大梦想家

先请编译和运行下面程序：
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();
        }
    }
}

   我们知道javac命令，当不带参数运行javac 程序时，它将输出帮助说明，为什么上面程序不产生任何输出并挂起，永不完成呢？java文档上说,由于有些本地平台为标准输入和输出流所提供的缓冲区大小有限,如果不能及时写入子进程的输入流或者读取子进程的输出流,可能导致子进程阻塞,甚至陷入死锁。所以，上面的程序应改写为：
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();
        }
    }
}
下面是正确的输出：
D:\java>java   MediocreExecJavac
<ERROR>
Usage: javac <options> <source files>
where possible options include:
  -g                         Generate all debugging info
  -g:none                    Generate no debugging info
  -g:{lines,vars,source}     Generate only some debugging info
  -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
  -cp <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
  -endorseddirs <dirs>       Override location of endorsed standards path
  -d <directory>             Specify where to place generated class files
  -encoding <encoding>       Specify character encoding used by source files
  -source <release>          Provide source compatibility with specified release
  -target <release>          Generate class files for specific VM version
  -version                   Version information
  -help                      Print a synopsis of standard options
  -X                         Print a synopsis of nonstandard options
  -J<flag>                   Pass <flag> directly to the runtime system
</ERROR>
Process exitValue: 2
D:\java>
   下面是一个更一般的程序，它用两个线程同步清空标准错误流和标准输出流，并能根据你所使用的windows操作系统选择windows命令解释器command.com或cmd.exe,然后执行你提供的命令。
import java.util.*;
import java.io.*;
class StreamGobbler extends Thread
{
    InputStream is;
    String type;  //输出流的类型ERROR或OUTPUT
    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);
                System.out.flush();
            }
            } 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" );
            System.out.println("osName: " + osName);
            String[] cmd = new String[3];
            if(osName.equals("Windows XP") ||osName.equals("Windows 2000"))
            {
                cmd[0] = "cmd.exe" ;
                cmd[1] = "/C" ;
                cmd[2] = args[0];
            }
            else if( osName.equals( "Windows 98" ) )
            {
                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();
        }
    }
}
下面是一个测试结果：
D:\java>java  GoodWindowsExec "copy Test.java Test1.java"
osName: Windows XP
Execing cmd.exe /C copy Test.java Test1.java
OUTPUT>已复制         1 个文件。
ExitValue: 0
D:\java>
下面的测试都能通过(windows xp+jdk1.5)
D:\java>java   GoodWindowsExec dir
D:\java>java   GoodWindowsExec Test.java
D:\java>java   GoodWindowsExec regedit.exe
D:\java>java   GoodWindowsExec NOTEPAD.EXE
D:\java>java   GoodWindowsExec first.ppt
D:\java>java   GoodWindowsExec second.doc