CON22-J. Use the correct form of the double-checked locking idiom

The double checked locking idiom is sometimes used to provide lazy initialization in multithreaded code. In a multi-threading scenario, lazy initialization refers to reducing the cost of synchronization on each method access by deferring the synchronization to the moment when the object is actually initialized.

The code shown below is correctly synchronized, albeit slower. The double-checked locking pattern strives to make it faster.

// Correct multithreaded version using synchronization
class Foo { 
  private Helper helper = null;
  
  public synchronized Helper getHelper() {
    if (helper == null) {
      helper = new Helper();
    }
    return helper;
  }
  // Other functions and members...
}

This code ensures that in a multithreaded context, only one instance of the Helper object can exist at a particular time. The double checked locking idiom eliminates the need to synchronize every time the getHelper() method is invoked, to achieve performance gains. If implemented incorrectly, it may offer no such benefits and lead to erroneous or ineffective synchronization.

According to the Java Memory Model (discussion reference) [[Pugh 04]]:

... writes that initialize the Helper object and the write to the helper field can be done or perceived out of order. As a result, a thread which invokes getHelper() could see a non-null reference to a helper object, but see the default values for fields of the helper object, rather than the values set in the constructor.

Even if the compiler does not reorder those writes, on a multiprocessor the processor or the memory system may reorder those writes, as perceived by a thread running on another processor.

Noncompliant Code Example

This noncompliant code example uses the incorrect form of the double checked locking idiom.

// "Double-Checked Locking" idiom
class Foo { 
  private Helper helper = null;
  public Helper getHelper() {
    if (helper == null) { 
      synchronized(this) {
        if (helper == null) 
          helper = new Helper();
      }    
    }
    return helper;
  }
  // other functions and members...
}

Compliant Solution

This compliant solution declares the Helper object as volatile.

// Works with acquire/release semantics for volatile
// Broken under JDK 1.4 and earlier
class Foo {
  private volatile Helper helper = null;
  
  public Helper getHelper() { 
    if (helper == null) {
      synchronized(this) {
        if (helper == null) {
          helper = new Helper(); // If the helper is null, create a new instance
        }
      }
    }
    return helper; // If helper is non-null, return its instance
  }
}

JDK 5.0 allows a write of a volatile variable to be reordered with respect to a previous read or write. A read of a volatile variable cannot be reordered with respect to any following read or write. Because of this, the double checked locking idiom can work when helper is declared volatile. If a thread initializes the Helper object, a happens-before relationship is established between this thread and another that retrieves and returns the instance. [[Pugh 04]] and [[Manson 04]]

Exceptions

EX1: Explicitly synchronized code does not require the use of double-checked locking.

EX2: "Although the double-checked locking idiom cannot be used for references to objects, it can work for 32-bit primitive values (e.g., int's or float's). Note that it does not work for long's or double's, since unsynchronized reads/writes of 64-bit primitives are not guaranteed to be atomic." [[Pugh 04]]

Risk Assessment

Using incorrect forms of the double checked locking idiom can lead to synchronization issues.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[API 06]]
[[Pugh 04]]
[[MITRE 09]] CWE ID 609 "Double-Checked Locking"

CON21-J. Facilitate thread reuse by using Thread Pools      11. Concurrency (CON)      CON23-J. Address the shortcomings of the Singleton design pattern