Socket是什么
socket起源于Unix，可以理解成一个文件，可以执行“读，写，打开，关闭”等操作，实际上是对TCP/IP协议的封装，太复杂，这篇简单看下Java层的实现。
Java的Socket默认是TCP协议，会建立一个稳定的长连接。
Socket的生命周期
Socket的构造过程像是工厂方法模式，Socket是一个抽象产品，具体产品是在SocketImpl类实现的。不同工厂可以根据需求实现不一样的具体产品，比如SocksSocketImpl，DualStackPlainSocketImpl等都是SocketImpl的子类。
/**
* Sets impl to the system-default type of SocketImpl.
* @since 1.4
*/
void setImpl() {
if (factory != null) {
impl = factory.createSocketImpl();
checkOldImpl();
} else {
// No need to do a checkOldImpl() here, we know it's an up to date
// SocketImpl!
impl = new SocksSocketImpl();
}
if (impl != null)
impl.setSocket(this);
}
Java Socket通信.png
1. create()
/**
* Creates a socket with a boolean that specifies whether this
* is a stream socket (true) or an unconnected UDP socket (false).
*/
protected synchronized void create(boolean stream) throws IOException {
this.stream = stream;
if (!stream) {
ResourceManager.beforeUdpCreate();
// only create the fd after we know we will be able to create the socket
fd = new FileDescriptor();
try {
socketCreate(false);
} catch (IOException ioe) {
ResourceManager.afterUdpClose();
fd = null;
throw ioe;
}
} else {
fd = new FileDescriptor();
socketCreate(true);
}
if (socket != null)
socket.setCreated();
if (serverSocket != null)
serverSocket.setCreated();
}
create()方法创建文件描述符（FileDescriptor），后续可以对这个socket进行读写操作。
传入的参数boolean stream用来区分协议，true标识TCP协议，false标识UDP协议。
文件描述符只能在本机标识这个socket，要进行网络通信，还需要进一步执行bind()。
2. bind()
/**
* Binds the socket to a local address.
* <P>
* If the address is {@code null}, then the system will pick up
* an ephemeral port and a valid local address to bind the socket.
*
* @param bindpoint the {@code SocketAddress} to bind to
* @throws IOException if the bind operation fails, or if the socket
* is already bound.
* @throws IllegalArgumentException if bindpoint is a
* SocketAddress subclass not supported by this socket
* @throws SecurityException if a security manager exists and its
* {@code checkListen} method doesn't allow the bind
* to the local port.
*
* @since 1.4
* @see #isBound
*/
public void bind(SocketAddress bindpoint) throws IOException {
if (isClosed())
throw new SocketException("Socket is closed");
if (!oldImpl && isBound())
throw new SocketException("Already bound");
if (bindpoint != null && (!(bindpoint instanceof InetSocketAddress)))
throw new IllegalArgumentException("Unsupported address type");
InetSocketAddress epoint = (InetSocketAddress) bindpoint;
if (epoint != null && epoint.isUnresolved())
throw new SocketException("Unresolved address");
if (epoint == null) {
epoint = new InetSocketAddress(0);
}
InetAddress addr = epoint.getAddress();
int port = epoint.getPort();
checkAddress (addr, "bind");
SecurityManager security = System.getSecurityManager();
if (security != null) {
security.checkListen(port);
}
getImpl().bind (addr, port);
bound = true;
}
bind()方法传入一个SocketAddress类型，给这个socket绑定一个地址，不考虑协议的情况下，需要IP地址+端口才能唯一标识一个socket。
在没有指定地址的情况下，系统会暂时分配一个空闲的端口给socket进行连接。
bind()方法是在Impl类实现的，可以看下socketBind()方法，具体实现应该是在native层。
3. connect()
如果是客户端，下一步会执行connet()方法尝试与服务端连接。
/**
* Connects this socket to the server with a specified timeout value.
* A timeout of zero is interpreted as an infinite timeout. The connection
* will then block until established or an error occurs.
*
* @param endpoint the {@code SocketAddress}
* @param timeout the timeout value to be used in milliseconds.
* @throws IOException if an error occurs during the connection
* @throws SocketTimeoutException if timeout expires before connecting
* @throws java.nio.channels.IllegalBlockingModeException
* if this socket has an associated channel,
* and the channel is in non-blocking mode
* @throws IllegalArgumentException if endpoint is null or is a
* SocketAddress subclass not supported by this socket
* @since 1.4
* @spec JSR-51
*/
public void connect(SocketAddress endpoint, int timeout) throws IOException {
if (endpoint == null)
throw new IllegalArgumentException("connect: The address can't be null");
if (timeout < 0)
throw new IllegalArgumentException("connect: timeout can't be negative");
if (isClosed())
throw new SocketException("Socket is closed");
if (!oldImpl && isConnected())
throw new SocketException("already connected");
if (!(endpoint instanceof InetSocketAddress))
throw new IllegalArgumentException("Unsupported address type");
InetSocketAddress epoint = (InetSocketAddress) endpoint;
InetAddress addr = epoint.getAddress ();
int port = epoint.getPort();
checkAddress(addr, "connect");
SecurityManager security = System.getSecurityManager();
if (security != null) {
if (epoint.isUnresolved())
security.checkConnect(epoint.getHostName(), port);
else
security.checkConnect(addr.getHostAddress(), port);
}
if (!created)
createImpl(true);
if (!oldImpl)
impl.connect(epoint, timeout);
else if (timeout == 0) {
if (epoint.isUnresolved())
impl.connect(addr.getHostName(), port);
else
impl.connect(addr, port);
} else
throw new UnsupportedOperationException("SocketImpl.connect(addr, timeout)");
connected = true;
/*
* If the socket was not bound before the connect, it is now because
* the kernel will have picked an ephemeral port & a local address
*/
bound = true;
}
connect()方法传入两个参数，SocketAddress代表目标地址，timeout代表超时时间，单位是毫秒，默认是0。
这个地址要跟上一个bind()方法区分，bind传入的是本地要绑定的地址，connect()传入的是要连接的服务端地址。具体实现也是在native层。
4. listen()
如果是服务端，下一步是调用listen()方法，进入被动监听，等待连接请求的过程。
/**
* Sets the maximum queue length for incoming connection indications
* (a request to connect) to the {@code count} argument. If a
* connection indication arrives when the queue is full, the
* connection is refused.
*
* @param backlog the maximum length of the queue.
* @exception IOException if an I/O error occurs when creating the queue.
*/
protected abstract void listen(int backlog) throws IOException;
传入的参数backlog表示请求队列的最大长度。这部分也是native代码，具体可以看下socketListen()方法。
下一步需要对客户端的请求进行接收和处理。
5. accept()
accept()方法对客户端请求进行接收，具体实现在Impl类，具体可以看下socketAccept()里面的native调用。
/**
* Listens for a connection to be made to this socket and accepts
* it. The method blocks until a connection is made.
*
* <p>A new Socket {@code s} is created and, if there
* is a security manager,
* the security manager's {@code checkAccept} method is called
* with {@code s.getInetAddress().getHostAddress()} and
* {@code s.getPort()}
* as its arguments to ensure the operation is allowed.
* This could result in a SecurityException.
*
* @exception IOException if an I/O error occurs when waiting for a
* connection.
* @exception SecurityException if a security manager exists and its
* {@code checkAccept} method doesn't allow the operation.
* @exception SocketTimeoutException if a timeout was previously set with setSoTimeout and
* the timeout has been reached.
* @exception java.nio.channels.IllegalBlockingModeException
* if this socket has an associated channel, the channel is in
* non-blocking mode, and there is no connection ready to be
* accepted
*
* @return the new Socket
* @see SecurityManager#checkAccept
* @revised 1.4
* @spec JSR-51
*/
public Socket accept() throws IOException {
if (isClosed())
throw new SocketException("Socket is closed");
if (!isBound())
throw new SocketException("Socket is not bound yet");
Socket s = new Socket((SocketImpl) null);
implAccept(s);
return s;
}
在没有请求的情况下，线程会阻塞等待，有客户端请求才会唤醒。
6. close()
当发生异常或者通信结束，调用close()方法关闭socket。
protected void close() throws IOException {
synchronized(fdLock) {
if (fd != null) {
if (!stream) {
ResourceManager.afterUdpClose();
}
if (fdUseCount == 0) {
if (closePending) {
return;
}
closePending = true;
/*
* We close the FileDescriptor in two-steps - first the
* "pre-close" which closes the socket but doesn't
* release the underlying file descriptor. This operation
* may be lengthy due to untransmitted data and a long
* linger interval. Once the pre-close is done we do the
* actual socket to release the fd.
*/
try {
socketPreClose();
} finally {
socketClose();
}
fd = null;
return;
} else {
/*
* If a thread has acquired the fd and a close
* isn't pending then use a deferred close.
* Also decrement fdUseCount to signal the last
* thread that releases the fd to close it.
*/
if (!closePending) {
closePending = true;
fdUseCount--;
socketPreClose();
}
}
}
}
}
