User Defined Java Expression

User Defined Java Expression

Description This transform allows you to enter Java expressions that are used to calculate new values or replace values in fields. Any custom jar libraries that your expressions depend on need to be placed in the folder plugins/transforms/janino/lib	Supported Engines Hop Engine Spark Flink Dataflow
Hop Engine
Spark
Flink
Dataflow

Usage

If you have a Java expression like :

C=A+B

Then you can simply enter the right side of the expression in the dialog:

A+B

The values are exposed to the expressions as the Java objects they are :

Data type	Java Class
BigNumber	BigDecimal
Binary	byte[]
Date	java.util.Date
Integer	java.lang.Long
Number	java.lang.Double
String	java.lang.String

Options

Option	Description
Transform name	Specify a name for this transform. This must be unique wihin the pipeline.
New field	The new field in the data stream that contains the result of the expression. If you want to overwrite an existing field, you need to define the same field here and in the "Replace value" option.
Java expression	The Java expression to evaluate. See examples below.
Value type	The field’s data type.
Length	The maximum length of a string in the new field.
Precision	The number of decimal places used to express numbers.
Replace value	Set this to be identical to the "New field" name when you want to replace the value in an existing field.

Examples

Add 2 integers, A and B

A+B

Concatenate 2 Strings : firstname and name and put a space in between

firstname+" "+name

or if you really care about performance, this might be faster:

new StringBuffer(firstname).append(" ").append(name).toString()

Use native Java and API functions

System.getProperty("os.name")

Business rules (If / Then / Else)

a<c?true:false

This can be more complicated

a<c?(a==1?1:2):3

even with OR and AND and other operators and functions

Using Constants

If you use a constant, you may need to define the right type in some expressions otherwise it could throw an error. Eg if you use "0" that is "int" but Apache Hop "Integer" is "java.lang.Long", so you’ll get an exception:

Incompatible expression types "int" and "java.lang.Long"

To solve this, use:

test == null ? new Long(0) : test

The above expression checks if "test" is null and replaces it with a zero Long. Otherwise, it return "test" unchanged.

Cut a string from end and test for null and minimal length

Imagine you have input strings "location" like

Orlando FL
New York NY

and you want to separate the state and city. You could use the following expressions:

For state (get the last 2 characters):

location != null && location.length()>2 ? location.substring(location.length()-2, location.length()) : null

For city (get the beginning without the last 2 characters and trim):

location != null && location.length()>2 ? location.substring(0, location.length()-2).trim() : location

Functionality of a LIKE operator (contains string)

The following example returns 1 when abc is within the source string, otherwise 2. It returns also 2 when the source string is null. If you prefer the return values to be of value type Integer, use "new Long(1)" and "new Long(2)".

samplestr != null && samplestr.indexOf("abc")>-1 ? 1 : 2

Blocking specific code

As a simple security measure you can block the execution of code containing specific strings. This can be done by adding exclusions to the codeExclusions.xml file located at <Hop Installation>/plugins/transforms/janino

Example:

    <exclusions>
        <exclusion>System.</exclusion>
        <exclusion>HopVfs.</exclusion>
    </exclusions>

Last updated 2025-09-04 18:23:22 +0200