FileOutputStream automatically overwrite files problem

The following code does not output any data , because in bufferedReader before reading text messages
FileOutputStream have D: \ Debug.log cover empty file .. did not call the write method ..
delete FileOutputStream will be able to read normal ... this is why

import java.io.BufferedReader;
  import java.io.BufferedWriter;
  import java.io.File;
  import java.io.FileInputStream;
  import java.io.FileNotFoundException;
  import java.io.FileOutputStream;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.io.OutputStreamWriter;
  import javax.swing.JOptionPane;
  
  public class _Writer
  {
  	/**
  	 * @param args
  	 */
  	public static void main(String[] args)
  	{
  		File hosts = new File("D:\\Debug.log");
  		FileOutputStream os = null;
  		FileInputStream is = null;
  		BufferedReader br = null;
  		BufferedWriter bw = null;
  		try
  		{
  			is = new FileInputStream(hosts);
  			os = new FileOutputStream(hosts);
  			br = new BufferedReader(new InputStreamReader(is));
  			bw = new BufferedWriter(new OutputStreamWriter(os));
  			String str = null;
  			while ((str = br.readLine()) != null)
  			{
  				System.out.println(str);
  			}
  		} catch (FileNotFoundException e)
  		{
  			// TODO Auto-generated catch block
  			e.printStackTrace();
  			JOptionPane.showMessageDialog(null, "hosts文件不存在!");
  		} catch (IOException e)
  		{
  			// TODO Auto-generated catch block
  			e.printStackTrace();
  			JOptionPane.showMessageDialog(null, "IO异常!");
  		} finally
  		{
  			try
  			{
  				if (os != null)
  					os.close();
  				if (is != null)
  					is.close();
  				if (br != null)
  					br.close();
  				if (bw != null)
  					bw.close();
  			} catch (IOException e)
  			{
  				// TODO Auto-generated catch block
  				e.printStackTrace();
  				JOptionPane.showMessageDialog(null, "CLOSE异常!");
  			}
  		}
  	}
  }
  

------ Solution ------------------------------------- -------
you think this is why ah ? Why do they read and write , or the same file. What is your purpose ?
------ Solution ---------------------------------------- ----
new FileOutputStream (hosts); into new FileOutputStream (hosts, true); indicate additional documents
------ Solution --- -----------------------------------------
as upstairs ,
os = new FileOutputStream (hosts); into
os = new FileOutputStream (hosts, true); enough
As for the reason , look at the source

    public FileOutputStream(String name) throws FileNotFoundException {
          this(name != null ? new File(name) : null, false);
      }

continue to track

   public FileOutputStream(File file, boolean append)
          throws FileNotFoundException
      {
          String name = (file != null ? file.getPath() : null);
          SecurityManager security = System.getSecurityManager();
          if (security != null) {
              security.checkWrite(name);
          }
          if (name == null) {
              throw new NullPointerException();
          }
          this.fd = new FileDescriptor();
          this.append = append;
  
          fd.incrementAndGetUseCount();
          open(name, append);
      }

find the open method

   /**
       * Opens a file, with the specified name, for overwriting or appending.
       * @param name name of file to be opened
       * @param append whether the file is to be opened in append mode
       */
      private native void open(String name, boolean append)
          throws FileNotFoundException;

stopped here . Here, let us re-write or input parameter is added .
Since the input , and I think this should be the open method of preparation according to the input to work, such as re-write it emptied. .
------ For reference only -------------------------------------- -



append files can not read information
------ For reference only --------------------------- ------------

  
     
append files can not read information  
not think so . .
I read that you first determine whether that file under your content. . After all, you emptied it several times .

  public static void main(String[] args)
      {
          File hosts = new File("D:\\Debug.log");
          FileOutputStream os = null;
          FileInputStream is = null;
          BufferedReader br = null;
          BufferedWriter bw = null;
          try
          {
              is = new FileInputStream(hosts);
              os = new FileOutputStream(hosts,true);
              br = new BufferedReader(new InputStreamReader(is));
              bw = new BufferedWriter(new OutputStreamWriter(os));
              String str = null;
              while ((str = br.readLine()) != null)
              {
                  System.out.println(str);
              }
          } catch (FileNotFoundException e)
          {
              // TODO Auto-generated catch block
              e.printStackTrace();
              JOptionPane.showMessageDialog(null, "hosts文件不存在!");
          } catch (IOException e)
          {
              // TODO Auto-generated catch block
              e.printStackTrace();
              JOptionPane.showMessageDialog(null, "IO异常!");
          } finally
          {
              try
              {
                  if (os != null)
                      os.close();
                  if (is != null)
                      is.close();
                  if (br != null)
                      br.close();
                  if (bw != null)
                      bw.close();
              } catch (IOException e)
              {
                  // TODO Auto-generated catch block
                  e.printStackTrace();
                  JOptionPane.showMessageDialog(null, "CLOSE异常!");
              }
          }
      }

------ For reference only ----------------------------------- ----
Thank you , I understand , OUTPUTSTREAM if not write also writes an empty file. This file can only be modified by overwriting the way