CON31-J. Avoid client-side locking when using classes that do not commit to their locking strategy

According to Goetz and colleagues [[Goetz 06]]:

Client-side locking entails guarding client code that uses some object X with the lock X uses to guard its own state. In order to use client-side locking, you must know what lock X uses.

While client-side locking is acceptable if the thread-safe class commits to its locking strategy and clearly documents it, Goetz cautions against its misuse [[Goetz 06]]:

If extending a class to add another atomic operation is fragile because it distributes the locking code for a class over multiple classes in an object hierarchy, client-side locking is even more fragile because it entails putting locking code for class C into classes that are totally unrelated to C. Exercise care when using client-side locking on classes that do not commit to their locking strategy.

The documentation of a class that supports client-side locking should explicitly state its applicability. For example, the class java.util.concurrent.ConcurrentHashMap<K,V> should not be used for client-side locking, because its documentation states [[API 06]]:

... even though all operations are thread-safe, retrieval operations do not entail locking, and there is not any support for locking the entire table in a way that prevents all access. This class is fully interoperable with Hashtable in programs that rely on its thread safety but not on its synchronization details.

In general, use client-side locking only when the documentation of the class recommends it. For example, the documentation of the wrapper method synchronizedList() of class java.util.Collections [[API 06]] states:

In order to guarantee serial access, it is critical that all access to the backing list is accomplished through the returned list. It is imperative that the user manually synchronize on the returned list when iterating over it. Failure to follow this advice may result in non-deterministic behavior.

Note that this advice is consistent with CON06-J. Do not synchronize on a collection view if the backing collection is accessible when the backing list is inaccessible to an untrusted client.

Noncompliant Code Example (Intrinsic Lock)

This noncompliant code example uses a thread-safe class Book that cannot be refactored. This might happen, for example, when the source code is not available for review or the class is part of a general library that cannot be extended.

final class Book {
  // May change its locking policy in the future to use private final locks
  private final String title;
  private Calendar dateIssued;
  private Calendar dateDue;

  Book(String title) {
    this.title = title; 
  }
  
  public synchronized void issue(int days) {
    dateIssued = Calendar.getInstance();
    dateDue = Calendar.getInstance();
    dateDue.add(dateIssued.DATE, days);	 
  }

  public synchronized Calendar getDueDate() {
    return dateDue;
  }
}

This class does not commit to its locking strategy (that is, it reserves the right to change its locking strategy without notice). Furthermore, it does not document that callers can safely use client-side locking. The client class BookWrapper uses client-side locking in the renew() method by synchronizing on a Book instance.

// Client
public class BookWrapper {
  private final Book book;

  BookWrapper(Book book) {
    this.book = book;
  }

  public void issue(int days) {
    book.issue(days);
  }

  public Calendar getDueDate() {
    return book.getDueDate();
  }

  public void renew() {
    synchronized(book) {
      if (book.getDueDate().after(Calendar.getInstance())) {
        throw new IllegalStateException("Book overdue");
      } else {
        book.issue(14); // Issue book for 14 days
      }
    }
  }
}

If class Book changes its synchronization policy in the future, the BookWrapper class's locking strategy might silently break. For instance, the Bookwrapper class's locking strategy breaks if Book is modified to use a private final lock object, as recommended by CON04-J. Use private final lock objects to synchronize classes that may interact with untrusted code. This is because threads that call BookWrapper.getDueDate() may perform operations on the thread-safe Book using its new locking policy. However, threads that call method renew() will always synchronize on the intrinsic lock of the Book instance. Consequently, the implementation will use two different locks.

Compliant Solution (Private Final Lock Object)

This compliant solution uses a private final lock object and synchronizes all its methods using this lock.

public final class BookWrapper {
  private final Book book;
  private final Object lock = new Object();

  BookWrapper(Book book) {
    this.book = book;
  }

  public void issue(int days) {
    synchronized(lock) {
      book.issue(days);
    }
  }

  public Calendar getDueDate() {
    synchronized(lock) {
      return book.getDueDate();
    }
  }

  public void renew() {
    synchronized(lock) {
      if (book.getDueDate().after(Calendar.getInstance())) {
        throw new IllegalStateException("Book overdue");
      } else {
        book.issue(14); // Issue book for 14 days
      }
    }
  }
}

The BookWrapper class's locking strategy is now independent of the locking policy of the Book instance.

Noncompliant Code Example (Class Extension and Accessible Member Lock)

Goetz and colleagues describe the fragility of class extension for adding functionality to thread-safe classes [[Goetz 06]]:

Extension is more fragile than adding code directly to a class, because the implementation of the synchronization policy is now distributed over multiple, separately maintained source files. If the underlying class were to change its synchronization policy by choosing a different lock to guard its state variables, the subclass would subtly and silently break, because it no longer used the right lock to control concurrent access to the base class's state.

In this noncompliant code example, the PrintableIPAddressList class extends the thread-safe class IPAddressList. PrintableIPAddressList locks on IpAddressList.ips in the method addAndPrintIPAddresses(). This is another example of client-side locking, because a subclass uses an object owned and locked by its superclass.

// This class may change its locking policy in the future, for example, 
// if new non-atomic methods are added
class IPAddressList {
  private final List<InetAddress> ips = Collections.synchronizedList(new ArrayList<InetAddress>());
  
  public List<InetAddress> getList() {
    return ips; // No defensive copies required as package-private visibility
  }

  public void addIPAddress(InetAddress address) {
    ips.add(address);
  }
}

class PrintableIPAddressList extends IPAddressList {
  public void addAndPrintIPAddresses(InetAddress address) {
    synchronized(getList()) {
      addIPAddress(address);
      InetAddress[] ia = (InetAddress[]) getList().toArray(new InetAddress[0]);      
      // ...
    }
  }
}

If the IPAddressList class is modified to use block synchronization on a private final lock object, as recommended by [CON04-J. Use private final lock objects to synchronize classes that may interact with untrusted code], the subclass PrintableIPAddressList will silently break. Moreover, if a wrapper such as Collections.synchronizedList() is used, it is difficult for a client to determine the type of the class being wrapped to extend it [[Goetz 06]].

Compliant Solution (Composition)

This compliant solution wraps an object of class IPAddressList and provides synchronized accessors that can be used to manipulate the state of the object.

Composition offers encapsulation benefits; usually with minimal overhead. Refer to OBJ07-J. Understand how a superclass can affect a subclass for more information on composition.

// Class IPAddressList remains unchanged
class PrintableIPAddressList {
  private final IPAddressList ips;
 
  public PrintableIPAddressList(IPAddressList list) {
    this.ips = list;
  }
  
  public synchronized void addIPAddress(InetAddress address) {
    ips.addIPAddress(address);
  }

  public synchronized void addAndPrintIPAddresses(InetAddress address) {  
    addIPAddress(address); 
    InetAddress[] ia = (InetAddress[]) ips.getList().toArray(new InetAddress[0]);     
    // ...
  }
}

In this case, composition allows the PrintableIPAddressList class to use its own intrinsic lock independent of the lock of the underlying list class. This does not require the underlying collection to be thread-safe because the PrintableIPAddressList wrapper prevents direct access to its methods by publishing its own synchronized equivalents. This approach provides consistent locking even if the underlying class changes its locking policy in the future [[Goetz 06]].

Risk Assessment

Using client-side locking when the thread-safe class does not commit to its locking strategy can cause data inconsistencies and deadlock.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[API 06]] Class Vector, Class WeakReference, Class ConcurrentHashMap<K,V>
[[JavaThreads 04]] 8.2 "Synchronization and Collection Classes"
[[Goetz 06]] 4.4.1. Client-side Locking, 4.4.2. Composition and 5.2.1. ConcurrentHashMap
[[Lee 09]] "Map & Compound Operation"

CON04-J. Ensure that calls to chained methods are atomic      11. Concurrency (CON)      CON32-J. Synchronize access to static fields that may be modified by untrusted code