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How do I know when data had been read from a port and it can be closed

Posted on 2012-04-05
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Last Modified: 2012-04-09
I am using this snippet of Java code to send data to a port. If I execute the two close commands the receiver reading from the port throws a bind error: "Unrecognized Windows Sockets error: 0: JVM_Bind"

If I never execute the two close commands, the reader gets the data just fine.

I assume I must close the PrintWriter and echoSocket eventually...but how do I know when the receiver has read all the data so I can close?

Java code:

Socket echoSocket = new Socket(servername, Tx.getPortNumber());
PrintWriter out = new PrintWriter(echoSocket.getOutputStream(), true);
out.println(SendMessage);
out.close();
echoSocket.close();
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Question by:cschene
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Expert Comment

by:CEHJ
ID: 37814489
the receiver reading from the port throws a bind error: "Unrecognized Windows Sockets error: 0: JVM_Bind"

Can you please post the full stack trace?

Generally speaking, when you have this kind of thing happening (although the error message seems slightly odd in this case), the fault is in the protocol. It sounds like you have not arranged the protocol in an orderly way,such that the receiver is 'surprised' that the sender has closed the socket. If you can't rearrange the protocol, you will be forced to handle the exception at the receiver as 'unexceptional'
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Author Comment

by:cschene
ID: 37814512
"Can you please post the full stack trace?"

How do I do that? I am processing the exception as follows:


catch(Exception e) {
                  x1.stopTimer();
                  x1.setPortData(e.getMessage() + " on port " + x1.getPortNumber());
                  System.err.println(e.getMessage());
                  }
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CEHJ earned 500 total points
ID: 37814519
Change

System.err.println(e.getMessage());

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to

e.printStackTrace();

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and post the output
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Author Comment

by:cschene
ID: 37815758
I figured out what my problem was...it was the parsing of the int...which I believe you answered as well.

What I am trying to do here is test a group of ports by sending a message to a port on a server (2112) and having the server send that message back to the client on the port that is specified in the message and repeat this until the client processes all ports and either verifies them as working or not working. We need this test because we have to keep certain ports open for our software to function.

The approach I am taking is (1) server waits on the Rx port (2112) for a message, (2) Server reads the message  and sends that exact message back to the client and then (4) the client processes the next port and so on until done.

This is experimental code:  I am just trying to figure out the proper approach and then I will throw away and re-write once I understand the design required.

if I run with the following commandline, the code will do a loopback test where it sends and receives on the same port. This works.

java CmServerPortTester -loopbacktest  -txport 2112 -rxport 2112

if I want to run a test of ports 2112, 500, 1024 I run two copies of the software. Note, server is defaulting to localhost...so I am looping on the same PC for testing.

server: java CmServerPortTester -server  -txport 2112 -rxport 2112
client: java CmServerPortTester -client  -txport 2112 -rxport 5000 -rxport 1024

So, it exchanges the first set of messages and verified one port, but the server side gets hung up after that.
CmServerPortTester.java
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LVL 86

Expert Comment

by:CEHJ
ID: 37817249
but the server side gets hung up after that.

Where? At what line?
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Author Comment

by:cschene
ID: 37817733
It is throwing when it hits this line


      Socket echoSocket = new Socket(servername, i);

In this Method: static void processServerPortTest(testPort x) {

I am not getting much detail on the exception. All I see is this: (how do I get more detail?)

Exception in thread "main" java.lang.NullPointerException
        at CmServerPortTester.processServerPortTest(CmServerPortTester.java:265)
        at CmServerPortTester.main(CmServerPortTester.java:66)
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LVL 86

Expert Comment

by:CEHJ
ID: 37817822
It is throwing when it hits this line

Be careful - 'throwing' != 'hung'

 at CmServerPortTester.processServerPortTest(CmServerPortTester.java:265)

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What line does that correspond to in the file you attached? (It's not the same)
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Author Comment

by:cschene
ID: 37825830
Sorry.....it was my mistake...pure cockpit error.

Thanks for the help...you have the points.
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