前言
上篇[【從入門到放棄-Java】并發程式設計-NIO使用]()簡單介紹了nio的基礎使用，本篇将深入源碼分析nio中channel的實作。
簡介
channel即通道，可以用來讀、寫資料，它是全雙工的可以同時用來讀寫操作。這也是它與stream流的最大差別。
channel需要與buffer配合使用，channel通道的一端是buffer，一端是資料源實體，如檔案、socket等。在nio中，通過channel的不同實作來處理 不同實體與資料buffer中的資料傳輸。
channel接口：
package java.nio.channels;
import java.io.IOException;
import java.io.Closeable;
/**
* A nexus for I/O operations.
*
* <p> A channel represents an open connection to an entity such as a hardware
* device, a file, a network socket, or a program component that is capable of
* performing one or more distinct I/O operations, for example reading or
* writing.
*
* <p> A channel is either open or closed. A channel is open upon creation,
* and once closed it remains closed. Once a channel is closed, any attempt to
* invoke an I/O operation upon it will cause a {@link ClosedChannelException}
* to be thrown. Whether or not a channel is open may be tested by invoking
* its {@link #isOpen isOpen} method.
*
* <p> Channels are, in general, intended to be safe for multithreaded access
* as described in the specifications of the interfaces and classes that extend
* and implement this interface.
*
*
* @author Mark Reinhold
* @author JSR-51 Expert Group
* @since 1.4
*/
public interface Channel extends Closeable {
/**
* Tells whether or not this channel is open.
*
* @return <tt>true</tt> if, and only if, this channel is open
*/
public boolean isOpen();
/**
* Closes this channel.
*
* <p> After a channel is closed, any further attempt to invoke I/O
* operations upon it will cause a {@link ClosedChannelException} to be
* thrown.
*
* <p> If this channel is already closed then invoking this method has no
* effect.
*
* <p> This method may be invoked at any time. If some other thread has
* already invoked it, however, then another invocation will block until
* the first invocation is complete, after which it will return without
* effect. </p>
*
* @throws IOException If an I/O error occurs
*/
public void close() throws IOException;
} 常見的channel實作有：
- FileChannel：檔案讀寫資料通道
- SocketChannel：TCP讀寫網絡資料通道
- ServerSocketChannel：服務端網絡資料讀寫通道，可以監聽TCP連接配接。對每一個新進來的連接配接都會建立一個SocketChannel。
- DatagramChannel：UDP讀寫網絡資料通道
FileChannel
FileChannel是一個抽象類，它繼承了AbstractInterruptibleChannel類，并實作了 SeekableByteChannel, GatheringByteChannel, ScatteringByteChannel接口。
具體的實作類主要是sun.nio.ch.FileChannelImpl。下面詳細分析下FileChannelImpl中每個方法的具體實作。
open
private FileChannelImpl(FileDescriptor var1, String var2, boolean var3, boolean var4, boolean var5, Object var6) {
//主要記載作業系統維護的檔案描述符
this.fd = var1;
//是否可讀
this.readable = var3;
//是否可寫
this.writable = var4;
//是否以追加的方式打開
this.append = var5;
this.parent = var6;
this.path = var2;
//底層使用native的read和write來處理檔案的
this.nd = new FileDispatcherImpl(var5);
}
//FileInputStream::getChannel 調用 FileChannelImpl.open(fd, path, true, false, this) 擷取隻讀channel
public static FileChannel open(FileDescriptor var0, String var1, boolean var2, boolean var3, Object var4) {
return new FileChannelImpl(var0, var1, var2, var3, false, var4);
}
//FileOutputStream::getChannel 調用 FileChannelImpl.open(fd, path, false, true, append, this) 擷取隻寫channel
public static FileChannel open(FileDescriptor var0, String var1, boolean var2, boolean var3, boolean var4, Object var5) {
return new FileChannelImpl(var0, var1, var2, var3, var4, var5);
}
private FileChannelImpl(FileDescriptor fd, String path, boolean readable,
boolean writable, boolean direct, Object parent)
{
this.fd = fd;
//是否可讀
this.readable = readable;
//是否可寫
this.writable = writable;
//對于從流建立的channel，在結束時要做不同的清理動作，（openJDK中才有，sun的jdk中沒有）
this.parent = parent;
//源檔案的path
this.path = path;
//是否使用DirectIO
this.direct = direct;
this.nd = new FileDispatcherImpl();
if (direct) {
assert path != null;
this.alignment = nd.setDirectIO(fd, path);
} else {
this.alignment = -1;
}
//當parent不存在時，則注冊一個cleaner，否則交由parent做清理動作。
// Register a cleaning action if and only if there is no parent
// as the parent will take care of closing the file descriptor.
// FileChannel is used by the LambdaMetaFactory so a lambda cannot
// be used here hence we use a nested class instead.
this.closer = parent != null ? null :
CleanerFactory.cleaner().register(this, new Closer(fd));
}
// Used by FileInputStream.getChannel(), FileOutputStream.getChannel
// and RandomAccessFile.getChannel()
public static FileChannel open(FileDescriptor fd, String path,
boolean readable, boolean writable,
boolean direct, Object parent)
{
return new FileChannelImpl(fd, path, readable, writable, direct, parent);
} - open方法主要是傳回一個新new的FileChannelImpl對象，初始化時設定fileDescriptor、readable、writable、append、parent、path等屬性，看變量名很容易了解，在此不贅述變量含義。
read
//實作自SeekableByteChannel接口的方法，将檔案中的内容讀取到給定的byteBuffer中
public int read(ByteBuffer dst) throws IOException {
//保證讀寫時，channel處于開啟狀态
ensureOpen();
//判斷是否可讀
if (!readable)
throw new NonReadableChannelException();
synchronized (positionLock) {
if (direct)
Util.checkChannelPositionAligned(position(), alignment);
int n = 0;
int ti = -1;
try {
//開始阻塞，并注冊為Interruptible，可以被中斷
beginBlocking();
//将目前線程添加到NativeThreadSet中，并傳回索引，友善後續操作。
//NativeThreadSet是一個線程安全的本地線程集合，友善管理，用來發送信号
ti = threads.add();
if (!isOpen())
return 0;
do {
//當未被系統中斷（即讀取完畢）或channel未被關閉，則一直讀，将内容寫入到byteBuffer（dst）中
n = IOUtil.read(fd, dst, -1, direct, alignment, nd);
} while ((n == IOStatus.INTERRUPTED) && isOpen());
return IOStatus.normalize(n);
} finally {
//把目前線程從set中移出
threads.remove(ti);
//結束，釋放鎖
endBlocking(n > 0);
assert IOStatus.check(n);
}
}
}
//實作自ScatteringByteChannel接口的方法，将檔案中的内容依次讀取到給定的byteBuffer數組中。
public long read(ByteBuffer[] dsts, int offset, int length)
throws IOException
{
if ((offset < 0) || (length < 0) || (offset > dsts.length - length))
throw new IndexOutOfBoundsException();
//保證讀寫時，channel處于開啟狀态
ensureOpen();
//判斷是否可讀
if (!readable)
throw new NonReadableChannelException();
synchronized (positionLock) {
if (direct)
Util.checkChannelPositionAligned(position(), alignment);
long n = 0;
int ti = -1;
try {
//開始阻塞，并注冊為Interruptible，可以被中斷
beginBlocking();
//将目前線程添加到NativeThreadSet中，并傳回索引，友善後續操作。
//NativeThreadSet是一個線程安全的本地線程集合，友善管理，用來發送信号
ti = threads.add();
if (!isOpen())
return 0;
do {
//當未被系統中斷（即讀取完畢）或channel未被關閉，則一直讀，将内容寫入到byteBuffer（dst）中
n = IOUtil.read(fd, dsts, offset, length,
direct, alignment, nd);
} while ((n == IOStatus.INTERRUPTED) && isOpen());
return IOStatus.normalize(n);
} finally {
//把目前線程從set中移出
threads.remove(ti);
//結束，釋放鎖
endBlocking(n > 0);
assert IOStatus.check(n);
}
}
} write
//實作自SeekableByteChannel接口的方法，将byteBuffer中的内容寫入到檔案中
public int write(ByteBuffer src) throws IOException {
//保證寫時，channel處于開啟狀态
ensureOpen();
//判斷是否可寫
if (!writable)
throw new NonWritableChannelException();
synchronized (positionLock) {
if (direct)
Util.checkChannelPositionAligned(position(), alignment);
int n = 0;
int ti = -1;
try {
//開始阻塞，并注冊為Interruptible，可以被中斷
beginBlocking();
//将目前線程添加到NativeThreadSet中，并傳回索引，友善後續操作。
//NativeThreadSet是一個線程安全的本地線程集合，友善管理，用來發送信号
ti = threads.add();
if (!isOpen())
return 0;
do {
//當未被系統中斷（即寫入完畢）或channel未被關閉，則一直寫，将内容寫入到檔案中
n = IOUtil.write(fd, src, -1, direct, alignment, nd);
} while ((n == IOStatus.INTERRUPTED) && isOpen());
return IOStatus.normalize(n);
} finally {
//把目前線程從set中移出
threads.remove(ti);
//結束，釋放鎖
assert IOStatus.check(n);
}
}
}
//實作自GatheringByteChannel接口的方法，将byteBuffer數組中的内容依次寫入到檔案中
public long write(ByteBuffer[] srcs, int offset, int length)
throws IOException
{
if ((offset < 0) || (length < 0) || (offset > srcs.length - length))
throw new IndexOutOfBoundsException();
//保證寫時，channel處于開啟狀态
ensureOpen();
//判斷是否可寫
if (!writable)
throw new NonWritableChannelException();
synchronized (positionLock) {
if (direct)
Util.checkChannelPositionAligned(position(), alignment);
long n = 0;
int ti = -1;
try {
//開始阻塞，并注冊為Interruptible，可以被中斷
beginBlocking();
//将目前線程添加到NativeThreadSet中，并傳回索引，友善後續操作。
//NativeThreadSet是一個線程安全的本地線程集合，友善管理，用來發送信号
ti = threads.add();
if (!isOpen())
return 0;
do {
//當未被系統中斷（即寫入完畢）或channel未被關閉，則一直寫，将内容寫入到檔案中
n = IOUtil.write(fd, srcs, offset, length,
direct, alignment, nd);
} while ((n == IOStatus.INTERRUPTED) && isOpen());
return IOStatus.normalize(n);
} finally {
//把目前線程從set中移出
threads.remove(ti);
//結束，釋放鎖
assert IOStatus.check(n);
}
}
} position
//實作自SeekableByteChannel接口的方法，擷取目前channel的position
public long position() throws IOException {
ensureOpen();
synchronized (positionLock) {
long p = -1;
int ti = -1;
try {
beginBlocking();
ti = threads.add();
if (!isOpen())
return 0;
boolean append = fdAccess.getAppend(fd);
do {
//append模式下，position在channel的末尾
// in append-mode then position is advanced to end before writing
p = (append) ? nd.size(fd) : nd.seek(fd, -1);
} while ((p == IOStatus.INTERRUPTED) && isOpen());
return IOStatus.normalize(p);
} finally {
threads.remove(ti);
endBlocking(p > -1);
assert IOStatus.check(p);
}
}
}
//實作自SeekableByteChannel接口的方法，設定目前channel的position為newPosition
public FileChannel position(long newPosition) throws IOException {
ensureOpen();
if (newPosition < 0)
throw new IllegalArgumentException();
synchronized (positionLock) {
long p = -1;
int ti = -1;
try {
beginBlocking();
ti = threads.add();
if (!isOpen())
return null;
do {
//設定目前position為newPosition
p = nd.seek(fd, newPosition);
} while ((p == IOStatus.INTERRUPTED) && isOpen());
return this;
} finally {
threads.remove(ti);
endBlocking(p > -1);
assert IOStatus.check(p);
}
}
} size
實作自SeekableByteChannel接口的方法，傳回目前實體（檔案）的大小
truncate
實作自SeekableByteChannel接口的方法，用來截取檔案至newSize大小
force
實作自SeekableByteChannel接口的方法，用來将channel中尚未寫入磁盤的資料強制落盤
transferTo
将fileChannel中的資料傳遞至另一個channel
transferFrom
從其它channel讀取資料至fileChannel
SocketChannel
/**
* Opens a socket channel.
*
* <p> The new channel is created by invoking the {@link
* java.nio.channels.spi.SelectorProvider#openSocketChannel
* openSocketChannel} method of the system-wide default {@link
* java.nio.channels.spi.SelectorProvider} object. </p>
*
* @return A new socket channel
*
* @throws IOException
* If an I/O error occurs
*/
public static SocketChannel open() throws IOException {
return SelectorProvider.provider().openSocketChannel();
} open方法是調用SelectorProvider中實作了java.nio.channels.spi.SelectorProvider#openSocketChannel的方法，底層實際是new SocketChannelImpl，調用native方法建立socket
connect
public boolean connect(SocketAddress sa) throws IOException {
//校驗Address是否合法
InetSocketAddress isa = Net.checkAddress(sa);
//擷取系統安全管理器
SecurityManager sm = System.getSecurityManager();
if (sm != null)
//校驗IP和端口是否被允許連接配接
sm.checkConnect(isa.getAddress().getHostAddress(), isa.getPort());
InetAddress ia = isa.getAddress();
//如果是本機位址，則擷取本機的host
if (ia.isAnyLocalAddress())
ia = InetAddress.getLocalHost();
try {
//加讀鎖
readLock.lock();
try {
//加寫鎖
writeLock.lock();
try {
int n = 0;
//是否阻塞
boolean blocking = isBlocking();
try {
//開啟connect前的校驗并設定為ST_CONNECTIONPENDING，如果blocking是true 即阻塞模式，則記錄目前線程的ID，以便接收信号處理。
beginConnect(blocking, isa);
do {
//調用native connect方法
n = Net.connect(fd, ia, isa.getPort());
} while (n == IOStatus.INTERRUPTED && isOpen());
} finally {
//結束連接配接
endConnect(blocking, (n > 0));
}
assert IOStatus.check(n);
return n > 0;
} finally {
//釋放寫鎖
writeLock.unlock();
}
} finally {
//釋放讀鎖
readLock.unlock();
}
} catch (IOException ioe) {
// connect failed, close the channel
close();
throw SocketExceptions.of(ioe, isa);
}
} configureBlocking
實作自SelectableChannel的接口方法，調用native方法設定socket的阻塞狀态
register
在AbstractSelectableChannel中定義，注冊要監聽的事件。
public final SelectionKey register(Selector sel, int ops, Object att)
throws ClosedChannelException
{
if ((ops & ~validOps()) != 0)
throw new IllegalArgumentException();
if (!isOpen())
throw new ClosedChannelException();
synchronized (regLock) {
if (isBlocking())
throw new IllegalBlockingModeException();
synchronized (keyLock) {
// re-check if channel has been closed
if (!isOpen())
throw new ClosedChannelException();
SelectionKey k = findKey(sel);
if (k != null) {
k.attach(att);
k.interestOps(ops);
} else {
// 向Selector中注冊事件
// New registration
k = ((AbstractSelector)sel).register(this, ops, att);
addKey(k);
}
return k;
}
}
} //實作自ReadableByteChannel接口的方法，從socket中讀取資料至ByteBuffer
@Override
public int read(ByteBuffer buf) throws IOException {
Objects.requireNonNull(buf);
readLock.lock();
try {
boolean blocking = isBlocking();
int n = 0;
try {
//檢查channel是否開啟并已經是connected的狀态。如果blocking是true 即阻塞模式，則記錄目前線程的ID，以便接收信号處理。
beginRead(blocking);
// check if input is shutdown
if (isInputClosed)
return IOStatus.EOF;
//如果是阻塞模式，則一直讀取直到資料讀取完畢；非阻塞模式則直接調用native方法不需要等待。
if (blocking) {
do {
n = IOUtil.read(fd, buf, -1, nd);
} while (n == IOStatus.INTERRUPTED && isOpen());
} else {
n = IOUtil.read(fd, buf, -1, nd);
}
} finally {
endRead(blocking, n > 0);
if (n <= 0 && isInputClosed)
return IOStatus.EOF;
}
return IOStatus.normalize(n);
} finally {
readLock.unlock();
}
}
//實作自ScatteringByteChannel接口的方法，從socket中依次讀取資料至ByteBuffer數組
@Override
public long read(ByteBuffer[] dsts, int offset, int length)
throws IOException
{
Objects.checkFromIndexSize(offset, length, dsts.length);
readLock.lock();
try {
boolean blocking = isBlocking();
long n = 0;
try {
beginRead(blocking);
// check if input is shutdown
if (isInputClosed)
return IOStatus.EOF;
//如果是阻塞模式，則一直讀取直到資料讀取完畢；非阻塞模式則直接調用native方法不需要等待。
if (blocking) {
do {
n = IOUtil.read(fd, dsts, offset, length, nd);
} while (n == IOStatus.INTERRUPTED && isOpen());
} else {
n = IOUtil.read(fd, dsts, offset, length, nd);
}
} finally {
endRead(blocking, n > 0);
if (n <= 0 && isInputClosed)
return IOStatus.EOF;
}
return IOStatus.normalize(n);
} finally {
readLock.unlock();
}
} //實作自ReadableByteChannel接口的方法，将ByteBuffer中的資料寫入socket
@Override
public int write(ByteBuffer buf) throws IOException {
Objects.requireNonNull(buf);
writeLock.lock();
try {
boolean blocking = isBlocking();
int n = 0;
try {
beginWrite(blocking);
//如果是阻塞模式，則一直讀取直到資料讀取完畢；非阻塞模式則直接調用native方法不需要等待。
if (blocking) {
do {
n = IOUtil.write(fd, buf, -1, nd);
} while (n == IOStatus.INTERRUPTED && isOpen());
} else {
n = IOUtil.write(fd, buf, -1, nd);
}
} finally {
endWrite(blocking, n > 0);
if (n <= 0 && isOutputClosed)
throw new AsynchronousCloseException();
}
return IOStatus.normalize(n);
} finally {
writeLock.unlock();
}
}
@Override
public long write(ByteBuffer[] srcs, int offset, int length)
throws IOException
{
Objects.checkFromIndexSize(offset, length, srcs.length);
writeLock.lock();
try {
boolean blocking = isBlocking();
long n = 0;
try {
beginWrite(blocking);
//如果是阻塞模式，則一直等待直到資料寫入完畢；非阻塞模式則直接調用native方法不需要等待。
if (blocking) {
do {
n = IOUtil.write(fd, srcs, offset, length, nd);
} while (n == IOStatus.INTERRUPTED && isOpen());
} else {
n = IOUtil.write(fd, srcs, offset, length, nd);
}
} finally {
endWrite(blocking, n > 0);
if (n <= 0 && isOutputClosed)
throw new AsynchronousCloseException();
}
return IOStatus.normalize(n);
} finally {
writeLock.unlock();
}
}
//實作自ReadableByteChannel接口的方法，将ByteBuffer數組中的資料依次寫入socket
/**
* Writes a byte of out of band data.
*/
int sendOutOfBandData(byte b) throws IOException {
writeLock.lock();
try {
boolean blocking = isBlocking();
int n = 0;
try {
beginWrite(blocking);
//如果是阻塞模式，則一直等待直到資料寫入完畢；非阻塞模式則直接調用native方法不需要等待。
if (blocking) {
do {
n = sendOutOfBandData(fd, b);
} while (n == IOStatus.INTERRUPTED && isOpen());
} else {
n = sendOutOfBandData(fd, b);
}
} finally {
endWrite(blocking, n > 0);
if (n <= 0 && isOutputClosed)
throw new AsynchronousCloseException();
}
return IOStatus.normalize(n);
} finally {
writeLock.unlock();
}
} ServerSocketChannel
socket
@Override
public ServerSocket socket() {
synchronized (stateLock) {
if (socket == null)
socket = ServerSocketAdaptor.create(this);
return socket;
}
} bind
@Override
public ServerSocketChannel bind(SocketAddress local, int backlog) throws IOException {
synchronized (stateLock) {
ensureOpen();
if (localAddress != null)
throw new AlreadyBoundException();
InetSocketAddress isa = (local == null)
? new InetSocketAddress(0)
: Net.checkAddress(local);
SecurityManager sm = System.getSecurityManager();
if (sm != null)
sm.checkListen(isa.getPort());
//綁定前做一些前置處理，如将tcp socket檔案描述符轉換成SDP
NetHooks.beforeTcpBind(fd, isa.getAddress(), isa.getPort());
//綁定IP和位址
Net.bind(fd, isa.getAddress(), isa.getPort());
//開始監聽，設定socket上最多可以挂起backlog個連接配接，若backlog小于1 則預設設定50個
Net.listen(fd, backlog < 1 ? 50 : backlog);
localAddress = Net.localAddress(fd);
}
return this;
} accept
@Override
public SocketChannel accept() throws IOException {
acceptLock.lock();
try {
int n = 0;
FileDescriptor newfd = new FileDescriptor();
InetSocketAddress[] isaa = new InetSocketAddress[1];
boolean blocking = isBlocking();
try {
begin(blocking);
do {
//阻塞等待接收用戶端連結
n = accept(this.fd, newfd, isaa);
} while (n == IOStatus.INTERRUPTED && isOpen());
} finally {
end(blocking, n > 0);
assert IOStatus.check(n);
}
if (n < 1)
return null;
//新接收的socket初始設定為阻塞模式（是以非阻塞模式的每次需要顯示設定）
// newly accepted socket is initially in blocking mode
IOUtil.configureBlocking(newfd, true);
InetSocketAddress isa = isaa[0];
//用新接收的socket建立SocketChannel
SocketChannel sc = new SocketChannelImpl(provider(), newfd, isa);
// check permitted to accept connections from the remote address
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
try {
sm.checkAccept(isa.getAddress().getHostAddress(), isa.getPort());
} catch (SecurityException x) {
sc.close();
throw x;
}
}
return sc;
} finally {
acceptLock.unlock();
}
} ServerSocketChannel并沒有read和write方法，隻是繼承了AbstractSelectableChannel，以便在selector中使用
DatagramChannel
public DatagramChannelImpl(SelectorProvider sp)
throws IOException
{
super(sp);
ResourceManager.beforeUdpCreate();
try {
//如果不支援IPv6則使用IPv4
this.family = Net.isIPv6Available()
? StandardProtocolFamily.INET6
: StandardProtocolFamily.INET;
//設定非流式的socket（tcp是流模式協定，udp是資料報模式協定）
this.fd = Net.socket(family, false);
this.fdVal = IOUtil.fdVal(fd);
} catch (IOException ioe) {
ResourceManager.afterUdpClose();
throw ioe;
}
} receive
public SocketAddress receive(ByteBuffer dst) throws IOException {
if (dst.isReadOnly())
throw new IllegalArgumentException("Read-only buffer");
readLock.lock();
try {
boolean blocking = isBlocking();
int n = 0;
ByteBuffer bb = null;
try {
SocketAddress remote = beginRead(blocking, false);
boolean connected = (remote != null);
SecurityManager sm = System.getSecurityManager();
if (connected || (sm == null)) {
// connected or no security manager
do {
n = receive(fd, dst, connected);
} while ((n == IOStatus.INTERRUPTED) && isOpen());
if (n == IOStatus.UNAVAILABLE)
return null;
} else {
// Cannot receive into user's buffer when running with a
// security manager and not connected
bb = Util.getTemporaryDirectBuffer(dst.remaining());
for (;;) {
do {
n = receive(fd, bb, connected);
} while ((n == IOStatus.INTERRUPTED) && isOpen());
if (n == IOStatus.UNAVAILABLE)
return null;
InetSocketAddress isa = (InetSocketAddress)sender;
try {
sm.checkAccept(isa.getAddress().getHostAddress(),
isa.getPort());
} catch (SecurityException se) {
// Ignore packet
bb.clear();
n = 0;
continue;
}
bb.flip();
dst.put(bb);
break;
}
}
//sender:發送方位址， Set by receive0 (## ugh)
assert sender != null;
return sender;
} finally {
if (bb != null)
Util.releaseTemporaryDirectBuffer(bb);
endRead(blocking, n > 0);
assert IOStatus.check(n);
}
} finally {
readLock.unlock();
}
} send
public int send(ByteBuffer src, SocketAddress target)
throws IOException
{
Objects.requireNonNull(src);
InetSocketAddress isa = Net.checkAddress(target, family);
writeLock.lock();
try {
boolean blocking = isBlocking();
int n = 0;
try {
//當connect後，remote會設定為連接配接的位址
SocketAddress remote = beginWrite(blocking, false);
if (remote != null) {
// connected
if (!target.equals(remote)) {
throw new AlreadyConnectedException();
}
do {
n = IOUtil.write(fd, src, -1, nd);
} while ((n == IOStatus.INTERRUPTED) && isOpen());
} else {
// not connected
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
InetAddress ia = isa.getAddress();
if (ia.isMulticastAddress()) {
sm.checkMulticast(ia);
} else {
sm.checkConnect(ia.getHostAddress(), isa.getPort());
}
}
do {
n = send(fd, src, isa);
} while ((n == IOStatus.INTERRUPTED) && isOpen());
}
} finally {
endWrite(blocking, n > 0);
assert IOStatus.check(n);
}
return IOStatus.normalize(n);
} finally {
writeLock.unlock();
}
} @Override
public DatagramChannel connect(SocketAddress sa) throws IOException {
InetSocketAddress isa = Net.checkAddress(sa, family);
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
InetAddress ia = isa.getAddress();
if (ia.isMulticastAddress()) {
sm.checkMulticast(ia);
} else {
sm.checkConnect(ia.getHostAddress(), isa.getPort());
sm.checkAccept(ia.getHostAddress(), isa.getPort());
}
}
readLock.lock();
try {
writeLock.lock();
try {
synchronized (stateLock) {
ensureOpen();
if (state == ST_CONNECTED)
throw new AlreadyConnectedException();
int n = Net.connect(family,
fd,
isa.getAddress(),
isa.getPort());
if (n <= 0)
throw new Error(); // Can't happen
// connected
remoteAddress = isa;
state = ST_CONNECTED;
// refresh local address
localAddress = Net.localAddress(fd);
// flush any packets already received.
boolean blocking = isBlocking();
if (blocking) {
IOUtil.configureBlocking(fd, false);
}
try {
ByteBuffer buf = ByteBuffer.allocate(100);
while (receive(buf) != null) {
buf.clear();
}
} finally {
if (blocking) {
IOUtil.configureBlocking(fd, true);
}
}
}
} finally {
writeLock.unlock();
}
} finally {
readLock.unlock();
}
return this;
} udp是資料報模式的協定，是沒有connect的。這裡的connect實際上是在底層忽略了與其他位址的資料傳輸。
在connect後，就可以像socketChannel似得使用read和write了
總結
本文學習了各種channel的實作，主要是對底層native方法的一些封裝，針對不同屬性的實體（檔案、socket），使用對應的channel與byteBuffer傳輸資料。再通過byteBuffer與byte資料進行轉換。
channel的實作中，封裝了大量的native方法，重要的底層實作全在native中，後續可以深入學習下。
本文中出現的byteBuffer和selector将在接下來的文章中，單獨分析。