Joist uses code-generated static inner classes, called “Shims”, to access the fields of domain objects.

Each shim is an instance assigned to a static final field in FooCodegen.Shims.fieldName.

Because shims are instances, they can be put in lists and passed around as meta-data to the data mapper for meta-data-based mapping. This means that Joist’s code generation does not have the complex/messy task of generating low-level SQL SELECT/UPDATE strings, but instead simply generates the meta-data that, at runtime, drives the meta data-based mapping.


For example:

    public abstract class FooCodegen {
      private String name;

      // public getters/setters
      public String getName() {
      public void setName(String name) {
 = name;

      // technically public, but "don't do that"
      public static class Shims {
        public static final Shim<Child, java.lang.String> name = new Shim<Child, java.lang.String>() {
          public void set(Child instance, java.lang.String name) {
            ((ChildCodegen) instance).name = name;
          public String get(Child instance) {
            return ((ChildCodegen) instance).name;
    } is then referenced in FooAlias’s list of columns for it to load/save from Foo when needed.