29.5. Predicates

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You can construct predicates by testing expressions for equality. For example, the predicate, simple("${header.timeOfDay} == '14:30'"), tests whether the timeOfDay header in the incoming message is equal to 14:30.
In addition, whenever the resultType is specified as a Boolean the expression is evaluated as a predicate instead of an expression. This allows the predicate syntax to be used for these expressions.


You can also test various parts of an exchange (headers, message body, and so on) using simple predicates. Simple predicates have the following general syntax:
${LHSVariable} Op RHSValue
Where the variable on the left hand side, LHSVariable, is one of the variables shown in Table 29.1, “Variables for the Simple Language” and the value on the right hand side, RHSValue, is one of the following:
  • Another variable, ${RHSVariable}.
  • A string literal, enclosed in single quotes, ' '.
  • A numeric constant, enclosed in single quotes, ' '.
  • The null object, null.
The simple language always attempts to convert the RHS value to the type of the LHS value.
While the simple language will attempt to convert the RHS, depending on the operator the LHS may need to be cast into the appropriate Type before the comparison is made.


For example, you can perform simple string comparisons and numerical comparisons as follows:
simple("${in.header.user} == 'john'")

simple("${in.header.number} > '100'")  // String literal can be converted to integer
You can test whether the left hand side is a member of a comma-separated list, as follows:
simple("${in.header.type} in 'gold,silver'")
You can test whether the left hand side matches a regular expression, as follows:
simple("${in.header.number} regex '\d{4}'")
You can test the type of the left hand side using the is operator, as follows:
simple("${in.header.type} is 'java.lang.String'")
simple("${in.header.type} is 'String'") // You can abbreviate java.lang. types
You can test whether the left hand side lies in a specified numerical range (where the range is inclusive), as follows:
simple("${in.header.number} range '100..199'")


You can also combine predicates using the logical conjunctions, && and ||.
For example, here is an expression using the && conjunction (logical and):
simple("${in.header.title} contains 'Camel' && ${in.header.type} == 'gold'")
And here is an expression using the || conjunction (logical inclusive or):
simple("${in.header.title} contains 'Camel' || ${in.header.type} == 'gold'")
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