When Runtime.exec() won't
Navigate yourself around pitfalls related to the Runtime.exec() method
By Michael C. Daconta, JavaWorld.com,
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:
java.lang.Runtime.getRuntime()方法得到當前JAVA運行時環境，這是獲得Runtime對象的唯一方法，通過調用Runtime對象的exec()方法可以執行外部程序，有以下方法形式
· 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.
調用這些方法後，將會返回一個操作系統級的進程給JAVA VM裏的Process類。你可以傳三種參數給這些方法
1、 帶命令行參數的執行程序做爲一個參數傳給exec方法
2、 將外部執行程序的命令和參數數組傳給exec方法
3、 環境變量數組傳給exec方法
傳環境變量的格式是name=value。如果用一個參數的exec方法，要注意命令和參數以空格分隔
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):
Runtime.exec()會拋出IllegalThreadStateException異常。下面有個例子是要看Runtime.exec()執行後的返回值
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();
}
}
}
將會拋出如下異常
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?
爲什麼會拋出異常呢？因爲外部進程還未執行完時，就用exitValue()取返回值的話，就會拋出IllegalThreadStateException異常。爲什麼這個方法不等到外部命令執行結束返回一個有用的結果呢？
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.
Process類有個waitFor()方法也能返回值，這就意味着只要用其中的一個就可以了。什麼情況下使用waitFor或exitValue？當你不想你的程序阻塞在一個可能永遠無法完成的外部進程時，就用exitValue，否則就用waitFor。建議使用exitValue，我更喜歡通過傳一個布爾值來決定使用waitFor或exitValue
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:
下面的代碼改進了Listing4.1以等待外部進程結束
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?
不幸的是，程序掛起不能結束了，爲什麼會這樣？
Why Runtime.exec() hangs
The JDK's Javadoc documentation provides the answer to this question:
JDK's Javadoc documentation這樣說的：
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.
因爲當前操作系統平臺只提供有限大小的緩存給輸入輸出流（TMD，怎麼跟輸入輸出流拉上關係了）因爲不能很快的讀寫輸入輸出流，可能會導致子進程阻塞甚至死鎖
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.
JDK文檔中告訴你需要處理外部進程流，但是沒告訴你怎麼做
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).
Runtime.exec() and the Process APIs看上去很簡單，用起來卻很容易出錯，The JConfig library提供了完整的API例子，來處理文件和進程操作
(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).
下面按照JDK文檔的建議來處理javac 進程的輸出流。首先你要知道的是在命令行下，當你不帶參數運行javac時，它會輸出javac的用法。
你可以在等待進程退出之前處理javac的輸出流。
Listing4.3展示了這種處理方法，但它不是很好的解決方法，這只是一個普通的解決方案。更好的解決方案是把標準錯誤流和標準輸出流騰空。最好的解決方案是同時騰空這兩種流（將會在下面討論）
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();
}
}
}
執行Listing4.3輸出的結果是
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.
MediocreExecJavac返回值是2，通常，返回0表示成功，非0表示錯誤。返回值代表的意思與操作系統有關。在win32系統下2代表文件未找到。
綜上所述，要避免掛起，你需要處理輸入輸出流
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:
在windows操作系統下，許多程序員偶爾發現使用Runtime.exec()運行非可執行命令dir或copy時，會掉進第3種陷阱，Listing4.4描述了這個問題
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();
}
}
}
執行Listing4.4出現下面的結果
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).
之前已提到過2代表文件未找到，在這裏則是說dir.exe命令未找到。因爲dir命令不是可獨立執行程序。爲了運行dir，必須先運行windows命令解釋程序 command.com或者cmd.exe，具體運行哪個與操作系統有關。Listing 4.5是運行windows命令解釋程序的例子
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
運行Listing4.5輸出以下結果
OUTPUT> Volume in drive E has no label.
OUTPUT> Volume Serial Number is
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:
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:
OUTPUT>10/22/00 09:
... (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.
GoodWindowsExec程序根據操作系統名決定執行哪個命令解釋程序。當執行了命令解釋程序後，用StreamGobbler類在一個獨立的線程中處理了傳入的流。使用type決定騰空哪種流
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.
如何判斷執行的外部命令是可執行程序還是命令解釋程序？下面會提到一個工具來判斷
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.
重點需要注意的是命名問題，得到process的輸出流的方法是調用getInputStream()。要記住的是API的觀點是站在JAVA程序的角度看的，而不是外部程序的角度。 因此外部程序的輸出流是java程序的輸入流。
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():
最後一個陷阱是你會錯誤的認爲Runtime.exec()能執行任何你在命令行中可以執行的命令。Runtime.exec()有很多限制並且不跨平臺。這個陷阱的原因是：用戶試圖用exec()方法接受一個可以在命令行下執行的單獨的字符串。使人困惑的本質是command是exec()方法的參數名字。因此程序員錯誤的認爲在命令行可以運行的，也可以通過參數傳給exec方法運行。
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.
這個程序的本意思是想將結果重定向到test.txt文件中。然而test.txt文件並不存在。Jecho程序只是把命令行參數寫入了標準流。用戶認爲在DOS命令行下能重定向到test.txt文件中，也就主觀的認爲通過調用exec方法執行命令也可以達到這樣的效果。如果你想把流重定向到另外一個程序中，你必須使用java.io包來處理。
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.
運行完GoodWinRedirect，test.txt就會有了。通過獨立於Runtime.exec()方法來處理外部進程輸出流來完成了重定向功能。我們建立了一個OutputStream，打開 文件，接收外部產生的輸出流，然後寫入到文件。Listing 4.7程序添加了一個帶三個參數的新的構造器，三個參數是輸入流，標識輸入流的類型參數、我們要重定向輸出流。
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():
總結：
1、 外部程序未退出前，你無法得到外部程序返回的退出狀態值
2、 你必須處理外部程序的輸入、輸出和錯誤流
3、 必須使用Runtime.exec()執行程序
4、 不能使用Runtime.exec()執行一個像一個命令行的命令(You cannot use Runtime.exec() like a command line)
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