When Runtime.exec() won't
Navigate yourself around pitfalls related to the Runtime.exec() method
Summary
In this installment of Java Traps, Michael Daconta discusses one new pitfall and revisits another from his previous column. Originating in the java.lang package, the pitfall specifically involves problems with the Runtime.exec() method. Daconta also corrects an error from Pitfall 3 and offers a simpler solution. (2,500 words)
s part of the Java language, the java.lang
package is implicitly imported into every Java program. This package's
pitfalls surface often, affecting most programmers. This month, I'll
discuss the traps lurking in the Runtime.exec() method.
Pitfall 4: When Runtime.exec() won't
The class java.lang.Runtime features a static method called getRuntime(), which retrieves the current Java Runtime Environment.
That is the only way to obtain a reference to the Runtime object. With that reference, you can run external programs by invoking the Runtime class's exec() method. Developers often call this method to launch a browser for displaying a help page in HTML.
There are four overloaded versions of the exec() command:
-
public Process exec(String command); -
public Process exec(String [] cmdArray); -
public Process exec(String command, String [] envp); -
public Process exec(String [] cmdArray, String [] envp);
For each of these
methods, a command -- and possibly a set of arguments -- is passed to
an operating-system-specific function call. This subsequently creates
an operating-system-specific process (a running program) with a
reference to a Process class returned to the Java VM. The Process class is an abstract class, because a specific subclass of Process exists for each operating system.
You can pass three possible input parameters into these methods:
- A single string that represents both the program to execute and any arguments to that program
- An array of strings that separate the program from its arguments
- An array of environment variables
Pass in the environment variables in the form name=value. If you use the version of exec()
with a single string for both the program and its arguments, note that
the string is parsed using white space as the delimiter via the StringTokenizer class.
Stumbling into an IllegalThreadStateException
The first pitfall relating to Runtime.exec() is the IllegalThreadStateException.
The prevalent first test of an API is to code its most obvious methods.
For example, to execute a process that is external to the Java VM, we
use the exec() method. To see the value that the external process returns, we use the exitValue() method on the Process class. In our first example, we will attempt to execute the Java compiler (javac.exe):
Listing 4.1 BadExecJavac.java
import java.util.*;
import java.io.*;
public class BadExecJavac
{
public static void main(String args[])
{
try
{
Runtime rt = Runtime.getRuntime();
Process proc = rt.exec("javac");
int exitVal = proc.exitValue();
System.out.println("Process exitValue: " + exitVal);
} catch (Throwable t)
{
t.printStackTrace();
}
}
}
A run of BadExecJavac produces:
E:\classes\com\javaworld\jpitfalls\article2>java BadExecJavac
java.lang.IllegalThreadStateException: process has not exited
at java.lang.Win32Process.exitValue(Native Method)
at BadExecJavac.main(BadExecJavac.java:13)
If an external process has not yet completed, the exitValue() method will throw an IllegalThreadStateException;
that's why this program failed. While the documentation states this
fact, why can't this method wait until it can give a valid answer?
A more thorough look at the methods available in the Process class reveals a waitFor() method that does precisely that. In fact, waitFor() also returns the exit value, which means that you would not use exitValue() and waitFor() in conjunction with each other, but rather would choose one or the other. The only possible time you would use exitValue() instead of waitFor()
would be when you don't want your program to block waiting on an
external process that may never complete. Instead of using the waitFor() method, I would prefer passing a boolean parameter called waitFor into the exitValue() method to determine whether or not the current thread should wait. A boolean would be more beneficial because exitValue()
is a more appropriate name for this method, and it isn't necessary for
two methods to perform the same function under different conditions.
Such simple condition discrimination is the domain of an input
parameter.
Therefore, to avoid this trap, either catch the IllegalThreadStateException or wait for the process to complete.
Now, let's fix the
problem in Listing 4.1 and wait for the process to complete. In Listing
4.2, the program again attempts to execute javac.exe and then waits for the external process to complete:
Listing 4.2 BadExecJavac2.java
import java.util.*;
import java.io.*;
public class BadExecJavac2
{
public static void main(String args[])
{
try
{
Runtime rt = Runtime.getRuntime();
Process proc = rt.exec("javac");
int exitVal = proc.waitFor();
System.out.println("Process exitValue: " + exitVal);
} catch (Throwable t)
{
t.printStackTrace();
}
}
}
Unfortunately, a run of BadExecJavac2 produces no output. The program hangs and never completes. Why does the javac process never complete?
Why Runtime.exec() hangs
The JDK's Javadoc documentation provides the answer to this question:
Because some native
platforms only provide limited buffer size for standard input and
output streams, failure to promptly write the input stream or read the
output stream of the subprocess may cause the subprocess to block, and
even deadlock.
Is this just a case of
programmers not reading the documentation, as implied in the oft-quoted
advice: read the fine manual (RTFM)? The answer is partially yes. In
this case, reading the Javadoc would get you halfway there; it explains
that you need to handle the streams to your external process, but it
does not tell you how.
Another variable is at
play here, as is evident by the large number of programmer questions
and misconceptions concerning this API in the newsgroups: though Runtime.exec()
and the Process APIs seem extremely simple, that simplicity is
deceiving because the simple, or obvious, use of the API is prone to
error. The lesson here for the API designer is to reserve simple APIs
for simple operations. Operations prone to complexities and
platform-specific dependencies should reflect the domain accurately. It
is possible for an abstraction to be carried too far. The JConfig library provides an example of a more complete API to handle file and process operations (see Resources below for more information).
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:
Listing 4.4 BadExecWinDir.java
import java.util.*;
import java.io.*;
public class BadExecWinDir
{
public static void main(String args[])
{
try
{
Runtime rt = Runtime.getRuntime();
Process proc = rt.exec("dir");
InputStream stdin = proc.getInputStream();
InputStreamReader isr = new InputStreamReader(stdin);
BufferedReader br = new BufferedReader(isr);
String line = null;
System.out.println("<OUTPUT>");
while ( (line = br.readLine()) != null)
System.out.println(line);
System.out.println("</OUTPUT>");
int
exitVal =
proc.waitFor();
System.out.println("Process exitValue: " + exitVal);
} catch (Throwable t)
{
t.printStackTrace();
}
}
}
A run of BadExecWinDir produces:
E:\classes\com\javaworld\jpitfalls\article2>java BadExecWinDir
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 BadExecWinDir.main(BadExecWinDir.java:12)
As stated earlier, the error value of 2 means "file not found," which, in this case, means that the executable named dir.exe
could not be found. That's because the directory command is part of the
Windows command interpreter and not a separate executable. To run the
Windows command interpreter, execute either command.com or cmd.exe,
depending on the Windows operating system you use. Listing 4.5 runs a
copy of the Windows command interpreter and then executes the
user-supplied command (e.g., dir).
Listing 4.5 GoodWindowsExec.java
import java.util.*;
import java.io.*;
class StreamGobbler extends Thread
{
InputStream is;
String type;
StreamGobbler(InputStream is, String type)
{
this.is = is;
this.type = type;
}
public void run()
{
try
{
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
String line=null;
while ( (line = br.readLine()) != null)
System.out.println(type
+ ">" + line);
} catch (IOException ioe)
{
ioe.printStackTrace();
}
}
}
public class GoodWindowsExec
{
public static void main(String args[])
{
if (args.length < 1)
{
System.out.println("USAGE:
java GoodWindowsExec <cmd>");
System.exit(1);
}
try
{
String osName = System.getProperty("os.name" );
String[] cmd = new String[3];
if( osName.equals( "Windows NT" ) )
{
cmd[0] = "cmd.exe" ;
cmd[1] = "/C" ;
cmd[2] = args[0];
}
else if( osName.equals( "Windows 95" ) )
{
cmd[0] = "command.com" ;
cmd[1] = "/C" ;
cmd[2] = args[0];
}
Runtime rt = Runtime.getRuntime();
System.out.println("Execing " + cmd[0] + " " + cmd[1]
+ " " + cmd[2]);
Process proc = rt.exec(cmd);
// any error message?
StreamGobbler errorGobbler = new
StreamGobbler(proc.getErrorStream(),
"ERROR");
// any output?
StreamGobbler outputGobbler = new
StreamGobbler(proc.getInputStream(),
"OUTPUT");
// kick them off
errorGobbler.start();
outputGobbler.start();
// any error???
int exitVal = proc.waitFor();
System.out.println("ExitValue:
" + exitVal);
} catch (Throwable t)
{
t.printStackTrace();
}
}
}
Running GoodWindowsExec with the dir command generates:
E:\classes\com\javaworld\jpitfalls\article2>java GoodWindowsExec "dir *.java"
Execing cmd.exe /C dir *.java
OUTPUT> Volume in drive E has no label.
OUTPUT> Volume Serial Number is 5C5F-0CC9
OUTPUT>
OUTPUT> Directory of E:\classes\com\javaworld\jpitfalls\article2
OUTPUT>
OUTPUT>10/23/00 09:01p
805 BadExecBrowser.java
OUTPUT>10/22/00 09:35a
770 BadExecBrowser1.java
OUTPUT>10/24/00 08:45p
488 BadExecJavac.java
OUTPUT>10/24/00 08:46p
519 BadExecJavac2.java
OUTPUT>10/24/00 09:13p
930 BadExecWinDir.java
OUTPUT>10/22/00 09:21a
2,282 BadURLPost.java
OUTPUT>10/22/00 09:20a
2,273 BadURLPost1.java
... (some output omitted for brevity)
OUTPUT>10/12/00 09:29p
151 SuperFrame.java
OUTPUT>10/24/00 09:23p
1,814 TestExec.java
OUTPUT>10/09/00 05:47p 23,543
TestStringReplace.java
OUTPUT>10/12/00 08:55p
228 TopLevel.java
OUTPUT> 22
File(s) 46,661 bytes
OUTPUT>
19,678,420,992 bytes free
ExitValue: 0
Running GoodWindowsExec
with any associated document type will launch the application
associated with that document type. For example, to launch Microsoft
Word to display a Word document (i.e., one with a .doc extension), type:
>java GoodWindowsExec "yourdoc.doc"
Notice that GoodWindowsExec uses the os.name
system property to determine which Windows operating system you are
running -- and thus determine the appropriate command interpreter.
After executing the command interpreter, handle the standard error and
standard input streams with the StreamGobbler class. StreamGobbler empties any stream passed into it in a separate thread. The class uses a simple String type to denote the stream it empties when it prints the line just read to the console.
Thus, to avoid the third pitfall related to Runtime.exec(),
do not assume that a command is an executable program; know whether you
are executing a standalone executable or an interpreted command. At the
end of this section, I will demonstrate a simple command-line tool that
will help you with that analysis.
It is important to note that the method used to obtain a process's output stream is called getInputStream().
The thing to remember is that the API sees things from the perspective
of the Java program and not the external process. Therefore, the
external program's output is the Java program's input. And that logic
carries over to the external program's input stream, which is an output
stream to the Java program.
Runtime.exec() is not a command line
One final pitfall to cover with Runtime.exec() is mistakenly assuming that exec() accepts any String that your command line (or shell) accepts. Runtime.exec() is much more limited and not cross-platform. This pitfall is caused by users attempting to use the exec() method to accept a single String as a command line would. The confusion may be due to the fact that command is the parameter name for the exec()
method. Thus, the programmer incorrectly associates the parameter
command with anything that he or she can type on a command line,
instead of associating it with a single program and its arguments. In
listing 4.6 below, a user tries to execute a command and redirect its
output in one call to exec():
Listing 4.6 BadWinRedirect.java
import java.util.*;
import java.io.*;
// StreamGobbler omitted for brevity
public class BadWinRedirect
{
public static void main(String args[])
{
try
{
Runtime rt = Runtime.getRuntime();
Process
proc = rt.exec("java jecho 'Hello World' > test.txt");
// any error message?
StreamGobbler errorGobbler = new
StreamGobbler(proc.getErrorStream(),
"ERROR");
// any output?
StreamGobbler outputGobbler = new
StreamGobbler(proc.getInputStream(),
"OUTPUT");
// kick them off
errorGobbler.start();
outputGobbler.start();
// any error???
int exitVal = proc.waitFor();
System.out.println("ExitValue:
" + exitVal);
} catch (Throwable t)
{
t.printStackTrace();
}
}
}
Running BadWinRedirect produces:
E:\classes\com\javaworld\jpitfalls\article2>java BadWinRedirect
OUTPUT>'Hello World' > test.txt
ExitValue: 0
The program BadWinRedirect attempted to redirect the output of an echo program's simple Java version into the file test.txt. However, we find that the file test.txt does not exist. The jecho program simply takes its command-line arguments and writes them to the standard output stream. (You will find the source for jecho in the source code available for download in Resources.)
In Listing 4.6, the user assumed that you could redirect standard
output into a file just as you could on a DOS command line.
Nevertheless, you do not redirect the output through this approach. The
incorrect assumption here is that the exec() method acts like a shell interpreter; it does not. Instead, exec()
executes a single executable (a program or script). If you want to
process the stream to either redirect it or pipe it into another
program, you must do so programmatically, using the java.io package. Listing 4.7 properly redirects the standard output stream of the jecho process into a file.
Listing 4.7 GoodWinRedirect.java
import java.util.*;
import java.io.*;
class StreamGobbler extends Thread
{
InputStream is;
String type;
OutputStream os;
StreamGobbler(InputStream is, String type)
{
this(is, type, null);
}
StreamGobbler(InputStream is, String type, OutputStream redirect)
{
this.is = is;
this.type = type;
this.os = redirect;
}
public void run()
{
try
{
PrintWriter pw = null;
if (os != null)
pw = new PrintWriter(os);
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
String line=null;
while ( (line = br.readLine()) != null)
{
if (pw != null)
pw.println(line);
System.out.println(type
+ ">" + line);
}
if (pw != null)
pw.flush();
} catch (IOException ioe)
{
ioe.printStackTrace();
}
}
}
public class GoodWinRedirect
{
public static void main(String args[])
{
if (args.length < 1)
{
System.out.println("USAGE
java GoodWinRedirect <outputfile>");
System.exit(1);
}
try
{
FileOutputStream fos = new FileOutputStream(args[0]);
Runtime rt = Runtime.getRuntime();
Process proc = rt.exec("java jecho 'Hello World'");
// any error message?
StreamGobbler errorGobbler = new
StreamGobbler(proc.getErrorStream(),
"ERROR");
// any output?
StreamGobbler outputGobbler = new
StreamGobbler(proc.getInputStream(),
"OUTPUT", fos);
// kick them off
errorGobbler.start();
outputGobbler.start();
// any error???
int exitVal = proc.waitFor();
System.out.println("ExitValue: " + exitVal);
fos.flush();
fos.close();
} catch (Throwable t)
{
t.printStackTrace();
}
}
}
Running GoodWinRedirect produces:
E:\classes\com\javaworld\jpitfalls\article2>java GoodWinRedirect test.txt
OUTPUT>'Hello World'
ExitValue: 0
After running GoodWinRedirect, test.txt
does exist. The solution to the pitfall was to simply control the
redirection by handling the external process's standard output stream
separately from the Runtime.exec() method. We create a separate OutputStream,
read in the filename to which we redirect the output, open the file,
and write the output that we receive from the spawned process's
standard output to the file. Listing 4.7 completes that task by adding
a new constructor to our StreamGobbler class. The new constructor takes three arguments: the input stream to gobble, the type String that labels the stream we are gobbling, and the output stream to which we redirect the input. This new version of StreamGobbler
does not break any of the code in which it was previously used, as we
have not changed the existing public API -- we only extended it.
Since the argument to Runtime.exec()
is dependent on the operating system, the proper commands to use will
vary from one OS to another. So, before finalizing arguments to Runtime.exec()
and writing the code, quickly test the arguments. Listing 4.8 is a
simple command-line utility that allows you to do just that.
Here's a useful exercise: try to modify TestExec to redirect the standard input or standard output to a file. When executing the javac
compiler on Windows 95 or Windows 98, that would solve the problem of
error messages scrolling off the top of the limited command-line
buffer.
Listing 4.8 TestExec.java
import java.util.*;
import java.io.*;
// class StreamGobbler omitted for brevity
public class TestExec
{
public static void main(String args[])
{
if (args.length < 1)
{
System.out.println("USAGE: java TestExec \"cmd\"");
System.exit(1);
}
try
{
String cmd = args[0];
Runtime rt = Runtime.getRuntime();
Process proc = rt.exec(cmd);
// any error message?
StreamGobbler errorGobbler = new
StreamGobbler(proc.getErrorStream(),
"ERR");
// any output?
StreamGobbler outputGobbler = new
StreamGobbler(proc.getInputStream(),
"OUT");
// kick them off
errorGobbler.start();
outputGobbler.start();
// any error???
int exitVal = proc.waitFor();
System.out.println("ExitValue: " + exitVal);
} catch (Throwable t)
{
t.printStackTrace();
}
}
}
Running TestExec to launch the Netscape browser and load the Java help documentation produces:
E:\classes\com\javaworld\jpitfalls\article2>java TestExec "e:\java\docs\index.html"
java.io.IOException: CreateProcess: e:\java\docs\index.html error=193
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 TestExec.main(TestExec.java:45)
Our first test failed with an error of 193.
The Win32 error for value 193 is "not a valid Win32 application." This
error tells us that no path to an associated application (e.g.,
Netscape) exists, and that the process cannot run an HTML file without
an associated application.
Therefore, we try the test again, this time giving it a full path to Netscape. (Alternately, we could add Netscape to our PATH environment variable.) A second run of TestExec produces:
E:\classes\com\javaworld\jpitfalls\article2>java TestExec
"e:\program files\netscape\program\netscape.exe e:\java\docs\index.html"
ExitValue: 0
This worked! The Netscape browser launches, and it then loads the Java help documentation.
One additional improvement to TestExec would include a command-line switch to accept input from standard input. You would then use the Process.getOutputStream() method to pass the input to the spawned external program.
To sum up, follow these rules of thumb to avoid the pitfalls in Runtime.exec():
- You cannot obtain an exit status from an external process until it has exited
- You must immediately handle the input, output, and error streams from your spawned external process
- You must use
Runtime.exec() to execute programs
- You cannot use
Runtime.exec() like a command line
Correction to Pitfall 3
In
the discussion of Pitfall 3 ("Don't mix floats and doubles when
generating text or XML messages") in my last column, I incorrectly
stated that the different string representation of a decimal number
after casting it from a float to a double was a bug. While this is a
pitfall, its cause is not a bug, but the fact that the decimal numbers
in question -- 100.28 and 91.09 -- do not represent precisely in
binary. I'd like to thank Thomas Okken and the others who straightened
me out. If you enjoy discussing the finer points of numerical methods,
you can email Thomas.
The combination of
forgetting my numerical methods class, the numerous bug reports on the
bug parade, and the automatic rounding of floats and doubles when
printing (but not after casting a float to a double) threw me. I
apologize for confusing anyone who read the article, especially to new
Java programmers. I present two better solutions to the problem:
The first possible solution is to always specify the desired rounding explicitly with NumberFormat.
In my case, I use the float and double to represent dollars and cents;
therefore, I need only two significant digits. Listing C3.1
demonstrates how to use the NumberFormat class to specify a maximum of two fraction digits.
Listing C3.1 FormatNumbers.java
import java.text.*;
public class FormatNumbers
{
public static void main(String [] args)
{
try
{
NumberFormat fmt = NumberFormat.getInstance();
fmt.setMaximumFractionDigits(2);
float f = 100.28f;
System.out.println("As
a float : " + f);
double d = f;
System.out.println("Cast
to a double : " + d);
System.out.println("Using
NumberFormat: " +
fmt.format(d));
} catch (Throwable t)
{
t.printStackTrace();
}
}
}
When we run the FormatNumbers program, it produces:
E:\classes\com\javaworld\jpitfalls\article2>java FormatNumbers
As a float : 100.28
Cast to a double : 100.27999877929688
Using NumberFormat: 100.28
As you can see --
regardless of whether we cast the float to a double -- when we specify
the number of digits we want, it properly rounds to that precision --
even if the number is infinitely repeating in binary. To circumvent
this pitfall, control the formatting of your doubles and floats when
converting to a String.
A second, simpler
solution would be to not use a float to represent cents. Integers
(number of pennies) can represent cents, with a legal range of 0 to 99.
You can check the range in the mutator method.