Activiti工作流框架:ProcessEngines.getDefaultProcessEngine()为null解决方案

可能原因1：Activiti的配置文件名称或路径出错

通过如下方法获取的流程引擎对象，需要保证Activiti配置文件名称为"activiti.cfg.xml"并且位于classpath下(可参考Activiti源码)。

// 创建流程引擎
 ProcessEngine processEngine = ProcessEngines.getDefaultProcessEngine();

如图所示为java project的目录结构，按照某些教程把activiti.cfg.xml放在了新建的resource文件夹下，但是resource并不是资源文件夹也是会出错，这时应该右键resource文件夹->build path->use as source folder指定为资源目录(右上角出现"#"图案)。

可能原因2：jar包缺失

只简单添加了 activiti/libs 目录下的所有jar包以及log4j、slf4j相关包

缺少mysql驱动包

缺少spring-beans、spring-core相关包

缺少mybatis相关包

缺少commons-xxx相关包

缺少juel相关包

缺少jackson相关包

...

建议将 activiti/wars/中的某个war包解压，导入其中lib下的所有Jar包，缺少包会报ClassNotFoundException，再根据情况添包。附上lib图：

其它原因：配置文件内容有误或没有事先建立配置文件指向的数据库(数据库服务要开启)

(1)在activiti.cfg.xml配置文件中，添加了<property name="jobExecutorActivate" value="false" /> 去掉该属性试试。

(2)检查databaseSchemaUpdate配置是否与数据库表存在冲突？

附测试源代码：

package com.bo.activiti;

import org.activiti.engine.ProcessEngine;
import org.activiti.engine.ProcessEngineConfiguration;
import org.activiti.engine.ProcessEngines;
import org.activiti.engine.RepositoryService;
import org.activiti.engine.RuntimeService;
import org.activiti.engine.TaskService;
import org.activiti.engine.task.Task;

public class First {
	public static void main(String[] args){
		// 创建流程引擎
 		ProcessEngine processEngine = ProcessEngines.getDefaultProcessEngine();
//		ProcessEngine processEngine = ProcessEngineConfiguration.createProcessEngineConfigurationFromResource("activiti.cfg.xml").buildProcessEngine();
		System.out.println(processEngine);
		// 得到流程存储服务组件
		RepositoryService repositoryService = processEngine.getRepositoryService();
		// 得到运行时服务组件
		RuntimeService runtimeService = processEngine.getRuntimeService();
		// 获取流程任务组件
		TaskService taskService = processEngine.getTaskService();
		// 部署流程定义文件
		repositoryService.createDeployment().addClasspathResource("bpmn/first.bpmn").deploy();
		// 启动流程
		runtimeService.startProcessInstanceByKey("myProcess");
		// 查询第一个任务
		Task task = taskService.createTaskQuery().singleResult();
		System.out.println("第一个任务完成前，当前任务名称:"+task.getName());
		// 完成第一个任务
		taskService.complete(task.getId());
		// 查询第二个任务
		task = taskService.createTaskQuery().singleResult();
		System.out.println("第二个任务完成前，当前任务名称:"+task.getName());
		// 完成第二个任务(流程结束)
		taskService.complete(task.getId());
		task = taskService.createTaskQuery().singleResult();
		System.out.println("流程结束后，查找任务：" + task);
		
	}
}

流程定义配置文件：first.bpmn

<?xml version="1.0" encoding="UTF-8"?>
<definitions xmlns="http://www.omg.org/spec/BPMN/20100524/MODEL" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:activiti="http://activiti.org/bpmn" xmlns:bpmndi="http://www.omg.org/spec/BPMN/20100524/DI" xmlns:omgdc="http://www.omg.org/spec/DD/20100524/DC" xmlns:omgdi="http://www.omg.org/spec/DD/20100524/DI" typeLanguage="http://www.w3.org/2001/XMLSchema" expressionLanguage="http://www.w3.org/1999/XPath" targetNamespace="http://www.activiti.org/test">
  <process id="myProcess" name="My process" isExecutable="true">
    <startEvent id="startevent1" name="Start"></startEvent>
    <userTask id="usertask2" name="Handle Request"></userTask>
    <userTask id="usertask3" name="Expense Request"></userTask>
    <endEvent id="endevent1" name="End"></endEvent>
    <sequenceFlow id="flow1" sourceRef="usertask2" targetRef="endevent1"></sequenceFlow>
    <sequenceFlow id="flow2" sourceRef="usertask3" targetRef="usertask2"></sequenceFlow>
    <sequenceFlow id="flow3" sourceRef="startevent1" targetRef="usertask3"></sequenceFlow>
  </process>
  <bpmndi:BPMNDiagram id="BPMNDiagram_myProcess">
    <bpmndi:BPMNPlane bpmnElement="myProcess" id="BPMNPlane_myProcess">
      <bpmndi:BPMNShape bpmnElement="startevent1" id="BPMNShape_startevent1">
        <omgdc:Bounds height="35.0" width="35.0" x="170.0" y="220.0"></omgdc:Bounds>
      </bpmndi:BPMNShape>
      <bpmndi:BPMNShape bpmnElement="usertask2" id="BPMNShape_usertask2">
        <omgdc:Bounds height="55.0" width="105.0" x="380.0" y="210.0"></omgdc:Bounds>
      </bpmndi:BPMNShape>
      <bpmndi:BPMNShape bpmnElement="usertask3" id="BPMNShape_usertask3">
        <omgdc:Bounds height="55.0" width="105.0" x="240.0" y="210.0"></omgdc:Bounds>
      </bpmndi:BPMNShape>
      <bpmndi:BPMNShape bpmnElement="endevent1" id="BPMNShape_endevent1">
        <omgdc:Bounds height="35.0" width="35.0" x="520.0" y="220.0"></omgdc:Bounds>
      </bpmndi:BPMNShape>
      <bpmndi:BPMNEdge bpmnElement="flow1" id="BPMNEdge_flow1">
        <omgdi:waypoint x="485.0" y="237.0"></omgdi:waypoint>
        <omgdi:waypoint x="520.0" y="237.0"></omgdi:waypoint>
      </bpmndi:BPMNEdge>
      <bpmndi:BPMNEdge bpmnElement="flow2" id="BPMNEdge_flow2">
        <omgdi:waypoint x="345.0" y="237.0"></omgdi:waypoint>
        <omgdi:waypoint x="380.0" y="237.0"></omgdi:waypoint>
      </bpmndi:BPMNEdge>
      <bpmndi:BPMNEdge bpmnElement="flow3" id="BPMNEdge_flow3">
        <omgdi:waypoint x="205.0" y="237.0"></omgdi:waypoint>
        <omgdi:waypoint x="240.0" y="237.0"></omgdi:waypoint>
      </bpmndi:BPMNEdge>
    </bpmndi:BPMNPlane>
  </bpmndi:BPMNDiagram>
</definitions>

流程定义设计图：

Activiti配置文件：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
		xsi:schemaLocation="http://www.springframework.org/schema/beans
				 http://www.springframework.org/schema/beans/spring-beans.xsd">
	
	<!-- Activiti的引擎配置管理器 -->
	<bean id="processEngineConfiguration" class="org.activiti.engine.impl.cfg.StandaloneProcessEngineConfiguration">
		<!-- 指定数据源 -->
		<property name="jdbcUrl" value="jdbc:mysql://localhost:3306/activiti" />
		<property name="jdbcDriver" value="com.mysql.jdbc.Driver" />
		<property name="jdbcUsername" value="root" />
		<property name="jdbcPassword" value="root" />
		<property name="databaseSchemaUpdate" value="true" />
	</bean>
	
</beans>

程序运行结果：

log4j:WARN No appenders could be found for logger (org.activiti.engine.ProcessEngines).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
org.activiti.engine.impl.ProcessEngineImpl@11e32b6
第一个任务完成前，当前任务名称:Expense Request
第二个任务完成前，当前任务名称:Handle Request
流程结束后，查找任务：null