running files in java

[agent_x91]

I know how to open streams to read or write to files, but is it possible to make a java application which can run or open another file?

ie. when program 1 is executed, it also executes program 2.

[NogDog]

http://javaalmanac.com/egs/java.lang/Exec.html

[HaganeNoKokoro]

There's a twist to this (at least on Windows machines). It works the way they say there for real executables, but for dos-type commands like "dir" you would have to make the command your java executes "cmd /c dir"

[agent_x91]

thanks, that's really helpful! are those commands like command-line commands?

[ray326]

Indeed they are all "command lines" for commands but the ones HaganeNoKokoro refers to (in Windows) are actually functions embedded in the command interpreter, cmd.exe or command.com.

[agent_x91]

I'm confused about this command thing. I tried what they said on the website NogDog posted, but it always finds an IOException.

What am I doing wrong? can someone give me an example of a working command?

this is the code i copied from the website:




try {
// Execute a command without arguments
String command = "ls";
Process child = Runtime.getRuntime().exec(command);

// Execute a command with an argument
command = "ls /tmp";
child = Runtime.getRuntime().exec(command);
} catch (IOException e) {
}





I assumed I would need to import java.io.*; as well.

It compiles correctly, but never works.

help!:( :confused: :confused: :confused:

[ray326]

What OS are you running this on? Those particular commands are for *nix systems, DOS/Win. Also IIRC you need to specify the full path to the command *unless* exec() spawns a shell and passes your command to it. The shell is what finds stuff on the PATH. Try printing the IOException to see but it may be "file not found."

[agent_x91]

I have suse linux, but I haven't used it yet.

My OS is Windows XP Professional.

[ray326]

XP don't know "ls" from shine-ola (and I don't know XP from shine-ola) so try "dir" or "cmd dir" or the old DOS style "cmd /c dir" and see what happens. And print out that execption instead of just eating it.

[agent_x91]

I've already tried another command and sometimes I've still gotten the error, sometimes it hasn't thrown an Exception, but still has done nothing. But I'll try printing it and see what I find now.

[agent_x91]

okay, the IOException when I triedt he command "dir":

java.io.IOException: CreateProcess: dir error=2

when I tried "cmd dir", nothing was printed at all - i guess it didn't throw a IOException, but it didn't do anything else either.

and when I tried "cmd /c dir", it was also blank...

[HaganeNoKokoro]

This will print out the dir command output:

import java.io.*;

class RunProc {
public static void main(String args[]) {
try {
Runtime rt = Runtime.getRuntime();
Process pr = rt.exec("cmd /c dir");

InputStream inStd = pr.getInputStream();
InputStreamReader inStdR = new InputStreamReader(inStd);
BufferedReader bStd = new BufferedReader(inStdR);

String line=null;

while((line=bStd.readLine()) != null) {
System.out.println(line);
}
int exitVal = pr.waitFor();
System.out.println("Exited with error code "+exitVal);

} catch(Exception e) {
System.out.println(e.toString());
e.printStackTrace();
}
}
}

[agent_x91]

ah, the information must be grabbed by a stream? maybe that's where i went wrong... okay I'll try your example and modify it. thanks for the help.

By the way, I've seen it before and I'm sure it does something simple, but what does the "Exception.printStackTrace()" function print exactly?

[agent_x91]

thanks a lot, the application works this time. I'll fiddle about with it and check how it works lol. thanks again.

[HaganeNoKokoro]

It gives more detail about what happened when the exception was thrown

example: when trying to rt.exec("dir");

toString() generates
java.io.IOException: CreateProcess: dir error=2

printStackTrace() generates
java.io.IOException: CreateProcess: dir error=2
at java.lang.Win32Process.create(Native Method)
at java.lang.Win32Process.<init>(Unknown Source)
at java.lang.Runtime.execInternal(Native Method)
at java.lang.Runtime.exec(Unknown Source)
at java.lang.Runtime.exec(Unknown Source)
at java.lang.Runtime.exec(Unknown Source)
at java.lang.Runtime.exec(Unknown Source)
at RunProc.main(RunProc.java:7)

[agent_x91]

ah right thanks I'll remember that

[ray326]

If you're not using Eclipse for your Java development then I'd recommend you give it a try. It not only will help with compile errors, it will also show you your options on the fly. E.g. in your exception handler when you type 'e.' it will give you a list of all of e's public attributes and methods including toString(), printStackTrace(), getMessage() and all the others.