a connection indication arrives when the queue is full, the connection is refused.

If the application has specified a server socket factory, that factory's createSocketImpl method is called to create the actual socket implementation. Otherwise a "plain" socket is created.

If there is a security manager, its checkListen method is called with the port argument as its argument to ensure the operation is allowed. This could result in a SecurityException.

Parameters:
port - the port number, or 0 to use a port number that is automatically allocated.
Throws:
IOException - if an I/O error occurs when opening the socket.
SecurityException - if a security manager exists and its checkListen method doesn't allow the operation.
IllegalArgumentException - if the port parameter is outside the specified range of valid port values, which is between 0 and 65535, inclusive.
See Also:
SocketImpl, SocketImplFactory.createSocketImpl(), setSocketFactory(java.net.SocketImplFactory), SecurityManager.checkListen(int)

ServerSocket

public ServerSocket(int port,
                    int backlog)
             throws IOException

Creates a server socket and binds it to the specified local port number, with the specified backlog. A port number of 0 means that the port number is automatically allocated, typically from an ephemeral port range. This port number can then be retrieved by calling getLocalPort.

The maximum queue length for incoming connection indications (a request to connect) is set to the backlog parameter. If a connection indication arrives when the queue is full, the connection is refused.

If the application has specified a server socket factory, that factory's createSocketImpl method is called to create the actual socket implementation. Otherwise a "plain" socket is created.

If there is a security manager, its checkListen method is called with the port argument as its argument to ensure the operation is allowed. This could result in a SecurityException. The backlog argument is the requested maximum number of pending connections on the socket. Its exact semantics are implementation specific. In particular, an implementation may impose a maximum length or may choose to ignore the parameter altogther. The value provided should be greater than 0. If it is less than or equal to 0, then an implementation specific default will be used.

Parameters:
port - the port number, or 0 to use a port number that is automatically allocated.
backlog - requested maximum length of the queue of incoming connections.
Throws:
IOException - if an I/O error occurs when opening the socket.
SecurityException - if a security manager exists and its checkListen method doesn't allow the operation.
IllegalArgumentException - if the port parameter is outside the specified range of valid port values, which is between 0 and 65535, inclusive.
See Also:
SocketImpl, SocketImplFactory.createSocketImpl(), setSocketFactory(java.net.SocketImplFactory), SecurityManager.checkListen(int)

ServerSocket

public ServerSocket(int port,
                    int backlog,
                    InetAddress bindAddr)
             throws IOException

Create a server with the specified port, listen backlog, and local IP address to bind to. The bindAddr argument can be used on a multi-homed host for a ServerSocket that will only accept connect requests to one of its addresses. If bindAddr is null, it will default accepting connections on any/all local addresses. The port must be between 0 and 65535, inclusive. A port number of 0 means that the port number is automatically allocated, typically from an ephemeral port range. This port number can then be retrieved by calling getLocalPort.

If there is a security manager, this method calls its checkListen method with the port argument as its argument to ensure the operation is allowed. This could result in a SecurityException. The backlog argument is the requested maximum number of pending connections on the socket. Its exact semantics are implementation specific. In particular, an implementation may impose a maximum length or may choose to ignore the parameter altogther. The value provided should be greater than 0. If it is less than or equal to 0, then an implementation specific default will be used.

Parameters:
port - the port number, or 0 to use a port number that is automatically allocated.
backlog - requested maximum length of the queue of incoming connections.
bindAddr - the local InetAddress the server will bind to
Throws:
SecurityException - if a security manager exists and its checkListen method doesn't allow the operation.
IOException - if an I/O error occurs when opening the socket.
IllegalArgumentException - if the port parameter is outside the specified range of valid port values, which is between 0 and 65535, inclusive.
Since:
1.1
See Also:
SocketOptions, SocketImpl, SecurityManager.checkListen(int)

Methods

bind

public void bind(SocketAddress endpoint)
          throws IOException

Binds the ServerSocket to a specific address (IP address and port number).

If the address is null, then the system will pick up an ephemeral port and a valid local address to bind the socket.

Parameters:
endpoint - The IP address and port number to bind to.
Throws:
IOException - if the bind operation fails, or if the socket is already bound.
SecurityException - if a SecurityManager is present and its checkListen method doesn't allow the operation.
IllegalArgumentException - if endpoint is a SocketAddress subclass not supported by this socket
Since:
1.4

bind

public void bind(SocketAddress endpoint,
                 int backlog)
          throws IOException

Binds the ServerSocket to a specific address (IP address and port number).

If the address is null, then the system will pick up an ephemeral port and a valid local address to bind the socket.

The backlog argument is the requested maximum number of pending connections on the socket. Its exact semantics are implementation specific. In particular, an implementation may impose a maximum length or may choose to ignore the parameter altogther. The value provided should be greater than 0. If it is less than or equal to 0, then an implementation specific default will be used.

Parameters:
endpoint - The IP address and port number to bind to.
backlog - requested maximum length of the queue of incoming connections.
Throws:
IOException - if the bind operation fails, or if the socket is already bound.
SecurityException - if a SecurityManager is present and its checkListen method doesn't allow the operation.
IllegalArgumentException - if endpoint is a SocketAddress subclass not supported by this socket
Since:
1.4

getInetAddress

public InetAddress getInetAddress()

Returns the local address of this server socket.

If the socket was bound prior to being closed, then this method will continue to return the local address after the socket is closed.

If there is a security manager set, its checkConnect method is called with the local address and -1 as its arguments to see if the operation is allowed. If the operation is not allowed, the loopback address is returned.

Returns:
the address to which this socket is bound, or the loopback address if denied by the security manager, or null if the socket is unbound.
See Also:
SecurityManager.checkConnect(java.lang.String, int)

getLocalPort

public int getLocalPort()

Returns the port number on which this socket is listening.

If the socket was bound prior to being closed, then this method will continue to return the port number after the socket is closed.

Returns:
the port number to which this socket is listening or -1 if the socket is not bound yet.

getLocalSocketAddress

public SocketAddress getLocalSocketAddress()

Returns the address of the endpoint this socket is bound to.

If the socket was bound prior to being closed, then this method will continue to return the address of the endpoint after the socket is closed.

If there is a security manager set, its checkConnect method is called with the local address and -1 as its arguments to see if the operation is allowed. If the operation is not allowed, a SocketAddress representing the loopback address and the local port to which the socket is bound is returned.

Returns:
a SocketAddress representing the local endpoint of this socket, or a SocketAddress representing the loopback address if denied by the security manager, or null if the socket is not bound yet.
Since:
1.4
See Also:
getInetAddress(), getLocalPort(), bind(SocketAddress), SecurityManager.checkConnect(java.lang.String, int)

accept

public Socket accept()
              throws IOException

Listens for a connection to be made to this socket and accepts it. The method blocks until a connection is made.

A new Socket s is created and, if there is a security manager, the security manager's checkAccept method is called with s.getInetAddress().getHostAddress() and s.getPort() as its arguments to ensure the operation is allowed. This could result in a SecurityException.

Returns:
the new Socket
Throws:
IOException - if an I/O error occurs when waiting for a connection.
SecurityException - if a security manager exists and its checkAccept method doesn't allow the operation.
SocketTimeoutException - if a timeout was previously set with setSoTimeout and the timeout has been reached.
IllegalBlockingModeException - if this socket has an associated channel, the channel is in non-blocking mode, and there is no connection ready to be accepted
See Also:
SecurityManager.checkAccept(java.lang.String, int)

implAccept

protected final void implAccept(Socket s)
                         throws IOException

Subclasses of ServerSocket use this method to override accept() to return their own subclass of socket. So a FooServerSocket will typically hand this method an empty FooSocket. On return from implAccept the FooSocket will be connected to a client.

Parameters:
s - the Socket
Throws:
IllegalBlockingModeException - if this socket has an associated channel, and the channel is in non-blocking mode
IOException - if an I/O error occurs when waiting for a connection.
Since:
1.1

close

public void close()
           throws IOException

Closes this socket. Any thread currently blocked in accept() will throw a SocketException.

If this socket has an associated channel then the channel is closed as well.

Specified by:
close in interface AutoCloseable
Specified by:
close in interface Closeable
Throws:
IOException - if an I/O error occurs when closing the socket.

getChannel

public ServerSocketChannel getChannel()

Returns the unique ServerSocketChannel object associated with this socket, if any.

A server socket will have a channel if, and only if, the channel itself was created via the ServerSocketChannel.open method.

Returns:
the server-socket channel associated with this socket, or null if this socket was not created for a channel
Since:
1.4

isBound

public boolean isBound()

Returns the binding state of the ServerSocket.

Returns:
true if the ServerSocket successfully bound to an address
Since:
1.4

isClosed

public boolean isClosed()

Returns the closed state of the ServerSocket.

Returns:
true if the socket has been closed
Since:
1.4

setSoTimeout

public void setSoTimeout(int timeout)
                  throws SocketException

Enable/disable SO_TIMEOUT with the specified timeout, in milliseconds. With this option set to a non-zero timeout, a call to accept() for this ServerSocket will block for only this amount of time. If the timeout expires, a java.net.SocketTimeoutException is raised, though the ServerSocket is still valid. The option must be enabled prior to entering the blocking operation to have effect. The timeout must be > 0. A timeout of zero is interpreted as an infinite timeout.

Parameters:
timeout - the specified timeout, in milliseconds
Throws:
SocketException - if there is an error in the underlying protocol, such as a TCP error.
Since:
1.1
See Also:
getSoTimeout()

getSoTimeout

public int getSoTimeout()
                 throws IOException

Retrieve setting for SO_TIMEOUT. 0 returns implies that the option is disabled (i.e., timeout of infinity).

Returns:
the SO_TIMEOUT value
Throws:
IOException - if an I/O error occurs
Since:
1.1
See Also:
setSoTimeout(int)

setReuseAddress

public void setReuseAddress(boolean on)
                     throws SocketException

Enable/disable the SO_REUSEADDR socket option.

When a TCP connection is closed the connection may remain in a timeout state for a period of time after the connection is closed (typically known as the TIME_WAIT state or 2MSL wait state). For applications using a well known socket address or port it may not be possible to bind a socket to the required SocketAddress if there is a connection in the timeout state involving the socket address or port.

Enabling SO_REUSEADDR prior to binding the socket using bind(SocketAddress) allows the socket to be bound even though a previous connection is in a timeout state.

When a ServerSocket is created the initial setting of SO_REUSEADDR is not defined. Applications can use getReuseAddress() to determine the initial setting of SO_REUSEADDR.

The behaviour when SO_REUSEADDR is enabled or disabled after a socket is bound (See isBound()) is not defined.

Parameters:
on - whether to enable or disable the socket option
Throws:
SocketException - if an error occurs enabling or disabling the SO_REUSEADDR socket option, or the socket is closed.
Since:
1.4
See Also:
getReuseAddress(), bind(SocketAddress), isBound(), isClosed()

getReuseAddress

public boolean getReuseAddress()
                        throws SocketException

Tests if SO_REUSEADDR is enabled.

Returns:
a boolean indicating whether or not SO_REUSEADDR is enabled.
Throws:
SocketException - if there is an error in the underlying protocol, such as a TCP error.
Since:
1.4
See Also:
setReuseAddress(boolean)

toString

public String toString()

Returns the implementation address and implementation port of this socket as a String.

If there is a security manager set, its checkConnect method is called with the local address and -1 as its arguments to see if the operation is allowed. If the operation is not allowed, an InetAddress representing the loopback address is returned as the implementation address.

Overrides:
toString in class Object
Returns:
a string representation of this socket.

setSocketFactory

public static void setSocketFactory(SocketImplFactory fac)
                             throws IOException

Sets the server socket implementation factory for the application. The factory can be specified only once.

When an application creates a new server socket, the socket implementation factory's createSocketImpl method is called to create the actual socket implementation.

Passing null to the method is a no-op unless the factory was already set.

If there is a security manager, this method first calls the security manager's checkSetFactory method to ensure the operation is allowed. This could result in a SecurityException.

Parameters:
fac - the desired factory.
Throws:
IOException - if an I/O error occurs when setting the socket factory.
SocketException - if the factory has already been defined.
SecurityException - if a security manager exists and its checkSetFactory method doesn't allow the operation.
See Also:
SocketImplFactory.createSocketImpl(), SecurityManager.checkSetFactory()

setReceiveBufferSize

public void setReceiveBufferSize(int size)
                          throws SocketException

Sets a default proposed value for the SO_RCVBUF option for sockets accepted from this ServerSocket. The value actually set in the accepted socket must be determined by calling Socket.getReceiveBufferSize() after the socket is returned by accept().

The value of SO_RCVBUF is used both to set the size of the internal socket receive buffer, and to set the size of the TCP receive window that is advertized to the remote peer.

It is possible to change the value subsequently, by calling Socket.setReceiveBufferSize(int). However, if the application wishes to allow a receive window larger than 64K bytes, as defined by RFC1323 then the proposed value must be set in the ServerSocket before it is bound to a local address. This implies, that the ServerSocket must be created with the no-argument constructor, then setReceiveBufferSize() must be called and lastly the ServerSocket is bound to an address by calling bind().

Failure to do this will not cause an error, and the buffer size may be set to the requested value but the TCP receive window in sockets accepted from this ServerSocket will be no larger than 64K bytes.

Parameters:
size - the size to which to set the receive buffer size. This value must be greater than 0.
Throws:
SocketException - if there is an error in the underlying protocol, such as a TCP error.
IllegalArgumentException - if the value is 0 or is negative.
Since:
1.4
See Also:
getReceiveBufferSize()

getReceiveBufferSize

public int getReceiveBufferSize()
                         throws SocketException

Gets the value of the SO_RCVBUF option for this ServerSocket, that is the proposed buffer size that will be used for Sockets accepted from this ServerSocket.

Note, the value actually set in the accepted socket is determined by calling Socket.getReceiveBufferSize().

Returns:
the value of the SO_RCVBUF option for this Socket.
Throws:
SocketException - if there is an error in the underlying protocol, such as a TCP error.
Since:
1.4
See Also:
setReceiveBufferSize(int)

setPerformancePreferences

public void setPerformancePreferences(int connectionTime,
                                      int latency,
                                      int bandwidth)

Sets performance preferences for this ServerSocket.

Sockets use the TCP/IP protocol by default. Some implementations may offer alternative protocols which have different performance characteristics than TCP/IP. This method allows the application to express its own preferences as to how these tradeoffs should be made when the implementation chooses from the available protocols.

Performance preferences are described by three integers whose values indicate the relative importance of short connection time, low latency, and high bandwidth. The absolute values of the integers are irrelevant; in order to choose a protocol the values are simply compared, with larger values indicating stronger preferences. If the application prefers short connection time over both low latency and high bandwidth, for example, then it could invoke this method with the values (1, 0, 0). If the application prefers high bandwidth above low latency, and low latency above short connection time, then it could invoke this method with the values (0, 1, 2).

Invoking this method after this socket has been bound will have no effect. This implies that in order to use this capability requires the socket to be created with the no-argument constructor.

Parameters:
connectionTime - An int expressing the relative importance of a short connection time
latency - An int expressing the relative importance of low latency
bandwidth - An int expressing the relative importance of high bandwidth
Since:
1.5

setOption

public <T> ServerSocket setOption(SocketOption<T> name,
                                  T value)
                           throws IOException

Sets the value of a socket option.

Type Parameters:
T - The type of the socket option value
Parameters:
name - The socket option
value - The value of the socket option. A value of null may be valid for some options.
Returns:
this ServerSocket
Throws:
UnsupportedOperationException - if the server socket does not support the option.
IllegalArgumentException - if the value is not valid for the option.
IOException - if an I/O error occurs, or if the socket is closed.
NullPointerException - if name is null
SecurityException - if a security manager is set and if the socket option requires a security permission and if the caller does not have the required permission. StandardSocketOptions do not require any security permission.
Since:
9

getOption

public <T> T getOption(SocketOption<T> name)
                throws IOException

Returns the value of a socket option.

Type Parameters:
T - The type of the socket option value
Parameters:
name - The socket option
Returns:
The value of the socket option.
Throws:
UnsupportedOperationException - if the server socket does not support the option.
IOException - if an I/O error occurs, or if the socket is closed.
NullPointerException - if name is null
SecurityException - if a security manager is set and if the socket option requires a security permission and if the caller does not have the required permission. StandardSocketOptions do not require any security permission.
Since:
9

supportedOptions

public Set<SocketOption<?>> supportedOptions()

Returns a set of the socket options supported by this server socket. This method will continue to return the set of options even after the socket has been closed.

Returns:
A set of the socket options supported by this socket. This set may be empty if the socket's SocketImpl cannot be created.
Since:
9

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https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/net/ServerSocket.html