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Applying a lock over a call to a method performing network transactions or declaring such a method synchronized can be problematic. Depending on the speed and reliability of the connection, synchronization can stall the program indefinitely causing a huge performance hit. At other times, it can result in temporary or permanent deadlock.

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Code Block

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// Class Page is defined separately. It stores and returns the Page name via getName()

public final boolean SUCCESS = true;
public final boolean FAILURE = false;
Page[] pageBuff = new Page[MAX_PAGE_SIZE];

public synchronized boolean sendPage(Socket socket, String pageName){
 throws IOException try{
    // Get the output stream to write the Page to
    ObjectOutputStream out = new 
      ObjectOutputStream(socket.getOutputStream());

    Page targetPage = null;

    // Find the Page requested by the client
 (this operation requires synchronization)
  for(Page p : pageBuff) {
      if(p.getName().compareTo(pageName) == 0) {
        targetPage = p;
      }
    }

    // Page requested does not exist
    if(targetPage == null) {
      return FAILURE;

  } 

  // Send the Page to the client
    out.writeObject(targetPage);
  
  out.flush();
    out.close();
  } catch(IOException io){ /* forward to handler */ }
  return SUCCESS;
}

Compliant Solution

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Perform actions on data structures requiring synchronization
Create copies of objects to send
Perform network calls in a separate method that does not require any synchronization

In the following examplethis compliant solution, the synchronized method getPage() is called from SendReply() to find the appropriate Page requested by the client from the array pageBuff of type Page. The method sendReply() in turn calls the unsynchronized method sendPage() to deliver the Page.

Code Block

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public boolean sendReply(Socket socket, String pageName) { // No synchronization
  Page targetPage = getPage(pageName); 

  if(targetPage == null)
    return FAILURE;

  return sendPage(socket, targetPage);
}

private synchronized Page getPage(String pageName) { // Requires synchronization
  Page targetPage = null;

  for(Page p : pageBuff) {
    if(p.getName().equals(pageName)) {
      targetPage = p;
    }
  }
  return targetPage;
}

public boolean sendPage(Socket socket, Page page){
  try{
    // Get the output stream to write the Page to
    ObjectOutputStream out = new 
      ObjectOutputStream(socket.getOutputStream());

    // Send the Page to the client
    out.writeObject(page);

    out.flush();
    out.close();
    return SUCCESS;
  }catch(IOException io){
     // handleHandle exception    
  }
  return FAILURE;
}

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Old Version 16

New Version 17

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Compliant Solution