일반 BufferedReader.readLine() hangs

증상은 BufferedReader 의 readLine() 함수만 호출하면 멈춰버리는 현상

원인은 readLine() 함수가 carridge return이나 line feed가 나올때까지 대기하기 때문이라고 하는데 해결은 아래와 같이 BufferedReader.ready() 함수를 이용하여 준비상태를 확인해주는 것이다

I debugged a simple program to learn JavaCompiler in new JDK, but found BufferedReader.readLine() hangs for some reason. BufferedReader.readLine() returns null toindicate that you have reached the end of the data stream (i.e. the remote has closed). At all other times, readLine() will block until there is a complete
line (i.e. a message terminated by a newline). If you need to know that there is data before attempting to read, try to use BufferedReader.isReady() method.

Here is the sample code:
        Runtime run = Runtime.getRuntime();
        Process p = run.exec("java");

        BufferedInputStream in = new BufferedInputStream(p.getInputStream());
        BufferedReader br = new BufferedReader(new InputStreamReader(in));
       
        if (br.ready()) {
            String line;
            while ((line = br.readLine()) != null) {
                System.out.println(line);
            }
        } 

다른 방법에 대해서도 나와있긴한데 별로 좋은 방법처럼 보이지는 않지만 혹시나해서 스크랩해둔다

스레드를 이용해서 waitFor() 를 강제 수행하고 있다

package dfi.exloadv3;

 

import java.util.*;

import java.io.*;

 

public class ShellCmd  extends Thread {

 

    private Process pProcess = null;

    private String resource_info = "";

    private int    return_code = -1;

    private boolean debug_mode = false;

    private String  cmd = null;

       private String cmdlog_file = null;

       private String cmderr_file = null;

   

    public void setCmd( String cmd )

    {

        this.cmd = cmd;

    }

    public void setCmdLogFile( String cmdlog_file )

    {

        this.cmdlog_file = cmdlog_file;

    }

    public void setCmdErrFile( String cmderr_file )

    {

        this.cmderr_file = cmderr_file;

    }

   

    public void run()

    {

        StringBuffer result = new StringBuffer();

        String line = "";

        BufferedReader stdout = null;

        BufferedReader stderr = null;

        BufferedWriter bwout = null;

        BufferedWriter bwerr = null;

        try

        {

               if( this.cmd != null && this.cmd.length() > 0 )

               {

                     if( cmdlog_file == null || cmdlog_file.length() <= 0  )

                     {

                            File tfile = new File( cmd );

                            if( tfile.exists() == true )

                            {

                                 cmdlog_file = cmd+ ".log";

                            }

                            else

                                   cmdlog_file = "/dev/null";

                     }

                     if( cmderr_file == null || cmderr_file.length() <= 0  )

                     {

                            File tfile = new File( cmd );

                            if( tfile.exists() == true )

                            {

                                   cmderr_file = cmd+ ".log";

                            }

                            else

                                   cmderr_file = "/dev/null";

                     }

                    

                   pProcess = Runtime.getRuntime().exec(  this.cmd   ); 

                stdout = new BufferedReader( new InputStreamReader(pProcess.getInputStream()) );

                  stderr = new BufferedReader(new InputStreamReader(pProcess.getErrorStream()));

               bwout = new BufferedWriter(new OutputStreamWriter( new FileOutputStream( this.cmdlog_file )));

               bwerr = new BufferedWriter(new OutputStreamWriter( new FileOutputStream( this.cmderr_file )));

                   while( (line = stdout.readLine()) != null )

                     {

//                       result.append( line + "n" ); 

                         bwout.write(line + "n" );

                     } 

                while( (line = stderr.readLine()) != null )

                     {

//                       result.append( line + "n" ); 

                      bwerr.write(line + "n" );

                     }  

                resource_info = result.toString();

               }

               else

               {

                     System.out.println("    [Error] Coammnd Emtpy");

               }

        }

        catch( Exception e)

        {

               System.out.println( e.toString() );

        }

        finally{

               try

               {

                      if( bwout != null )

                            bwout.close();

                      if( bwerr != null )

                            bwerr.close();

                      if( stdout != null )

                            stdout.close();

                      if( stderr != null )

                            stderr.close();

               }catch (IOException e)

               {

                    

               }

        }

    }

   

    public boolean WaitProcess()

    {

        boolean bRet = false;

        try

        {

            while( true )

            {

                if( this.isAlive() == false )

                {

//                      System.out.println( this.getName() + "    Status : "+ this.getState());

                   pProcess.waitFor();                    

//                        System.out.println( " result : " + resource_info ) ;

                   return_code = pProcess.exitValue();

                    System.out.println("  return code : " + return_code );

                    if( return_code == 0 )

                         bRet = true;

                   break;

                  }

            } 

        }

        catch( Exception e)

        {

                 System.out.println( e.toString() );

        }

        return bRet;

    }

 

    public static void main(String[] args) 

    {

          String cmd = null;

        

          ShellCmd jt = new ShellCmd();

          cmd = "ls";

          jt.setCmd( cmd );

          jt.start();

          jt.WaitProcess(); 

    }

}