[jira] [Created] (NET-596) NullPointerException when disconnecting TelnetClient twice with JDK 7

["Vincent Bories-Azeau (JIRA)" <ji...@apache.org>]

Vincent Bories-Azeau created NET-596:
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             Summary: NullPointerException when disconnecting TelnetClient twice with JDK 7
                 Key: NET-596
                 URL: https://issues.apache.org/jira/browse/NET-596
             Project: Commons Net
          Issue Type: Bug
          Components: Telnet
    Affects Versions: 3.5
         Environment: Both telnet client & server with JDK 7.
Bug not reproduced with JDK 8 (there's another bug with this JDK though, as mentioned in the description).
            Reporter: Vincent Bories-Azeau


When using the TelnetClient class, a {{NullPointerException}} may occur when calling the {{disconnect}} method twice, in the {{_closeOutputStream}} method called under the hood, if the Telnet connection is lost (for instance, server is hardly shut down).

1. The first call to {{disconnect}} resets completely the TelnetClient instance.
2. The second call to {{disconnect}} leads to the NPE exception, because the {_output_} property is {{null}}, in the {{_closeOutputStream}} method.

*NOTE: the NPE does not occur with JDK 8, because, the first call to {{disconnect}} throws an I/O exception (socket is closed), leaving the TelnetClient instance with a non-null {{\_output\_}} property. Then a second call to disconnect does not throw a NPE. It seems the JDK 8 behaves differently when a client socket loses a connection. So there is also a bug with JDK 8, as disconnection shall close quietly resources without an I/O exception, and without leaving non-null resources, and then disconnect the client socket. The {{SocketClient.disconnect}} is a good implementation to start with.*

The problem is that the TelnetOutputStream class closes the Socket output stream under the hood, but doesn't check if it is null and doesn't reset it to null once done. _The implementation of the TelnetOutputStream is quite strange, as there is a cycling dependency between this class and the TelnetClient class. The {{TelnetClient}} class shall handle itself the close of its internal resources, and disconnect the client socket. But this responsibility is delegates to the TelnetOutputStream._

Here's the stack trace of the NPE exception:
{quote}
|java.lang.NullPointerException
     at org.apache.commons.net.telnet.TelnetClient._closeOutputStream(TelnetClient.java:83)
      at org.apache.commons.net.telnet.TelnetOutputStream.close(TelnetOutputStream.java:163)
      at org.apache.commons.net.telnet.TelnetClient.disconnect(TelnetClient.java:124)
{quote}




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