CON43-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. The code shown below ensures that only one instance of the Helper object can exist at a particular time in a multithreaded context.

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

The double checked locking idiom eliminates the synchronization 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

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. [[Pugh 04]]

// 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();
        }
      }
    }
    return helper;
  }
}

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"

CON15-J. Ensure actively held locks are released on exceptional conditions      11. Concurrency (CON)      CON44-J. Do not expect sleep() and yield() methods to have any synchronization semantics