Question

Is there any way I can save all the things that is happening on my Windows 7 Command Prompt in a file. So that I can see what are the things that got printed on the console. I am running a multithreaded Java Program from the command prompt as-

java -server -Xms512m -Xmx512m -XX:PermSize=512m -XX:MaxPermSize=512m -Duser.timezone=GMT-7 –jar BatchMain.jar -taskId V3-PERSONALIZATIONGEO-SAMPLE-TASK -noofthreads 1 -timeout 5 -numberOfIP 1000 -privateIPAddress false

And it prints lot of things on to the command prompt, And I want to store all these things that are getting printed on the console into a file.

Answer 1

You can either capture the output directly in your program, or on the console.

On the console

If you want to get absolutely everything (both STDOUT and STDERR), you need to redirect STDOUT (from System.out) and STDERR (from System.err). You can either redirect both to the same file (add > log.txt 2>&1 to the end of your console command), or you can log them to different files (add > BatchMain.log 2> BatchMain.err to the end of your console).

Log to a single file, log.txt:

java -server -Xms512m -Xmx512m -XX:PermSize=512m -XX:MaxPermSize=512m -Duser.timezone=GMT-7 –jar BatchMain.jar -taskId V3-PERSONALIZATIONGEO-SAMPLE-TASK -noofthreads 1 -timeout 5 -numberOfIP 1000 -privateIPAddress false > log.txt 2>&1

Log to separate files, BatchMain.log and BatchMain.err:

java -server -Xms512m -Xmx512m -XX:PermSize=512m -XX:MaxPermSize=512m -Duser.timezone=GMT-7 –jar BatchMain.jar -taskId V3-PERSONALIZATIONGEO-SAMPLE-TASK -noofthreads 1 -timeout 5 -numberOfIP 1000 -privateIPAddress false > BatchMain.log 2> BatchMain.err

Directly within your program

In your main(String[] args) method (or some other appropriate place), set System.out and System.err to print directly to files (or the same file, if you wish). For example:

package com.example.logging;

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.PrintStream;

public class LogToFiles {

    public static void main(String[] args) throws FileNotFoundException {
        System.setOut(new PrintStream("example.log"));
        System.setErr(new PrintStream(new FileOutputStream("example.err"), true));

        System.out.println("System.out example");
        System.err.println("System.err example");
    }

}

If you want the output to also be visible on the console, you can go a step further and insert your own intermediate PrintStream which does this.

Other things to consider

Note that Java buffers System.out, so if you log both STDOUT and STDERR to the same file, you could end up with nonsensical output that has the two PrintStreams interwoven, or something in STDOUT could be printed much later than a related message that was printed to STDERR. One workaround is to call System.out.flush() as appropriate, but this could degrade performance if you’re logging a lot of text to System.out.

Rather than printing everything to the console with System.out.println() and trying to capture that output, I’d recommend using Java Logging or Log4J, or going a step further and using Apache Commons Logging, which can be configured to use either of the first two systems internally. The really awesome part is that you don’t have to divine which class printed each log message, because you’ll get that automatically.

If you use a logging framework, you can also adjust the verbosity of your logging output by setting a single variable–which will come in handy at release time.