A composer class is used to map a single complex bean field to multiple database columns. This composition is needed for complex fields that are themselves objects with fields and behavior.
Consider the following example: In your CustomerBean CMP entity bean, you have an attribute called CustomerAddress. The type of the CustomerAddress attribute is a complex type that you defined in a class called Address.class, which contains multiple Java™ fields: street, city, and zip. In order to map the complex CustomerAddress type to different columns in a table, you need a new composer class that defines which fields in the composed type class (Address.class) map to which columns in the database. The composer class composes the Address.class making it possible for you to map the CustomerAddress attribute in CustomerBean.
Use the New Composer wizard to add a new composer definition to your type list. After you create the new composer definition, you can use the composer class in the Mapping editor.
To create a new composer definition: