EXP01-J. Never dereference null pointers

Null pointer dereferencing occurs when a null variable is treated as if it were a valid object reference and used without checking its state. This condition results in a NullPointerException, which could result in denial of service. Programs must not dereference null pointers.

Noncompliant Code Example

This noncompliant example shows a bug in Tomcat version 4.1.24, initially discovered by Reasoning [[Reasoning 2003]]. The cardinality method was designed to return the number of occurrences of object obj in collection col. One valid use of the cardinality method is to determine how many objects in the collection are null. However, because membership in the collection is checked using the expression obj.equals(elt), a null pointer dereference is guaranteed whenever obj is null.

public static int cardinality(Object obj, final Collection col) {
  int count = 0;
  Iterator it = col.iterator();
  while (it.hasNext()) {
    Object elt = it.next();
    if ((null == obj && null == elt) || obj.equals(elt)) {  // null pointer dereference
      count++;
    }
  }
  return count;
}

Compliant Solution

This compliant solution eliminates the null pointer dereference.

public static int cardinality(Object obj, final Collection col) {
  int count = 0;
  Iterator it = col.iterator();
  while (it.hasNext()) {
    Object elt = it.next();
    if ((null == obj && null == elt) || 
        (null != obj && obj.equals(elt))) {
      count++;
    }
  }
  return count;
}

Note that explicit null checks as shown here are one acceptable approach to eliminating null pointer dereferences;

Risk Assessment

Dereferencing a null pointer can lead to a denial of service. For example, Java Web Start applications and applets particular to JDK version 1.6, prior to update 4, were affected by a bug that had some noteworthy security consequences. In some isolated cases, the application or applet's attempt to establish an HTTPS connection with a server generated a NullPointerException [[SDN 2008]]. The resulting failure to establish a secure HTTPS connection with the server caused a denial of service: clients were temporarily forced to use an insecure http channel for data exchange. In multithreaded programs, null pointer dereferences can violate cache coherency policies and can cause resource leaks.

Automated Detection

Null pointer dereferences can happen in path-dependent ways. Limitations of automatic detection tools can require manual inspection of code [[Hovemeyer 2007]] to detect instances of null pointer dereferences. Annotations for method parameters that must be non-null can reduce the need for manual inspection by assisting automated null pointer dereference detection; use of these annotations is strongly encouraged.

The Coverity Prevent Version 5.0 FORWARD_NULL checker can detect the instance where reference is checked against null but then dereferenced anyway.

Related Guidelines

Bibliography

EXP04-J. Ensure that autoboxed values have the intended type      02. Expressions (EXP)      03. Numeric Types and Operations (NUM)