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 code example is designed to rename a given file if the file to be renamed is present, perform operations on the renamed file, and then delete the renamed file. However, due to the short-circuit behavior of the || operator, the renameTo() method appearing as the second argument to || is not executed when the exists() method appearing as the first argument to || returns true. Because of this, the renamed file may or may not exist, which may result in an attempt to use and then delete a nonexistent file. 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, which is not guaranteed to exist. fNew will only 
      // exist if file 'new.txt' exists, which has not been checked by this program.
      // ...
      // fNew may not exist as renameTo() may not have been successfully executed and 
      // the existence of 'new.txt' has not been checked.
      fNew.delete();
    }
  }
} 

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 comparing sm with null). If both d and sm are not null, the security check will be performed. This is a case of trying to combine together two null checks into one statement.

The result of this expression is that if d is equal to null, the 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 this is not possible, 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 code 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 of autoboxing when removing elements from a Collection      03. Expressions (EXP)      EXP07-J. Do not diminish the benefits of constants by assuming their values in expressions