I am mostly concerned with Linux but answers involving windows are welcome.
When I use System.out.println or cout<< what is actually happening and what happens when I do a cout in a gui application (does it go anywhere)?
One case that I am interested in is the Netbeans IDE. When I run a java program in Netbeans what makes it possible for the IDE to "steal" the output from the program and display it?
Update/Sidenote
http://www.linfo.org/standard_output.html
One of the features of standard output is that it has a default
destination but can easily be redirected (i.e., diverted) to another
destination. That default destination is the display screen on the
computer that initiated the program. Because the standard streams are
plain text, they are by definition human readable.
What is meant by "initiate the program"?
I'm not very familiar with how the execution of a program begins but in the case of my netbeans example it's pretty clear that the IDE initiated the program. So what does that mean? When the program is being setup to be executed is there some meta data that is floating around letting the OS know that Netbeans is initiating the program?
When the program gets executed, three special file descriptors: stdin, stdout and stderr are associated to some device to determine how input and output is managed. If you execute a program from a terminal shell, stdin is associated to the keyboard, stdout and stderr to the terminal window. When you execute the program in a development environment usually stdout and stderr are displayed in some special console tabs. In other situations the output goes to some log file or maybe get discarded...
System.out and cout are the objects representing the stdout stream in Java and C++.
Related
If the input is interactive, i.e. from the console, I want to print a command prompt e.g. ">"
But if it is redirected e.g. from a file, then I do not want to prompt.
System.in is an abstract InputStream which does not appear to have any method for this.
Maybe you could use instanceof if the concrete type is different?
As well, if System.out is redirected to a file I also do not want to prompt
AFAIK, there is no way to do this in pure Java, and even doing it in JNI / JNA would be complicated.
An alternative might be to use the new Console API introduced in JDK 1.6. This allows you to try to get a reader/writer for the console. If it succeeds, the result is guaranteed to be interactive ... in the sense that you mean.
A second alternative would be to do the "check for interactivity" in the wrapper script that you use to launch your application, and pass the information to Java via the system properties. For instance, on a GNU/Linux system the tty(1) command can be used to tell if stdin is a connected to a "tty" device.
Note that there are other ways to deal with a requirement to avoid unwanted prompts when running non-interactively:
If System.out is redirected to a file I also do not want to prompt.
(I think you might mean that System.in is redirected. That is the normal way to non-interactively run an application that normally takes interactive input from the user ...)
The alternatives include:
You could modify the program to write the prompts to (say) System.err and redirect that to a different place.
You could modify the program to have options that mean "don't prompt" or "take input from a file".
Since Java 6 there is the method java.lang.System.console():
Whether a virtual machine has a console is dependent upon the
underlying platform and also upon the manner in which the virtual
machine is invoked. If the virtual machine is started from an
interactive command line without redirecting the standard input and
output streams then its console will exist and will typically be
connected to the keyboard and display from which the virtual machine
was launched. If the virtual machine is started automatically, for
example by a background job scheduler, then it will typically not have
a console.
If this virtual machine has a console then it is represented by a
unique instance of this class which can be obtained by invoking the
java.lang.System.console() method. If no console device is available
then an invocation of that method will return null.
In UNIX, I'm supposed to write a Java file that will print "EXIT 1" to the standard error, and then exit with a status of 1.
Here is my approach..
System.err.println("EXIT 1");
System.exit(1);
Is this what I'm supposed to do?
If so, how am I supposed to use it in the Unix shells? When I compile and run it in the bash, it just prints "EXIT 1" (so it does the same thing as System.out.println, why should I use "err"?). What is the "standard error" here?
Every running program has these three streams:
Standard input (stdin), which normally comes from the keyboard. Exposed as System.in
Standard out (stdout), which normally goes to the console. Exposed as
System.out
Standard error (stderr), which normally also goes to the console. Exposed as System.err
Your program is correct – it does print to stderr. But under normal circumstances, the stderr stream goes to the console just like the stdout stream, so they are visually indistinguishable.
However, the reason you should use stderr instead of stdout for error messages, is redirection. That means that you send stderr to a file instead of the console. Meanwhile, stdout will be unaffected, because the two streams are independent.
For example, you can do this in bash, cmd, PowerShell, etc:
$ java Program 2> errors.txt
Now, all output with System.err.println() will end up in errors.txt, while System.out.println() will still go to the screen. This can help with debugging.
There are three data streams associated with nearly every process:
Standard Input: This is the stream of input into a program, either from a terminal, a console, piped output from another process, or some other means.
Standard Error: This is where all debugging and error messages should go. This is so that this sort of information can easily be separately captured from the regular output of a program. Web servers do this, by sending error messages to an error_log file via stderr, while the normal log file would be e. g. access_log.
Standard Output: This is the where all typical, expected output that a user running a program should expect to see said output appear.
Standard Output (stdout) and Standard Error (stderr) are nearly always the first and second output streams coming from a process, respectively. This allows me to do something like /path/to/my/neat/program > logs/program.log 2> logs/program.err and have output and errors nicely sorted.
I have a R script I need to call from Java and run. I tried this code: Runtime.getRuntime().exec("Rscript pathTo/R/myScript.R"). I run it from windows command it worked fine, but when I run java class with this code in Eclipse, nothing happens. The console doesnt show anything no error no logs. Can someone tell me how to run this script from Java?
By default, a Process launched from java has its standard input, standard output and standard error redirected to pipes, which you can access from within java. Unless you read from the standard output and error pipes and transfer the text to the output of the Java application yourself, no output will become visible. Furthermore, if the internal buffer of the pipe gets full, then the child application might even block while waiting for root to write its data. So the process probably will hang and never terminate.
Since Java 7, you can have the child process inherit its I/O channels from your Java application using ProcessBuilder.inheritIO. That saves you all the trouble to read from those streams yourself.
When I try to use java.lang.System.console(), I get a null pointer. I can still write to out and read from in, but this only works when I run straight from my IDE. When I run the .jar file directly, nothing happens. How can I create a console like I'd see using std::cout for use in Java?
Edit:
I was hoping to just create one, rather than understand why I don't have one, since I need one for my program's operation.
Perhaps you're trying to get the console by double-clicking in the jar
Try creating a batch file that opens the console for you.
You can however create a console using Swing and redirect standard input/output there.
Source: Create Java console inside a GUI panel
How are you running the JAR file exactly? That would be the expected behavior for double-clicking its icon in Windows Explorer, as Kelly alluded to, but not for firing it up from the command line.
From the Console entry in the API (emphasis mine):
Whether a virtual machine has a console is dependent upon the underlying platform and also upon the manner in which the virtual machine is invoked. If the virtual machine is started from an interactive command line without redirecting the standard input and output streams then its console will exist and will typically be connected to the keyboard and display from which the virtual machine was launched. If the virtual machine is started automatically, for example by a background job scheduler, then it will typically not have a console.
java.lang.System.out and java.lang.System.in are the input/output streams for console access. Java won't create a "console" but allows you to interact with the IO streams provided by the operating system.
When you run it from a jar file (like clicking on it from a folder) you'll get the I/O streams of the GUI which don't display anywhere.
Try creating a batch file with a 'java -jar ' command in it. When you run the batch file, you should see the command window. I'm assuming windows here. Another way is to run cmd.exe directly with arguments that keep the window open, i.e. "cmd.exe /c".
Instead of running the jar file directly, open a console (you didn't specify an operating system, but this would be the command line on Windows and a console on *Nix, or Terminal on OS X). Then, run java -jar /path/to/your.jar.
The equivalent of std::cout in Java would be System.out as you probably already know.
EDIT: With regards to your edit, there is code out there to do this. For example, you can use Swing. There's even a StackOverflow answer with more than one working code sample.
See JConsole, which is a general console in java, used for instance by groovy. Or see groovy directly.
If the input is interactive, i.e. from the console, I want to print a command prompt e.g. ">"
But if it is redirected e.g. from a file, then I do not want to prompt.
System.in is an abstract InputStream which does not appear to have any method for this.
Maybe you could use instanceof if the concrete type is different?
As well, if System.out is redirected to a file I also do not want to prompt
AFAIK, there is no way to do this in pure Java, and even doing it in JNI / JNA would be complicated.
An alternative might be to use the new Console API introduced in JDK 1.6. This allows you to try to get a reader/writer for the console. If it succeeds, the result is guaranteed to be interactive ... in the sense that you mean.
A second alternative would be to do the "check for interactivity" in the wrapper script that you use to launch your application, and pass the information to Java via the system properties. For instance, on a GNU/Linux system the tty(1) command can be used to tell if stdin is a connected to a "tty" device.
Note that there are other ways to deal with a requirement to avoid unwanted prompts when running non-interactively:
If System.out is redirected to a file I also do not want to prompt.
(I think you might mean that System.in is redirected. That is the normal way to non-interactively run an application that normally takes interactive input from the user ...)
The alternatives include:
You could modify the program to write the prompts to (say) System.err and redirect that to a different place.
You could modify the program to have options that mean "don't prompt" or "take input from a file".
Since Java 6 there is the method java.lang.System.console():
Whether a virtual machine has a console is dependent upon the
underlying platform and also upon the manner in which the virtual
machine is invoked. If the virtual machine is started from an
interactive command line without redirecting the standard input and
output streams then its console will exist and will typically be
connected to the keyboard and display from which the virtual machine
was launched. If the virtual machine is started automatically, for
example by a background job scheduler, then it will typically not have
a console.
If this virtual machine has a console then it is represented by a
unique instance of this class which can be obtained by invoking the
java.lang.System.console() method. If no console device is available
then an invocation of that method will return null.