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[API 06|AA. Java References#API 06]\] states:

{quote}
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.
{quote} 

Note that the advice suggested above is compliant with [CON40-J. Do not synchronize on a collection view if the backing collection is accessible] when the backing list is inaccessible from a caller that can potentially inflict some harm.


h2. Noncompliant Code Example (intrinsic lock)

This noncompliant code example uses a thread-safe class {{Book}} that cannot be refactored. This could 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.

{code}
final class Book {
  // MayCould change its locking policy in the future
  // to use private internalfinal 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;
  }
}
{code}

 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.

{code:bgColor=#FFCCCC}
// 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().afterbefore(Calendar.getInstance())) {
        throw new IllegalStateException("Book overdue");
      } else {
        book.issue(14); // Issue book for 14 days
      }
    }
  }
}

{code}

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 will definitely break if {{Book}} is modified to use an internal private lock, 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 {{getDueDate()}} of class {{BookWrapper}} 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.


h2. Compliant Solution (intrinsic lock)

This compliant solution uses an internal private lock object and synchronizes all its methods using this lock.

{code:bgColor=#ccccff}
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().afterbefore(Calendar.getInstance())) {
        throw new IllegalStateException("Book overdue");
      } else {
        book.issue(14); // Issue book for 14 days
      }
    }
  }
}
{code}

Consequently, the {{BookWrapper}} 

// This class could 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 visibility is package-private
  }

  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]);
      // ...
    }
  }
}

// Class IPAddressList remains unchanged
class PrintableIPAddressList {
  private final IPAddressList ips;

  public PrintableIPAddressList(IPAddressList list) {
    this.ips = list;}} instance. This solution incurs a very small performance penalty but the resulting code is much more robust \[[Goetz 06|AA. Java References#Goetz 06]\].


h2. Noncompliant Code Example (class extension and accessible member lock)

Goetz et al. describe the insecurity of class extension for adding functionality to thread-safe classes \[[Goetz 06|AA. Java References#Goetz 06]\]:

{quote}
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.
{quote}

In this noncompliant code example, the class {{PrintableIPAddressList}} extends the thread-safe class {{IPAddressList}}. It locks on {{IpAddressList}}'s member {{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.

{code:bgColor=#FFCCCC}
// This class may change its locking policy in the future, for example, 
// when 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 synchronized void addIPAddress(InetAddress address) {
    ips.addaddIPAddress(address);
  }
}

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

If the class {{IPAddressList}} 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, when a wrapper such as {{Collections.synchronizedList()}} is used, it is unwieldy for a client to determine the type of the class ({{List}}) that is being wrapped to extend it \[[Goetz 06|AA. Java References#Goetz 06]\]. 


h2. Compliant Solution (Composition)

Composition offers encapsulation benefits at the cost of performance. The performance impact is usually minimal. Refer to [OBJ07-J. Understand how a superclass can affect a subclass] for more information on implementing 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.

{code:bgColor=#ccccff}
// 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]);     
    // ...
  }
}
{code}

This approach allows the {{CompositeCollection}} class to use its own intrinsic lock in a way that is completely independent of the lock of the underlying list class.  This allows the underlying collection to be not thread-safe because the {{CompositeCollection}} 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. \[[Goetz 06|AA. Java References#Goetz 06]\]


h2. Risk Assessment

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

|| Rule || Severity || Likelihood || Remediation Cost || Priority || Level ||
| CON31- J | low | probable | medium | {color:green}{*}P4{*}{color} | {color:green}{*}L3{*}{color} |



h3. Automated Detection

TODO


h3. Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the [CERT website|https://www.kb.cert.org/vulnotes/bymetric?searchview&query=FIELD+KEYWORDS+contains+CON38-J].

h2. References

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

----
[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_left.png!|VOID CON06-J. Do not defer a thread that is holding a lock]&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_up.png!|11. Concurrency (CON)]&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_right.png!|CON08-J. Do not call alien methods that synchronize on the same objects as any callers in the execution chain]
}