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Java: Trying to log a string to file, but it doesn't work

Posted on 2011-03-11
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Last Modified: 2013-11-24
Hi Guys - I'm creating a program that has a need to log HTTP traffic.  The traffic is read from socket to buffer to string.  Then the below method is invoked to write that string to file.  The code is pretty straight forward.

It uses java.io.filewriter to do the work.  filewriter has a method - write() that takes a String as an argument and according to the JavaDocs, should write it to the file linked to the filewriter.

The File object referenced (request) is created as a global variable, so the file exists.  When I compile and run the code, the file gets created and my debug shos that the method and proper code is being invoked, yet the file is empty.

Am I not using the FileWriter correctly?  What could be the problem?  Maybe there is a better way to do this?

In case you plan to suggest using Apache Commons, please don't - I can't use it for this project.
private void logIt(String toWrite, int a) throws IOException
    {
        //Determine if we are logging Client request traffic
        //or server reply traffic
        if (a == 0){
            FileWriter writer = new FileWriter(request, true);
            //Write to requestlog.txt
            writer.write(toWrite);
            debug("Client Request traffic logged");
        }else {
            //Write to requestlog.txt
            FileWriter writer = new FileWriter(reply, true);
            writer.write(toWrite);
            debug("Server Reply traffic logged");
        }
    }

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Question by:southpau1
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Accepted Solution

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CEHJ earned 250 total points
ID: 35113179
You need to close the Writer each time
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Assisted Solution

by:objects
objects earned 250 total points
ID: 35113180
you need to close the writer
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Expert Comment

by:objects
ID: 35113187
you'll also find it simpler to use a PrintWriter


            PrintWriter writer = new PrintWriter(new FileWriter(reply, true));
            writer.println(toWrite);
            writer.close();
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Author Comment

by:southpau1
ID: 35113225
Ok that worked!  I close and it logs.  But now I get these errors and my entire program doesn't work..
java.lang.NullPointerException
	at java.io.Writer.write(Writer.java:140)
	at com.mba.proxylight.RequestProcessor.logIt(RequestProcessor.java:421)
	at com.mba.proxylight.RequestProcessor.access$1400(RequestProcessor.java:24)
	at com.mba.proxylight.RequestProcessor$RequestProcessorRunner.processChannel(RequestProcessor.java:237)
	at com.mba.proxylight.RequestProcessor$RequestProcessorRunner.run(RequestProcessor.java:121)
	at java.lang.Thread.run(Thread.java:662)

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Expert Comment

by:CEHJ
ID: 35113242
You're calling write without initializing the Writer again
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Expert Comment

by:objects
ID: 35113247
looks like you're passing null to your logit method, should test for that

if (request!=null) {
        if (a == 0){
            PrintWriter writer = new PrintWriter(new FileWriter(request, true));
            writer.println(toWrite);
            writer.close();
            debug("Client Request traffic logged");
        }else {
            PrintWriter writer = new PrintWriter(new FileWriter(reply, true));
            writer.println(toWrite);
            writer.close();
            debug("Server Reply traffic logged");
        }
}
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Expert Comment

by:objects
ID: 35113255
you can also simplify that a little

      if (toWrite!=null) {
            PrintWriter writer = new PrintWriter(new FileWriter(a==0 ? request : reply), true));
            writer.println(toWrite);
            writer.close();
      }
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Expert Comment

by:CEHJ
ID: 35113270
Try
private void logIt(String toWrite, int a) throws IOException {
        //Determine if we are logging Client request traffic
        //or server reply traffic
        FileWriter writer = null;

        try {
            if (a == 0) {
                writer = new FileWriter(request, true);
                //Write to requestlog.txt
                writer.write(toWrite);
                debug("Client Request traffic logged");
            } else {
                //Write to requestlog.txt
                writer = new FileWriter(reply, true);
                writer.write(toWrite);
                debug("Server Reply traffic logged");
            }
        } finally {
            try {
                writer.close();
            } catch (Exception e) { /* ignore */
            }
        }
    }

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Expert Comment

by:CEHJ
ID: 35113280
Don't hide/trap null pointers. Find out why null is being passed if it is and change it
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by:CEHJ
ID: 35113296
:)
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