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Actions have four parts to them (responsibliities, I suppose):

  • execution
  • supporting methods for perform the validation etc
  • target object that they act upon
  • event publishing

Our programming model could provide several syntaxes to put these responsibilities in different places.  It already supports two (standard actions, and mixins).To compare these currently provides two different syntaxes: standard, and mixins.  This page recaps on those options and suggests several others.  To compare the syntaxes, we'll use a concrete example:

...

Code Block
public class Customer {                                                              // target

    @Action
    public staticCustomer class PlaceOderEvent extends ActionDomainEvent<Customer> {}  placeOrder(Product p, int quantity) { ... }                      // action execution

    public boolean hidePlaceOrder() {     

 ... }   @Action(domainEvent = PlaceOrderEvent.class)                                     // eventsupporting publishingmethods
    public CustomerString placeOrder(Product p, int quantitydisablePlaceOrder() { ... }
    public String disable1PlaceOrder(Product p) { ... }
    public Collection<Product> choices0PlaceOrder() {     // action execution

... }
    public booleanProduct hidePlaceOrderdefault0PlaceOrder() { ... }
    public int default1PlaceOrder() {                                   // supporting methods... }
    public String disablePlaceOrdervalidate1PlaceOrder(int quantity) { ... }
    public String disable1PlaceOrdervalidatePlaceOrder(Product p, int quantity) { ... }
}


Mixins syntax

Mixins change the target, by allowing this set of methods to be moved to a different object.  In other words, the target responsibility changes.

So the real target is simply:

Code Block
public class Customer {}


and the action itself moves onto the mixin:

Code Block
@Action      public Collection<Product> choices0PlaceOrder() { ... }
    public Product default0PlaceOrder() { ... }
    public int default1PlaceOrder                                         
public class Customer_placeOrder() { ...  }
    public String validate1PlaceOrder(int quantity) { ... }
    public String validatePlaceOrder(Product p, int quantity) { ... }
}

...

Mixins change the target, by allowing this set of methods to be moved to a different object.  In other words, the target responsibility changes.

So the real target is simply:

Code Block
public class Customer {}

and the action itself moves onto the mixin:

Code Block
@Action      

    private final Customer target;                                                // target
    
public class Customer_placeOrder()Customer target) { ... }

    public Customer act(Product p, int quantity) { ... }                          // action execution

    privatepublic finalboolean Customer target;  hideAct() { ... }                                              // targetsupporting methods
    public Customer_placeOrder(Customer targetString disableAct() { ... }

    public staticString class PlaceOrderEvent extends ActionDomainEvent<Customer> {}disable1Act(Product p) { ... }
    public Collection<Product> choices0Act() { ... }
    public Product default0Act() { ... }
    public int default1Act() { ...}
    public String validate1Act(int quantity) { ... }
  

  public String @Action(domainEvent = PlaceOrderEvent.class)                             validateAct(Product p, int quantity) { ... }
}

Notes:

  • Instead of @Action, the @Mixin(method="act") could also be used, with additional annotations on the act(...) method.  I've chosen the version with the least boilerplate here.
  • Mixins are also used for derived properties or collections (ie query-only actions with no side-effects).  These are specified using @Property or @Collection 
  • We now have two classes here: the mixin, and the domain event within.

Analysis

Actions have three parts (or responsibilities) to them:

  • execution
  • the target object that they act upon
  • the set of parameters/arguments that are passed to the execution and to the supporting methods that perform validation etc.

(Actually, there's also event publishing, and an earlier version of this page also discussed that ... but we didn't see any point in changing how that worked).

The standard model and the mixin model have a quite different "feel" to them, though they only subtly change where these responsibilities reside: for the standard model, the target object is implicit (ie "this") whereas with mixins the target object is explicit (passed into the constructor).  In other respects the programming models are the same.

Playing around with where these responsibilities live allow us to create a number of other programming models.  The table below summarises and names these options::


targetbehaviourparameter
values		Notes
standardimplicitY
The target is implicit ("this"), and the set of parameter values (arguments) are only implicit in the signatures of the execute action and the supporting methods
mixinsYY
The target is explicit, being the constructor of the mixin.
Parameters model

YSeparate class that captures the set of parameters that are passed to the supporting methods
Parameters on Act

YMinor variation
Parameters Everywhere

YAnother variation
Mixins + ParametersYYYCombines the concepts of a mixin along with a parameters model
Targetless Mixins +
Targeted Parameters
Y		Y
Y		Splits out state and behaviour
Command handlers 
Commands
Y		Y
Y		Variation that splits behaviour into separate interfaces

The rest of the page describes these options in more detail.

Parameters model syntax (proposed)

Per this thread on slack, we could introduce a Parameters object (in Java 14+, this might be a record) to bring together all of the parameters into a single object.  This would make it easier to avoid issues with numbering etc.

This syntax changes the way in which supporting methods are associated back to the main execution method.

Code Block
languagejava
public class Customer {                                                           // target

    @Value @Accessors(fluent = true)                   
    public class PlaceOrderParameters {                                           // to assist supporting methods 
        Product product;
        int quantity;
    }

    @Action
    public Customer placeOrder(Product p, int quantity) { ... }                   // execution    

    public boolean hidePlaceOrder() { ... }                                       // supporting methods use PlaceOrderParameters
    public String disablePlaceOrder() { ... }
    public String disable1PlaceOrder(PlaceOrderParameters params) { ... }
    public Collection<Product> choices0PlaceOrder() { ... }              
    public Product default0PlaceOrder() { ... }
    public int default1PlaceOrder() { ... }
    public String validate1PlaceOrder(PlaceOrderParameters params) { ... }
    public String validatePlaceOrder(PlaceOrderParameters params) { ... }
}

Notes:

  • The @Value @Accessors(fluent=true) allows us to use a syntax that is very similar to Java 14 records.
  • There is some duplication here: the list of the parameter types appears both in the placeOrder(...) method, as well as in the PlacerOrdersParameters class.

The above would also be supported with mixins:

Code Block
@Action
public class Customer_placeOrder {

    private final Customer target;                                                // target
    public Customer_placeOrder(Customer target) { ... }

    @Value @Accessors(fluent = true)             
    public static class PlaceOrderParameters {                                    // to assist supporting methods
        Product product;
        int quantity;
    }

    public Customer act(Product p, int quantity) { ... }                          // execution

    public boolean hideAct() { ... }                                              // supporting methods
    public String disableAct() { ... }
    public String disable1Act(PlaceOrderParameters params) { ... }
    public Collection<Product> choices0Act() { ... }              
    public Product default0Act() { ... }
    public int default1Act() { ... }
    public String validate1Act(PlaceOrderParameters params) { ... }
    public String validateAct(PlaceOrderParameters params) { ... }
}

Notes:

  • we now have three classes here: the mixin, the domain event, and the parameters object.

Parameters on Act syntax (proposed)

This is a variant of the previous, but uses the parameters class in the action as well:

Code Block
public class Customer {                                                           // target

    @Value @Accessors(fluent = true)                   
    public class PlaceOrderParameters {                                           // to assist supporting methods 
        @Parameter() @MemberOrder(1)
        Product product;
        @Parameter() @MemberOrder(2)
        int quantity;
    }

    @Action
    public Customer placeOrder(PlaceOrderParameters params) { ... }               // execution    

    public boolean hidePlaceOrder() { ... }                                       // supporting methods use PlaceOrderParameters
    public String disablePlaceOrder() { ... }
    public String disable1PlaceOrder(PlaceOrderParameters params) { ... }
    public Collection<Product> choices0PlaceOrder() { ... }              
    public Product default0PlaceOrder() { ... }
    public int default1PlaceOrder() { ... }
    public String validate1PlaceOrder(PlaceOrderParameters params) { ... }
    public String validatePlaceOrder(PlaceOrderParameters params) { ... }
}

Notes:

  • this removes the duplication between the placeOrder(...) parameter list and the list of members in PlaceOrderParameters class.
  • the @Parameter and @MemberOrder syntax would be required by the framework to identify PlaceOrderParameters as a container of parameters (as opposed to a reference object or custom value type)

As a mixin, this becomes:

Code Block
@Action
public class Customer_placeOrder {

    private final Customer target;                                                // target
    public Customer_placeOrder(Customer target) { ... }

    @Value @Accessors(fluent = true)             
    public static class PlaceOrderParameters {                                    // to assist supporting methods
        @Parameter()
        Product product;
        @Parameter()
        int quantity;
    }

    @Action
    public Customer act(PlaceOrderParameters params) { ... }                             // execution

    public boolean hideAct() { ... }                                              // supporting methods
    public String disableAct() { ... }
    public String disable1Act(PlaceOrderParameters params) { ... }
    public Collection<Product> choices0Act() { ... }              
    public Product default0Act() { ... }
    public int default1Act() { ... }
    public String validate1Act(PlaceOrderParameters params) { ... }
    public String validateAct(PlaceOrderParameters params) { ... }
}

Notes:

  • we still have three classes here (mixin, parameters and domain event), but we have removed the duplication between the act(...) parameter list and the list of members of PlaceOrderParameters class

Parameters everywhere syntax (proposed)

The previous syntax only passes in parameters to some of the supporting methods.  For consistency, we could imagine it being passed in always.

Just focusing on the mixin syntax, this would become:

Code Block
@Action
public class Customer_placeOrder {

    private final Customer target;                                                // target
    public Customer_placeOrder(Customer target) { ... }

    @Value @Accessors(fluent = true)             
    public static class PlaceOrderParameters {                                    // to assist supporting methods
        @Parameter()
        Product product;
        @Parameter()
        int quantity;
    }

    @Action
    public Customer act(PlaceOrderParameters params) { ... }                      // execution

    public boolean hideAct(PlaceOrderParameters params) { ... }                   // supporting methods
    public String disableAct(PlaceOrderParameters params) { ... }
    public String disable1Act(PlaceOrderParameters params) { ... }
    public Collection<Product> choices0Act(PlaceOrderParameters params) { ... }              
    public Product default0Act(PlaceOrderParameters params) { ... }
    public int default1Act(PlaceOrderParameters params) { ... }
    public String validate1Act(PlaceOrderParameters params) { ... }
    public String validateAct(PlaceOrderParameters params) { ... }
}

Discussion

With the parameters object passed in everywhere, I could see myself starting to move functionality onto that object.  So as an idiom, we might see the following sort of code (in a mixin):

Code Block
@Action
public class Customer_placeOrder {

    private final Customer target;                                                
    public Customer_placeOrder(Customer target) { ... }

    @Value @Accessors(fluent = true)             
    public static class PlaceOrderParameters { ... }                                       // see below.                                   

    public static class PlaceOrderEvent extends ActionDomainEvent<Customer> {}                                      

    @Action
    public Customer act(PlaceOrderParameters params) { return params.act(this); } 

    public boolean hideAct(PlaceOrderParameters params) { return params.hide(this); }                                              
    public String disableAct(PlaceOrderParameters params) { return params.disable(this); }
    public String disable1Act(PlaceOrderParameters params) { return params.disable1(this); }
    public Collection<Product> choices0Act(PlaceOrderParameters params) { return params.choices0(this); }              
    public Product default0Act(PlaceOrderParameters params) { return params.default0(this); }
    public int default1Act(PlaceOrderParameters params) { return params.default1(this); }
    public String validate1Act(PlaceOrderParameters params) { return params.validate1(this); }
    public String validateAct(PlaceOrderParameters params) { params.validate(this); }
}

which would then beef up the parameters object:

Code Block
@Action
public class Customer_placeOrder {

    private final Customer target;                                         // target
    ...

    @Value @Accessors(fluent = true)             
    public static class PlaceOrderParameters {                                    

        @Parameter()
        Product product;
        @Parameter()
        int quantity;

	    public Customer act(Customer customer) { ... }                     // execution

    	public boolean hideAct(Customer customer) { ... }                  // supporting methods                
	    public String disableAct(Customer customer) { ... }
	    public String disable1Act(Customer customer) { ... }
	    public Collection<Product> choices0Act(Customer customer) { ... }              
	    public Product default0Act(Customer customer) { ... }
	    public int default1Act(Customer customer) { ... }
	    public String validate1Act(Customer customer) { ... }
	    public String validateAct(Customer customer) { ...}
    }

    ...

    @Action
    public Customer act(PlaceOrderParameters params) { return params.act(this); }                         // remainder is just boilerplate

    public boolean hideAct(PlaceOrderParameters params) { return params.hide(this); }                                              
    public String disableAct(PlaceOrderParameters params) { return params.disable(this); }
    public String disable1Act(PlaceOrderParameters params) { return params.disable1(this); }
    public Collection<Product> choices0Act(PlaceOrderParameters params) { return params.choices0(this); }              
    public Product default0Act(PlaceOrderParameters params) { return params.default0(this); }
    public int default1Act(PlaceOrderParameters params) { return params.default1(this); }
    public String validate1Act(PlaceOrderParameters params) { return params.validate1(this); }
    public String validateAct(PlaceOrderParameters params) { params.validate(this); }
}

Notes:

  • the target is still outside of the parameters object
  • Event publishing also outside
  • Everything else has moved inside the parameters object
  • This implies that we would need dependency injection for the parameters object
  • The rest of the code in the mixin is just boilerplate.  It's possible that the Lombok @Delegate annotation might be used to remove some of this boilerplate, didn't investigate further.

Mixins and Parameters combined (proposed)

The previous section describes an idiom to work within the new Parameter object programming model.  But the next step along the journey would be to formally recognise this pattern.  This would amount to collapsing the mixin concept and the parameters concept into the same thing.  Said another way, mixins start to become stateful, keeping track of the parameter argument values as well as the target object:

Code Block
@Action
public class Customer_placeOrder {

    private final Customer target;                                              // target
    ...

    @Parameter() @MemberOrder(1)                                                // supporting methods support
    Product product;
    @Parameter() @MemberOrder(2)
    int quantity;

    @Action
    public Customer act() { ... }                                               // execution 

    public boolean hideAct() { ... }                                            // supporting methods
    public String disableAct() { ... }
    public String disable1Act() { ... }
    public Collection<Product> choices0Act() { ... }              
    public Product default0Act() { ... }
    public int default1Act() { ... }
    public String validate1Act() { ... }
    public String validateAct() { ... }
}

Notes:

  • here the supporting methods would simply read from the fields of the mixin that represent the parameters of the mixin itself.
  • the domain event class is still separate
  • @MemberOrder is required because the JVM does not guarantee the order in the bytecode is the same as in the source file.

Target-less Mixins + Targeted Parameters

Traditionally mixins hold all of the behaviour and a little bit of the state - namely the target object.    Meanwhile parameters hold the rest of the state, but without the target.

Another way to divide the responsibilities would be to move the target from the mixin, and add it into the parameters object.  In other words, the former would just be the behaviour, the latter would be just the state.

Thus we have an extended parameters object, that also takes the target:

Code Block
@Value @Accessors(fluent = true)             
public class PlaceOrderParameters {
    @Target                          // a new annotation
    Customer customer;

    @Parameter()
    Product product;
    @Parameter()
    int quantity;
}

Meanwhile the mixin provides just the behaviour, of both the action and also the various supporting methods.  The supporting methods all need to take the PlaceOrderParameters, because it now contains the target, at least 

Code Block
@Action
public class Customer_placeOrder {

    @Action
    public Customer act(PlaceOrderParameters params) { ... }     // event publishing
    public Customer act(Product p, int quantity) { ... }                          // action execution

    public boolean hideAct() { ... }                                              // supporting methodsexecution

    public Stringboolean disableActhideAct(PlaceOrderParameters params) { ... }                          // supporting methods
    public String disable1ActdisableAct(ProductPlaceOrderParameters pparams) { ... }
    public String Collection<Product> choices0Act(disable1Act(PlaceOrderParameters params) { ... }
    public ProductCollection<Product> default0Actchoices0Act(PlaceOrderParameters params) { ... }
          public int default1Act() { ...}
    public StringProduct validate1Actdefault0Act(intPlaceOrderParameters quantityparams) { ... }
    public Stringint validateActdefault1Act(Product p, int quantityPlaceOrderParameters params) { ... }
}

Notes:

  • Instead of @Action, the @Mixin(method="act") could also be used, with additional annotations on the act(...) method.  I've chosen the version with the least boilerplate here.
  • Mixins are also used for derived properties or collections (ie query-only actions with no side-effects).  These are specified using @Property or @Collection 
  • We now have two classes here: the mixin, and the domain event within.

Parameters syntax (proposed)

Per this thread on slack, we could introduce a Parameters object (in Java 14+, this might be a record) to bring together all of the parameters into a single object.  This would make it easier to avoid issues with numbering etc.

This syntax changes the way in which supporting methods are associated back to the main execution method.

    public String validate1Act(PlaceOrderParameters params) { ... }
    public String validateAct(PlaceOrderParameters params) { ... }
}

Command Handlers

Building on the previous example, having split up the behaviour from the state completely, we realise that there's no need to keep all of the methods of the mixin together.

We could rename the "parameters object" as a command:

Code Block
@Value @Accessors(fluent = true)             
public class PlaceOrderCommand {    
    @Target                        
    Customer customer;

    @Parameter()
    Product product;
    @Parameter()
    int quantity;
}

and then we could have a number of handlers, for example for the execution:

Code Block
public class CustomerPlaceOrderHandler {

    @Action
    public Customer act(PlaceOrderCommand command) { ... }
Code Block
languagejava
public class Customer {                                                           // target

execution - infer the @Valuename @Accessors(fluentof =the true)action from                  
    the type
}

and for the preconditions (no need for the "Act" suffix):

Code Block
public class PlaceOrderParametersCustomerPlaceOrderValidationHandler {

    public boolean hide(PlaceOrderCommand command) { ... }                                 // to assist supporting methods 
    public String disable(PlaceOrderCommand command) Product product;
        int quantity;
    }

{ ... }
    public staticString class PlacerOrderEvent extends ActionDomainEvent<Customer> {}     disable1(PlaceOrderCommand command) { ... }
    public String validate1(PlaceOrderCommand command) { ... }
    public String validate(PlaceOrderCommand command) { ... }
}

and for the UI hints:

Code Block
public class CustomerPlaceOrderUiHintsHandler {

    public Collection<Product> choices0(PlaceOrderCommand command) { ... 

}    @Action(domainEvent = PlaceOrderEvent.class)        
    public Product default0(PlaceOrderCommand command) { ... }
    public int default1(PlaceOrderCommand command) { ... }
}

Command Handler Contracts

Command handlers in other frameworks often have a single method, called something like "apply" or "accept".  We can't quite get there because we not only need to execute the action, but also do the validation and UI hint stuff.

We could though introduce some API to define this contract.

Code Block
public class CustomerPlaceOrderHandler implements   // event publishingCommandActHandler<PlaceOrderCommand> {

    public Customer placeOrderact(Product p, int quantityPlaceOrderCommand command) { ... }              
}

To hide entire action:

Code Block
public class CustomerPlaceOrderHideActHandler implements  // execution    

CommandHideActHandler<PlaceOrderCommand> {
    public boolean hidePlaceOrderhide(PlaceOrderCommand command) { ... }                                       // supporting methods use PlaceOrderParameters
    
}

To hide individual parameters:

Code Block
public Stringclass disablePlaceOrder()CustomerPlaceOrderHideParamHandler {implements ...CommandHideParamHandler<PlaceOrderCommand> }{
    public String disable1PlaceOrder(PlaceOrderParameters paramshide(PlaceOrderCommand command, int paramNum) { ... }
}

to disable entire action:

Code Block
public class CustomerPlaceOrderDisableActHandler implements CommandDisableActHandler<PlaceOrderCommand> {
    public Collection<Product>String choices0PlaceOrderdisable(PlaceOrderCommand command) { ... }      
}

To disable individual parameters:

Code Block
public class CustomerPlaceOrderDisableHandler implements CommandDisableParamHandler<PlaceOrderCommand> {
        
public String disable(PlaceOrderCommand command, public Product default0PlaceOrder(int paramNum) { ... }
}

To validate entire parameter set:

Code Block
public class CustomerPlaceOrderValidateActHandler implements CommandValidateActHandler<PlaceOrderCommand> {
    public intString default1PlaceOrdervalidate(PlaceOrderCommand command) { ... }
   }

To validate individual parameters:

Code Block
 public Stringclass validate1PlaceOrder(PlaceOrderParameters params) { ... }CustomerPlaceOrderValidateParamHandler implements CommandValidateParamHandler<PlaceOrderCommand> {
    public String validatePlaceOrder(PlaceOrderParameters paramsvalidate(PlaceOrderCommand command, int paramNum) { ... }
}

Notes:

  • The @Value @Accessors(fluent=true) allows us to use a syntax that is very similar to Java 14 records.
  • There is some duplication here: the list of the parameter types appears both in the placeOrder(...) method, as well as in the PlacerOrdersParameters class.

And we keep going for the UI hints.

To return choicesThe above would also be supported with mixins:

Code Block
@Action
public class Customer_placeOrder {

CustomerPlaceOrderChoicesParamHandler implements CommandChoicesParamHandler<PlaceOrderCommand> {
    public Collection<Object> choices(PlaceOrderCommand command, int paramNum) { ... }                    private final Customer target;       // bit ugly
}

To provide an autoComplete:

Code Block
public class CustomerPlaceOrderAutoCompleteHandler implements CommandAutoCompleteParamHandler<PlaceOrderCommand> {
    public Collection<Object> autoComplete(PlaceOrderCommand command, int paramNum, String search) { ... }           // bit ugly
}

To return defaults: 

Code Block
public class CustomerPlaceOrderDefaultParamHandler implements CommandDefaultParamHandler<PlaceOrderCommand> {
  // target
 public Object defaultOf(PlaceOrderCommand public Customer_placeOrder(Customer targetcommand, int paramNum) { ... }

        @Value @Accessors(fluent = true)             
    public static class PlaceOrderParameters {         // 'default' is a reserved word
}

Of course, there's nothing to prevent a single class from implementing all of these interfaces:

Code Block
@Action
public class CustomerPlaceOrderHandler 
       implements CommandActHandler<PlaceOrderCommand>,
           // to assist supporting methods
   CommandHideActHandler<PlaceOrderCommand>,
     Product product;
        int quantity;
   CommandHideParamHandler<PlaceOrderCommand>, }

     public static class PlaceOrderEvent extends ActionDomainEvent<Customer> {}       CommandDisableActHandler<PlaceOrderCommand>, 
                  CommandDisableParamHandler<PlaceOrderCommand>,
            

    @Action(domainEvent = PlaceOrderEvent.class) CommandValidateActHandler<PlaceOrderCommand>,
                  CommandValidateParamHandler<PlaceOrderCommand>,
                // event publishingCommandChoicesParamHandler<PlaceOrderCommand>,
    public Customer act(Product p, int quantity) { ... }        CommandAutoCompleteParamHandler<PlaceOrderCommand>,
                  //CommandDefaultParamHandler<PlaceOrderCommand> execution{

    public booleanCustomer hideActact(PlaceOrderCommand command) { ... }   
    public boolean hide(PlaceOrderCommand command) {         ... }                          // supporting methods
    public String disableAct(hide(PlaceOrderCommand command, int paramNum) { ... }
    public String disable1Actdisable(PlaceOrderParametersPlaceOrderCommand paramscommand) { ... } {
    public Collection<Product>String choices0Act(disable(PlaceOrderCommand command, int paramNum) { ... }
    public String validate(PlaceOrderCommand command) { ... }
    
public String validate(PlaceOrderCommand command, public Product default0Act(int paramNum) { ... }
    public Collection<Object> choices(PlaceOrderCommand command, int default1Act(paramNum) { ... }
    public String validate1Act(PlaceOrderParameters params 
    public Collection<Object> autoComplete(PlaceOrderCommand command, int paramNum, String search) { ... } 
    public StringObject validateAct(PlaceOrderParameters paramsdefaultOf(PlaceOrderCommand command, int paramNum) { ... }
}

Notes:

  • we now have three classes here: the mixin, the domain event, and the parameters object.

Parameters on Act syntax (proposed)

...