What happens if you forget to close a Socket ?
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.Socket;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.Socket;
public class ConnLeak {
public static void main(String[] args) throws InterruptedException, IOException {
for (;;) {
Thread.sleep(1000);
leak();
}
}
static void leak() throws IOException {
System.out.println("connecting ");
String hostName = "localhost";
int portNumber = 8080;
Socket echoSocket = new Socket(hostName, portNumber);
BufferedReader in = new BufferedReader(new InputStreamReader(echoSocket.getInputStream()));
System.out.println(in.readLine());
// in.close(); // WE FORGOT TO CLOSE!
// echoSocket.close(); // WE FORGOT TO CLOSE!
System.out.println("done");
}
}
centos@localhost ~]$ netstat -an | grep WAIT tcp6 0 0 127.0.0.1:8080 127.0.0.1:50008 FIN_WAIT2 tcp6 0 0 127.0.0.1:8080 127.0.0.1:50020 FIN_WAIT2 tcp6 0 0 127.0.0.1:8080 127.0.0.1:50016 FIN_WAIT2 tcp6 0 0 127.0.0.1:50014 127.0.0.1:8080 CLOSE_WAIT tcp6 0 0 127.0.0.1:8080 127.0.0.1:50006 FIN_WAIT2 tcp6 0 0 127.0.0.1:8080 127.0.0.1:50012 FIN_WAIT2 tcp6 0 0 127.0.0.1:50020 127.0.0.1:8080 CLOSE_WAIT tcp6 578 0 ::1:8080 ::1:41240 CLOSE_WAIT tcp6 0 0 127.0.0.1:8080 127.0.0.1:50014 FIN_WAIT2 tcp6 0 0 127.0.0.1:8080 127.0.0.1:50018 FIN_WAIT2 tcp6 0 0 127.0.0.1:50022 127.0.0.1:8080 CLOSE_WAIT tcp6 0 0 127.0.0.1:50016 127.0.0.1:8080 CLOSE_WAIT tcp6 0 0 127.0.0.1:50006 127.0.0.1:8080 CLOSE_WAIT tcp6 0 0 127.0.0.1:8080 127.0.0.1:50022 FIN_WAIT2 tcp6 0 0 127.0.0.1:8080 127.0.0.1:50010 FIN_WAIT2 tcp6 0 0 127.0.0.1:50018 127.0.0.1:8080 CLOSE_WAIT tcp6 0 0 127.0.0.1:50010 127.0.0.1:8080 CLOSE_WAIT tcp6 0 0 127.0.0.1:50008 127.0.0.1:8080 CLOSE_WAIT tcp6 0 0 127.0.0.1:50012 127.0.0.1:8080 CLOSE_WAIT
you end up with a pile of socket file descriptors in FIN_WAIT2+CLOSE_WAIT
If instead you DO close the socket, you have only fd in TIME_WAIT
In the LEAKING case, you can observe the leaking fd:
sudo lsof -p 10779
lsof: WARNING: can't stat() fuse.gvfsd-fuse file system /run/user/1000/gvfs
Output information may be incomplete.
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
java 10779 centos cwd DIR 253,0 4096 52165832 /home/centos
java 10779 centos rtd DIR 253,0 243 64 /
java 10779 centos txt REG 253,0 7734 33728521 /home/centos/jdk1.8.0_141/bin/java
java 10779 centos mem REG 253,0 106070960 56227 /usr/lib/locale/locale-archive
java 10779 centos mem REG 253,0 115814 52241403 /home/centos/jdk1.8.0_141/jre/lib/amd64/libnet.so
java 10779 centos mem REG 253,0 66216625 3748173 /home/centos/jdk1.8.0_141/jre/lib/rt.jar
java 10779 centos mem REG 253,0 124327 52241426 /home/centos/jdk1.8.0_141/jre/lib/amd64/libzip.so
java 10779 centos mem REG 253,0 62184 56246 /usr/lib64/libnss_files-2.17.so
java 10779 centos mem REG 253,0 225914 52241483 /home/centos/jdk1.8.0_141/jre/lib/amd64/libjava.so
java 10779 centos mem REG 253,0 66472 52241404 /home/centos/jdk1.8.0_141/jre/lib/amd64/libverify.so
java 10779 centos mem REG 253,0 44448 42091 /usr/lib64/librt-2.17.so
java 10779 centos mem REG 253,0 1139680 56236 /usr/lib64/libm-2.17.so
java 10779 centos mem REG 253,0 17013932 19043494 /home/centos/jdk1.8.0_141/jre/lib/amd64/server/libjvm.so
java 10779 centos mem REG 253,0 2127336 42088 /usr/lib64/libc-2.17.so
java 10779 centos mem REG 253,0 19776 56234 /usr/lib64/libdl-2.17.so
java 10779 centos mem REG 253,0 102990 52241266 /home/centos/jdk1.8.0_141/lib/amd64/jli/libjli.so
java 10779 centos mem REG 253,0 144792 56254 /usr/lib64/libpthread-2.17.so
java 10779 centos mem REG 253,0 164264 34688 /usr/lib64/ld-2.17.so
java 10779 centos mem REG 253,0 32768 41856127 /tmp/hsperfdata_centos/10779
java 10779 centos 0u CHR 136,1 0t0 4 /dev/pts/1
java 10779 centos 1u CHR 136,1 0t0 4 /dev/pts/1
java 10779 centos 2u CHR 136,1 0t0 4 /dev/pts/1
java 10779 centos 3r REG 253,0 66216625 3748173 /home/centos/jdk1.8.0_141/jre/lib/rt.jar
java 10779 centos 4u unix 0xffff8801eaabfc00 0t0 327936 socket
java 10779 centos 5u IPv6 327938 0t0 TCP localhost:50132->localhost:webcache (CLOSE_WAIT)
java 10779 centos 6u IPv6 327940 0t0 TCP localhost:50134->localhost:webcache (CLOSE_WAIT)
java 10779 centos 7u IPv6 327942 0t0 TCP localhost:50136->localhost:webcache (CLOSE_WAIT)
java 10779 centos 8u IPv6 327146 0t0 TCP localhost:50138->localhost:webcache (CLOSE_WAIT)
java 10779 centos 9u IPv6 329030 0t0 TCP localhost:50140->localhost:webcache (CLOSE_WAIT)
java 10779 centos 10u IPv6 329036 0t0 TCP localhost:50142->localhost:webcache (CLOSE_WAIT)
java 10779 centos 11u IPv6 326450 0t0 TCP localhost:50146->localhost:webcache (CLOSE_WAIT)
java 10779 centos 12u IPv6 328073 0t0 TCP localhost:50148->localhost:webcache (CLOSE_WAIT)
java 10779 centos 13u IPv6 329065 0t0 TCP localhost:50150->localhost:webcache (ESTABLISHED)
A Java Flight Recorder measurement reveals the source of the leak:
You can view those sockets in /proc/10779/fd/ folder (use ls -ltra, they are links)
and display more info with
cat /proc/10779/net/sockstat
sockets: used 881 TCP: inuse 9 orphan 0 tw 9 alloc 94 mem 75 UDP: inuse 8 mem 1 UDPLITE: inuse 0 RAW: inuse 0 FRAG: inuse 0 memory 0
(see also https://www.cyberciti.biz/faq/linux-find-all-file-descriptors-used-by-a-process/ for procfs info )
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