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Do not use the null value in any instance where an object is required, including the following cases:

Calling the instance method of a null object
Accessing or modifying the field of a null object
Taking the length of null as if it were an array
Accessing or modifying the elements of null as if it were an array
Throwing null as if it were a Throwable value

Using a null in cases where an object is required results in a NullPointerException being thrown, which interrupts execution of the program or thread. Code conforming to this coding standard will consequently terminate because ERR08-J. Do not catch NullPointerException or any of its ancestors requires that NullPointerException is not caught.

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 and elt is not null.

public static int cardinality(Object obj, final Collection<?> col) {
  int count = 0;
  if (col == null) {
    return count;
  }
  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 by adding an explicit check:

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

Noncompliant Code Example

This noncompliant code example defines an isProperName () method that returns true if the specified String argument is a valid name (two capitalized words separated by one or more spaces):

public boolean isProperName(String s) {
  String names[] = s.split(" ");
  if (names.length != 2) {
    return false;
  }
  return (isCapitalized(names[0]) && isCapitalized(names[1]));
}

Method isProperName () is noncompliant because it may be called with a null argument, resulting in a null pointer dereference.

Compliant Solution (Wrapped Method)

This compliant solution includes the same isProperName() method implementation as the previous noncompliant example, but it is now a private method with only one caller in its containing class.

public class Foo {
  private boolean isProperName(String s) {
    String names[] = s.split(" ");
    if (names.length != 2) {
      return false;
    }
    return (isCapitalized(names[0]) && isCapitalized(names[1]));
  }

  public boolean testString(String s) {
    if (s == null) return false;
    else return isProperName(s);
  }
}

The calling method, testString(), guarantees that isProperName () is always called with a valid string reference. As a result, the class conforms with this rule even though a public isProperName() method would not. Guarantees of this sort can be used to eliminate null pointer dereferences.

Compliant Solution (Optional Type)

This compliant solution uses an Optional String instead of a String object that may be null. The Optional class (java.util.Optional [API 2014]) was introduced in Java 8 and can be used to mitigate against null pointer dereferences .

public boolean isProperName(Optional<String> os) {
  String names[] = os.orElse("").split(" ");
  return (names.length != 2) ? false : 
         (isCapitalized(names[0]) && isCapitalized(names[1]));
}

The Optional class contains methods that can be used to make programs shorter and more intuitive [ Urma 2014 ].

Exceptions

EXP01-J-EX0: A method may dereference an object-typed parameter without guarantee that it is a valid object reference provided that the method documents that it (potentially) throws a NullPointerException, either via the throws clause of the method or in the method comments. However, this exception should be relied on sparingly.

Risk Assessment

Dereferencing a null pointer can lead to a denial of service. In multithreaded programs, null pointer dereferences can violate cache coherency policies and can cause resource leaks.

Rule	Severity	Likelihood	Remediation Cost	Priority	Level
EXP01-J	Low	Likely	High	P3	L3

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.

Tool	Version	Checker	Description
The Checker Framework		Nullness Checker Initialization Checker Map Key Checker	Null pointer errors (see Chapter 3) Ensure all fields are set in the constructor (see Chapter 3.8) Track which values are keys in a map (see Chapter 4)
CodeSonar		JAVA.DEEPNULL.PARAM.EACTUAL JAVA.DEEPNULL.EFIELD JAVA.DEEPNULL.FIELD JAVA.NULL.PARAM.ACTUAL JAVA.NULL.DEREF JAVA.DEEPNULL.DEREF JAVA.DEEPNULL.RET.EMETH JAVA.DEEPNULL.RET.METH JAVA.NULL.RET.ARRAY JAVA.NULL.RET.BOOL JAVA.NULL.RET.OPT JAVA.STRUCT.UPD JAVA.STRUCT.DUPD JAVA.STRUCT.UPED JAVA.DEEPNULL.PARAM.ACTUAL	Actual Parameter Element may be null Field Element may be null (deep) Field may be null (deep) Null Parameter Dereference Null Pointer Dereference Null Pointer Dereference (deep) Return Value may Contain null Element Return Value may be null Return null Array Return null Boolean Return null Optional Unchecked Parameter Dereference Unchecked Parameter Dereference (deep) Unchecked Parameter Element Dereference (deep) null Passed to Method (deep)
Coverity	v7.5	FORWARD_NULL NULL_RETURNS REVERSE_INULL FB.BC_NULL_INSTANCEOF FB.NP_ALWAYS_NULL FB.NP_ALWAYS_NULL_EXCEPTION FB.NP_ARGUMENT_MIGHT_BE_NULL FB.NP_BOOLEAN_RETURN_NULL FB.NP_CLONE_COULD_RETURN_NULL FB.NP_CLOSING_NULL FB.NP_DEREFERENCE_OF_ READLINE_VALUE FB.NP_DOES_NOT_HANDLE_NULL FB.NP_EQUALS_SHOULD_HANDLE_ NULL_ARGUMENT FB.NP_FIELD_NOT_INITIALIZED_ IN_CONSTRUCTOR FB.NP_GUARANTEED_DEREF FB.NP_GUARANTEED_DEREF_ON_ EXCEPTION_PATH FB.NP_IMMEDIATE_DEREFERENCE_ OF_READLINE FB.NP_LOAD_OF_KNOWN_NULL_ VALUE FB.NP_NONNULL_FIELD_NOT_ INITIALIZED_IN_CONSTRUCTOR FB.NP_NONNULL_PARAM_VIOLATION FB.NP_NONNULL_RETURN_VIOLATION FB.NP_NULL_INSTANCEOF FB.NP_NULL_ON_SOME_PATH FB.NP_NULL_ON_SOME_PATH_ EXCEPTION FB.NP_NULL_ON_SOME_PATH_ FROM_RETURN_VALUE FB.NP_NULL_ON_SOME_PATH_ MIGHT_BE_INFEASIBLE FB.NP_NULL_PARAM_DEREF FB.NP_NULL_PARAM_DEREF_ALL_ TARGETS_DANGEROUS FB.NP_NULL_PARAM_DEREF_ NONVIRTUAL FB.NP_PARAMETER_MUST_BE_NON - NULL_BUT_MARKED_AS_NULLABLE FB.NP_STORE_INTO_NONNULL_FIELD FB.NP_TOSTRING_COULD_ RETURN_NULL FB.NP_UNWRITTEN_FIELD FB.NP_UNWRITTEN_PUBLIC_OR_ PROTECTED_FIELD FB.RCN_REDUNDANT_COMPARISON_ OF_NULL_AND_NONNULL_VALUE FB.RCN_REDUNDANT_COMPARISON_ TWO_NULL_VALUES FB.RCN_REDUNDANT_NULLCHECK_ OF_NONNULL_VALUE FB.RCN_REDUNDANT_NULLCHECK_ OF_NULL_VALUE FB.RCN_REDUNDANT_NULLCHECK_ WOULD_HAVE_BEEN_A_NPE	Implemented
Fortify		Missing_Check_against_Null Null_Dereference Redundant_Null_Check	Implemented
Findbugs		NP_DEREFERENCE_OF_READLINE_VALUE NP_NULL_PARAM_DEREF NP_TOSTRING_COULD_RETURN_NULL	Implemented
Parasoft Jtest		CERT.EXP01.NP CERT.EXP01.NCMD	Avoid NullPointerException Ensure that dereferenced variables match variables which were previously checked for "null"
PVS-Studio		V6008, V6073, V6093
SonarQube		S2259 S2225 S2447 S2637	Null pointers should not be dereferenced "toString()" and "clone()" methods should not return null Null should not be returned from a "Boolean" method "@NonNull" values should not be set to null
SpotBugs		NP_DEREFERENCE_OF_READLINE_VALUE NP_IMMEDIATE_DEREFERENCE_OF_READLINE NP_ALWAYS_NULL NP_NULL_ON_SOME_PATH NP_NULL_ON_SOME_PATH_EXCEPTION NP_NULL_PARAM_DEREF NP_NULL_PARAM_DEREF_NONVIRTUAL NP_NULL_PARAM_DEREF_ALL_TARGETS_DANGEROUS NP_TOSTRING_COULD_RETURN_NULL	Implemented

Related Vulnerabilities

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.

Related Guidelines

SEI CERT C Coding Standard	EXP34-C. Do not dereference null pointers
ISO/IEC TR 24772:2010	Null Pointer Dereference [XYH]
MITRE CWE	CWE-476, NULL Pointer Dereference

Android Implementation Details

Android applications are more sensitive to NullPointerException because of the constraint of the limited mobile device memory. Static members or members of an Activity may become null when memory runs out.

Bibliography

[API 2006]	Method `doPrivileged()`
[API 2014]	Class `java.util.Optional`
[Hovemeyer 2007]
[Reasoning 2003]	"Defect ID 00-0001" "Null Pointer Dereference"
[SDN 2008]	Bug ID 6514454
[ Seacord 2015 ]	EXP01-J. Never dereference null pointers LiveLesson
[Urma 2014]	Tired of Null Pointer Exceptions? Consider Using Java SE 8's Optional!