Expressions in the DMN Engine

Decision tables allow specifying different types of expressions. This section describes which types of expressions exist. It lists which expression languages are supported and demonstrates how to change the used expression language for an expression.

Expressions in DMN

As shown in the decision table reference, three types of expressions are supported:

  • Input Expression: sets the input value for an input column of the decision table
  • Input Entry: used to determine if a rule of the decision table is applicable
  • Output Entry: returns a value which is added to the output of a matched rule of the decision table

You can read more on this in the DMN 1.1 reference. In the DMN 1.1 XML, expressions can be found in the following XML elements inputExpression, inputEntry and outputEntry:

<definitions xmlns="http://www.omg.org/spec/DMN/20151101/dmn11.xsd" id="definitions" name="definitions" namespace="http://camunda.org/schema/1.0/dmn">
  <decision id="decision" name="Decision">
    <decisionTable>
      <input id="input">
        <!-- the input expression determines the input value of a column -->
        <inputExpression>
          <text>age</text>
        </inputExpression>
      </input>
      <output id="output"/>
      <rule id="rule1">
      <!-- the input entry determines if the rule is applicable -->
        <inputEntry>
          <text>[18..30]</text>
        </inputEntry>
        <!-- the output entry determines the rule if it is applicable -->
        <outputEntry>
          <text>"okay"</text>
        </outputEntry>
      </rule>
    </decisionTable>
  </decision>
</definitions>

Supported Expression Languages

The Camunda DMN engine supports two expression languages out of the box:

Depending on the JDK you use there may also be a Javascript implementation available like Rhino or Nashhorn.

You can also use every other script language which provides a JSR-223 implementation. This includes groovy, python and ruby. To use these languages you have to add the corresponding dependency to your project.

For example, to use groovy as language for expressions add this dependency to your project pom.xml:

<dependency>
  <groupId>org.codehaus.groovy</groupId>
  <artifactId>groovy-all</artifactId>
  <!-- please update this version if needed -->
  <version>2.4.5</version>
</dependency>

Default Expression Languages

The default expression languages of the different expression types in the DMN engine are as follows:

  • Input Expression: JUEL
  • Input Entry: FEEL
  • Output Entry: JUEL

The default language can be changed by setting it directly in the DMN 1.1 XML as global expression language with the expressionLanguage attribute of the definitions element:

<!-- this sets the default expression language for all expressions -->
<!-- in this file to javascript -->
<definitions xmlns="http://www.omg.org/spec/DMN/20151101/dmn11.xsd" id="definitions" name="definitions" namespace="http://camunda.org/schema/1.0/dmn" expressionLanguage="javascript">
  <decision  id="decision" name="Decision">
    <decisionTable>
      <!-- ... -->
    </decisionTable>
  </decision>
</definitions>

Additionally, it is possible to change the default expression language in the default DMN engine configuration as described in the user guide.

Configuring the Expression Language

It is also possible to set the language for each expression individually using the expressionLanguage attribute:

<definitions xmlns="http://www.omg.org/spec/DMN/20151101/dmn11.xsd" id="definitions" name="definitions" namespace="http://camunda.org/schema/1.0/dmn">
  <decision id="decision" name="Decision">
    <decisionTable>
      <input id="input">
        <!-- use javascript for this input expression -->
        <inputExpression expressionLanguage="javascript">
          <text>age</text>
        </inputExpression>
      </input>
      <output id="output"/>
      <rule id="rule1">
        <!-- use juel for this input entry -->
        <inputEntry expressionLanguage="juel">
          <text><![CDATA[cellInput >= 18 && cellInput <= 30]]></text>
        </inputEntry>
        <!-- use javascript for this output entry -->
        <outputEntry expressionLanguage="javascript">
          <text>"okay"</text>
        </outputEntry>
      </rule>
    </decisionTable>
  </decision>
</definitions>

If you want to use another Java Unified Expression Language or FEEL implementation, you can replace the default implementations in the DMN engine configuration. This way you can also change the JSR-223 script engine resolving, for example if you want to configure the script engine before using it.

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