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<h2 class="first-section-title"><a name="531"></a><a name="ch16lev1sec5"></a>Component Usage Guidelines</h2><p class="first-para">Whether a third-party component is open source or not, one of your goals should be minimizing business risk. A large part of that is keeping switching costs small so that if a mistake is made, it will be easier to correct.</p>
<p class="para">
<b class="bold">Minimize the classes directly using third-party components.</b> I typically accomplish this by using a proxy pattern. For example, consider a Java FTP <a name="532"></a><a name="IDX-220"></a>client. I mentioned previously that I switched out this component for one with lower memory requirements. I was able to switch components easily because most of my application did not use the FTP component directly. One class in my application called <span class="fixed">FtpClient</span> performed all FTP requests.</p>
<p class="para">Since <span class="fixed">FtpClient</span> was the only class that used the open source component, I could switch products easily with no impact to the rest of the application. <a class="internaljump" href="#ch16list04">Listing 16.4</a> shows the source for this client.</p>
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<span class="example-title"><span class="example-titlelabel">Listing 16.4: </span>Sample Component Delegate</span><a name="533"></a><a name="ch16list04"></a>
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<pre class="literallayout">
    1:import com.enterprisedt.net.ftp.*;
    2:
    3:public class FtpClient
    4:{
    5:  public FtpClient ( String hostName,
    6:                     String userName,
    7:                     String password)
    8:         throws ApplicationException
    9:  {
   10:    // arg check omitted.
   11:    try {
   12:       _ftpClient = new FTPClient(hostName);
   13:     _ftpClient.login(userName, password);
   14:     _ftpClient.setTimeout(DEFAULT_TIMEOUT_IN_MILLIS);
   15:     _ftpClient.setConnectMode(FTPConnectMode.ACTIVE);
   16:    }
   17:    catch (Throwable e)
   18:    {
   19:      throw new ApplicationException(
   20:          "Error creating FTP client. ", e);
   21:    }
   22:  }
   23:
   24:  public void put( String localFileName,
   25:                   String hostFileName)
   26:       throws ApplicationException
   27:  {
   28:    // arg check omitted
   29:    try{_ftpClient.put(localFileName, hostFileName);}
   30:    catch (Throwable e)
   31:    {
   32:      throw new ApplicationException(
   33:          "Error with FTP put: local=" + localFileName +
   34:          ", remote=" + hostFileName, e);
   35:    }
   36:  }
   37:<a name="534"></a><a name="IDX-221"></a>
   38:  public void get(String hostFileName)
   39:       throws ApplicationException
   40:  {
   41:    // arg check omitted
   42:    try{_ftpClient.get(hostFileName);}
   43:    catch (Throwable e)
   44:    {
   45:      throw new ApplicationException(
   46:          "Error with FTP get: file=" + hostFileName, e);
   47:    }
   48:
   49:  private FTPClient _ftpClient = null;
   50:  }
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<p class="para">You can use this technique with virtually any software component. Remember that you also must front any component-based exception so that your application relies on internal classes, not external ones.</p>
<a name="535"></a><a name="IDX-222"></a>
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