Usе Expression Language

The NiFi Expression Language always starts with a start delimiter ${ and ends with an end delimiter }. Between the start and end delimiters is the text of the expression itself. In its simplest form, an expression can only consist of an attribute name. For example, ${topic.name} will return the value of the topic name attribute.

To manipulate the value of an attribute, variable, or parameter, the Expression Language uses various functions.

You can replace part of an attribute’s value by calling the replace function — ${topic.name:replace('1', '2')}. In this case, we refer to the topic name attribute, and then change the value of this attribute from 1 to 2.

When a function call is made on an attribute, the attribute is the subject on which the function operates. It is possible to combine 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. There is no limit to the number of functions that can be linked together.

Any FlowFile attribute can be referenced using an expression language. However, if the attribute name contains a "special character", the attribute name must be escaped by enclosing it in quotation marks. The following characters are considered "special characters" — $, |, {, }, (, ), [, ], ,, :, ;, /, *, ', space, \t, \r, \n.

In addition, a number is considered a "special character" if it is the first character of an attribute name. If any of these special characters are present in an attribute, it is enclosed in single or double quotes.

There are also some functions that assume the absence of a subject. These functions are called by a simple function call at the beginning of an expression, such as ${hostname()}. Attempting to evaluate a function with a subject will result in an error.

When using an Expression Language to refer to a property by name, there is a specific hierarchy in which NiFi will look for the value.

The current hierarchy in NiFi looks like this:

NiFi will search and return the first occurrence of the matching property. If no matching property is found, null is returned.

Setting the value of processor properties is more convenient when using the Expression Language editor. As soon as the expression starts with ${, the editor starts to highlight the parentheses and curly braces so that the user can easily tell which open brace or curly brace matches which close brace or curly brace.

The editor also provides context-sensitive help, showing a list of all functions that can be used at the current cursor position. To activate this feature, press Ctrl+Space.

The user can also type part of a function name and then press Ctrl+Space to see all available functions that start with the same prefix:

If you continue typing, you can narrow down the displayed functions. You can also select one of the functions from the list by double-clicking it or using the arrow keys to highlight the desired function and press Enter.

Each function subject value, argument value, and function return value has a specific data type. The type of the function subject (Subject Type), the type of arguments (Arguments), if any, or the return type of the function (Return Type), are specified in the description of each of functions.

The Expression Language supports four different data types:

After evaluating the Expression Language functions, all attributes are stored as type String.

Expression Language is widely used in the NiFi Server interface to set processor properties. Not all processor properties support Expression Language. Information about whether the Expression Language is supported for this processor property is indicated at the top of the window for entering a value under processor settings on the PROPERTIES tab.

Also, when you click on the icon next to the property name, you can see which scope for the Expression Language is supported: