StreamGobbler的详细解释

这是摘自《More Java pitfalls 中文版》上的例子。

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


以上的意思是说如果不使用StreamGobbler把inputstream清空的话,就会造成进程被堵死,从而导致waitFor()一直等下去,永远结束不了.所以StreamGobbler作为单独的线程会自动清理inputstream防止他被堵住