SEI CERT Oracle Coding Standard for Java

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Synchronizing a method performing 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.

Noncompliant Code Example

This noncompliant code example involves the method sendPage() that sends a Page object containing information being passed between a client and a server. The method is synchronized to protect access to 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.

// 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)
  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 (does not require any synchronization)
  out.writeObject(targetPage);

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

Compliant Solution

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

  • Perform actions on data structures requiring synchronization
  • Create copies of the objects that are required to be sent
  • Perform network calls in a separate method that does not require any synchronization

In this 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.

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;
}

Risk Assessment

If synchronized methods and statements contain network transactional logic, temporary or permanent deadlocks may result.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

CON20- J

low

probable

high

P2

L3

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[Grosso 01]] Chapter 10: Serialization
[[JLS 05]] Chapter 17, Threads and Locks
[[Rotem 08]] Falacies of Distributed Computing Explained

CON19-J. Notify all waiting threads instead of a single thread      11. Concurrency (CON)      CON21-J. Facilitate thread reuse by using Thread Pools

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