EXP06-J. Be aware of the short-circuit behavior of the conditional AND and OR operators

The conditional AND and OR operators (&& and ||, respectively) exhibit short-circuit behavior. That is, the second operand is not evaluated if the result can be deduced solely by evaluating the first operand.

Exercise caution if the operands following the first operand contain side effects. In the following code, the value of i is incremented only when i >= 0.

int i = /* initialize to user supplied value */
int max = /* initialize to maximum value */
if ( (i >= 0) && ( (i++) <= max) ) {
  /* code */
}

Although the behavior is well defined, it is not immediately obvious whether i gets incremented or not. Even in the absence of side effects, it is possible to write confusing expressions that do not execute as desired. This guideline exemplifies both these conditions.

Noncompliant Code Example

This noncompliant example is designed to rename a given file if it is present, perform operations on it, and then delete it. However, the renameTo() method does not execute when the exists() method returns true, and an unsuspecting developer would incorrectly attempt to delete the fNew file instead of the original one. This problem is exacerbated by the fact that File.delete() does not throw an exception but returns an error code on failure, which is often silently ignored or perceived as unnecessary. (See EXP02-J. Do not ignore values returned by methods)

class BadRenameFile {
  public static void main(String[] args) {
    File fOriginal = new File("original.txt");
    File fNew = new File("new.txt");
    if(fOriginal.exists() || fOriginal.renameTo(fNew)) {
      // do something with fNew
      fNew.delete(); // fNew does not exist as renameTo() was not executed
    }
  }
} 

Compliant Solution

Knowledge of the short-circuit behavior can be used to enforce the desired specification. This program traps an error if the file does not exist or when it cannot be renamed to the new file name. Operations on the new file follow.

class RenameFile {
  public static void main(String[] args) {
    File fOriginal = new File("original.txt");
    File fNew = new File("new.txt");

    if(!fOriginal.exists() || !fOriginal.renameTo(fNew)) {
      // handle error
    }
    // do something with fNew
    if(!fNew.delete()) {
      // handle error  
    }
  }
} 

Noncompliant Code Example

This noncompliant example differs from the previous one in that, there are no side effects in the right hand side operand. Nevertheless, an unaware programmer can get caught in the short-circuit behavior of the conditional AND and OR operators. The programmer has combined two expressions in the if statement. The first checks whether the d object is null and the second checks if the default security manager has been installed (by comparinf sm with null) depending on which the security check will be performed. This is a case of trying to combine together two null checks into one statement. A conditional && is used as using a conditional || would mean that whenever d is null, the complete expression can still succeed depending on the value of sm (see the next noncompliant example). This violates the invariants of d as it is desired that operations on it be prohibited if it is null.

Unfortunately, when d is equal to null as shown, the current if expression evaluates to false and the security check is not executed.

// d = null
SecurityManager sm = System.getSecurityManager();	  
if(d != null && sm != null) {
  FilePermission perm = new FilePermission("file.dat", "read");
  sm.checkPermission(perm);
  /* do something with d */
}
/* read the file (skips security check) */

Noncompliant Code Example

In this example, the programmer switches the ordering of the two subexpressions and uses a || operator to ensure the security check is carried out but does not realize that the second check is evaluated if the first one succeeds. The result is the inadvertent modification of d.

SecurityManager sm = System.getSecurityManager();	  
if(sm != null || d != null) {
  FilePermission perm = new FilePermission("file.dat", "read");
  sm.checkPermission(perm);
  // do something with d
}
// read the file

Compliant Solution

Decouple distinct operations that use the conditional AND and OR operators from expressions constituting decision statements. When inevitable, be aware of the short-circuit behavior and code accordingly.

if(d != null) { /* do something with d */ }

if(sm != null) {
  FilePermission perm = new FilePermission("file.dat", "read");
  sm.checkPermission(perm);
}
/* read the file (default security check is carried out) */

Exceptions

EXP06-J-EX1: Sometimes programmers who are aware of the short-circuit behavior use it to their advantage, as Flanagan [[Flanagan 05]] exemplifies:

if (data != null && i < data.length && data[i] != -1) ... 

This snippet sequentially executes the subexpressions while avoiding an array indexing exception resulting from the checks that execute prior to the last subexpression.

Risk Assessment

Failing to understand the short-circuit behavior of the logical AND and OR operators may cause unintended program behavior.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[JLS 05]] Sections 15.23 "Conditional-And Operator &&" and 15.24 "Conditional-Or Operator ||"
[[Flanagan 05]] 2.5.6. Boolean Operators

EXP05-J. Be careful about the wrapper class and autoboxing      03. Expressions (EXP)      03. Expressions (EXP)