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 According to The Java Language Specification (JLS), §15.7, "Evaluation Order" [JLS 2015] Wiki MarkupAccording to the Java Language Specification \[[JLS 05|AA. Java References#JLS 05]\] section 15.7 "Evaluation Order":

The Java programming language guarantees that the operands of operators appear to be evaluated in a specific evaluation order, namely, from left to right.

Section 15§15.7.3, "Evaluation Respects Parentheses and Precedence", on the other hand states adds:

Java programming language implementations must respect the order of evaluation as indicated explicitly by parentheses and implicitly by operator precedence.

These When an expression contains side effects, these two requirements can be counter intuitive when expressions contain side-effects. This is because such expressions follow the left to right evaluation order irrespective of operator precedence, associativity rules and indicative parenthesesyield unexpected results. Evaluation of the operands proceeds left to right, without regard to operator precedence rules and indicative parentheses; evaluation of the operators, however, obeys precedence rules and parentheses.

Expressions must not write to memory that they subsequently read and also must not write to any memory twice. Note that memory reads and writes can occur either directly in the expression from assignments or indirectly through side effects in methods called in the expression.

Noncompliant Code Example (Order of Evaluation)

This noncompliant code example shows how side - effects in expressions can lead to unanticipated outcomes. The programmer intends to write access control logic based on different threshold levelsthresholds. Each user has a rating that must be above the threshold to be granted access. As shown, a simple function can be used to calculate the rating. The get() method is expected to return a non-zero factor when the user is authorized, value for authorized users and a zero value when notfor unauthorized users.

In this case, the programmer expects the rightmost subexpression to evaluate The programmer in this example incorrectly assumes that the rightmost subexpression is evaluated first because the * operator has a higher precedence than the + operator . The parentheses reinforce this belief. These ideas lead and because the subexpression is parenthesized. This assumption leads to the incorrect conclusion that the right hand side evaluates to zero whenever the get() method returns zero. The programmer expects number to be that number is assigned 0 because of the rightmost number = get() subexpression. Consequently, the test in the left-hand subexpression is expected to reject the unprivileged user because the rating value (of number) is below the threshold of 10.

However, the program grants access to the unauthorized user . The reason is that because evaluation of the side-effect-infested subexpressions follows the left-to-right ordering rule which should not be confused with the tenets of operator precedence, associativity and indicative parentheses.

class BadPrecedence {
  public static void main(String[] args) {
    int number = 17;
    int[] threshold = new int[20];
    threshold[0] = 10;
    number = (number > threshold[0] ? 0 : -2) 
             + ((31 * ++number) * (number = get()));
    // ... 
    if (number == 0) {
      System.out.println("Access granted");
    } else {
      System.out.println("Denied access"); // number = -2
    }
  }

  public static int get() {
    int number = 0;
    // Assign number to nonnonzero zero value if authorized, else 0
    return number;
  }
}

Compliant Solution

Noncompliant Code Example (Order of Evaluation)

This noncompliant code example reorders the previous expression so that the left-to-right evaluation order of the operands corresponds with the programmer's intent. Although this code performs as expected, it still represents poor practice by writing to number three times in a single expressionBy diligently following the left to right evaluation order, a programmer can expect this compliant solution to evaluate to the expected final outcome depending on the value returned by the get() method.

number = ((31 * ++number) * (number=get())) + (number > threshold[0] ? 0 : -2);

...

Compliant Solution (Order of Evaluation)

This compliant solution uses an equivalent expression code with no side -effects. This allows the expression to effects and performs not more than one write per expression. The resulting expression can be reordered without concern for the evaluation order of the component expressions, making the code easier to understand and maintain.

final int authnum = get();
number = ((31 * (number + 1)) * get(authnum)) + (get()authnum > threshold[0] ? 0 : -2);

Exceptions

EXP05-J-EX0: The increment and decrement operators (++) and (--) read a numeric variable, and then assign a new value to the variable. Although these operators read and modify a value, they are well-understood and are an exception to this rule. This exception does not apply if a value modified by an increment or decrement operator is subsequently read or written.

EXP05-J-EX1: The conditional-or || and conditional-and && operators have well-understood semantics. Writes followed by subsequent writes or reads do not violate this rule if they occur in different operands of || or &&. Consider the following code example:

public void exampleMethod(InputStream in) {
  int i;
  // Process chars until '' found
  while ((i = in.read()) != -1 && i != '\'' && 
         (i = in.read()) != -1 && i != '\'') {
    // ...
  }

}

This rule is not violated by the controlling expression of the while loop because the rule is not violated by any operand to the conditional-and && operators. The subexpressions (i = in.read()) != -1 have one assignment and one side effect (the reading of a character from in).

Risk Assessment

Failing Failure to keep in mind understand the evaluation order of expressions containing side effects can result in unexpected output.

Automated Detection

TODO

Related Vulnerabilities

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

References

\[[JLS 05|AA. Java References#JLS 05]\] Section 15.7 "Evaluation Order" and 15.7.3 "Evaluation Respects Parentheses and Precedence"

Detection of all expressions involving both side effects and multiple operator precedence levels is straightforward. Determining the correctness of such uses is infeasible in the general case; heuristic warnings could be useful.

Related Guidelines

Bibliography

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Image Added Image Added Image AddedEXP09-J. Use parentheses for precedence of operation      04. Expressions (EXP)      EXP31-J. Avoid side effects in assertions