Module java.base
Package javax.net.ssl

# Class SSLEngine

java.lang.Object
javax.net.ssl.SSLEngine

public abstract class SSLEngine extends Object
A class which enables secure communications using protocols such as the Secure Sockets Layer (SSL) or IETF RFC 2246 "Transport Layer Security" (TLS) protocols, but is transport independent.

The secure communications modes include:

- Integrity Protection. SSL/TLS/DTLS protects against modification of messages by an active wiretapper.
- Authentication. In most modes, SSL/TLS/DTLS provides peer authentication. Servers are usually authenticated, and clients may be authenticated as requested by servers.
- Confidentiality (Privacy Protection). In most modes, SSL/TLS/DTLS 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 cipher suite 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 `SSLSocket` class provides much of the same security functionality, but all of the inbound and outbound data is automatically transported using the underlying `Socket`, which by design uses a blocking model. While this is appropriate for many applications, this model does not provide the scalability required by large servers.

The primary distinction of an `SSLEngine` is that it operates on inbound and outbound byte streams, independent of the transport mechanism. It is the responsibility of the `SSLEngine` user to arrange for reliable I/O transport to the peer. By separating the SSL/TLS/DTLS abstraction from the I/O transport mechanism, the `SSLEngine` can be used for a wide variety of I/O types, such as `non-blocking I/O (polling)`, `selectable non-blocking I/O`, `Socket` and the traditional Input/OutputStreams, local `ByteBuffers` or byte arrays, future asynchronous I/O models , and so on.

At a high level, the `SSLEngine` appears thus:

                       app data

                    |           ^
                    |     |     |
                    v     |     |
               +----+-----|-----+----+
               |          |          |
               |       SSL|Engine    |
       wrap()  |          |          |  unwrap()
               | OUTBOUND | INBOUND  |
               |          |          |
               +----+-----|-----+----+
                    |     |     ^
                    |     |     |
                    v           |

                       net data

Application data (also known as plaintext or cleartext) is data which is produced or consumed by an application. Its counterpart is network data, which consists of either handshaking and/or ciphertext (encrypted) data, and destined to be transported via an I/O mechanism. Inbound data is data which has been received from the peer, and outbound data is destined for the peer.

(In the context of an `SSLEngine`, the term "handshake data" is taken to mean any data exchanged to establish and control a secure connection. Handshake data includes the SSL/TLS/DTLS messages "alert", "change_cipher_spec," and "handshake.")

There are five distinct phases to an `SSLEngine`.

1. Creation - The `SSLEngine` has been created and initialized, but has not yet been used. During this phase, an application may set any `SSLEngine`-specific settings (enabled cipher suites, whether the `SSLEngine` should handshake in client or server mode, and so on). Once handshaking has begun, though, any new settings (except client/server mode, see below) will be used for the next handshake.
2. Initial Handshake - The initial handshake is a procedure by which the two peers exchange communication parameters until an SSLSession is established. Application data can not be sent during this phase.
3. Application Data - Once the communication parameters have been established and the handshake is complete, application data may flow through the `SSLEngine`. Outbound application messages are encrypted and integrity protected, and inbound messages reverse the process.
4. Rehandshaking - Either side may request a renegotiation of the session at any time during the Application Data phase. New handshaking data can be intermixed among the application data. Before starting the rehandshake phase, the application may reset the SSL/TLS/DTLS communication parameters such as the list of enabled ciphersuites and whether to use client authentication, but can not change between client/server modes. As before, once handshaking has begun, any new `SSLEngine` configuration settings will not be used until the next handshake.
5. Closure - When the connection is no longer needed, the client and the server applications should each close both sides of their respective connections. For `SSLEngine` objects, an application should call `closeOutbound()` and send any remaining messages to the peer. Likewise, an application should receive any remaining messages from the peer before calling `closeInbound()`. The underlying transport mechanism can then be closed after both sides of the `SSLEngine` have been closed. If the connection is not closed in an orderly manner (for example `closeInbound()` is called before the peer's write closure notification has been received), exceptions will be raised to indicate that an error has occurred. Once an engine is closed, it is not reusable: a new `SSLEngine` must be created.

An `SSLEngine` is created by calling `SSLContext.createSSLEngine()` from an initialized `SSLContext`. Any configuration parameters should be set before making the first call to `wrap()`, `unwrap()`, or `beginHandshake()`. These methods all trigger the initial handshake.

Data moves through the engine by calling `wrap()` or `unwrap()` on outbound or inbound data, respectively. Depending on the state of the `SSLEngine`, a `wrap()` call may consume application data from the source buffer and may produce network data in the destination buffer. The outbound data may contain application and/or handshake data. A call to `unwrap()` will examine the source buffer and may advance the handshake if the data is handshaking information, or may place application data in the destination buffer if the data is application. The state of the underlying SSL/TLS/DTLS algorithm will determine when data is consumed and produced.

Calls to `wrap()` and `unwrap()` return an `SSLEngineResult` which indicates the status of the operation, and (optionally) how to interact with the engine to make progress.

The `SSLEngine` produces/consumes complete SSL/TLS/DTLS packets only, and does not store application data internally between calls to `wrap()/unwrap()`. Thus input and output `ByteBuffer`s must be sized appropriately to hold the maximum record that can be produced. Calls to `SSLSession.getPacketBufferSize()` and `SSLSession.getApplicationBufferSize()` should be used to determine the appropriate buffer sizes. The size of the outbound application data buffer generally does not matter. If buffer conditions do not allow for the proper consumption/production of data, the application must determine (via `SSLEngineResult`) and correct the problem, and then try the call again.

For example, `unwrap()` will return a `SSLEngineResult.Status.BUFFER_OVERFLOW` result if the engine determines that there is not enough destination buffer space available. Applications should call `SSLSession.getApplicationBufferSize()` and compare that value with the space available in the destination buffer, enlarging the buffer if necessary. Similarly, if `unwrap()` were to return a `SSLEngineResult.Status.BUFFER_UNDERFLOW`, the application should call `SSLSession.getPacketBufferSize()` to ensure that the source buffer has enough room to hold a record (enlarging if necessary), and then obtain more inbound data.

       SSLEngineResult r = engine.unwrap(src, dst);
       switch (r.getStatus()) {
       case BUFFER_OVERFLOW:
           // Could attempt to drain the dst buffer of any already obtained
           // data, but we'll just increase it to the size needed.
           int appSize = engine.getSession().getApplicationBufferSize();
           ByteBuffer b = ByteBuffer.allocate(appSize + dst.position());
           dst.flip();
           b.put(dst);
           dst = b;
           // retry the operation.
           break;
       case BUFFER_UNDERFLOW:
           int netSize = engine.getSession().getPacketBufferSize();
           // Resize buffer if needed.
           if (netSize > src.capacity()) {
               ByteBuffer b = ByteBuffer.allocate(netSize);
               src.flip();
               b.put(src);
               src = b;
           }
           // Obtain more inbound network data for src,
           // then retry the operation.
           break;
       // other cases: CLOSED, OK.
       }

Unlike `SSLSocket`, all methods of SSLEngine are non-blocking. `SSLEngine` implementations may require the results of tasks that may take an extended period of time to complete, or may even block. For example, a TrustManager may need to connect to a remote certificate validation service, or a KeyManager might need to prompt a user to determine which certificate to use as part of client authentication. Additionally, creating cryptographic signatures and verifying them can be slow, seemingly blocking.

For any operation which may potentially block, the `SSLEngine` will create a `Runnable` delegated task. When `SSLEngineResult` indicates that a delegated task result is needed, the application must call `getDelegatedTask()` to obtain an outstanding delegated task and call its `run()` method (possibly using a different thread depending on the compute strategy). The application should continue obtaining delegated tasks until no more exist, and try the original operation again.

At the end of a communication session, applications should properly close the SSL/TLS/DTLS link. The SSL/TLS/DTLS protocols have closure handshake messages, and these messages should be communicated to the peer before releasing the `SSLEngine` and closing the underlying transport mechanism. A close can be initiated by one of: an SSLException, an inbound closure handshake message, or one of the close methods. In all cases, closure handshake messages are generated by the engine, and `wrap()` should be repeatedly called until the resulting `SSLEngineResult`'s status returns "CLOSED", or `isOutboundDone()` returns true. All data obtained from the `wrap()` method should be sent to the peer.

`closeOutbound()` is used to signal the engine that the application will not be sending any more data.

A peer will signal its intent to close by sending its own closure handshake message. After this message has been received and processed by the local `SSLEngine`'s `unwrap()` call, the application can detect the close by calling `unwrap()` and looking for a `SSLEngineResult` with status "CLOSED", or if `isInboundDone()` returns true. If for some reason the peer closes the communication link without sending the proper SSL/TLS/DTLS closure message, the application can detect the end-of-stream and can signal the engine via `closeInbound()` that there will no more inbound messages to process. Some applications might choose to require orderly shutdown messages from a peer, in which case they can check that the closure was generated by a handshake message and not by an end-of-stream condition.

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(String [])` method, and queried using the `getEnabledCipherSuites()` method. Initially, a default set of cipher suites will be enabled on a new engine 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 cipher suite be selected.

Each SSL/TLS/DTLS connection must have one client and one server, thus each endpoint must decide which role to assume. This choice determines who begins the handshaking process as well as which type of messages should be sent by each party. The method `setUseClientMode(boolean)` configures the mode. Note that the default mode for a new `SSLEngine` is provider-specific. Applications should set the mode explicitly before invoking other methods of the `SSLEngine`. Once the initial handshaking has started, an `SSLEngine` 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 {code String} format for comparison.

>          String networkString = sslEngine.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")) {
>              ...
>          }

Applications might choose to process delegated tasks in different threads. When an `SSLEngine` is created, the current `AccessControlContext` is saved. All future delegated tasks will be processed using this context: that is, all access control decisions will be made using the context captured at engine creation.

Concurrency Notes: There are two concurrency issues to be aware of:

1. The `wrap()` and `unwrap()` methods may execute concurrently of each other.
2. The SSL/TLS/DTLS protocols employ ordered packets. Applications must take care to ensure that generated packets are delivered in sequence. If packets arrive out-of-order, unexpected or fatal results may occur.

   For example:

                     synchronized (outboundLock) {
                         sslEngine.wrap(src, dst);
                         outboundQueue.put(dst);
                     }

   As a corollary, two threads must not attempt to call the same method (either `wrap()` or `unwrap()`) concurrently, because there is no way to guarantee the eventual packet ordering.

Since:
  1.5
See Also:
  - `SSLContext`
  - `SSLSocket`
  - `SSLServerSocket`
  - `SSLSession`
  - `Socket`

- ## Constructor Summary

  Constructors
  Modifier
  Constructor
  Description
  `protected`
  `SSLEngine()`
  Constructor for an `SSLEngine` providing no hints for an internal session reuse strategy.
  `protected`
  `SSLEngine(String peerHost, int peerPort)`
  Constructor for an `SSLEngine`.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `abstract void`
  `beginHandshake()`
  Initiates handshaking (initial or renegotiation) on this SSLEngine.
  `abstract void`
  `closeInbound()`
  Signals that no more inbound network data will be sent to this `SSLEngine`.
  `abstract void`
  `closeOutbound()`
  Signals that no more outbound application data will be sent on this `SSLEngine`.
  `String`
  `getApplicationProtocol()`
  Returns the most recent application protocol value negotiated for this connection.
  `abstract Runnable`
  `getDelegatedTask()`
  Returns a delegated `Runnable` task for this `SSLEngine`.
  `abstract String[]`
  `getEnabledCipherSuites()`
  Returns the names of the SSL cipher suites which are currently enabled for use on this engine.
  `abstract String[]`
  `getEnabledProtocols()`
  Returns the names of the protocol versions which are currently enabled for use with this `SSLEngine`.
  `abstract boolean`
  `getEnableSessionCreation()`
  Returns true if new SSL sessions may be established by this engine.
  `String`
  `getHandshakeApplicationProtocol()`
  Returns the application protocol value negotiated on a SSL/TLS handshake currently in progress.
  `BiFunction<SSLEngine,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/DTLS handshake.
  `abstract SSLEngineResult.HandshakeStatus`
  `getHandshakeStatus()`
  Returns the current handshake status for this `SSLEngine`.
  `abstract boolean`
  `getNeedClientAuth()`
  Returns true if the engine will require client authentication.
  `String`
  `getPeerHost()`
  Returns the host name of the peer.
  `int`
  `getPeerPort()`
  Returns the port number of the peer.
  `abstract SSLSession`
  `getSession()`
  Returns the `SSLSession` in use in this `SSLEngine`.
  `SSLParameters`
  `getSSLParameters()`
  Returns the SSLParameters in effect for this SSLEngine.
  `abstract String[]`
  `getSupportedCipherSuites()`
  Returns the names of the cipher suites which could be enabled for use on this engine.
  `abstract String[]`
  `getSupportedProtocols()`
  Returns the names of the protocols which could be enabled for use with this `SSLEngine`.
  `abstract boolean`
  `getUseClientMode()`
  Returns true if the engine is set to use client mode when handshaking.
  `abstract boolean`
  `getWantClientAuth()`
  Returns true if the engine will request client authentication.
  `abstract boolean`
  `isInboundDone()`
  Returns whether `unwrap(ByteBuffer, ByteBuffer)` will accept any more inbound data messages.
  `abstract boolean`
  `isOutboundDone()`
  Returns whether `wrap(ByteBuffer, ByteBuffer)` will produce any more outbound data messages.
  `abstract void`
  `setEnabledCipherSuites(String[] suites)`
  Sets the cipher suites enabled for use on this engine.
  `abstract void`
  `setEnabledProtocols(String[] protocols)`
  Set the protocol versions enabled for use on this engine.
  `abstract void`
  `setEnableSessionCreation(boolean flag)`
  Controls whether new SSL sessions may be established by this engine.
  `void`
  `setHandshakeApplicationProtocolSelector(BiFunction<SSLEngine,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 engine to require client authentication.
  `void`
  `setSSLParameters(SSLParameters params)`
  Applies SSLParameters to this engine.
  `abstract void`
  `setUseClientMode(boolean mode)`
  Configures the engine to use client (or server) mode when handshaking.
  `abstract void`
  `setWantClientAuth(boolean want)`
  Configures the engine to request client authentication.
  `SSLEngineResult`
  `unwrap(ByteBuffer src, ByteBuffer dst)`
  Attempts to decode SSL/TLS/DTLS network data into a plaintext application data buffer.
  `SSLEngineResult`
  `unwrap(ByteBuffer src, ByteBuffer[] dsts)`
  Attempts to decode SSL/TLS/DTLS network data into a sequence of plaintext application data buffers.
  `abstract SSLEngineResult`
  `unwrap(ByteBuffer src, ByteBuffer[] dsts, int offset, int length)`
  Attempts to decode SSL/TLS/DTLS network data into a subsequence of plaintext application data buffers.
  `abstract SSLEngineResult`
  `wrap(ByteBuffer[] srcs, int offset, int length, ByteBuffer dst)`
  Attempts to encode plaintext bytes from a subsequence of data buffers into SSL/TLS/DTLS network data.
  `SSLEngineResult`
  `wrap(ByteBuffer[] srcs, ByteBuffer dst)`
  Attempts to encode plaintext bytes from a sequence of data buffers into SSL/TLS/DTLS network data.
  `SSLEngineResult`
  `wrap(ByteBuffer src, ByteBuffer dst)`
  Attempts to encode a buffer of plaintext application data into SSL/TLS/DTLS network data.

  ### Methods declared in class java.lang.Object

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

- ## Constructor Details

  - ### SSLEngine

    protected SSLEngine()
    Constructor for an `SSLEngine` providing no hints for an internal session reuse strategy.
    See Also:
      - `SSLContext.createSSLEngine()`
      - `SSLSessionContext`

  - ### SSLEngine

    protected SSLEngine(String peerHost, int peerPort)
    Constructor for an `SSLEngine`.

    `SSLEngine` implementations may use the `peerHost` and `peerPort` parameters as hints for their internal session reuse strategy.

    Some cipher suites (such as Kerberos) require remote hostname information. Implementations of this class should use this constructor to use Kerberos.

    The parameters are not authenticated by the `SSLEngine`.

    Parameters:
      `peerHost` - the name of the peer host
      `peerPort` - the port number of the peer
    See Also:
      - `SSLContext.createSSLEngine(String, int)`
      - `SSLSessionContext`

- ## Method Details

  - ### getPeerHost

    public String getPeerHost()
    Returns the host name of the peer.

    Note that the value is not authenticated, and should not be relied upon.

    Returns:
      the host name of the peer, or null if nothing is available.

  - ### getPeerPort

    public int getPeerPort()
    Returns the port number of the peer.

    Note that the value is not authenticated, and should not be relied upon.

    Returns:
      the port number of the peer, or -1 if nothing is available.

  - ### wrap

    public SSLEngineResult wrap(ByteBuffer src, ByteBuffer dst) throws SSLException
    Attempts to encode a buffer of plaintext application data into SSL/TLS/DTLS network data.

    An invocation of this method behaves in exactly the same manner as the invocation:

    >      engine.wrap(new ByteBuffer [] { src }, 0, 1, dst);

    Parameters:
      `src` - a `ByteBuffer` containing outbound application data
      `dst` - a `ByteBuffer` to hold outbound network data
    Returns:
      an `SSLEngineResult` describing the result of this operation.
    Throws:
      `SSLException` - A problem was encountered while processing the data that caused the `SSLEngine` to abort. See the class description for more information on engine closure.
      `ReadOnlyBufferException` - if the `dst` buffer is read-only.
      `IllegalArgumentException` - if either `src` or `dst` is null.
      `IllegalStateException` - if the client/server mode has not yet been set.
    See Also:
      - `wrap(ByteBuffer [], int, int, ByteBuffer)`

  - ### wrap

    public SSLEngineResult wrap(ByteBuffer[] srcs, ByteBuffer dst) throws SSLException
    Attempts to encode plaintext bytes from a sequence of data buffers into SSL/TLS/DTLS network data.

    An invocation of this method behaves in exactly the same manner as the invocation:

    >      engine.wrap(srcs, 0, srcs.length, dst);

    Parameters:
      `srcs` - an array of `ByteBuffers` containing the outbound application data
      `dst` - a `ByteBuffer` to hold outbound network data
    Returns:
      an `SSLEngineResult` describing the result of this operation.
    Throws:
      `SSLException` - A problem was encountered while processing the data that caused the `SSLEngine` to abort. See the class description for more information on engine closure.
      `ReadOnlyBufferException` - if the `dst` buffer is read-only.
      `IllegalArgumentException` - if either `srcs` or `dst` is null, or if any element in `srcs` is null.
      `IllegalStateException` - if the client/server mode has not yet been set.
    See Also:
      - `wrap(ByteBuffer [], int, int, ByteBuffer)`

  - ### wrap

    public abstract SSLEngineResult wrap(ByteBuffer[] srcs, int offset, int length, ByteBuffer dst) throws SSLException
    Attempts to encode plaintext bytes from a subsequence of data buffers into SSL/TLS/DTLS network data. This "gathering" operation encodes, in a single invocation, a sequence of bytes from one or more of a given sequence of buffers. Gathering wraps are often useful when implementing network protocols or file formats that, for example, group data into segments consisting of one or more fixed-length headers followed by a variable-length body. See `GatheringByteChannel` for more information on gathering, and `GatheringByteChannel.write(ByteBuffer[], int, int)` for more information on the subsequence behavior.

    Depending on the state of the SSLEngine, this method may produce network data without consuming any application data (for example, it may generate handshake data.)

    The application is responsible for reliably transporting the network data to the peer, and for ensuring that data created by multiple calls to wrap() is transported in the same order in which it was generated. The application must properly synchronize multiple calls to this method.

    If this `SSLEngine` has not yet started its initial handshake, this method will automatically start the handshake.

    This method will attempt to produce SSL/TLS/DTLS records, and will consume as much source data as possible, but will never consume more than the sum of the bytes remaining in each buffer. Each `ByteBuffer`'s position is updated to reflect the amount of data consumed or produced. The limits remain the same.

    The underlying memory used by the `srcs` and `dst ByteBuffer`s must not be the same.

    See the class description for more information on engine closure.

    Parameters:
      `srcs` - an array of `ByteBuffers` containing the outbound application data
      `offset` - The offset within the buffer array of the first buffer from which bytes are to be retrieved; it must be non-negative and no larger than `srcs.length`
      `length` - The maximum number of buffers to be accessed; it must be non-negative and no larger than `srcs.length` - `offset`
      `dst` - a `ByteBuffer` to hold outbound network data
    Returns:
      an `SSLEngineResult` describing the result of this operation.
    Throws:
      `SSLException` - A problem was encountered while processing the data that caused the `SSLEngine` to abort. See the class description for more information on engine closure.
      `IndexOutOfBoundsException` - if the preconditions on the `offset` and `length` parameters do not hold.
      `ReadOnlyBufferException` - if the `dst` buffer is read-only.
      `IllegalArgumentException` - if either `srcs` or `dst` is null, or if any element in the `srcs` subsequence specified is null.
      `IllegalStateException` - if the client/server mode has not yet been set.
    See Also:
      - `GatheringByteChannel`
      - `GatheringByteChannel.write(ByteBuffer[], int, int)`

  - ### unwrap

    public SSLEngineResult unwrap(ByteBuffer src, ByteBuffer dst) throws SSLException
    Attempts to decode SSL/TLS/DTLS network data into a plaintext application data buffer.

    An invocation of this method behaves in exactly the same manner as the invocation:

    >      engine.unwrap(src, new ByteBuffer [] { dst }, 0, 1);

    Parameters:
      `src` - a `ByteBuffer` containing inbound network data.
      `dst` - a `ByteBuffer` to hold inbound application data.
    Returns:
      an `SSLEngineResult` describing the result of this operation.
    Throws:
      `SSLException` - A problem was encountered while processing the data that caused the `SSLEngine` to abort. See the class description for more information on engine closure.
      `ReadOnlyBufferException` - if the `dst` buffer is read-only.
      `IllegalArgumentException` - if either `src` or `dst` is null.
      `IllegalStateException` - if the client/server mode has not yet been set.
    See Also:
      - `unwrap(ByteBuffer, ByteBuffer [], int, int)`

  - ### unwrap

    public SSLEngineResult unwrap(ByteBuffer src, ByteBuffer[] dsts) throws SSLException
    Attempts to decode SSL/TLS/DTLS network data into a sequence of plaintext application data buffers.

    An invocation of this method behaves in exactly the same manner as the invocation:

    >      engine.unwrap(src, dsts, 0, dsts.length);

    Parameters:
      `src` - a `ByteBuffer` containing inbound network data.
      `dsts` - an array of `ByteBuffer`s to hold inbound application data.
    Returns:
      an `SSLEngineResult` describing the result of this operation.
    Throws:
      `SSLException` - A problem was encountered while processing the data that caused the `SSLEngine` to abort. See the class description for more information on engine closure.
      `ReadOnlyBufferException` - if any of the `dst` buffers are read-only.
      `IllegalArgumentException` - if either `src` or `dsts` is null, or if any element in `dsts` is null.
      `IllegalStateException` - if the client/server mode has not yet been set.
    See Also:
      - `unwrap(ByteBuffer, ByteBuffer [], int, int)`

  - ### unwrap

    public abstract SSLEngineResult unwrap(ByteBuffer src, ByteBuffer[] dsts, int offset, int length) throws SSLException
    Attempts to decode SSL/TLS/DTLS network data into a subsequence of plaintext application data buffers. This "scattering" operation decodes, in a single invocation, a sequence of bytes into one or more of a given sequence of buffers. Scattering unwraps are often useful when implementing network protocols or file formats that, for example, group data into segments consisting of one or more fixed-length headers followed by a variable-length body. See `ScatteringByteChannel` for more information on scattering, and `ScatteringByteChannel.read(ByteBuffer[], int, int)` for more information on the subsequence behavior.

    Depending on the state of the SSLEngine, this method may consume network data without producing any application data (for example, it may consume handshake data.)

    The application is responsible for reliably obtaining the network data from the peer, and for invoking unwrap() on the data in the order it was received. The application must properly synchronize multiple calls to this method.

    If this `SSLEngine` has not yet started its initial handshake, this method will automatically start the handshake.

    This method will attempt to consume one complete SSL/TLS/DTLS network packet, but will never consume more than the sum of the bytes remaining in the buffers. Each `ByteBuffer`'s position is updated to reflect the amount of data consumed or produced. The limits remain the same.

    The underlying memory used by the `src` and `dsts ByteBuffer`s must not be the same.

    The inbound network buffer may be modified as a result of this call: therefore if the network data packet is required for some secondary purpose, the data should be duplicated before calling this method. Note: the network data will not be useful to a second SSLEngine, as each SSLEngine contains unique random state which influences the SSL/TLS/DTLS messages.

    See the class description for more information on engine closure.

    Parameters:
      `src` - a `ByteBuffer` containing inbound network data.
      `dsts` - an array of `ByteBuffer`s to hold inbound application data.
      `offset` - The offset within the buffer array of the first buffer from which bytes are to be transferred; it must be non-negative and no larger than `dsts.length`.
      `length` - The maximum number of buffers to be accessed; it must be non-negative and no larger than `dsts.length` - `offset`.
    Returns:
      an `SSLEngineResult` describing the result of this operation.
    Throws:
      `SSLException` - A problem was encountered while processing the data that caused the `SSLEngine` to abort. See the class description for more information on engine closure.
      `IndexOutOfBoundsException` - If the preconditions on the `offset` and `length` parameters do not hold.
      `ReadOnlyBufferException` - if any of the `dst` buffers are read-only.
      `IllegalArgumentException` - if either `src` or `dsts` is null, or if any element in the `dsts` subsequence specified is null.
      `IllegalStateException` - if the client/server mode has not yet been set.
    See Also:
      - `ScatteringByteChannel`
      - `ScatteringByteChannel.read(ByteBuffer[], int, int)`

  - ### getDelegatedTask

    public abstract Runnable getDelegatedTask()
    Returns a delegated `Runnable` task for this `SSLEngine`.

    `SSLEngine` operations may require the results of operations that block, or may take an extended period of time to complete. This method is used to obtain an outstanding `Runnable` operation (task). Each task must be assigned a thread (possibly the current) to perform the `run` operation. Once the `run` method returns, the `Runnable` object is no longer needed and may be discarded.

    Delegated tasks run in the `AccessControlContext` in place when this object was created.

    A call to this method will return each outstanding task exactly once.

    Multiple delegated tasks can be run in parallel.

    Returns:
      a delegated `Runnable` task, or null if none are available.

  - ### closeInbound

    public abstract void closeInbound() throws SSLException
    Signals that no more inbound network data will be sent to this `SSLEngine`.

    If the application initiated the closing process by calling `closeOutbound()`, under some circumstances it is not required that the initiator wait for the peer's corresponding close message. (See section 7.2.1 of the TLS specification (RFC 2246) for more information on waiting for closure alerts.) In such cases, this method need not be called.

    But if the application did not initiate the closure process, or if the circumstances above do not apply, this method should be called whenever the end of the SSL/TLS/DTLS data stream is reached. This ensures closure of the inbound side, and checks that the peer followed the SSL/TLS/DTLS close procedure properly, thus detecting possible truncation attacks.

    This method is idempotent: if the inbound side has already been closed, this method does not do anything.

    `wrap()` should be called to flush any remaining handshake data.

    Throws:
      `SSLException` - if this engine has not received the proper SSL/TLS/DTLS close notification message from the peer.
    See Also:
      - `isInboundDone()`
      - `isOutboundDone()`

  - ### isInboundDone

    public abstract boolean isInboundDone()
    Returns whether `unwrap(ByteBuffer, ByteBuffer)` will accept any more inbound data messages.
    Returns:
      true if the `SSLEngine` will not consume anymore network data (and by implication, will not produce any more application data.)
    See Also:
      - `closeInbound()`

  - ### closeOutbound

    public abstract void closeOutbound()
    Signals that no more outbound application data will be sent on this `SSLEngine`.

    This method is idempotent: if the outbound side has already been closed, this method does not do anything.

    `wrap(ByteBuffer, ByteBuffer)` should be called to flush any remaining handshake data.

    See Also:
      - `isOutboundDone()`

  - ### isOutboundDone

    public abstract boolean isOutboundDone()
    Returns whether `wrap(ByteBuffer, ByteBuffer)` will produce any more outbound data messages.

    Note that during the closure phase, a `SSLEngine` may generate handshake closure data that must be sent to the peer. `wrap()` must be called to generate this data. When this method returns true, no more outbound data will be created.

    Returns:
      true if the `SSLEngine` will not produce any more network data
    See Also:
      - `closeOutbound()`
      - `closeInbound()`

  - ### getSupportedCipherSuites

    public abstract String[] getSupportedCipherSuites()
    Returns the names of the cipher suites which could be enabled for use on this engine. 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 Cryptography Architecture Standard Algorithm Name Documentation, and may also include other cipher suites that the provider supports.

    Returns:
      an array of cipher suite 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 engine. When an SSLEngine 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 Cryptography Architecture Standard Algorithm Name Documentation, and may also include other cipher suites that the provider supports.

    Returns:
      an array of cipher suite names
    See Also:
      - `getSupportedCipherSuites()`
      - `setEnabledCipherSuites(String [])`

  - ### setEnabledCipherSuites

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

    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 Cryptography Architecture Standard Algorithm Name Documentation. 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 cipher suite may never be used on a engine.

    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.
    See Also:
      - `getSupportedCipherSuites()`
      - `getEnabledCipherSuites()`

  - ### getSupportedProtocols

    public abstract String[] getSupportedProtocols()
    Returns the names of the protocols which could be enabled for use with this `SSLEngine`.
    Returns:
      an array of protocols supported

  - ### getEnabledProtocols

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

    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)
    Set the protocol versions enabled for use on this engine.

    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 `SSLSession` in use in this `SSLEngine`.

    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.

    Unlike `SSLSocket.getSession()` this method does not block until handshaking is complete.

    Until the initial handshake has completed, this method returns a session object which reports an invalid cipher suite of "SSL_NULL_WITH_NULL_NULL".

    Returns:
      the `SSLSession` for this `SSLEngine`
    See Also:
      - `SSLSession`

  - ### getHandshakeSession

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

    TLS/DTLS 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/DTLS 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.

    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:
      - `SSLSocket`
      - `SSLSession`
      - `ExtendedSSLSession`
      - `X509ExtendedKeyManager`
      - `X509ExtendedTrustManager`

  - ### beginHandshake

    public abstract void beginHandshake() throws SSLException
    Initiates handshaking (initial or renegotiation) on this SSLEngine.

    This method is not needed for the initial handshake, as the `wrap()` and `unwrap()` methods will implicitly call this method if handshaking has not already begun.

    Note that the peer may also request a session renegotiation with this `SSLEngine` by sending the appropriate session renegotiate handshake message.

    Unlike the `SSLSocket#startHandshake()` method, this method does not block until handshaking is completed.

    To force a complete SSL/TLS/DTLS session renegotiation, the current session should be invalidated prior to calling this method.

    Some protocols may not support multiple handshakes on an existing engine and may throw an `SSLException`.

    Throws:
      `SSLException` - if a problem was encountered while signaling the `SSLEngine` to begin a new handshake. See the class description for more information on engine closure.
      `IllegalStateException` - if the client/server mode has not yet been set.
    See Also:
      - `SSLSession.invalidate()`

  - ### getHandshakeStatus

    public abstract SSLEngineResult.HandshakeStatus getHandshakeStatus()
    Returns the current handshake status for this `SSLEngine`.
    Returns:
      the current `SSLEngineResult.HandshakeStatus`.

  - ### setUseClientMode

    public abstract void setUseClientMode(boolean mode)
    Configures the engine 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 engine.

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

    Implementation Note:
      The JDK SunJSSE provider implementation default for this mode is false.
    Parameters:
      `mode` - true if the engine 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 engine is set to use client mode when handshaking.
    Implementation Note:
      The JDK SunJSSE provider implementation returns false unless `setUseClientMode(boolean)` is used to change the mode to true.
    Returns:
      true if the engine should do handshaking in "client" mode
    See Also:
      - `setUseClientMode(boolean)`

  - ### setNeedClientAuth

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

    An engine'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 engine will begin its closure procedure.

    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 engine will require client authentication. This option is only useful to engines 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 engine to request client authentication. This option is only useful for engines in the server mode.

    An engine'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 engine will request client authentication. This option is only useful for engines 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 engine. 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 engine.
    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 SSLEngine. The ciphersuites and protocols of the returned SSLParameters are always non-null.
    Returns:
      the SSLParameters in effect for this SSLEngine.
    Since:
      1.6

  - ### setSSLParameters

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

    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 engine will configure its server names with that value.
    - If `params.getSNIMatchers()` is non-null, the engine 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

  - ### 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<SSLEngine,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:

    > `SSLEngine`
    >   The function's first argument allows the current `SSLEngine` 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:

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

    API Note:
      This method should be called by TLS server applications before the TLS handshake begins. Also, this `SSLEngine` 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 disable the callback functionality.
    Throws:
      `UnsupportedOperationException` - if the underlying provider does not implement the operation.
    Since:
      9

  - ### getHandshakeApplicationProtocolSelector

    public BiFunction<SSLEngine,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