Variable mappings allow you to map the value of a variable definition within a Dozer configuration into a target field instead of using the value of a source field.  This is equivalent to constant mapping in other mapping frameworks, where can you assign a literal value to a target field.  To use a variable mapping, simply define a variable within your mapping configuration and then map from the VariableMapper class into your target field of choice:

<mappings xmlns="http://dozermapper.github.io/schema/bean-mapping" 
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://dozermapper.github.io/schema/bean-mapping https://dozermapper.github.io/schema/bean-mapping.xsd">
  <configuration>
    <variables>
      <variable name="CUST_ID">ACME-SALES</variable>
    </variables>
  </configuration>
  <mapping>
    <class-a>org.apache.camel.component.dozer.VariableMapper</class-a>
    <class-b>org.example.Order</class-b>
    <field custom-converter-id="_variableMapping" custom-converter-param="${CUST_ID}">
      <a>literal</a>
      <b>custId</b>
    </field>
  </mapping>
</mappings>

Custom mappings allow you to define your own logic for how a source field is mapped to a target field.  They are similar in function to Dozer customer converters, with two notable differences:

A custom mapping is declared by using the built-in '_customMapping' converter in your mapping configuration.  The parameter to this converter has the following syntax:

[class-name][,method-name]

Method name is optional - the Dozer component will search for a method that matches the input and output types required for a mapping.  An example custom mapping and configuration are provided below.

public class CustomMapper {
    // All customer ids must be wrapped in "[ ]"
    public Object mapCustomer(String customerId) {
        return "[" + customerId + "]";
    }
} 

<mappings xmlns="http://dozermapper.github.io/schema/bean-mapping" 
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://dozermapper.github.io/schema/bean-mapping https://dozermapper.github.io/schema/bean-mapping.xsd">
  <mapping>
    <class-a>org.example.A</class-a>
    <class-b>org.example.B</class-b>
    <field custom-converter-id="_customMapping"
        custom-converter-param="org.example.CustomMapper,mapCustomer">
      <a>header.customerNum</a>
      <b>custId</b>
    </field>
  </mapping>
</mappings>

Expression mappings allow you to use the powerful language capabilities of Camel to evaluate an expression and assign the result to a target field in a mapping.  Any language that Camel supports can be used in an expression mapping.  Basic examples of expressions include the ability to map a Camel message header or exchange property to a target field or to concatenate multiple source fields into a target field.  The syntax of a mapping expression is:

[language]:[expression]

An example of mapping a message header into a target field:

<mappings xmlns="http://dozermapper.github.io/schema/bean-mapping" 
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://dozermapper.github.io/schema/bean-mapping https://dozermapper.github.io/schema/bean-mapping.xsd">
  <mapping>
    <class-a>org.apache.camel.component.dozer.ExpressionMapper</class-a>
    <class-b>org.example.B</class-b>
    <field custom-converter-id="_expressionMapping" custom-converter-param="simple:\${header.customerNumber}">
      <a>expression</a>
      <b>custId</b>
    </field>
  </mapping>
</mappings>

Note that any properties within your expression must be escaped with "\" to prevent an error when Dozer attempts to resolve variable values defined using the EL.