java selector 源码_Java NIO——Selector机制源码分析---转
一直不明白pipe是如何喚醒selector的，所以又去看了jdk的源碼(openjdk下載)，整理了如下：
以Java nio自帶demo : OperationServer.java???OperationClient.java(見附件)
其中server端的核心代碼：
public voidinitSelector() {try{
selector=SelectorProvider.provider().openSelector();this.serverChannel1 =ServerSocketChannel.open();
serverChannel1.configureBlocking(false);
InetSocketAddress isa= new InetSocketAddress("localhost", this.port1);
serverChannel1.socket().bind(isa);
serverChannel1.register(selector, SelectionKey.OP_ACCEPT);
}catch(IOException e) {//TODO Auto-generated catch block
e.printStackTrace();
}
}
從頭開始，
先看看SelectorProvider.provider()做了什么：
public staticSelectorProvider provider() {synchronized(lock) {if (provider != null)returnprovider;returnAccessController.doPrivileged(new PrivilegedAction() {publicSelectorProvider run() {if(loadProviderFromProperty())returnprovider;if(loadProviderAsService())returnprovider;
provider=sun.nio.ch.DefaultSelectorProvider.create();returnprovider;
}
});
}
}
其中provider?= sun.nio.ch.DefaultSelectorProvider.create();會根據操作系統來返回不同的實現類，windows平臺就返回WindowsSelectorProvider；
而if?(provider?!=?null)?returnprovider;
保證了整個server程序中只有一個WindowsSelectorProvider對象；
再看看WindowsSelectorProvider.?openSelector():
public AbstractSelector openSelector() throwsIOException {return new WindowsSelectorImpl(this);
}newWindowsSelectorImpl(SelectorProvider)代碼：
WindowsSelectorImpl(SelectorProvider sp)throwsIOException {super(sp);
pollWrapper= newPollArrayWrapper(INIT_CAP);
wakeupPipe=Pipe.open();
wakeupSourceFd=((SelChImpl)wakeupPipe.source()).getFDVal();//Disable the Nagle algorithm so that the wakeup is more immediate
SinkChannelImpl sink =(SinkChannelImpl)wakeupPipe.sink();
(sink.sc).socket().setTcpNoDelay(true);
wakeupSinkFd=((SelChImpl)sink).getFDVal();
pollWrapper.addWakeupSocket(wakeupSourceFd,0);
}
其中Pipe.open()是關鍵，這個方法的調用過程是：
Java代碼
public static Pipe open() throwsIOException {returnSelectorProvider.provider().openPipe();
}
SelectorProvider 中：public Pipe openPipe() throwsIOException {return new PipeImpl(this);
}
再看看怎么new PipeImpl()的：
Java代碼
PipeImpl(SelectorProvider sp) {long pipeFds = IOUtil.makePipe(true);int readFd = (int) (pipeFds >>> 32);int writeFd = (int) pipeFds;
FileDescriptor sourcefd= newFileDescriptor();
IOUtil.setfdVal(sourcefd, readFd);
source= newSourceChannelImpl(sp, sourcefd);
FileDescriptor sinkfd= newFileDescriptor();
IOUtil.setfdVal(sinkfd, writeFd);
sink= newSinkChannelImpl(sp, sinkfd);
}
其中IOUtil.makePipe(true)是個native方法：
/**
* Returns two file descriptors for a pipe encoded in a long.
* The read end of the pipe is returned in the high 32 bits,
* while the write end is returned in the low 32 bits.
*/
staticnativelong?makePipe(boolean?blocking);
具體實現：
JNIEXPORT jlong JNICALL
Java_sun_nio_ch_IOUtil_makePipe(JNIEnv*env, jobject this, jboolean blocking)
{int fd[2];if (pipe(fd) < 0) {
JNU_ThrowIOExceptionWithLastError(env,"Pipe failed");return 0;
}if (blocking ==JNI_FALSE) {if ((configureBlocking(fd[0], JNI_FALSE) < 0)|| (configureBlocking(fd[1], JNI_FALSE) < 0)) {
JNU_ThrowIOExceptionWithLastError(env,"Configure blocking failed");
close(fd[0]);
close(fd[1]);return 0;
}
}return ((jlong) fd[0] << 32) | (jlong) fd[1];
}static intconfigureBlocking(intfd, jboolean blocking)
{int flags =fcntl(fd, F_GETFL);int newflags = blocking ? (flags & ~O_NONBLOCK) : (flags |O_NONBLOCK);return (flags == newflags) ? 0: fcntl(fd, F_SETFL, newflags);
}
正如這段注釋：
/**
* Returns two file descriptors for a pipe encoded in a long.
* The read end of the pipe is returned in the high 32 bits,
* while the write end is returned in the low 32 bits.
*/
High32位存放的是通道read端的文件描述符FD(file descriptor)，low 32 bits存放的是write端的文件描述符。所以取到makepipe()返回值后要做移位處理。
pollWrapper.addWakeupSocket(wakeupSourceFd, 0);
這行代碼把返回的pipe的write端的FD放在了pollWrapper中(后面會發現，這么做是為了實現selector的wakeup())
ServerSocketChannel.open()的實現：
public static ServerSocketChannel open() throwsIOException {returnSelectorProvider.provider().openServerSocketChannel();
}
SelectorProvider：public ServerSocketChannel openServerSocketChannel() throwsIOException {return new ServerSocketChannelImpl(this);
}
可見創建的ServerSocketChannelImpl也有WindowsSelectorImpl的引用。
ServerSocketChannelImpl(SelectorProvider sp) throwsIOException {super(sp);this.fd = Net.serverSocket(true); //打開一個socket，返回FD
this.fdVal =IOUtil.fdVal(fd);this.state =ST_INUSE;
}
然后通過serverChannel1.register(selector, SelectionKey.OP_ACCEPT);把selector和channel綁定在一起，也就是把new ServerSocketChannel時創建的FD與selector綁定在了一起。
到此，server端已啟動完成了，主要創建了以下對象：
WindowsSelectorProvider：單例
WindowsSelectorImpl中包含：
pollWrapper：保存selector上注冊的FD，包括pipe的write端FD和ServerSocketChannel所用的FD
wakeupPipe：通道(其實就是兩個FD，一個read，一個write)
再到Server?中的run():
selector.select();主要調用了WindowsSelectorImpl中的這個方法：
protected int doSelect(long timeout) throwsIOException {if (channelArray == null)throw newClosedSelectorException();this.timeout = timeout; //set selector timeout
processDeregisterQueue();if(interruptTriggered) {
resetWakeupSocket();return 0;
}//Calculate number of helper threads needed for poll. If necessary//threads are created here and start waiting on startLock
adjustThreadsCount();
finishLock.reset();//reset finishLock//Wakeup helper threads, waiting on startLock, so they start polling.//Redundant threads will exit here after wakeup.
startLock.startThreads();//do polling in the main thread. Main thread is responsible for//first MAX_SELECTABLE_FDS entries in pollArray.
try{
begin();try{
subSelector.poll();
}catch(IOException e) {
finishLock.setException(e);//Save this exception
}//Main thread is out of poll(). Wakeup others and wait for them
if (threads.size() > 0)
finishLock.waitForHelperThreads();
}finally{
end();
}//Done with poll(). Set wakeupSocket to nonsignaled for the next run.
finishLock.checkForException();
processDeregisterQueue();int updated =updateSelectedKeys();//Done with poll(). Set wakeupSocket to nonsignaled for the next run.
resetWakeupSocket();returnupdated;
}
其中subSelector.poll()是核心，也就是輪訓pollWrapper中保存的FD；具體實現是調用native方法poll0：
private int poll() throws IOException{ //poll for the main thread
returnpoll0(pollWrapper.pollArrayAddress,
Math.min(totalChannels, MAX_SELECTABLE_FDS),
readFds, writeFds, exceptFds, timeout);
}private native int poll0(long pollAddress, intnumfds,int[] readFds, int[] writeFds, int[] exceptFds, longtimeout);//These arrays will hold result of native select().//The first element of each array is the number of selected sockets.//Other elements are file descriptors of selected sockets.
private final int[] readFds = new int [MAX_SELECTABLE_FDS + 1];//保存發生read的FD
private final int[] writeFds = new int [MAX_SELECTABLE_FDS + 1]; //保存發生write的FD
private final int[] exceptFds = new int [MAX_SELECTABLE_FDS + 1]; //保存發生except的FD
這個poll0()會監聽pollWrapper中的FD有沒有數據進出，這會造成IO阻塞，直到有數據讀寫事件發生。比如，由于pollWrapper中保存的也有ServerSocketChannel的FD，所以只要ClientSocket發一份數據到ServerSocket,那么poll0()就會返回；又由于pollWrapper中保存的也有pipe的write端的FD，所以只要pipe的write端向FD發一份數據，也會造成poll0()返回；如果這兩種情況都沒有發生，那么poll0()就一直阻塞，也就是selector.select()會一直阻塞；如果有任何一種情況發生，那么selector.select()就會返回，所有在OperationServer的run()里要用while?(true) {，這樣就可以保證在selector接收到數據并處理完后繼續監聽poll();
這時再來看看WindowsSelectorImpl.?Wakeup():
publicSelector wakeup() {synchronized(interruptLock) {if (!interruptTriggered) {
setWakeupSocket();
interruptTriggered= true;
}
}return this;
}//Sets Windows wakeup socket to a signaled state.
private voidsetWakeupSocket() {
setWakeupSocket0(wakeupSinkFd);
}private native void setWakeupSocket0(intwakeupSinkFd);
JNIEXPORTvoidJNICALL
Java_sun_nio_ch_WindowsSelectorImpl_setWakeupSocket0(JNIEnv*env, jclass this,
jint scoutFd)
{/*Write one byte into the pipe*/
const char byte = 1;
send(scoutFd,&byte, 1, 0);
}
可見wakeup()是通過pipe的write?端send(scoutFd, &byte, 1, 0)，發生一個字節1，來喚醒poll()。所以在需要的時候就可以調用selector.wakeup()來喚醒selector。
原文：http://goon.iteye.com/blog/1775421
補充linux操作系統下的DefaultSelectorProvider的實現，可以看到，如果內核版本>=2.6則，具體的SelectorProvider為EPollSelectorProvider，否則為默認的PollSelectorProvider
//sun.nio.ch.DefaultSelectorProvider
public staticSelectorProvider create() {
PrivilegedAction pa= new GetPropertyAction("os.name");
String osname=(String) AccessController.doPrivileged(pa);if ("SunOS".equals(osname)) {return newsun.nio.ch.DevPollSelectorProvider();
}//use EPollSelectorProvider for Linux kernels >= 2.6
if ("Linux".equals(osname)) {
pa= new GetPropertyAction("os.version");
String osversion=(String) AccessController.doPrivileged(pa);
String[] vers= osversion.split("\\.", 0);if (vers.length >= 2) {try{int major = Integer.parseInt(vers[0]);int minor = Integer.parseInt(vers[1]);if (major > 2 || (major == 2 && minor >= 6)) {return newsun.nio.ch.EPollSelectorProvider();
}
}catch(NumberFormatException x) {//format not recognized
}
}
}return newsun.nio.ch.PollSelectorProvider();
}
總結
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