从贫血到充血Domain Model

关于Domain Model的讨论已经非常多了，炒炒冷饭，这里是自己的一些做法。以Workitem（工作流里的工作项）作为例子最开始的做法：一个实体类叫做Workitem，指的是一个工作项或者称为任务项一个DAO类叫做WorkitemDao一个业务逻辑类叫做WorkitemManager(或者叫做WorkitemService)主要看看WorkitemManager，因为主要逻辑集中在这里

public class WorkitemManager {        private WorkItemDAO workItemDAO;    public void setWorkItemDAO(WorkItemDAO workItemDAO) {        this.workItemDAO = workItemDAO;    }       /**     * 提交工作项     * @param workitemId 工作项ID     */    public void commitWorkitem(String workitemId){            WorkItem workitem = workItemDAO.getWorkItem(workitemId);            //当前工作项结束        workitem.complete();        int sID = workitem.getSequenceId();        //找到所对应的节点        InstActivity instActivity=workitem.getInstActivity();        //查找是否存在下一工作项        WorkItem sequenceWorkitem = workItemDAO.findSequenceWorkItem(instActivity.getId(), sID + 1);        //如果不存在则触发节点流转        if (sequenceWorkitem == null) {            instActivity.signal();        }        //否则把下一工作项激活        else {            sequenceWorkitem.setExecutive();        }    }   }

Workitem类里有一些状态转换的逻辑，这样避免直接调用get/set属性方法

public class Workitem{        private int state = WorkitemInfo.PREPARE;        /**     * 委派工作项     */    public void commission() {        if (state != WorkitemInfo.EXECUTE && state != WorkitemInfo.SIGNINED                && state != WorkitemInfo.TOREAD&& state != WorkitemInfo.SUSPEND)            throw new WorkflowException(Messages.CANNOT_ALTER_WORKITEM_STATE);        setState(WorkitemInfo.COMMISSIONED);        setCommitted(new Timestamp(System.currentTimeMillis()));    }    /**     * 完成工作项     */    public void complete() {        if (state != WorkitemInfo.SIGNINED)            throw new WorkflowException(Messages.CANNOT_ALTER_WORKITEM_STATE);        setState(WorkitemInfo.COMPLETE);        setCompleted(new Timestamp(System.currentTimeMillis()));    }}

接下来的做法：三个类不变，将WorkitemManager打平，将逻辑移动到Workitem

public class WorkitemManager {        private WorkItemDAO workItemDAO;    public void setWorkItemDAO(WorkItemDAO workItemDAO) {        this.workItemDAO = workItemDAO;    }       /**     * 提交工作项     * @param workitemId 工作项ID     */    public void commitWorkitem(String workitemId){            WorkItem workitem = workItemDAO.getWorkItem(workitemId);            //当前工作项提交        workitem.commit();    }   }

实际上此时WorkitemManager的功能非常有限，仅仅是事务边界和获取workitem对象，甚至在一些情况下可以省略。通过一个Container类将spring的applicationContext进行封装，然后通过getBean()的静态方法即可访问被spring所管理的bean。实际是将workItemDAO隐式注入了Workitem。

public class Workitem{        /**     * 提交工作项     */    public void commit() {        if (state != WorkitemInfo.EXECUTE && state != WorkitemInfo.SIGNINED                && state != WorkitemInfo.TOREAD&& state != WorkitemInfo.SUSPEND)            throw new WorkflowException(Messages.CANNOT_ALTER_WORKITEM_STATE);        setState(WorkitemInfo.COMMISSIONED);        setCommitted(new Timestamp(System.currentTimeMillis()));        int sID = workitem.getSequenceId();        WorkItemDAO workItemDAO=(WorkItemDAO)Container.getBean("workItemDAO");        //查找是否存在下一工作项        WorkItem sequenceWorkitem = workItemDAO.findSequenceWorkItem(instActivity.getId(), sID + 1);        //如果不存在则触发节点流转        if (sequenceWorkitem == null) {            instActivity.signal();        }        //否则把下一工作项激活        else {            sequenceWorkitem.setExecutive();        }    }}

这样带来的好处是业务逻辑全部被封装到Domain Model，Domain Model之间的交互变得非常的简单，没有频繁的set/get，直接调用有业务语义的Domain Model的方法即可。问题在于单元测试时脱离不了spring的容器，workItemDAO需要stub。我觉得这个问题不大，问题是Domain Model开始变得臃肿，在业务逻辑复杂时代码行急剧膨胀。
现在的做法以上三个类保持不变，增加一个类WorkitemExecutor，将业务逻辑移步。

public class Workitem{        /**     * 提交工作项     */    public void commit() {        if (state != WorkitemInfo.EXECUTE && state != WorkitemInfo.SIGNINED                && state != WorkitemInfo.TOREAD&& state != WorkitemInfo.SUSPEND)            throw new WorkflowException(Messages.CANNOT_ALTER_WORKITEM_STATE);        setState(WorkitemInfo.COMMISSIONED);        setCommitted(new Timestamp(System.currentTimeMillis()));        WorkitemExecutor workitemExecutor=(WorkitemExecutor)Container.getBean("workitemExecutor");        workitemExecutor.commitWorkitem(this);    }}public class WorkitemExecutor {        private WorkItemDAO workItemDAO;    public void setWorkItemDAO(WorkItemDAO workItemDAO) {        this.workItemDAO = workItemDAO;    }       /**     * 提交工作项     * @param workitemId 工作项ID     */    public void commitWorkitem(Workitem workitem){        int sID = workitem.getSequenceId();        //找到所对应的节点        InstActivity instActivity=workitem.getInstActivity();        //查找是否存在下一工作项        WorkItem sequenceWorkitem = workItemDAO.findSequenceWorkItem(instActivity.getId(), sID + 1);        //如果不存在则触发节点流转        if (sequenceWorkitem == null) {            instActivity.signal();        }        //否则把下一工作项激活        else {            sequenceWorkitem.setExecutive();        }    }   }

将业务逻辑拆分成两部分，一部分在Workitem，另一部分委托给WorkitemExecutor。实际上是Domain Model将复杂逻辑的情况重新外包出去。调用的时候，面向的接口还是Domain Model的方法。注意到WorkitemExecutor和WorkitemManager的API是非常相似的。实际可以这样认为，传统的方式 Client->(Business Facade)->service(Business Logic 部分依赖Domain Model)->Data Access(DAO)。现在的方式Client->(Business Facade)->Domain Model->service->Data Access(DAO)。另外，在返回client端的查询的时候还是倾向于直接调用DAO，而不是通过Domain Model。