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; if(osName.equals("Windows XP") ||osName.equals("Windows 2000")) { cmd = "cmd.exe" ; cmd = "/C" ; cmd = args; } else if( osName.equals( "Windows 98" ) ) { cmd = "command.com" ; cmd = "/C" ; cmd = args; } Runtime rt = Runtime.getRuntime(); System.out.println("Execing " + cmd + " " + cmd+ " " + cmd); 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>javaGoodWindowsExec "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
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