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This revision made January 06, 2012 21:29, by Kin-man Chung
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Operators

Several operators are introduced in EL 3.0.

String Concatenation

Syntax

 A & B
  1. Coerce A and B to String. Note: according 1.18.2, a null is coerced to "".
  2. Return the concatenated string of A and B.

Operator Precedence

The precedence for & is lower than "+" and "-" (binary) but higher than logical operators (== != etc.). It is left-associative, i.e. a & b & c is (a & b)&c.

Example

1 & 2 returns "12"

"1" & "2" returns "12"

Assignment

Syntax

expr-a = expr-b

  • If expr-a is an identifier (name-a) and name-a is an EL variable (i.e. it is defined in the VariableMapper), or if name-a is not resolved by the ELResolver
    • Set name-a in the VariableMapper with the ValueExpression for expr-b, without evaluation.
    • Evaluate expr-b to value-b.
  • Else
    • Evaluate expr-b to value-b.
    • Evaluate expr-a, up to (but not including) the last property resolution.
    • If expr-a is a . or [] operator, evaluate the base to base-a, and the property to prop-a.
      • If base-a is null, throw a BeanNotFoundException.
      • If prop-a is null, throw a PropertyNotFoundException.
    • Invoke ValueExpression.setValue for expr-a: expr-a.setValue(context, value-b);
  • If ValueEpxression.getValue was called to initiate this expression evaluation, return value-b. In other words, the result of evaluating the assignment is the value of the right hand side.
  • If ValueExpression.setValue was called to initiate this expression evaluation, throw a PropertyNotWritableException. An assignment expression cannot be the target of setValue.

Note that EL variables are bound at parse time, so their assignment in an expression only takes effect after the expression is evaluated. This can lead to some unexpected behaviors. For example, consider the following.

  elp = new ELProcessor();
  elp.setVariable("x", "10");
  int n = elp.getValue("(x = 20) + x}";

The value of n is 30 instead of 40, because the value of x is 10 when the expression is parsed.

Operator Precedence

It is lower than the ?: operator, but higher than the semicolon operator. It is also right-associative, i.e. a = b = c is a = (b = c).

Example

  x = a.b
  If x does not exist then same as calling defineVariable("x", "a.b");
  x.y = a.b
  Set x.y = a.b. Error if x does not exist.

SemiColon Operator

Syntax

expr-a ";" expr-b

  • expr-a is evaluated and the resulting value is discarded.
  • If ValueEpxression.getValue was called to initiate this expression evaluation, expr-b is evaluated and the resulting value is returned.
  • If ValueExpression.setValue was called to initiate this expression evaluation, expr-b.setValue is called.

Operator Precedence

The semiColon operator has the lowest precedence of all operators.

Example

Difference compared to previous revision
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