什么情况，出现异常

Exception in thread "main" java.net.SocketException: Connection reset

at java.net.SocketInputStream.read(SocketInputStream.java:209)

at java.net.SocketInputStream.read(SocketInputStream.java:141)

at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284)

at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:326)

at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:178)

at java.io.InputStreamReader.read(InputStreamReader.java:184)

at java.io.BufferedReader.fill(BufferedReader.java:161)

at java.io.BufferedReader.readLine(BufferedReader.java:324)

at java.io.BufferedReader.readLine(BufferedReader.java:389)

at socket.socket1.main(socket1.java:21)