SelectionKey：通道向selector注冊后會創建一個SelectionKey，SelectionKey維系通道和selector的關系.SelectionKey包含兩個整數集一個為interest集合，一個為ready集合.interest集合指定Selector需要監聽的事件.ready集合為Selector為SelectorKey監聽后已經準備就緒的可以進行操作的事件.ready集合特別需要注意，這個里面可能有阻塞的行為，如OP_READ事件，只是暗示可讀，但是真正的數據此時還沒有到來，此時就會阻塞了。
epoll有三個方法epoll_create(),epoll_ctl(),epoll_wait():
int epoll_create(int size)
創建epoll文件，用于存放epoll_ctl注冊的event。
int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event)
注冊fd到epfd中，op為操作數(add,mod,delete) ，epoll_event為注冊感興趣的事件，這個里面也注冊了回調函數，當對應的fd的設備ready時，就調用回調函數，將這個fd加入epfd的ready set當中，epoll_wait()一直就在那里等待ready set。
int epoll_wait(int epfd, struct epoll_event *events, int maxevents, int timeout)
等待ready set，當準備好有數據的時候返回數據的個數，epoll_event為感興趣的事件集合，maxevents為事件集合的個數。
JDK中向Selector注冊事件流程(以linux epoll為例)：
關鍵代碼如下：
AbstractSelectableChannel.register():
public final SelectionKey register(Selector sel, int ops,
Object att)
throws ClosedChannelException
{
if (!isOpen()) //channel關閉拋異常
throw new ClosedChannelException();
if ((ops & ~validOps()) != 0)//不合理的注冊值，拋異常
throw new IllegalArgumentException();
synchronized (regLock) {//有鎖就有可能有線程的阻塞和切換 關鍵點1
if (blocking)
throw new IllegalBlockingModeException();
SelectionKey k = findKey(sel); //查看原來有沒有注冊過
if (k != null) {//注冊過直接設置后返回
k.interestOps(ops);
k.attach(att);
}
if (k == null) {
// New registration
k = ((AbstractSelector)sel).register(this, ops, att);//沒有注冊的話執行selector的register
addKey(k);
}
return k;
}
}
SelectionImpl.register():
protected final SelectionKey register(AbstractSelectableChannel ch,
int ops,
Object attachment)
{
if (!(ch instanceof SelChImpl))
throw new IllegalSelectorException();
SelectionKeyImpl k = new SelectionKeyImpl((SelChImpl)ch, this);
k.attach(attachment);
synchronized (publicKeys) {//又是鎖，可能阻塞或者線程切換
implRegister(k);
}
k.interestOps(ops);
return k;
}
EPollSelectorImpl.implRegister():
protected void implRegister(SelectionKeyImpl ski) {
SelChImpl ch = ski.channel;
fdToKey.put(Integer.valueOf(ch.getFDVal()), ski);//關鍵點2 fd和Selectionkey對應的map
pollWrapper.add(ch);
keys.add(ski);
}
EPollArrayWrapper.add():
void add(SelChImpl channel) {
synchronized (updateList) {
updateList.add(new Updator(channel, EPOLL_CTL_ADD));//關鍵點3
}
}
關鍵點1.register()方法中有同步語句，可能當前線程就阻塞了，線程切換會有性能的損耗。
關鍵點2.fdToKey是個key為fd，value為selectionKey的map
關鍵點3.
Updator是個內部類：
private static class Updator {
SelChImpl channel;
int opcode;
int events;
Updator(SelChImpl channel, int opcode, int events) {
this.channel = channel;
this.opcode = opcode;
this.events = events;
}
Updator(SelChImpl channel, int opcode) {
this(channel, opcode, 0);
}
}
代表channel以及對channel的操作類型以及操作的事件，新注冊的操作類型為add，操作事件為0，代表沒有事件。
在SelectionImpl.register()中最后還要執行SelectionKeyImpl.interestOps()方法注冊操作事件(前面添加的時候操作事件是為0的)
SelectionKeyImpl:
public SelectionKey interestOps(int ops) {
ensureValid();//檢測是否可用
return nioInterestOps(ops);
}
SelectionKey nioInterestOps(int ops) { // package-private
if ((ops & ~channel().validOps()) != 0)
throw new IllegalArgumentException();
//真正的進行epoll感興趣事件的注冊
channel.translateAndSetInterestOps(ops, this);
interestOps = ops;
return this;
}
public void translateAndSetInterestOps(int ops, SelectionKeyImpl sk) {
int newOps = 0;
//進行java的ops和epoll的ops轉換，每個具體的channel是有區別的
if ((ops & SelectionKey.OP_READ) != 0)
newOps |= PollArrayWrapper.POLLIN;
if ((ops & SelectionKey.OP_WRITE) != 0)
newOps |= PollArrayWrapper.POLLOUT;
if ((ops & SelectionKey.OP_CONNECT) != 0)
newOps |= PollArrayWrapper.POLLCONN;
sk.selector.putEventOps(sk, newOps);
}
void setInterest(SelChImpl channel, int mask) {
synchronized (updateList) {
// if the previous pending operation is to add this file descriptor
// to epoll then update its event set
if (updateList.size() > 0) {//關鍵點1
Updator last = updateList.getLast();
//這個肯定是剛才那個注冊的channel，直接進行事件的更新
if (last.channel == channel && last.opcode == EPOLL_CTL_ADD) {
last.events = mask;
return;
}
}
// update existing registration 程序運行到這里的話，說明前面已經有更新的updator加入了，這里只好新加入一個
updateList.add(new Updator(channel, EPOLL_CTL_MOD, mask));
}
}
關鍵點1.這里是考慮到并發的問題了，但是我有個疑問為什么注冊要分兩個步驟執行，為什么不直接在EPollSelectorImpl.implRegister()加入 updateList.add(new Updator(channel, EPOLL_CTL_ADD, mask))呢？
至此register()動作已經完成。
select()
代碼如下：
public int select(long timeout)
throws IOException
{
if (timeout < 0)
throw new IllegalArgumentException("Negative timeout");
return lockAndDoSelect((timeout == 0) ? -1 : timeout);
}
private int lockAndDoSelect(long timeout) throws IOException {
synchronized (this) {
if (!isOpen())
throw new ClosedSelectorException();
synchronized (publicKeys) {
synchronized (publicSelectedKeys) {
return doSelect(timeout);
}
}
}
}
protected int doSelect(long timeout)
throws IOException
{
if (closed)
throw new ClosedSelectorException();
//反注冊過程 刪除取消的通道的key(interest keys，ready keys(也就是selected keys)，channel的key set)
processDeregisterQueue();
try {
begin();
pollWrapper.poll(timeout);//關鍵點1
} finally {
end();
}
processDeregisterQueue();
int numKeysUpdated = updateSelectedKeys();
if (pollWrapper.interrupted()) {
// Clear the wakeup pipe
pollWrapper.putEventOps(pollWrapper.interruptedIndex(), 0);
synchronized (interruptLock) {
pollWrapper.clearInterrupted();
IOUtil.drain(fd0);
interruptTriggered = false;
}
}
return numKeysUpdated;
}
void processDeregisterQueue() throws IOException {
// Precondition: Synchronized on this, keys, and selectedKeys
//遍歷取消的key
Set cks = cancelledKeys();
synchronized (cks) {
Iterator i = cks.iterator();
while (i.hasNext()) {
SelectionKeyImpl ski = (SelectionKeyImpl)i.next();
try {
//底層的取消實現
implDereg(ski);
} catch (SocketException se) {
IOException ioe = new IOException(
"Error deregistering key");
ioe.initCause(se);
throw ioe;
} finally {
//刪除取消的key
i.remove();
}
}
}
}
protected void implDereg(SelectionKeyImpl ski) throws IOException {
assert (ski.getIndex() >= 0);
SelChImpl ch = ski.channel;
int fd = ch.getFDVal();
fdToKey.remove(new Integer(fd));//hashmap中去除
pollWrapper.release(ch);//這步很關鍵
ski.setIndex(-1);
keys.remove(ski);//總的key set去除
selectedKeys.remove(ski);//已經準備好的set去除
deregister((AbstractSelectionKey)ski);//移除channel的集合
SelectableChannel selch = ski.channel();
if (!selch.isOpen() && !selch.isRegistered())
((SelChImpl)selch).kill();
}
void release(SelChImpl channel) {
//空閑隊列刪除了
synchronized (updateList) {
// flush any pending updates
for (Iterator<Updator> it = updateList.iterator(); it.hasNext();) {
if (it.next().channel == channel) {
it.remove();
}
}
// remove from the idle set (if present)
idleSet.remove(channel);
//調用native 通知本channel對應的fd被被刪除
// remove from epoll (if registered)
epollCtl(epfd, EPOLL_CTL_DEL, channel.getFDVal(), 0);
}
}