Develop Reference

Run a python or other language code inside Java

We are building a grading system, and part of its job is to take input files, and from the given directory, our system will compile and run a (non-java) source code file written by the students. Then, the system will display the output from that run.
It is not limited to python, any other language that can meet the requirement is OK.
Is there is any method for providing for the location of the python code and the input file, and then run that code, returning the output file. If not, how can i achieve this goal?

Take a look at the Runtime class. In particular, look at Runtime.exec(). It should enable you to execute external applications from within your Java program, as well as passing command line arguments, and specifying working directory.
Note that the python or any other program must have some well-defined way to get its input and write its output, such as passing filenames on the command line, or reading from stdin.

An application to get Userinput and to run commands background in windows

I want to create one standalone executable file which accepts some user input and it should use those inputs as arguments to run a command at CMD as background process. For GUI i thought of using swings and to run cmd batch file . Is it possible to merge java files and batch file to form an exe.? or i should go for someother scripting languages to execute command at background process and someother programming languages for UI design ? Thanks in advance.

Theoretically you could do this all in Java. As you mentioned, you could use Swing to read in the user input. Instead of trying to get ahold of the input data from a separate batch file, you could just use Java's Runtime class to execute a command using the inputs as arguments.
And, if you need this all to run from an executable, you could use a utility like Launch4J or JSmooth to wrap the Java application.

run interactive command line application from java

I normally use java.lang.ProcessBuilder and java.lang.Process to run external command line programs, and it works fine for run-and-done commands. For example, this would run "myProgram" with argument "myArg" in the working directory:
List<String> commandLine = new ArrayList<String>();
commandLine.add("myProgram");
commandLine.add("myArg");
ProcessBuilder builder = new ProcessBuilder(commandLine);
builder.redirectErrorStream(true);
Process process = builder.start();
However, say I wanted to run a script or program or something that had interactive input (it prompted me for more input after starting). Can I do that in Java with code similar to that above, or do I need a different approach? Or is there some library that can help me with this?

According to the documentation you should be able to redirect the input and output streams. This tells it to use the System.in/System.out from the parent process:
builder.redirectInput(ProcessBuilder.Redirect.INHERIT);
builder.redirectOutput(ProcessBuilder.Redirect.INHERIT);
If you want to write things to the processes's input:
If the source is Redirect.PIPE (the initial value), then the standard input of a subprocess can be written to using the output stream returned by Process.getOutputStream(). If the source is set to any other value, then Process.getOutputStream() will return a null output stream.

Redirecting stdin and stdout is certainly one option for simple command-line programs.
Using a "robot" class is another, if you actually need to script keystrokes (for example, in a test script):
http://www.java-tips.org/java-se-tips/java.awt/how-to-use-robot-class-in-java.html
http://download.java.net/jdk7/archive/b123/docs/api/java/awt/Robot.html
Writing a simple .bat file or shell script, that calls your Java program and uses "<" and ">" redirection operators is yet a third option.
It all depends on exactly what you're looking for :)

System.out.println without printing to file when using " > filename"

I have a peculiar problem. I have a Java program that is run with the command :
cat input_file_name | ./script_name > file_output_name
the script_name script just does : java myprogram
My question is : how can I print out something in the console without it being "put in the file_output_name file" (since the > file puts all System.out.prints in that file)
I know this is possible because there are some already that come from some class in a library that I'm using from my java program. However I can't find the exact source of those prints so I don't know how it is coded.
Thank you for any help.

The simple answer is to write those messages to System.err rather than System.out.
This will work since > redirects standard output but not standard error. (You can redirect standard error, but the shell syntax is different; e.g. foo 2> /tmp/somefile.)
A more complicated alternative is to change your program so that it can be called with the name of the output file as an argument. Then open the file, wrap it in a PrintWriter and write to that stream rather than System.out. With a bit of refactoring, you should be able to structure your program so that it can be used either way.

The easiest way is to use System.err.println() instead of System.out.
It will go to a different "device" (stderr instead of stdout), and it won't be redirected.

With what you've shown, you're only redirecting the standard out (stdout). You can write something to the standard error (stderr) instead to have it show on the console yet. In Java, this can be done by System.err.println(...), and related methods.

Notice that if a Java program writes to standard output, it can't control where does its output get redirected, that depends on how the program is invoked from the command line.
Having that clear, you can redirect the program's standard output only to the console, by simply removing the > file_output_name part from the command.
Or, you can redirect the output of the program to both the console and a file by using the tee command, take a look at this article which explains in detail how to do it.

get the data from batch file to java program

i want to read the data from the batch file to java program. my java program is calling the batch file & it is giving the output for following command
C:>FIND "check" d:\c.txt
---------- D:\C.TXT
check
i want to read this "check" in my java program.
Thanks,
Murali

You haven't shown us how do you execute this batch. If you use Runtime.exec() then have a look at this example of capturing output: http://www.rgagnon.com/javadetails/java-0014.html

You can redirect the output of the FIND command to a dynamically named file and have it read by Java.
C:\FIND "check" d:\c.txt > yourUniqueFileName.txt
Then read the file yourUniqueFileName.text, parse and delete on the end (or not).

You can use ProcessBuilder or Process to exec() and then capture the output. More info here. You could redirect to a file and then read the file - however you're at the mercy of diskspace/permissioning issues, plus you should uniquely name your file etc.
Note that you'll need to be careful when capturing output from a spawned process. See this answer for more details.