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Concurrency related issues manifest themselves when assumptions are made about the multithreaded behavior of derived classes. An Overriding thread-safe methods with methods that are unsafe for concurrent use can result in improper synchronization when a client that depends on the thread-safety promised by the parent inadvertently operates on an instance of the subclass. For example, an overridden synchronized method's contract may can be violated if when a subclass defines provides an implementation that is unsafe for concurrent use. Such overriding can easily result in errors that are difficult to diagnose. Consequently, programs must not override thread-safe methods with methods that are unsafe for concurrent use.

The guideline CON04-J. Synchronize using an private final lock object recommends documenting the locking strategy of classes designed for inheritance should always be documented. This information is useful for deciding the locking strategy of subclassescan subsequently be used to determine an appropriate locking strategy for subclasses (see LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code and LCK11-J. Avoid client-side locking when using classes that do not commit to their locking strategy).

Noncompliant Code Example (

...

Synchronized Method)

This noncompliant code example defines a overrides the synchronized doSomething() method in class Base. the Base class with an unsynchronized method in the Derived class:

class Base {
  public synchronized void doSomething() {
    // ...	
  }
}

class Derived extends Base {
  @Override public void doSomething() {
    // ...		
  }
}

The doSomething() method of the Base class Base can be safely used by multiple threads. However, if a subclass Derived overrides the method but leaves it unsynchronized, its instance cannot be safely used by multiple threads. , but instances of the Derived subclass cannot.

This programming error can be difficult to diagnose This problem is hard to notice because threads that accept instances of Base can also accept instances of its subclasses. Consequently, the threads may incorrectly assume that the subclasses are thread-safeclients could be unaware that they are operating on a thread-unsafe instance of a subclass of a thread-safe class.

Compliant Solution (

...

Synchronized Method)

This compliant solution synchronizes the doSomething() method of the subclass. :

class Base {
  public synchronized void doSomething() {
    // ...	
  }
}

class Derived extends Base {
  @Override public synchronized void doSomething() {
    // ...		
  }
}

This compliant solution does not violate CON04solution also complies with LCK00-J. Synchronize using an Use private final lock objectobjects to synchronize classes that may interact with untrusted code because the accessibility of the class is package-private. Package-private which accessibility is allowable permitted when untrusted code cannot infiltrate the package.

Compliant Solution (Private Final Lock Object)

This compliant solution ensures that the Derived class is thread-safe by overriding the synchronized doSomething() method of the Base class with a method that synchronizes on a private final lock object.

class Base {

  public synchronized void doSomething() {
    // ...
  }
}

class Derived extends Base {
  private final Object lock = new Object();

  @Override public void doSomething() {
    synchronized (lock) {
      // ...
    }
  }
}

This is an acceptable solution, provided the locking policy of the Derived class is consistent with that of the Base class.

Noncompliant Code Example (

...

Private Lock)

This noncompliant code example defines a doSomething() method in the Base class Base that uses an internal a private final lock , in accordance with CON04LCK00-J. Synchronize using an Use private final lock objectobjects to synchronize classes that may interact with untrusted code.

class Base {
  private final Object lock = new Object();

  public void doSomething() {
    synchronized (lock) {
      // ...	
    }
  }
}

class Derived extends Base {
  Logger logger = // Initialize

  @Override public void doSomething() {
    try {
      super.doSomething();
    } finally {
      logger.log(Level.FINE, "Did something"); 
    }
  }
}

The doSomething() method of class Derived cannot be safely used by multiple threads because it is not thread-safe. It is possible that for multiple threads may to cause the entries to be logged in an order different than the one that differs from the order in which the tasks are performed. Consequently, the doSomething() method of the Derived class cannot be used safely by multiple threads because it is not thread-safe.

Compliant Solution (

...

Private Lock)

This compliant solution solution synchronizes the doSomething() method of the subclass . using its own private final lock object:

class Base {
  private final Object lock = new Object();

  public void doSomething() {
    synchronized (lock) {
      // ...	
    }
  }
}

class Derived extends Base {
  Logger logger = // Initialize

  private final Object lock = new Object();

  @Override public void doSomething() {
    synchronized (lock) {
      try {
        super.doSomething();
      } finally {
        logger.log(Level.FINE, "Did something"); 
      }
    }
  }
}

Note that the objects of Base and Derived objects maintain distinct locks that are inaccessible from each othersother's classes. Consequently, Derived can provide thread-safety guarantees independent of Base.

Risk Assessment

Overriding Overriding thread-safe methods with methods that are not thread-safe unsafe for concurrent access can result in unexpected behavior.

Automated Detection

...

TODO

Related Vulnerabilities

Sound automated detection is infeasible; heuristic checks could be useful.

Tool	Version	Checker	Description
Parasoft Jtest	Include Page Parasoft_V Parasoft_V	CERT.TSM00.OSNS	Avoid overriding synchronized methods with non-synchronized methods

Bibliography

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

References

\[[API 06|AA. Java References#API 06]\]
\[[SDN 08|AA. Java References#SDN 08]\] Sun bug database, [Bug ID 4294756|http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4294756]

VOID CON09-J. Do not invoke alien methods that rely on invariants protected by the same lock object      11. Concurrency (CON)      CON11-J. Do not assume that declaring an object reference volatile guarantees visibility of its members