Java Apache FTPClient API : Why isn't logout() in finally clause?

Question

http://commons.apache.org/proper/commons-net/apidocs/org/apache/commons/net/ftp/FTPClient.html

我注意到finally子句中的disconnects()示例，但对logout()没有做同样的事情

FTPClient ftp = new FTPClient();
FTPClientConfig config = new FTPClientConfig();
config.setXXX(YYY); // change required options
ftp.configure(config );
boolean error = false;
try {
  int reply;
  ftp.connect("ftp.foobar.com");
  System.out.println("Connected to " + server + ".");
  System.out.print(ftp.getReplyString());

  // After connection attempt, you should check the reply code to verify
  // success.
  reply = ftp.getReplyCode();

  if(!FTPReply.isPositiveCompletion(reply)) {
    ftp.disconnect();
    System.err.println("FTP server refused connection.");
    System.exit(1);
  }
  ... // transfer files
  ftp.logout();
} catch(IOException e) {
  error = true;
  e.printStackTrace();
} finally {
  if(ftp.isConnected()) {
    try {
      ftp.disconnect();
    } catch(IOException ioe) {
      // do nothing
    }
  }
  System.exit(error ? 1 : 0);
}

有人知道为什么我们在捕获异常时不需要 logout() 吗？

Answer 1

Anyone know why we don't need to logout() when we catch an exception?

内码ftp.logout()功能如下:

public boolean  logout() throws IOException
{
         return FTPReply.isPositiveCompletion(quit());
}

quit() 函数使用 sendCommand(FTPCommand.QUIT) 将命令发送到 FTP 服务器。如果发生连接异常，我们很可能无法连接FTP服务器。调用 logout() 将尝试再次写入 FTP 服务器并创建资源，并抛出额外的异常。另外，虽然disconnect()函数也会抛出异常，但它会关闭输入、输出、套接字并释放logout()所占用的资源。函数没有:从下面的 disconnect() 源代码可以明显看出功能:

 public void disconnect() throws IOException
 {
      if (_socket_ != null) _socket_.close();
      if (_input_ != null) _input_.close();
      if (_output_ != null) _output_.close();
      if (_socket_ != null) _socket_ = null;
      _input_ = null;
      _output_ = null;
       _controlInput_ = null;
       _controlOutput_ = null;
       _newReplyString = false;
       _replyString = null;
 }