JBPM与软件架构模式设计之命令模式

前面我们介绍了JBPM的长事务设计，其中用到了职责链模式和命令模式，其中职责链模式我们已经介绍过了，今天我们来看看JBPM是怎样使用命令模式的。
命令模式定义
将请求封装成一个对象，使你可以使用不同的请求对客户端进行参数化；对请求排队和记录请求队列，并支持对请求的撤销操作。
命令模式UML类图

命令模式代码示例

package org.jbpm.pvm.internal.svc;

public abstract class Command {

protected Reciever reviever;

public Command(Reciever reviever)
{
this .reviever = reviever;
}

public abstract void Execution();
}

package org.jbpm.pvm.internal.svc;

public class ConcreteCommand extends Command{

public ConcreteCommand(Reciever reviever) {
super (reviever);
// TODO Auto-generated constructor stub
}

@Override
public void Execution() {
// TODO Auto-generated method stub
reviever.Action();
}

}

package org.jbpm.pvm.internal.svc;

public class Invoker {
private Command command;

public void SetCommand(Command command)
{
this .command = command;
}
public void ExecutionCommand()
{
this .command.Execution();
}
}

package org.jbpm.pvm.internal.svc;

public class Reciever {
public void Action(){}
}

package org.jbpm.pvm.internal.svc;

public class Client {

/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
Reciever r = new Reciever();
Command c = new ConcreteCommand(r);
Invoker i = new Invoker();
i.SetCommand(c);
i.ExecutionCommand();

}

}

命令模式的优点
可以比较容易的设置一个命令队列
在需要的情况下可以很容易的将命令记录下来
允许接受请求的一方决定是否否决命令
很容易实现对请求的撤销和重做

JBPM的命令模式实现

package org.jbpm.api;

import org.jbpm.api.history.HistoryComment;
import org.jbpm.api.history.HistoryActivityInstance;
import org.jbpm.api.task.Participation;
import org.jbpm.api.task.Task;

import java.util.List;
import java.util.Map;
import java.util.Set;

public interface TaskService {
Task getTask(String taskId);
}

package org.jbpm.pvm.internal.svc;

import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

import org.jbpm.api.TaskQuery;
import org.jbpm.api.TaskService;
import org.jbpm.api.history.HistoryComment;
import org.jbpm.api.task.Participation;
import org.jbpm.api.task.Task;
import org.jbpm.pvm.internal.cmd.AddParticipationCmd;
import org.jbpm.pvm.internal.cmd.AddReplyCommentCmd;
import org.jbpm.pvm.internal.cmd.AddTaskCommentCmd;
import org.jbpm.pvm.internal.cmd.AssignTaskCmd;
import org.jbpm.pvm.internal.cmd.CompositeCmd;
import org.jbpm.pvm.internal.cmd.DeleteTaskCmd;
import org.jbpm.pvm.internal.cmd.CompleteTaskCmd;
import org.jbpm.pvm.internal.cmd.CreateTaskQueryCmd;
import org.jbpm.pvm.internal.cmd.DeleteCommentCmd;
import org.jbpm.pvm.internal.cmd.GetOutcomes;
import org.jbpm.pvm.internal.cmd.GetParticipantsCmd;
import org.jbpm.pvm.internal.cmd.GetSubTasksCmd;
import org.jbpm.pvm.internal.cmd.GetTaskCmd;
import org.jbpm.pvm.internal.cmd.GetTaskCommentsCmd;
import org.jbpm.pvm.internal.cmd.GetTaskVariableNamesCmd;
import org.jbpm.pvm.internal.cmd.GetTaskVariablesCmd;
import org.jbpm.pvm.internal.cmd.NewTaskCmd;
import org.jbpm.pvm.internal.cmd.RemoveParticipantCmd;
import org.jbpm.pvm.internal.cmd.SaveTaskCmd;
import org.jbpm.pvm.internal.cmd.SetTaskVariablesCmd;
import org.jbpm.pvm.internal.query.TaskQueryImpl;
import org.jbpm.pvm.internal.task.TaskImpl;


public class TaskServiceImpl extends AbstractServiceImpl implements TaskService {



public Task getTask(String taskId) {
return commandService.execute( new GetTaskCmd(taskId));
}
}

public interface CommandService {

String NAME_TX_REQUIRED_COMMAND_SERVICE = " txRequiredCommandService " ;
String NAME_NEW_TX_REQUIRED_COMMAND_SERVICE = " newTxRequiredCommandService " ;

/**
* @throws JbpmException if command throws an exception.
*/
< T > T execute(Command < T > command);
}

package org.jbpm.pvm.internal.svc;

import org.jbpm.pvm.internal.cmd.CommandService;



public class AbstractServiceImpl {

protected CommandService commandService;

public CommandService getCommandService() {
return commandService;
}

public void setCommandService(CommandService commandService) {
this .commandService = commandService;
}
}

public interface Command < T > extends Serializable {

T execute(Environment environment) throws Exception;
}

public class GetTaskCmd extends Command < Task > {

private static final long serialVersionUID = 1L ;

protected String taskId;

public GetTaskCmd(String taskId) {
if (taskId == null ) {
throw new JbpmException( " taskId is null " );
}
this .taskId = taskId;
}

public Task execute(Environment environment) throws Exception {
DbSession dbSession = environment.get(DbSession. class );
return dbSession.get(TaskImpl. class , Long.parseLong(taskId));
}
}

JBPM使用命令模式的设计的优点请参见 JBPM长事务设计