var fsb = new FancyStringBuilder();

fsb.l("package %s;", targetPackage);
fsb.n();
fsb.l("import lombok.Data;");
fsb.n();
fsb.l("/**").l(spec.getDescription()).l("*/");
fsb.l("@Data");
fsb.l("public class %s ", spec.getName()).a("{").bb();
for (var field : spec.getFields()) {
    fsb
        .l("/**")
        .l(field.getDescription()).l("*/")
        .l("private final %s %s;", field.getType(), field.getName())
        .n();
}
fsb.eb().l("}");

return fsb.toString();

import static butvinm.fsb.FancyStringBuilder.fsb;

...
System.out.println(fsb().l("Hello, %s!", username));