踩一踩OSWorkflow和jBMP：39行代码实现一个很好很强大的工作流引

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现成的工作流引擎有很多，我以前曾浅尝过OSWorkflow和jBPM，但都未能深入研究。总感觉它们过于复杂，术语也特别多，让我对它们逐渐失去了兴趣（还有那些流程设计器，个人觉得完全没有存在的必要：1.一般的用户用不来；2.程序员直接写代码（流程定义代码量一般几十行就够了），要流程设计器干嘛？）。另外有一个开源的基于Grails的工作流项目：http://www.grailsflow.org/ ，尽管它宣称“GrailsFlow： Your Workflow just got easier！”，我感觉它还是做得太复杂，凭我多年的IT从业经验，竟然没看懂，我也不想继续把时间浪费在它上面。

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工作流引擎，从本质上说，是状态机，只要控制好了各个状态之间的转换就行了。我使用Groovy/Grails有一段时间了，感觉Grails的webflow设计得不错，简单易懂，很好很强大，只可惜它只适合做在线支付之类的流程。但是webflow却给了我一些启发，尤其是它那优美的DSL方式的流程定义，让我印象深刻。利用Groovy的BuilderSupport，Closure，还有我已经整合到Grails中的db4o持久化框架，很容易快速写出一个工作流引擎。

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我也用一把DSL，一个流程定义的例子如下：

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   start {
       on('Submit') {o->
           println o.state
           // change o.state to a new state ...
       } 
   }
   state1 {
       on('Action11') {o->
           // do something with o and then change o.state to a new state ...
       }
   }
   state2 {
       on('Action21') {o->
           // do something with o and then change o.state to a new state ...
       }
       on('Action22') {o->
           // do something with o and then change o.state to a new state ...           
       }
   }

怎么样，够简洁吧？

在某个状态下，请求某个操作后，要执行的具体动作在对应的closure中写好了。这些closures中的代码犹如诸葛亮的锦囊妙计，工作流引擎依计行事。

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现在设想我们把DSL写在workflows目录下的demo.groovy（扩展名随便你怎么写）中，怎样使工作流引擎读懂这个流程呢？这需要一点点Groovy Builder的知识，自己看Groovy官方文档或Google。

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我们的FlowBuilder长得像这个样子：

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class FlowBuilder extends BuilderSupport {
    private Map flow = [:]
    private String _state
    protected void setParent(Object parent,Object child){}
    protected Object createNode(Object name){
        createNode(name,null)
    }
    protected Object createNode(Object name,Object value){
        createNode(name,null,value)
    }
    protected Object createNode(Object name,Map attributes){
        createNode(name,attributes,Map attributes,Object value){}
    protected void nodeCompleted(Object parent,Object node) {}
    def invokeMethod(String name,args) {  
        switch(name) {
            case 'flow':
                super.invokeMethod name,args
                break    
            case 'on':
                flow[_state][args[0]] = args[1]
                break
            default:
                _state = _state ? name : 'start' // the first state is always 'start'
                flow[_state] = flow[_state] ?: [:]
                super.invokeMethod name,args
                break
        }
    }
}

简单测试一下：

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def builder = new FlowBuilder()
def s = '{->'+'''
   // DSL可从文件中读取
   start {
       on('Submit') {o->
           println o.state
           // change o.state to a new state ...
       } 
   }
   state1 {
       on('Action11') {o->
           // do something with o and then change o.state to a new state ...
       }
   }
   state2 {
       on('Action21') {o->
           // do something with o and then change o.state to a new state ...
       }
       on('Action22') {o->
           // do something with o and then change o.state to a new state ...           
       }
   }
'''+'}'
this.class.classLoader.rootLoader.addURL( new URL("file:///E:/lib/bsf-2.4.0.jar") )
def closure = new org.apache.bsf.BSFManager().eval("groovy",s)
builder.flow closure
println builder.flow

输出以下内容：?

[start:[submit:_$_run_closure1_closure2_closure5@31f2a7],state1:[action11:_$_run_closure1_closure3_closure6@131c89c],state2:[action21:_$_run_closure1_closure4_closure7@1697b67,action22:_$_run_closure1_closure4_closure8@24c4a3]]

这表明，流程定义已经被成功地装入一Map中。

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其实，代码可以写得更精简：直接在文件中写成Map的形式（这可以看作另一种形式的DSL），以下是文章开头的图片所对应的工作流流程定义：

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[
    start : [
        'Save as Draft' : {o,p,m->
	        o.state = 'draft'
            o.principals = [m]
        },Submit : {o,m->
	        o.state = 'assign'
	        o.principals = [p.principal]
        }
    ],draft : [
        Save : {o,m->
	        o.state = 'draft'
            o.principals = [o.originator]
        },assign : [
        Submit : {o,m->
            o.state = 'working'
        }
    ],working : [
        Ask : {o,m->
            o.state = 'has_questions'
            o.principals = [o.originator]
        },m->
            o.state = 'report_ready'
            o.principals = [o.originator]
        }
    ],has_questions : [
        Answer : {o,m->
            o.state = 'working'
            o.principals = [o.previousPrincipal]
        }
    ],report_ready : [
        OK : {o,m->
            o.state = 'end'
            o.principals = [o.originator]
        },NG : {o,_ds : [
        report_type : {
            return [['sas','SAS report'],['cogonos','Cogonos report']]    
        },principal : {
	        def principals = []
	    	User.findAll(sort:'username').each {
			    principals << [it.username,it.profile?.name ?: it.username]
		    }
	        principals
        }
    ]
]

?这样连FlowBuilder也省了，一行eval代码就解析并载入了流程定义。。。

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以上内容还没涉及到我的工作流引擎，其实有了groovy语言的closure的强大支持，实现这个引擎实在是小菜一碟：

class WorkflowService {
    static transactional = true
    def formService
	def process(params) {
	    def params_values = JSON.parse(params.values)
        def workflowBean = WorkflowBean.find(id:params_values._bid)
        def workflow
        if(workflowBean) {
            workflow = getWorkflow(workflowBean)
        } else {
            def workflowDef = WorkflowDefinition.find(id:params_values._fid)
            if(workflowDef) {
	            workflow = getWorkflow(workflowDef)
                workflowBean = new WorkflowBean(state:'start',definition:workflowDef)
            }
        }
	    def state = workflowBean.state
		def action = workflow."$state"?."${params_values._action}"
		if(action && action instanceof Closure) {
			action.call(workflowBean,params_values,params.session.user.username)
			workflowBean.save()
			notifyPrincipals(workflowBean)
			return [ok:true]
		}
        def arrayStore = [:]
        workflow?._ds?.each {
        	arrayStore[it.key] = it.value.call()
        }
	    def values = [_fid:workflowBean?.definition?.id,_bid:workflowBean?.id,_state:workflowBean?.state]
	    workflowBean?.data?.each {
		    values[it.key] = it.value
	    }
        [workflowBean:workflowBean,form:formService.getFlowForm(workflowBean,params.servletContext),values:values as JSON,arrayStore:arrayStore as JSON,_actions:workflow?."${workflowBean?.state}"?.keySet() as JSON]
    }
}

只有39行代码 -- 不能望39K女项背，我只能做个39L男 :-)?