# Class SSLSocket

java.lang.Object
java.net.Socket
javax.net.ssl.SSLSocket

All Implemented Interfaces:
  `Closeable, AutoCloseable`

public abstract class SSLSocket extends Socket
This class extends `Socket` and provides secure sockets using protocols such as the "Secure Sockets Layer" (SSL) or IETF "Transport Layer Security" (TLS) protocols.

Such sockets are normal stream sockets, but they add a layer of security protections over the underlying network transport protocol, such as TCP. Those protections include:

- Integrity Protection. SSL protects against modification of messages by an active wiretapper.
- Authentication. In most modes, SSL provides peer authentication. Servers are usually authenticated, and clients may be authenticated as requested by servers.
- Confidentiality (Privacy Protection). In most modes, SSL encrypts data being sent between client and server. This protects the confidentiality of data, so that passive wiretappers won't see sensitive data such as financial information or personal information of many kinds.

These kinds of protection are specified by a "cipher suite", which is a combination of cryptographic algorithms used by a given SSL connection. During the negotiation process, the two endpoints must agree on a ciphersuite that is available in both environments. If there is no such suite in common, no SSL connection can be established, and no data can be exchanged.

The cipher suite used is established by a negotiation process called "handshaking". The goal of this process is to create or rejoin a "session", which may protect many connections over time. After handshaking has completed, you can access session attributes by using the getSession method. The initial handshake on this connection can be initiated in one of three ways:

- calling `startHandshake` which explicitly begins handshakes, or
- any attempt to read or write application data on this socket causes an implicit handshake, or
- a call to `getSession` tries to set up a session if there is no currently valid session, and an implicit handshake is done.

If handshaking fails for any reason, the `SSLSocket` is closed, and no further communications can be done.

There are two groups of cipher suites which you will need to know about when managing cipher suites:

- Supported cipher suites: all the suites which are supported by the SSL implementation. This list is reported using getSupportedCipherSuites.
- Enabled cipher suites, which may be fewer than the full set of supported suites. This group is set using the setEnabledCipherSuites method, and queried using the getEnabledCipherSuites method. Initially, a default set of cipher suites will be enabled on a new socket that represents the minimum suggested configuration.

Implementation defaults require that only cipher suites which authenticate servers and provide confidentiality be enabled by default. Only if both sides explicitly agree to unauthenticated and/or non-private (unencrypted) communications will such a ciphersuite be selected.

When an `SSLSocket` is first created, no handshaking is done so that applications may first set their communication preferences: what cipher suites to use, whether the socket should be in client or server mode, etc. However, security is always provided by the time that application data is sent over the connection.

You may register to receive event notification of handshake completion. This involves the use of two additional classes. HandshakeCompletedEvent objects are passed to HandshakeCompletedListener instances, which are registered by users of this API. An `SSLSocket` is created by `SSLSocketFactory`, or by `accept`ing a connection from a `SSLServerSocket`.

A SSL socket must choose to operate in the client or server mode. This will determine who begins the handshaking process, as well as which messages should be sent by each party. Each connection must have one client and one server, or handshaking will not progress properly. Once the initial handshaking has started, a socket can not switch between client and server modes, even when performing renegotiations.

The ApplicationProtocol `String` values returned by the methods in this class are in the network byte representation sent by the peer. The bytes could be directly compared, or converted to its Unicode `String` format for comparison.

>         String networkString = sslSocket.getHandshakeApplicationProtocol();
>         byte[] bytes = networkString.getBytes(StandardCharsets.ISO_8859_1);

>         //
>         // Match using bytes:
>         //
>         //   "http/1.1"                       (7-bit ASCII values same in UTF-8)
>         //   MEETEI MAYEK LETTERS "HUK UN I"  (Unicode 0xabcd->0xabcf)
>         //
>         String HTTP1_1 = "http/1.1";
>         byte[] HTTP1_1_BYTES = HTTP1_1.getBytes(StandardCharsets.UTF_8);

>         byte[] HUK_UN_I_BYTES = new byte[] {
>             (byte) 0xab, (byte) 0xcd,
>             (byte) 0xab, (byte) 0xce,
>             (byte) 0xab, (byte) 0xcf};

>         if ((Arrays.compare(bytes, HTTP1_1_BYTES) == 0 )
>                 || Arrays.compare(bytes, HUK_UN_I_BYTES) == 0) {
>            ...
>         }

>         //
>         // Alternatively match using string.equals() if we know the ALPN value
>         // was encoded from a String using a certain character set,
>         // for example UTF-8.  The ALPN value must first be properly
>         // decoded to a Unicode String before use.
>         //
>         String unicodeString = new String(bytes, StandardCharsets.UTF_8);
>         if (unicodeString.equals(HTTP1_1)
>                 || unicodeString.equals("\uabcd\uabce\uabcf")) {
>             ...
>         }

API Note:
  When the connection is no longer needed, the client and server applications should each close both sides of their respective connection. This can be done either in one shot by calling `Socket.close()`, or by closing each side individually using `Socket.shutdownOutput()` / `Socket.shutdownInput()` which is useful for protocol versions that can support half-closed connections.

  Note that in some cases, closing the input stream may depend on the peer's output stream being closed first. If the connection is not closed in an orderly manner (for example `Socket.shutdownInput()` is called before the peer's write closure notification has been received), exceptions may be raised to indicate that an error has occurred.

  Once an `SSLSocket` is closed, it is not reusable: a new `SSLSocket` must be created.

Since:
  1.4
See Also:
  - `Socket`
  - `SSLServerSocket`
  - `SSLSocketFactory`

- ## Constructor Summary

  Constructors
  Modifier
  Constructor
  Description
  `protected`
  `SSLSocket()`
  Used only by subclasses.
  `protected`
  `SSLSocket(String host, int port)`
  Used only by subclasses.
  `protected`
  `SSLSocket(String host, int port, InetAddress clientAddress, int clientPort)`
  Used only by subclasses.
  `protected`
  `SSLSocket(InetAddress address, int port)`
  Used only by subclasses.
  `protected`
  `SSLSocket(InetAddress address, int port, InetAddress clientAddress, int clientPort)`
  Used only by subclasses.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `abstract void`
  `addHandshakeCompletedListener(HandshakeCompletedListener listener)`
  Registers an event listener to receive notifications that an SSL handshake has completed on this connection.
  `String`
  `getApplicationProtocol()`
  Returns the most recent application protocol value negotiated for this connection.
  `abstract String[]`
  `getEnabledCipherSuites()`
  Returns the names of the SSL cipher suites which are currently enabled for use on this connection.
  `abstract String[]`
  `getEnabledProtocols()`
  Returns the names of the protocol versions which are currently enabled for use on this connection.
  `abstract boolean`
  `getEnableSessionCreation()`
  Returns true if new SSL sessions may be established by this socket.
  `String`
  `getHandshakeApplicationProtocol()`
  Returns the application protocol value negotiated on a SSL/TLS handshake currently in progress.
  `BiFunction<SSLSocket, List<String>, String>`
  `getHandshakeApplicationProtocolSelector()`
  Retrieves the callback function that selects an application protocol value during a SSL/TLS/DTLS handshake.
  `SSLSession`
  `getHandshakeSession()`
  Returns the `SSLSession` being constructed during a SSL/TLS handshake.
  `abstract boolean`
  `getNeedClientAuth()`
  Returns true if the socket will require client authentication.
  `abstract SSLSession`
  `getSession()`
  Returns the SSL Session in use by this connection.
  `SSLParameters`
  `getSSLParameters()`
  Returns the SSLParameters in effect for this SSLSocket.
  `abstract String[]`
  `getSupportedCipherSuites()`
  Returns the names of the cipher suites which could be enabled for use on this connection.
  `abstract String[]`
  `getSupportedProtocols()`
  Returns the names of the protocols which could be enabled for use on an SSL connection.
  `abstract boolean`
  `getUseClientMode()`
  Returns true if the socket is set to use client mode when handshaking.
  `abstract boolean`
  `getWantClientAuth()`
  Returns true if the socket will request client authentication.
  `abstract void`
  `removeHandshakeCompletedListener(HandshakeCompletedListener listener)`
  Removes a previously registered handshake completion listener.
  `abstract void`
  `setEnabledCipherSuites(String[] suites)`
  Sets the cipher suites enabled for use on this connection.
  `abstract void`
  `setEnabledProtocols(String[] protocols)`
  Sets the protocol versions enabled for use on this connection.
  `abstract void`
  `setEnableSessionCreation(boolean flag)`
  Controls whether new SSL sessions may be established by this socket.
  `void`
  `setHandshakeApplicationProtocolSelector(BiFunction<SSLSocket, List<String>, String> selector)`
  Registers a callback function that selects an application protocol value for a SSL/TLS/DTLS handshake.
  `abstract void`
  `setNeedClientAuth(boolean need)`
  Configures the socket to require client authentication.
  `void`
  `setSSLParameters(SSLParameters params)`
  Applies SSLParameters to this socket.
  `abstract void`
  `setUseClientMode(boolean mode)`
  Configures the socket to use client (or server) mode when handshaking.
  `abstract void`
  `setWantClientAuth(boolean want)`
  Configures the socket to request client authentication.
  `abstract void`
  `startHandshake()`
  Starts an SSL handshake on this connection.

  ### Methods declared in class Socket

  `bind, close, connect, connect, getChannel, getInetAddress, getInputStream, getKeepAlive, getLocalAddress, getLocalPort, getLocalSocketAddress, getOOBInline, getOption, getOutputStream, getPort, getReceiveBufferSize, getRemoteSocketAddress, getReuseAddress, getSendBufferSize, getSoLinger, getSoTimeout, getTcpNoDelay, getTrafficClass, isBound, isClosed, isConnected, isInputShutdown, isOutputShutdown, sendUrgentData, setKeepAlive, setOOBInline, setOption, setPerformancePreferences, setReceiveBufferSize, setReuseAddress, setSendBufferSize, setSocketImplFactory, setSoLinger, setSoTimeout, setTcpNoDelay, setTrafficClass, shutdownInput, shutdownOutput, supportedOptions, toString`

  ### Methods declared in class Object

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

- ## Constructor Details

  - ### SSLSocket

    protected SSLSocket()
    Used only by subclasses. Constructs an uninitialized, unconnected TCP socket.

  - ### SSLSocket

    protected SSLSocket(String host, int port) throws IOException, UnknownHostException
    Used only by subclasses. Constructs a TCP connection to a named host at a specified port. This acts as the SSL client.
    Parameters:
      `host` - name of the host with which to connect, or `null` for the loopback address.
      `port` - number of the server's port
    Throws:
      `IOException` - if an I/O error occurs when creating the socket
      `UnknownHostException` - if the host is not known
      `IllegalArgumentException` - if the port parameter is outside the specified range of valid port values, which is between 0 and 65535, inclusive.

  - ### SSLSocket

    protected SSLSocket(InetAddress address, int port) throws IOException
    Used only by subclasses. Constructs a TCP connection to a server at a specified address and port. This acts as the SSL client.
    Parameters:
      `address` - the server's host
      `port` - its port
    Throws:
      `IOException` - if an I/O error occurs when creating the socket
      `IllegalArgumentException` - if the port parameter is outside the specified range of valid port values, which is between 0 and 65535, inclusive.
      `NullPointerException` - if `address` is null.

  - ### SSLSocket

    protected SSLSocket(String host, int port, InetAddress clientAddress, int clientPort) throws IOException, UnknownHostException
    Used only by subclasses. Constructs an SSL connection to a named host at a specified port, binding the client side of the connection a given address and port. This acts as the SSL client.
    Parameters:
      `host` - name of the host with which to connect, or `null` for the loopback address.
      `port` - number of the server's port
      `clientAddress` - the client's address the socket is bound to, or `null` for the `anyLocal` address.
      `clientPort` - the client's port the socket is bound to, or `zero` for a system selected free port.
    Throws:
      `IOException` - if an I/O error occurs when creating the socket
      `UnknownHostException` - if the host is not known
      `IllegalArgumentException` - if the port parameter or clientPort parameter is outside the specified range of valid port values, which is between 0 and 65535, inclusive.

  - ### SSLSocket

    protected SSLSocket(InetAddress address, int port, InetAddress clientAddress, int clientPort) throws IOException
    Used only by subclasses. Constructs an SSL connection to a server at a specified address and TCP port, binding the client side of the connection a given address and port. This acts as the SSL client.
    Parameters:
      `address` - the server's host
      `port` - its port
      `clientAddress` - the client's address the socket is bound to, or `null` for the `anyLocal` address.
      `clientPort` - the client's port the socket is bound to, or `zero` for a system selected free port.
    Throws:
      `IOException` - if an I/O error occurs when creating the socket
      `IllegalArgumentException` - if the port parameter or clientPort parameter is outside the specified range of valid port values, which is between 0 and 65535, inclusive.
      `NullPointerException` - if `address` is null.

- ## Method Details

  - ### getSupportedCipherSuites

    public abstract String[] getSupportedCipherSuites()
    Returns the names of the cipher suites which could be enabled for use on this connection. Normally, only a subset of these will actually be enabled by default, since this list may include cipher suites which do not meet quality of service requirements for those defaults. Such cipher suites might be useful in specialized applications.

    The returned array includes cipher suites from the list of standard cipher suite names in the JSSE Cipher Suite Names section of the Java Security Standard Algorithm Names Specification, and may also include other cipher suites that the provider supports.

    Returns:
      an array of cipher suite names
    External Specifications
      - Java Security Standard Algorithm Names
    See Also:
      - `getEnabledCipherSuites()`
      - `setEnabledCipherSuites(String [])`

  - ### getEnabledCipherSuites

    public abstract String[] getEnabledCipherSuites()
    Returns the names of the SSL cipher suites which are currently enabled for use on this connection. When an SSLSocket is first created, all enabled cipher suites support a minimum quality of service. Thus, in some environments this value might be empty.

    Note that even if a suite is enabled, it may never be used. This can occur if the peer does not support it, or its use is restricted, or the requisite certificates (and private keys) for the suite are not available, or an anonymous suite is enabled but authentication is required.

    The returned array includes cipher suites from the list of standard cipher suite names in the JSSE Cipher Suite Names section of the Java Security Standard Algorithm Names Specification, and may also include other cipher suites that the provider supports.

    Returns:
      an array of cipher suite names
    External Specifications
      - Java Security Standard Algorithm Names
    See Also:
      - `getSupportedCipherSuites()`
      - `setEnabledCipherSuites(String [])`

  - ### setEnabledCipherSuites

    public abstract void setEnabledCipherSuites(String[] suites)
    Sets the cipher suites enabled for use on this connection.

    Each cipher suite in the `suites` parameter must have been listed by getSupportedCipherSuites(), or the method will fail. Following a successful call to this method, only suites listed in the `suites` parameter are enabled for use.

    Note that the standard list of cipher suite names may be found in the JSSE Cipher Suite Names section of the Java Security Standard Algorithm Names Specification. Providers may support cipher suite names not found in this list or might not use the recommended name for a certain cipher suite.

    See `getEnabledCipherSuites()` for more information on why a specific ciphersuite may never be used on a connection.

    Parameters:
      `suites` - Names of all the cipher suites to enable
    Throws:
      `IllegalArgumentException` - when one or more of the ciphers named by the parameter is not supported, or when the parameter is null.
    External Specifications
      - Java Security Standard Algorithm Names
    See Also:
      - `getSupportedCipherSuites()`
      - `getEnabledCipherSuites()`

  - ### getSupportedProtocols

    public abstract String[] getSupportedProtocols()
    Returns the names of the protocols which could be enabled for use on an SSL connection.
    Returns:
      an array of protocols supported

  - ### getEnabledProtocols

    public abstract String[] getEnabledProtocols()
    Returns the names of the protocol versions which are currently enabled for use on this connection.

    Note that even if a protocol is enabled, it may never be used. This can occur if the peer does not support the protocol, or its use is restricted, or there are no enabled cipher suites supported by the protocol.

    Returns:
      an array of protocols
    See Also:
      - `setEnabledProtocols(String [])`

  - ### setEnabledProtocols

    public abstract void setEnabledProtocols(String[] protocols)
    Sets the protocol versions enabled for use on this connection.

    The protocols must have been listed by `getSupportedProtocols()` as being supported. Following a successful call to this method, only protocols listed in the `protocols` parameter are enabled for use.

    Parameters:
      `protocols` - Names of all the protocols to enable.
    Throws:
      `IllegalArgumentException` - when one or more of the protocols named by the parameter is not supported or when the protocols parameter is null.
    See Also:
      - `getEnabledProtocols()`

  - ### getSession

    public abstract SSLSession getSession()
    Returns the SSL Session in use by this connection. These can be long-lived, and frequently correspond to an entire login session for some user. The session specifies a particular cipher suite which is being actively used by all connections in that session, as well as the identities of the session's client and server.

    This method will initiate the initial handshake if necessary and then block until the handshake has been established.

    If an error occurs during the initial handshake, this method returns an invalid session object which reports an invalid cipher suite of "SSL_NULL_WITH_NULL_NULL".

    Returns:
      the `SSLSession`

  - ### getHandshakeSession

    public SSLSession getHandshakeSession()
    Returns the `SSLSession` being constructed during a SSL/TLS handshake.

    TLS protocols may negotiate parameters that are needed when using an instance of this class, but before the `SSLSession` has been completely initialized and made available via `getSession`. For example, the list of valid signature algorithms may restrict the type of certificates that can be used during TrustManager decisions, or the maximum TLS fragment packet sizes can be resized to better support the network environment.

    This method provides early access to the `SSLSession` being constructed. Depending on how far the handshake has progressed, some data may not yet be available for use. For example, if a remote server will be sending a Certificate chain, but that chain has yet not been processed, the `getPeerCertificates` method of `SSLSession` will throw a SSLPeerUnverifiedException. Once that chain has been processed, `getPeerCertificates` will return the proper value.

    Unlike `getSession()`, this method does not initiate the initial handshake and does not block until handshaking is complete.

    Returns:
      null if this instance is not currently handshaking, or if the current handshake has not progressed far enough to create a basic SSLSession. Otherwise, this method returns the `SSLSession` currently being negotiated.
    Throws:
      `UnsupportedOperationException` - if the underlying provider does not implement the operation.
    Since:
      1.7
    See Also:
      - `SSLEngine`
      - `SSLSession`
      - `ExtendedSSLSession`
      - `X509ExtendedKeyManager`
      - `X509ExtendedTrustManager`

  - ### addHandshakeCompletedListener

    public abstract void addHandshakeCompletedListener(HandshakeCompletedListener listener)
    Registers an event listener to receive notifications that an SSL handshake has completed on this connection.
    Parameters:
      `listener` - the HandShake Completed event listener
    Throws:
      `IllegalArgumentException` - if the argument is null.
    See Also:
      - `startHandshake()`
      - `removeHandshakeCompletedListener(HandshakeCompletedListener)`

  - ### removeHandshakeCompletedListener

    public abstract void removeHandshakeCompletedListener(HandshakeCompletedListener listener)
    Removes a previously registered handshake completion listener.
    Parameters:
      `listener` - the HandShake Completed event listener
    Throws:
      `IllegalArgumentException` - if the listener is not registered, or the argument is null.
    See Also:
      - `addHandshakeCompletedListener(HandshakeCompletedListener)`

  - ### startHandshake

    public abstract void startHandshake() throws IOException
    Starts an SSL handshake on this connection. Common reasons include a need to use new encryption keys, to change cipher suites, or to initiate a new session. To force complete reauthentication, the current session could be invalidated before starting this handshake.

    If data has already been sent on the connection, it continues to flow during this handshake. When the handshake completes, this will be signaled with an event. This method is synchronous for the initial handshake on a connection and returns when the negotiated handshake is complete. Some protocols may not support multiple handshakes on an existing socket and may throw an IOException.

    Throws:
      `IOException` - on a network level error
    See Also:
      - `addHandshakeCompletedListener(HandshakeCompletedListener)`

  - ### setUseClientMode

    public abstract void setUseClientMode(boolean mode)
    Configures the socket to use client (or server) mode when handshaking.

    This method must be called before any handshaking occurs. Once handshaking has begun, the mode can not be reset for the life of this socket.

    Servers normally authenticate themselves, and clients are not required to do so.

    Parameters:
      `mode` - true if the socket should start its handshaking in "client" mode
    Throws:
      `IllegalArgumentException` - if a mode change is attempted after the initial handshake has begun.
    See Also:
      - `getUseClientMode()`

  - ### getUseClientMode

    public abstract boolean getUseClientMode()
    Returns true if the socket is set to use client mode when handshaking.
    Returns:
      true if the socket should do handshaking in "client" mode
    See Also:
      - `setUseClientMode(boolean)`

  - ### setNeedClientAuth

    public abstract void setNeedClientAuth(boolean need)
    Configures the socket to require client authentication. This option is only useful for sockets in the server mode.

    A socket's client authentication setting is one of the following:

    - client authentication required
    - client authentication requested
    - no client authentication desired

    Unlike `setWantClientAuth(boolean)`, if this option is set and the client chooses not to provide authentication information about itself, the negotiations will stop and the connection will be dropped.

    Calling this method overrides any previous setting made by this method or `setWantClientAuth(boolean)`.

    Parameters:
      `need` - set to true if client authentication is required, or false if no client authentication is desired.
    See Also:
      - `getNeedClientAuth()`
      - `setWantClientAuth(boolean)`
      - `getWantClientAuth()`
      - `setUseClientMode(boolean)`

  - ### getNeedClientAuth

    public abstract boolean getNeedClientAuth()
    Returns true if the socket will require client authentication. This option is only useful to sockets in the server mode.
    Returns:
      true if client authentication is required, or false if no client authentication is desired.
    See Also:
      - `setNeedClientAuth(boolean)`
      - `setWantClientAuth(boolean)`
      - `getWantClientAuth()`
      - `setUseClientMode(boolean)`

  - ### setWantClientAuth

    public abstract void setWantClientAuth(boolean want)
    Configures the socket to request client authentication. This option is only useful for sockets in the server mode.

    A socket's client authentication setting is one of the following:

    - client authentication required
    - client authentication requested
    - no client authentication desired

    Unlike `setNeedClientAuth(boolean)`, if this option is set and the client chooses not to provide authentication information about itself, the negotiations will continue.

    Calling this method overrides any previous setting made by this method or `setNeedClientAuth(boolean)`.

    Parameters:
      `want` - set to true if client authentication is requested, or false if no client authentication is desired.
    See Also:
      - `getWantClientAuth()`
      - `setNeedClientAuth(boolean)`
      - `getNeedClientAuth()`
      - `setUseClientMode(boolean)`

  - ### getWantClientAuth

    public abstract boolean getWantClientAuth()
    Returns true if the socket will request client authentication. This option is only useful for sockets in the server mode.
    Returns:
      true if client authentication is requested, or false if no client authentication is desired.
    See Also:
      - `setNeedClientAuth(boolean)`
      - `getNeedClientAuth()`
      - `setWantClientAuth(boolean)`
      - `setUseClientMode(boolean)`

  - ### setEnableSessionCreation

    public abstract void setEnableSessionCreation(boolean flag)
    Controls whether new SSL sessions may be established by this socket. If session creations are not allowed, and there are no existing sessions to resume, there will be no successful handshaking.
    Parameters:
      `flag` - true indicates that sessions may be created; this is the default. false indicates that an existing session must be resumed
    See Also:
      - `getEnableSessionCreation()`

  - ### getEnableSessionCreation

    public abstract boolean getEnableSessionCreation()
    Returns true if new SSL sessions may be established by this socket.
    Returns:
      true indicates that sessions may be created; this is the default. false indicates that an existing session must be resumed
    See Also:
      - `setEnableSessionCreation(boolean)`

  - ### getSSLParameters

    public SSLParameters getSSLParameters()
    Returns the SSLParameters in effect for this SSLSocket. The ciphersuites and protocols of the returned SSLParameters are always non-null.
    Returns:
      the SSLParameters in effect for this SSLSocket.
    Since:
      1.6

  - ### setSSLParameters

    public void setSSLParameters(SSLParameters params)
    Applies SSLParameters to this socket.

    This means:

    - If `params.getCipherSuites()` is non-null, `setEnabledCipherSuites()` is called with that value.
    - If `params.getProtocols()` is non-null, `setEnabledProtocols()` is called with that value.
    - If `params.getNeedClientAuth()` or `params.getWantClientAuth()` return `true`, `setNeedClientAuth(true)` and `setWantClientAuth(true)` are called, respectively; otherwise `setWantClientAuth(false)` is called.
    - If `params.getServerNames()` is non-null, the socket will configure its server names with that value.
    - If `params.getSNIMatchers()` is non-null, the socket will configure its SNI matchers with that value.
    Parameters:
      `params` - the parameters
    Throws:
      `IllegalArgumentException` - if the setEnabledCipherSuites() or the setEnabledProtocols() call fails
    Since:
      1.6

  - ### getApplicationProtocol

    public String getApplicationProtocol()
    Returns the most recent application protocol value negotiated for this connection.

    If supported by the underlying SSL/TLS/DTLS implementation, application name negotiation mechanisms such as RFC 7301 , the Application-Layer Protocol Negotiation (ALPN), can negotiate application-level values between peers.

    Implementation Requirements:
      The implementation in this class throws `UnsupportedOperationException` and performs no other action.
    Returns:
      null if it has not yet been determined if application protocols might be used for this connection, an empty `String` if application protocols values will not be used, or a non-empty application protocol `String` if a value was successfully negotiated.
    Throws:
      `UnsupportedOperationException` - if the underlying provider does not implement the operation.
    Since:
      9
    External Specifications
      - RFC 7301: Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension

  - ### getHandshakeApplicationProtocol

    public String getHandshakeApplicationProtocol()
    Returns the application protocol value negotiated on a SSL/TLS handshake currently in progress.

    Like `getHandshakeSession()`, a connection may be in the middle of a handshake. The application protocol may or may not yet be available.

    Implementation Requirements:
      The implementation in this class throws `UnsupportedOperationException` and performs no other action.
    Returns:
      null if it has not yet been determined if application protocols might be used for this handshake, an empty `String` if application protocols values will not be used, or a non-empty application protocol `String` if a value was successfully negotiated.
    Throws:
      `UnsupportedOperationException` - if the underlying provider does not implement the operation.
    Since:
      9

  - ### setHandshakeApplicationProtocolSelector

    public void setHandshakeApplicationProtocolSelector(BiFunction<SSLSocket, List<String>, String> selector)
    Registers a callback function that selects an application protocol value for a SSL/TLS/DTLS handshake. The function overrides any values supplied using `SSLParameters.setApplicationProtocols` and it supports the following type parameters:

    > `SSLSocket`
    >   The function's first argument allows the current `SSLSocket` to be inspected, including the handshake session and configuration settings.
    > `List<String>`
    >   The function's second argument lists the application protocol names advertised by the TLS peer.
    > `String`
    >   The function's result is an application protocol name, or null to indicate that none of the advertised names are acceptable. If the return value is an empty `String` then application protocol indications will not be used. If the return value is null (no value chosen) or is a value that was not advertised by the peer, the underlying protocol will determine what action to take. (For example, ALPN will send a "no_application_protocol" alert and terminate the connection.)

    For example, the following call registers a callback function that examines the TLS handshake parameters and selects an application protocol name:

            serverSocket.setHandshakeApplicationProtocolSelector(
                (serverSocket, clientProtocols) -> {
                    SSLSession session = serverSocket.getHandshakeSession();
                    return chooseApplicationProtocol(
                        serverSocket,
                        clientProtocols,
                        session.getProtocol(),
                        session.getCipherSuite());
                });

    API Note:
      This method should be called by TLS server applications before the TLS handshake begins. Also, this `SSLSocket` should be configured with parameters that are compatible with the application protocol selected by the callback function. For example, enabling a poor choice of cipher suites could result in no suitable application protocol. See `SSLParameters`.
    Implementation Requirements:
      The implementation in this class throws `UnsupportedOperationException` and performs no other action.
    Parameters:
      `selector` - the callback function, or null to de-register.
    Throws:
      `UnsupportedOperationException` - if the underlying provider does not implement the operation.
    Since:
      9

  - ### getHandshakeApplicationProtocolSelector

    public BiFunction<SSLSocket, List<String>, String> getHandshakeApplicationProtocolSelector()
    Retrieves the callback function that selects an application protocol value during a SSL/TLS/DTLS handshake. See `setHandshakeApplicationProtocolSelector` for the function's type parameters.
    Implementation Requirements:
      The implementation in this class throws `UnsupportedOperationException` and performs no other action.
    Returns:
      the callback function, or null if none has been set.
    Throws:
      `UnsupportedOperationException` - if the underlying provider does not implement the operation.
    Since:
      9