Runtime.exec() sucks!!!!

Posted 2020-07-02 digdeep

自己项目中使用到了

Runtime rt = Runtime.getRuntime();
Process p = rt.exec("query session");
p.waitFor();

结果在不同的windows 操作系统中，程序的运行不一致，在windows server 2008上可以很好的运行，但是到了windows7上去卡死了！！！！！！！！！！！！！！！！！！！！！！

p.waitFor() 卡死了或者报错：

[ERROR] xxxxx Thread-0 - Cannot run program "query": CreateProcess error=2, ?????
????

如果改为：

Process p = rt.exec("cmd.exe query session");

则直接卡死了。如果将 p.waitFor() 注释掉，则后面的读取命令结果的语句

  InputStreamReader isr = new InputStreamReader(p.getInputStream());
            BufferedReader br = new BufferedReader(isr);
            String line=null;
            while ( (line = br.readLine()) != null){
            	resultList.add(line);
            }

br.readLine() 被阻塞，卡死了。

google的结果，直接转帖过来：

http://www.javaworld.com/article/2071275/core-java/when-runtime-exec---won-t.html

When Runtime.exec() won‘t

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Navigate yourself around pitfalls related to the Runtime.exec() method

 

As 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:

1. A single string that represents both the program to execute and any arguments to that program
2. An array of strings that separate the program from its arguments
3. 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)

我自己在windows7下测试报错：

java.lang.IllegalThreadStateException: process has not exited
	at java.lang.ProcessImpl.exitValue(Native Method)
	at com.diantusoft.test.BadExecJavac.main(BadExecJavac.java:10)

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 theexitValue() 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 executejavac.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.

因为一些操作系统平台为标准输入输出仅仅提供受限制的缓存，会导致不能正确的想 子进程 写数据和从 子进程中读取数据流，这样会导致 子进程被阻塞，甚至导致死锁。

本人测试的结果是 windows server 2008可以正常的处理，到了 windows7环境，去被阻塞了！！！！！没有任何的提示或者报错！！！！！！

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();
          }
    }
}

该程序能正常输出，但是会乱码，需要修改一句：InputStreamReader isr = new InputStreamReader(stderr, "gbk");

输出如下：

<ERROR>
用法：javac <选项> <源文件>
其中，可能的选项包括：
  -g                         生成所有调试信息
  -g:none                    不生成任何调试信息
  -g:{lines,vars,source}     只生成某些调试信息
  -nowarn                    不生成任何警告
  -verbose                   输出有关编译器正在执行的操作的消息
  -deprecation               输出使用已过时的 API 的源位置
  -classpath <路径>            指定查找用户类文件和注释处理程序的位置
  -cp <路径>                   指定查找用户类文件和注释处理程序的位置
  -sourcepath <路径>           指定查找输入源文件的位置
  -bootclasspath <路径>        覆盖引导类文件的位置
  -extdirs <目录>              覆盖安装的扩展目录的位置
  -endorseddirs <目录>         覆盖签名的标准路径的位置
  -proc:{none,only}          控制是否执行注释处理和/或编译。
  -processor <class1>[,<class2>,<class3>...]要运行的注释处理程序的名称；绕过默认的搜索进程
  -processorpath <路径>        指定查找注释处理程序的位置
  -d <目录>                    指定存放生成的类文件的位置
  -s <目录>                    指定存放生成的源文件的位置
  -implicit:{none,class}     指定是否为隐式引用文件生成类文件 
  -encoding <编码>             指定源文件使用的字符编码
  -source <版本>               提供与指定版本的源兼容性
  -target <版本>               生成特定 VM 版本的类文件
  -version                   版本信息
  -help                      输出标准选项的提要
  -Akey[=value]              传递给注释处理程序的选项
  -X                         输出非标准选项的提要
  -J<标志>                     直接将 <标志> 传递给运行时系统

</ERROR>
Process exitValue: 2

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 intoRuntime.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)

我实际运行的结果：

java.io.IOException: Cannot run program "dir": CreateProcess error=2, ??Õ?»µ½?¶
	at java.lang.ProcessBuilder.start(ProcessBuilder.java:470)
	at java.lang.Runtime.exec(Runtime.java:593)
	at java.lang.Runtime.exec(Runtime.java:431)
	at java.lang.Runtime.exec(Runtime.java:328)
	at com.diantusoft.test.BadExecWinDir.main(BadExecWinDir.java:10)
Caused by: java.io.IOException: CreateProcess error=2, ??Õ?»µ½?¶
	at java.lang.ProcessImpl.create(Native Method)
	at java.lang.ProcessImpl.<init>(ProcessImpl.java:177)
	at java.lang.ProcessImpl.start(ProcessImpl.java:28)
	at java.lang.ProcessBuilder.start(ProcessBuilder.java:452)
	... 4 more

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 inResources.) 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 thejavac 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 ofTestExec 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():

1. You cannot obtain an exit status from an external process until it has exited
2. You must immediately handle the input, output, and error streams from your spawned external process
3. You must use Runtime.exec() to execute programs
4. You cannot use Runtime.exec() like a command line