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Synchronizing It is often insecure to synchronize a method performing that performs network transactions 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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This noncompliant code example involves shows the method sendPage() that sends a Page object containing information being passed between from a server to a client and a server. The method is synchronized to protect access to so that the array pageBuff. Calling writeObject() within the synchronized sendPage can result in a deadlock condition in high latency networks or when network connections are inherently lossy is accessed safely, when multiple threads request concurrent access.

// 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 {
  // 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)
  Page targetPage = null;
  for(Page p : pageBuff) {
    if(p.getName().compareTo(pageName) == 0) {
      targetPage = p;
    }
  }

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

  // Send the Page to the client (does not require any synchronization)
  out.writeObject(targetPage);

  out.flush();
  out.close();
  return SUCCESS;
}

Calling writeObject() within the synchronized sendPage() method can result in delays and deadlock-like conditions in high latency networks or when network connections are inherently lossy.

Compliant Solution

This compliant solution entails separating the actions into a sequence of steps:

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

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);

  } catch(IOException io){
    // If recovery is not possible return FAILURE
    return FAILURE;    
  } finally {
    out.flush();
    out.close();
  }  
  return SUCCESS;
}

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