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When Runtime.exec() won't
Navigate yourself around pitfalls related to the Runtime.exec() method
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Page 3 of 4
Now, let's follow the JDK documentation and handle the output of the javac process. When you run javac without any arguments, it produces a set of usage statements that describe how to run the program and the meaning of all
the available program options. Knowing that this is going to the stderr stream, you can easily write a program to exhaust that stream before waiting for the process to exit. Listing 4.3 completes
that task. While this approach will work, it is not a good general solution. Thus, Listing 4.3's program is named MediocreExecJavac; it provides only a mediocre solution. A better solution would empty both the standard error stream and the standard output
stream. And the best solution would empty these streams simultaneously (I'll demonstrate that later).
Listing 4.3 MediocreExecJavac.java
import java.util.*;
import java.io.*;
public class MediocreExecJavac
{
public static void main(String args[])
{
try
{
Runtime rt = Runtime.getRuntime();
Process proc = rt.exec("javac");
InputStream stderr = proc.getErrorStream();
InputStreamReader isr = new InputStreamReader(stderr);
BufferedReader br = new BufferedReader(isr);
String line = null;
System.out.println("<ERROR>");
while ( (line = br.readLine()) != null)
System.out.println(line);
System.out.println("</ERROR>");
int exitVal = proc.waitFor();
System.out.println("Process exitValue: " + exitVal);
} catch (Throwable t)
{
t.printStackTrace();
}
}
}
A run of MediocreExecJavac generates:
E:\classes\com\javaworld\jpitfalls\article2>java MediocreExecJavac
<ERROR>
Usage: javac <options> <source files>
where <options> includes:
-g Generate all debugging info
-g:none Generate no debugging info
-g:{lines,vars,source} Generate only some debugging info
-O Optimize; may hinder debugging or enlarge class files
-nowarn Generate no warnings
-verbose Output messages about what the compiler is doing
-deprecation Output source locations where deprecated APIs are used
-classpath <path> Specify where to find user class files
-sourcepath <path> Specify where to find input source files
-bootclasspath <path> Override location of bootstrap class files
-extdirs <dirs> Override location of installed extensions
-d <directory> Specify where to place generated class files
-encoding <encoding> Specify character encoding used by source files
-target <release> Generate class files for specific VM version
</ERROR>
Process exitValue: 2
So, MediocreExecJavac works and produces an exit value of 2. Normally, an exit value of 0 indicates success; any nonzero value indicates an error. The meaning of these exit values depends on the particular operating
system. A Win32 error with a value of 2 is a "file not found" error. That makes sense, since javac expects us to follow the program with the source code file to compile.
Thus, to circumvent the second pitfall -- hanging forever in Runtime.exec() -- if the program you launch produces output or expects input, ensure that you process the input and output streams.
Assuming a command is an executable program
Under the Windows operating system, many new programmers stumble upon Runtime.exec() when trying to use it for nonexecutable commands like dir and copy. Subsequently, they run into Runtime.exec()'s third pitfall. Listing 4.4 demonstrates exactly that:
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Resources
- Download the source code for all the examples in this article
http://www.javaworld.com/jw-12-2000/traps/jw-1229-traps.zip - Java Pitfalls, Time Saving Solutions, and Workarounds to Improve Programs, Michael C. Daconta, Eric Monk, J. Paul Keller, Keith Bohnenberger (John Wiley & Sons, 2000)
http://www.amazon.com/exec/obidos/ASIN/0471361747/javaworld - Review the
JConfiglibrary or download it for evaluation
http://tolstoy.com/samizdat/jconfig.html - Read Michael Daconta's previous Java Traps column, "Steer Clear of Java Pitfalls" (JavaWorld, September 22, 2000)
http://www.javaworld.com/javaworld/jw-09-2000/jw-0922-pitfalls.html - Browse JavaWorld's Topical Index
http://www.javaworld.com/javaworld/topicalindex/jw-ti-index.html
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This artcle is very good. Thnaks for your helpBy Anonymous on February 9, 2010, 8:59 amThis artcle is very good. Thnaks for your help
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Nice article, it resolved my 'rar.exe' deaklock problemBy Anonymous on January 27, 2010, 6:50 amI write waitFor() BEFORE consuming StdOut/StdErr stream of the process, so the 'rar.exe LARGE_FILE' process is deadlock. Moved waitFor() AFTER consuming StdOut/StdErr...
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hai i need to execte the command "javac" by using this how can iBy Anonymous on January 17, 2010, 1:50 pmhai i need to execte the command "javac" by using this how can i do this
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Thank you for the great article! But something is missing, how dBy Anonymous on December 20, 2009, 5:15 amThank you for the great article! But something is missing, how do I properly send commands to an already runnnig process? Of course I have tried the process.getOutputStream(),...
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Application exit value 128By Anonymous on December 18, 2009, 8:43 amHello, i use the Runtime exec with the Class "StreamGobbler" to execute one of my application (exe) and the exit value of the watFor is 128. There is no such return...
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