Page History

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If the JVM interacts with a file system that operates over an unreliable network, file I/O might incur a large performance penalty. In such cases, avoid file I/O over the network when holding a lock. File operations (such as logging) that could block waiting for the output stream lock or for I/O to complete could be performed in a dedicated thread to speed up task processing. Logging requests can be added to a queue, given that the queue's {{put()}} operation incurs little overhead as compared to file I/O \[[Goetz 2006|AA. Bibliography#Goetz 06]\].

Noncompliant Code Example (Deferring a Thread)

This noncompliant code example defines a utility method that accepts a time argument.

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Because the method is synchronized, if the thread is suspended, other threads are unable to use the synchronized methods of the class. The current object's monitor is not released because the Thread.sleep() method does not have any synchronization semantics, as detailed in guideline THI00-J. Do not assume that the sleep(), yield() or getState() methods provide synchronization semantics.

Compliant Solution (Intrinsic Lock)

This compliant solution defines the doSomething() method with a timeout parameter instead of the time value. Using Object.wait() instead of Thread.sleep() allows setting a time out period during which a notification may awaken the thread.

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Ensure that a thread that holds locks on other objects releases them appropriately, before entering the wait state. Additional guidance on waiting and notification is available in guidelines THI03-J. Always invoke wait() and await() methods inside a loop and THI04-J. Notify all waiting threads instead of a single thread.

Noncompliant Code Example (Network I/O)

This noncompliant code example shows the method sendPage() that sends a Page object from a server to a client. The method is synchronized so that the pageBuff array is accessed safely when multiple threads request concurrent access.

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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 separates the process into a sequence of steps:

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

  if (targetPage == null)
    return false;

  return deliverPage(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;
}

// Return false if an error occurs, true if successful
public boolean deliverPage(Socket socket, Page page) {
  ObjectOutputStream out = null;
  boolean result = true;
  try {
    // Get the output stream to write the Page to
    out = new ObjectOutputStream(socket.getOutputStream());

    // Send the Page to the client
    out.writeObject(page);
  } catch (IOException io) {
    result = false;
  } finally {
    if (out != null) {
      try {
        out.flush();
        out.close();
      } catch (IOException e) {
        result = false;
      }
    }
  }
  return result;
}

Exceptions

LCK09-EX1: Classes that provide an appropriate termination mechanism to callers are allowed to violate this guideline. See guideline THI06-J. Ensure that threads performing blocking operations can be terminated.

LCK09-EX2: A method that requires multiple locks may hold several locks while waiting for the remaining locks to become available. This constitutes a valid exception, although the programmer must follow other applicable guidelines to avoid deadlock. See guideline LCK07-J. Avoid deadlock by requesting and releasing locks in the same order for more information.

Risk Assessment

Blocking or lengthy operations performed within synchronized regions could result in a deadlocked or unresponsive system.

Bibliography

\[[API 2006|AA. Bibliography#API 06]\] Class {{Object}}
\[[Grosso 2001|AA. Bibliography#Grosso 01]\] [Chapter 10: Serialization|http://oreilly.com/catalog/javarmi/chapter/ch10.html]
\[[JLS 2005|AA. Bibliography#JLS 05]\] [Chapter 17, Threads and Locks|http://java.sun.com/docs/books/jls/third_edition/html/memory.html]
\[[Rotem 2008|AA. Bibliography#Rotem 08]\] [Falacies of Distributed Computing Explained|http://www.rgoarchitects.com/Files/fallacies.pdf]

Related Vulnerabilities

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

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