The NiFi Expression Language always begins with the start delimiter ${ and ends with the end delimiter }. Between the start and end delimiters is the text of the Expression itself. In its most basic form, the Expression can consist of just an attribute name. For example, ${filename} will return the value of the "filename" attribute.

In a slightly more complex example, we can instead return a manipulation of this value. We can, for example, return an all upper-case version of the filename by calling the toUpper function: ${filename:toUpper()}. In this case, we reference the "filename" attribute and then manipulate this value by using the toUpper function. A function call consists of 5 elements. First, there is a function call delimiter :. Second is the name of the function - in this case, "toUpper". Next is an open parenthesis ((), followed by the function arguments. The arguments necessary are dependent upon which function is being called. In this example, we are using the toUpper function, which does not have any arguments, so this element is omitted. Finally, the closing parenthesis ()) indicates the end of the function call. There are many different functions that are supported by the Expression Language to achieve many different goals. Some functions provide String (text) manipulation, such as the toUpper function. Others, such as the equals and matches functions, provide comparison functionality. Functions also exist for manipulating dates and times and for performing mathematical operations. Each of these functions is described below, in the Functions section, with an explanation of what the function does, the arguments that it requires, and the type of information that it returns.

When we perform a function call on an attribute, as above, we refer to the attribute as the subject of the function, as the attribute is the entity on which the function is operating. We can then chain together multiple function calls, where the return value of the first function becomes the subject of the second function and its return value becomes the subject of the third function and so on. Continuing with our example, we can chain together multiple functions by using the expression ${filename:toUpper():equals('HELLO.TXT')}. There is no limit to the number of functions that can be chained together.

Any FlowFile attribute can be referenced using the Expression Language. However, if the attribute name contains a "special character," the attribute name must be escaped by quoting it. The following characters are each considered "special characters":

Additionally, a number is considered a "special character" if it is the first character of the attribute name. If any of these special characters is present in an attribute is quoted by using either single or double quotes. The Expression Language allows single quotes and double quotes to be used interchangeably. For example, the following can be used to escape an attribute named "my attribute": ${"my attribute"} or ${'my attribute'}.

In this example, the value to be returned is the value of the "my attribute" value, if it exists. If that attribute does not exist, the Expression Language will then look for a System Environment Variable named "my attribute." If unable to find this, it will look for a JVM System Property named "my attribute." Finally, if none of these exists, the Expression Language will return a null value.

There also exist some functions that expect to have no subject. These functions are invoked simply by calling the function at the beginning of the Expression, such as ${hostname()}. These functions can then be changed together, as well. For example, ${hostname():toUpper()}. Attempting to evaluate the function with subject will result in an error. In the Functions section below, these functions will clearly indicate in their descriptions that they do not require a subject.

Often times, we will need to compare the values of two different attributes to each other. We are able to accomplish this by using embedded Expressions. We can, for example, check if the "filename" attribute is the same as the "uuid" attribute: ${filename:equals( ${uuid} )}. Notice here, also, that we have a space between the opening parenthesis for the equals method and the embedded Expression. This is not necessary and does not affect how the Expression is evaluated in any way. Rather, it is intended to make the Expression easier to read. White space is ignored by the Expression Language between delimiters. Therefore, we can use the Expression ${ filename : equals(${ uuid}) } or ${filename:equals(${uuid})} and both Expressions mean the same thing. We cannot, however, use ${file name:equals(${uuid})}, because this results in file and name being interpreted as different tokens, rather than a single token, filename.