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Expression Language Basic Usage

Syntax

Expressions

Expressions are enclosed in curly braces {} and begin with the # symbol. Both dot notation and bracket notation are supported for accessing the properties of an object.

Example: {#context.attributes['user'].email}

note

Dot notation will not work with special characters:

{#request.headers.my-header} will result in an error.

Bracket notation should be used for property names that include a space or a hyphen, or start with a number:

{#request.headers['my-header']}

Lists

  • Expressions can be used to assign lists, for example, {({'admin', 'writer'})}

  • The outer enclosing brackets start and end the EL expression.

  • The parentheses indicates an object is being instantiated.

  • The list comprises the inner brackets and enclosed values, for example, {'admin', 'writer'}.

Object Properties

EL allows you to reference certain values injected into the EL context as object properties.

  • {#endpoints}: Contains information about the Gateway API's respective endpoints.
  • {#request}: Contains information about the current API request.
  • {#response}: Contains information about the current API response.

Attributes

The attributes object property contains attributes that are automatically created by the APIM Gateway during an API transaction or added during the execution phase through the Assign Attributes policy.

  • {#context.attributes}: Contains attributes associated with Proxy APIs.

Operators

EL supports various operators, such as arithmetic, logical, comparison, and ternary operators. Examples of commonly used operators include:

  • Arithmetic operators: +, -, *, /
  • Logical operators: && (logical and), || (logical or), ! (logical not)
  • Comparison operators: ==, !=, <, <=, >, >=
  • Ternary operators: condition ? expression1 : expression2

Functions

EL provides a variety of built-in functions to manipulate and transform data in expressions. Examples of commonly used functions include:

  • String functions: length(), substring(), replace()
  • #jsonPath: Evaluates a jsonPath on a specified object. This function invokes JsonPathUtils.evaluate(…​), which delegates to the Jayway JsonPath library. The best way to learn jsonPath syntax is by using the online evaluator.
  • #xpath: To evaluate an xpath on some provided object. For more information regarding XML and XPath, see XML Support - Dealing with XML Payloads.

jsonPath example

As an example of how jsonPath can be used with EL, if you have a JSON payload in the request body that contains the following data:

{
"store": {
"book": [
  {
    "category": "fiction",
    "author": "Herman Melville",
    "title": "Moby Dick",
    "isbn": "0-553-21311-3",
    "price": 8.99
  },
  {
    "category": "fiction",
    "author": "J. R. R. Tolkien",
    "title": "The Lord of the Rings",
    "isbn": "0-395-19395-8",
    "price": 22.99
  }
]
}
}

To extract the value of the price property for the book with title "The Lord of the Rings," you can use the following expression:

{#jsonPath(#request.content, "$.store.book[?(@.title=='The Lord of the Rings')].price")}
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