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套接字是通信端点的抽象。正如使用文件描述符访问文件，应用程序用套接字描述符访问套接字。套接字描述符在UNIX系统中被当作是一种文件描述符。事实上，许多处理文件描述符的函数（read和write）可以用于处理套接字描述符。 ——《unix环境高级编程》
WEB应用客户端（浏览器或APP等）与服务端的通信大多数是通过HTTP或HTTPS协议进行的。而套接字（Socket）应该算是OSI七层网络模型中应用层与传输层直接的抽象层，它是面向应用层的一个编程接口。而HTTP/HTTPS则位于应用层，就是通过套接字（Socket）进行信息通信的。所以，WEB应用的每个HTTP链接都与套接字（Socket）息息相关，特别是并发量大的应用，套接字（Socket）的使用至关重要。毕竟，它是应用出入的一个“口”。处理不当，则成为应用系统瓶颈。
套接字（Socket）的瓶颈，大多数情况，还是套接字描述符的一个瓶颈（文件打开数过多）。正如开头所说：套接字是通信端点的抽象，应用对互联网的通讯就是通过对这个抽象进行读写，就像读写普通文件一样。在unix操作系统中，读写文件，就需要系统分配文件标识：文件描述符。套接字（Socket）也不例外。
Talk is cheap,show your code:
我在本地（window）用Java写了Socket服务端代码，并启动服务：
public class SocketServer{
public static void main(String[] args) {
try{
ServerSocket server=null;
server = new ServerSocket(8000);
Socket socket=null;
while(true){
socket=server.accept();
BufferedReader br = new BufferedReader(new InputStreamReader(socket.getInputStream()));
PrintWriter pw = new PrintWriter(socket.getOutputStream());
System.out.println(br.readLine());
pw.println("ok");
pw.flush();
pw.close();
br.close();
}
}catch(Exception e){e.printStackTrace();}
}
}另外也用Java写了一个Socket的客户端，用打包成jar放到测试机（SUSE Linux）下进行测试，并观察其对套接字描述符的一个消耗情况：
public class SocketClient{
public static void main(String[] args) throws Exception {
Socket socket=new Socket("192.168.22.15",8000);
Socket socket2=new Socket("192.168.22.15",8000);
BufferedReader br=new BufferedReader(new InputStreamReader(socket.getInputStream()));
BufferedReader br2=new BufferedReader(new InputStreamReader(socket2.getInputStream()));
PrintWriter pw = new PrintWriter(socket.getOutputStream());
PrintWriter pw2 = new PrintWriter(socket2.getOutputStream());
pw.println("hello");
pw2.println("hello2");
pw.flush();
pw2.flush();
System.out.println(br.readLine());
System.out.println(br2.readLine());
Thread.sleep(20000);
br.close();
br2.close();
pw.close();
pw2.close();
socket.close();
socket2.close();
System.out.println("socket close");
Thread.sleep(20000);
System.out.println("function close");
}
}
以上代码运行期间，我会通过lsof -p <SocketClient进程号>|wc –l命令观察此SocketClient进程消耗的文件描述符数量。
启动SocketClient.jar20秒前（OK）：
结果:101
启动SocketClient.jar20秒到40秒之间（socket close）：
结果:99
启动SocketClient.jar40秒后（function close）：
结果:0
启动SocketClient.jar20秒前lsof -p <SocketClient进程号>详情：
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
java 5920 root cwd DIR 8,3 4096 486764 /usr/local/test
java 5920 root rtd DIR 8,3 4096 2 /
java 5920 root txt REG 8,3 59141 878312 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/bin/java
java 5920 root mem REG 8,3 116348 878486 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/libnet.so
java 5920 root mem REG 8,3 88544 90416 /lib64/libgcc_s.so.1
java 5920 root mem REG 8,3 84914 878492 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/libzip.so
java 5920 root mem REG 8,3 9975 878465 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/libdbgwrapper.so
java 5920 root mem REG 8,3 108248 90308 /lib64/libnsl-2.11.3.so
java 5920 root mem REG 8,3 224723 878477 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/libjava.so
java 5920 root mem REG 8,3 140261 878396 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/compressedrefs/libj9ute24.so
java 5920 root DEL REG 8,3 2183413 /var/run/nscd/dbZxaX8m
java 5920 root mem REG 8,3 90320 878429 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/default/libiverel24.so
java 5920 root mem REG 8,3 629488 878451 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/default/libjclscar_24.so
java 5920 root mem REG 8,3 225264 878449 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/default/libj9vrb24.so
java 5920 root mem REG 8,3 316148 878441 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/default/libj9jvmti24.so
java 5920 root mem REG 8,3 187277 878433 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/default/libj9dyn24.so
java 5920 root mem REG 8,3 841499 878434 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/default/libj9gc24.so
java 5920 root mem REG 8,3 109715 878430 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/default/libj9bcv24.so
java 5920 root mem REG 8,3 6235544 878438 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/default/libj9jit24.so
java 5920 root mem REG 8,3 93053 878446 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/default/libj9trc24.so
java 5920 root mem REG 8,3 74373 878399 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/compressedrefs/libj9zlib24.so
java 5920 root mem REG 8,3 205374 878432 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/default/libj9dmp24.so
java 5920 root mem REG 8,3 35728 436315 /usr/lib64/libnuma.so.1
java 5920 root mem REG 8,3 15691 878377 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/compressedrefs/libhythr.so
java 5920 root mem REG 8,3 46239 878376 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/compressedrefs/libhyprtshim24.so
java 5920 root mem REG 8,3 6957 878403 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/compressedrefs/libvmi.so
java 5920 root mem REG 8,3 47241 90327 /lib64/librt-2.11.3.so
java 5920 root mem REG 8,3 259675 878391 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/compressedrefs/libj9prt24.so
java 5920 root mem REG 8,3 22489 878385 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/compressedrefs/libj9hookable24.so
java 5920 root mem REG 8,3 70975 878394 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/compressedrefs/libj9thr24.so
java 5920 root mem REG 8,3 712429 878448 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/default/libj9vm24.so
java 5920 root mem REG 8,3 164469 878453 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/default/libjvm.so
java 5920 root mem REG 8,3 86110 878311 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/bin/j9vm/libjvm.so
java 5920 root mem REG 8,3 541821 90305 /lib64/libm-2.11.3.so
java 5920 root mem REG 8,3 1754140 90297 /lib64/libc-2.11.3.so
java 5920 root mem REG 8,3 19149 90303 /lib64/libdl-2.11.3.so
java 5920 root mem REG 8,3 42772 878460 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/jli/libjli.so
java 5920 root mem REG 8,3 135690 90323 /lib64/libpthread-2.11.3.so
java 5920 root mem REG 8,3 151051 90290 /lib64/ld-2.11.3.so
java 5920 root mem REG 8,3 217016 2906402 /var/run/nscd/passwd
java 5920 root mem REG 8,3 26050 576479 /usr/lib64/gconv/gconv-modules.cache
java 5920 root mem REG 8,3 257156 576144 /usr/lib/locale/zh_CN.utf8/LC_CTYPE
java 5920 root mem REG 8,3 14684 878401 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/compressedrefs/libjsig.so
java 5920 root 0u CHR 136,0 0t0 3 /dev/pts/0
java 5920 root 1u CHR 136,0 0t0 3 /dev/pts/0
java 5920 root 2u CHR 136,0 0t0 3 /dev/pts/0
java 5920 root 3r REG 8,3 416697 878426 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/amd64/default/jclSC160/vm.jar
java 5920 root 4r REG 8,3 10160 878493 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/annotation.jar
java 5920 root 5r REG 8,3 189574 878494 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/beans.jar
java 5920 root 6r REG 8,3 435869 878550 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/java.util.jar
java 5920 root 7r REG 8,3 216671 878556 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/jndi.jar
java 5920 root 8r REG 8,3 54586 878558 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/logging.jar
java 5920 root 9r REG 8,3 287085 878567 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/security.jar
java 5920 root 10r REG 8,3 128157 878569 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/sql.jar
java 5920 root 11r REG 8,3 1198418 878542 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ibmorb.jar
java 5920 root 12r REG 8,3 426065 878543 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ibmorbapi.jar
java 5920 root 13r REG 8,3 536255 878536 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ibmcfw.jar
java 5920 root 14r REG 8,3 20511988 878566 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/rt.jar
java 5920 root 15r REG 8,3 9601775 878496 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/charsets.jar
java 5920 root 16r REG 8,3 501898 878565 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/resources.jar
java 5920 root 17r REG 8,3 1161371 878544 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ibmpkcs.jar
java 5920 root 18r REG 8,3 133770 878534 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ibmcertpathfw.jar
java 5920 root 19r REG 8,3 27626 878538 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ibmjgssfw.jar
java 5920 root 20r REG 8,3 60208 878540 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ibmjssefw.jar
java 5920 root 21r REG 8,3 10966 878545 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ibmsaslfw.jar
java 5920 root 22r REG 8,3 97697 878537 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ibmjcefw.jar
java 5920 root 23r REG 8,3 932373 878539 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ibmjgssprovider.jar
java 5920 root 24r REG 8,3 442208 878541 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ibmjsseprovider2.jar
java 5920 root 25r REG 8,3 357168 878535 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ibmcertpathprovider.jar
java 5920 root 26r REG 8,3 152611 878546 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ibmxmlcrypto.jar
java 5920 root 27r REG 8,3 375 878560 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/management-agent.jar
java 5920 root 28r REG 8,3 8915002 878575 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/xml.jar
java 5920 root 29r REG 8,3 91126 878555 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/jlm.jar
java 5920 root 30r REG 8,3 363834 878551 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/javascript.jar
java 5920 root 31r REG 8,3 240130 886599 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/ibmjcefips.jar
java 5920 root 32r REG 8,3 1273 486857 /usr/local/test/socketClient.jar
java 5920 root 33r REG 8,3 554639 886594 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/dtfj.jar
java 5920 root 34r REG 8,3 153450 886604 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/ibmxmlencprovider.jar
java 5920 root 35r REG 8,3 277912 886606 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/jdmpview.jar
java 5920 root 36r REG 8,3 481038 886589 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/IBMKeyManagementServer.jar
java 5920 root 37r REG 8,3 232590 886601 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/ibmkeycert.jar
java 5920 root 38r REG 8,3 183719 886588 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/CmpCrmf.jar
java 5920 root 39r REG 8,3 206636 886598 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/ibmcmsprovider.jar
java 5920 root 40r REG 8,3 17081 886593 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/dtfj-interface.jar
java 5920 root 41r REG 8,3 50129 886605 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/jaccess.jar
java 5920 root 42r REG 8,3 20228 886590 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/IBMSecureRandom.jar
java 5920 root 43r REG 8,3 23831 886608 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/xmlencfw.jar
java 5920 root 44r REG 8,3 353513 886602 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/ibmpkcs11impl.jar
java 5920 root 45r REG 8,3 1202552 886596 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/gskikm.jar
java 5920 root 46r REG 8,3 148480 886595 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/dtfjview.jar
java 5920 root 47r REG 8,3 8230 886592 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/dnsns.jar
java 5920 root 48r REG 8,3 112283 886597 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/healthcenter.jar
java 5920 root 49r REG 8,3 67395 886603 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/ibmsaslprovider.jar
java 5920 root 50r REG 8,3 992455 886600 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/ibmjceprovider.jar
java 5920 root 51r REG 8,3 67801 886591 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/JavaDiagnosticsCollector.jar
java 5920 root 52r REG 8,3 468772 886607 /usr/lib64/jvm/java-1_6_0-ibm-1.6.0/jre/lib/ext/localedata.jar
java 5920 root 53r REG 8,3 1273 486857 /usr/local/test/socketClient.jar
java 5920 root 54u IPv6 1565663 0t0 TCP node1:48707->192.168.22.15:irdmi (CLOSE_WAIT)
java 5920 root 55u sock 0,7 0t0 1565661 can‘t identify protocol
java 5920 root 56u IPv6 1571368 0t0 TCP node1:48708->192.168.22.15:irdmi (CLOSE_WAIT)
node为TCP的就是两个SocketClient产生的套接字描述符的消耗，其它的99文件描述符，是跟套接字功能相关的文件打开。当Socket关闭的时候，node为TCP的套接字描述符就会别系统回收。所以，如果万一使用套接字的时候忘记关闭套接字（Scoket），不但消耗内存，更容易把系统对用户的最大文件打开数所消耗完，就会影响应用的正常运行。
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原文地址：http://blog.csdn.net/wichow/article/details/44975925
