It may be a silly question but I can't find anything on the web with the answer. What (if anything) determines where the args of a System.out.println(args) command will be displayed? I've been using an IO class created by my lecturer which created it's own GUI and wrote to specific areas of that by default but now that I am making my own programs I am struggling to get the text/images/whatever to display where I want it to.
From the Javadocs for System.out:
The "standard" output stream. This stream is already open and ready to accept output data. Typically this stream corresponds to display output or another output destination specified by the host environment or user.
In most computing environments, there are three standard streams associated with a process.
Note, however, that System.out can be reassigned in Java, by using System.setOut().
Depending on the operating system you're using, each process has associated a standard input, standard output and a standard error. If you execute your program in a console, the it will be your standard input, output and error.
For instance, in Linux you can redirect the standard output with redirection operators like this:
java MyClass > fileAsOutput.txt
Related
I am trying to test a command line app that waits for the user input after every step. I am able to test the app using System Rules provided by Stefan Birkner. Currently, I provide inputs from the beginning to the end which works like a charm and I can assert the final output from system log.
However, I want to test for the negative cases before the end of the app for which I give invalid inputs in the beginning to evaluate the error message. When invalid inputs are given, the console prints an error message and keeps waiting for the user to provide a valid input. How do I send Ctrl+C using as shown below:
systemInMock.provideLines(Ctrl+C);
systemInMock.provideLines accepts only strings. Is there a way to send Ctrl+C signal?
An example of my junit test is shown below:
#Test
public void testInValidMarker() throws Exception{
systemInMock.provideLines("abc","def","1");
Main.main(new String[]{});
assertTrue(systemOutRule.getLog().contains("Invalid marker, try again"));
}
Appreciate your help!
If I'm not mistaken, when you do ctrl+c, it doesn't actually get written to console. If that's true, then in no case will your program ever be given ctrl+c, so provideLines will never be in a position where it is given ctrl+c.
For proof, open up cmd and type in a program with program arguments (in my case, I use ant). If you type ant and then ctrl+c, the cursor is moved to a new line.
There are two ways you can control termination behavior:
You can use a shutdown hook (found from this previously asked question ). Doing this will allow you to handle what happens (potentially with issues).
Or you could create your own termination argument like -q or q, which would trigger an action to end the program (maybe a System.exit(1)). This way you can mock that input.
In UNIX/Linux, when you type CTRL-C, your shell intercepts it and sends the process a SIGINT signal -- see: How does Ctrl-C terminate a child process?
Therefore the System Rules project doesn't have anything to help you -- in this situation the process doesn't receive any character input.
By default, the whole JVM shuts down when it receives SIGINT. This is obviously bad news for a running test.
The SO question Signal handling using "TERM" -- may be of use.
A side effect of Java's portability is that for some OS features, it either abstracts things away until they're unrecognisable, or doesn't expose them at all.
I suspect what you're asking for can't be achieved.
If you're allowed to change the requirements slightly, you could ask the user to close with CTRL-D -- this closes stdin with EOF.
Although it's quite the overkill, you could launch a whole new JVM running your program, using ProcessBuilder. You might imagine you'd get an API to send arbitrary signals to that process. But for portability reasons, all you can do is process.destroy(), which sends SIGTERM.
Tried this as a comment, but it didn't read right. It's not exactly an "Answer" though.
So Java is really bad at console input. It reads an entire line at once and you can't do anything about it--there is no way to trap special characters or even see any of the input before the user hits return. Also I think a ctrl-d will close your input session--(Add that test to your use case if you don't use any other suggestion here because it can put you into a state you didn't expect!)
Three suggestions:
The simplest: If you can use a GUI and aren't really looking for an ongoing input/response REPL the simplest answer is usually to use JOptionPane to throw up a quick dialog. It's a one-line solution to get some user input, but not so good for an ongoing command-driven system.
If you can't use swing (If you are running headless) then you may have little choice, but you can use the JLine library. That will give you a lot more flexibility. This is how Groovysh does it's REPL. It will let you see each character as it is typed and do things like completion where a user might type part of a file name and hit tab and you put the rest in for him.
If you don't want to use JLine but want a REPL feel there is also a more complex GUI solution--create a swing console window. A trivial solution would just be a text input box to allow typing and a text area to display results, but there are certainly libraries out there with more complete console solutions.
The point here is that using Java standard input alone is just not a good solution for anything beyond a trivial/personal script--and even then I avoid it. Perhaps not the answer you asked for, but maybe it's the one you need :)
I get that this isn't possible to do with normal java, although if there are any libraries out this it would be very useful.
Essentially, I'm designing a console app and running into an issue that when output happens while something is typed in the input line, that input text will move up and appear before the line that just got output. Is it possible to fix this in some form so that the text you are inputting that stays at the bottom?
EX:
I'm typing something as input into my commandline app, and then the program prints something WHILE I'm typing - this causes what was originally on the input line to be scrolled up with whatever the output text was. When you are trying to type something in this can obviously be detrimental. I know it's possible to prevent this.. (Other programs have done it... EX: Minecraft Server)
(If I need to be more descriptive I can.)
You could use the help of threads. One that listens to user input, the other process the actual output. This problem is similar to basic race condition problems when multiple threads attempt to read and write to a shared resource.
Your shared resource is that console. You need to keep the Input/Output operations synchronized. Have a look at race condition.
What is the difference between the "standard streams" (System.out, System.in, System.err) and the "Console" (System.console()) in Java?
Every process (not just Java programs) have three streams: in, out, and error. In Java access to these has been simplified to System.in, System.out, and System.err. These are used for reading from or writing to the command line. For example, if you ever had a command line program that asked for input, which you typed and then pressed enter, that input went to standard in.
There are two out streams (out and err), because they report different things. For example, you might want to save the error output but not care about the generic output a program prints out. Or you might want to suppress the standard output so only errors are printed. Or you have a program with all kinds of problems and you want to suppress the errors so you can see what it's trying to do and not 10 million stack traces.
So, the short answer to your question is that the standard streams are ways of writing to and reading from the console.
In short, a console is the thing that might capture the streams and display them on a screen. Every program has access to the input/output streams, but not every program has access to a console.
Some programs lose access to the console as their streams are initially (or eventually redirected). Other programs are launched in configurations where they never had access to a console. In the latter case, everything written to System.out might as well have been written to /dev/null.
While this applies to Java, the Console (and the "other side" of the streams) are really operating system concepts.
Alright, I'll get into the meat of things straight away:
I want to run a perl script from a java app (via ProcessBuilder), which will then parse an html page and find out some required information. I then want to pass that information back to my java app, and display that information in a JTextArea.
My knowledge of perl is VERY limited. My original thought was to write this data to a txt file, and then read that file with my java program, which could then display it to JTextArea pretty easily. However, this seems like an ugly solution, compared to simply returning a string.
Please let me know if there is a better way to do this. perhaps a completely different method than what I'm thinking of. Thanks,
Aelfhere
I think you want something like this
You can pass strings between processes only by using some type of inter-process communication: either a pipe or shared memory or using network.
Why can you not do in Java what you want to do in Perl?
when you use a ProcessBuilder you instantiate a Process Object it's java representation of a batch execution
Tipically you can hook process streaming via Java API.
Extracted from Process JAVA API:
abstract
InputStream getErrorStream()
Gets the error stream of the subprocess
abstract
InputStream getInputStream()
Gets the input stream of the subprocess
abstract
OutputStream getOutputStream()
Gets the output stream of the subprocess
If perl script write on standard output stream then you can read that output.
Generally, If process doesn't write on standard output stream then you cannot read it.
Just a quick one here.
What are the benefits of using java.io.Console as opposed to using a BufferedReader wrapping an InputStreamReader for System.in?
Why would I use it?
Thanks for any advice!
Because it's code that is already written for you...no need to re-invent the wheel. Chances are, you're not going to get it any better than it already is.
You can use java.io.Console to present an interactive command-line to the user. You could do all that with System.in yourself, but you would have to implement things like noticing when the input was done, or readPassword, etc.
See java.io.Console is finally here!
One of the most popular feature
requests for J2SE in recent times has
been the request to improve console
support and provide a way to enter
passwords with echo disabled.
Developers know this feature 4050435
as it has been skulking in the Top 25
RFEs list for some time.
java.io.Console only works when you start a Java program from a command line without redirecting STDIN/STDOUT.
The main advantage I see with Console over System.in is that you have the readPassword() method, which won't echo the characters typed by the user (I couldn't find a way to do this with System.in).
You also have readLine() which will present a prompt and read a single line. You don't have to create your own LineNumberReader.
But, if you want your Java program to be able to read from STDIN when it's redirected from a file or pipe, you still have to use System.in.
Another trick I'm pretty sure you won't get with Console--I created my own input and output streams and replaced System.in/out with them. My implementation of the stream appended to a log file as well as echoing to the screen.
When I turned on my poor-man's "Debug Info", I could even have it tell me what program/line the sysout came from (It was slow though. It created an exception and examined the appropriate stack entry so it was off by default)
java.io.Console is used to take and read input from the user at runtime and output are displayed after processing the input from user.
For more and detailed information visit https://www.examsmyantra.com/article/58/java/java-io---console-input-and-output