Java nio 空輪詢bug

beanlam 發(fā)布于2019-08-19 11:01 / 1024人閱讀

這個(gè)應(yīng)該是很老的bug了，linux平臺(tái)，jdk6好像就修復(fù)了

bug 描述：https://bugs.java.com/bugdata...

This is an issue with poll (and epoll) on Linux. If a file descriptor for a connected socket is polled with a request event mask of 0, and if the connection is abruptly terminated (RST) then the poll wakes up with the POLLHUP (and maybe POLLERR) bit set in the returned event set. The implication of this behaviour is that Selector will wakeup and as the interest set for the SocketChannel is 0 it means there aren"t any selected events and the select method returns 0.

之前一直不知道event mask 為0 代表啥，知道翻到了資料

http://man7.org/linux/man-pag...

The field events is an input parameter, a bit mask specifying the
 events the application is interested in for the file descriptor fd.
 This field may be specified as zero, in which case the only events
 that can be returned in revents are POLLHUP, POLLERR, and POLLNVAL
 (see below).

大概知道了0代表了POLLHUP, POLLERR, and POLLNVA 這些event

繼續(xù)查閱https://blog.csdn.net/tilter/...

常量    說(shuō)明
POLLIN    普通或優(yōu)先級(jí)帶數(shù)據(jù)可讀
POLLRDNORM    普通數(shù)據(jù)可讀
POLLRDBAND    優(yōu)先級(jí)帶數(shù)據(jù)可讀
POLLPRI    高優(yōu)先級(jí)數(shù)據(jù)可讀
POLLOUT    普通數(shù)據(jù)可寫
POLLWRNORM    普通數(shù)據(jù)可寫
POLLWRBAND    優(yōu)先級(jí)帶數(shù)據(jù)可寫
POLLERR    發(fā)生錯(cuò)誤
POLLHUP    對(duì)方描述符掛起
POLLNVAL    描述字不是一個(gè)打開的文件

大概明白了Bug描述中 abruptly terminated (RST)的含義及發(fā)生場(chǎng)景

也就知道什么時(shí)候會(huì)發(fā)生空輪詢bug

個(gè)人記錄下

還有1問(wèn)題沒想明白，為啥先cancel，再select(or selectNow) 還是無(wú)法避免這個(gè)問(wèn)題？ ref: https://www.cnblogs.com/JAYIT...
網(wǎng)上說(shuō)，在多線程環(huán)境下，selectNow先發(fā)生，cancel后發(fā)生，這樣還是會(huì)存在問(wèn)題
但是，即使cancel后發(fā)生，channel也會(huì)被移到待移除channel集合了，下次再輪詢的時(shí)候，select也會(huì)出發(fā)這個(gè)集合里面的channel被清除
這樣，最多多一次空輪訓(xùn)。
目前給的方案都是open一個(gè)新的selector，把有效的channel全部注冊(cè)到新的selector，再輪詢新的selector