审批流程图
如下图，在此流程图中，存在两个UserTask节点，第一个节点是主管审批，第二个节点是产品经理审批，两个节点中间有一个排他网关，此网关用来对主管审批的结果进行判断，如果主管审批通过，则流程走到产品经理审批节点，如果主管审批拒绝，则流程走到结束节点。
主管审批节点通过UEL表达式${assignManager}动态赋值，产品经理审批节点通过UEL表达式${assignProductLineManager}动态赋值，网关节点通过UEL表达式${isPass}动态赋值。
流程流转
上文 《01 启动流程实例》 讲到在完成流程实例的启动后，流程将通过commandContext.getAgenda().planContinueProcessOperation(execution)流转到下一个节点。本文将以此代码为入口，研究流程如何流转到下一个节点。
上面的代码中，getAgent 方法返回来一个 ActivitiEngineAgenda 接口，此接口的实现类是DefaultActivitiEngineAgenda，它的内部成员属性如下，operations 在这里充当 FIFO 队列，用来存储将要执行的 AbstractOperation (本质上是一个 Runnable 接口)操作，而 CommandContext 则是命令上下文。
public class DefaultActivitiEngineAgenda implements ActivitiEngineAgenda {
private static final Logger logger = LoggerFactory.getLogger(DefaultActivitiEngineAgenda.class);
protected LinkedList<Runnable> operations = new LinkedList<Runnable>();
protected CommandContext commandContext;
public DefaultActivitiEngineAgenda(CommandContext commandContext) {
this.commandContext = commandContext;
}
// 省略部分代码
}ContinueProcessOperation 操作(第一次)
构造 ContinueProcessOperation
planContinueProcessOperation(execution) 在 DefaultActivitiEngineAgenda 中的实现如下，把传入的 execution 和 commandContext 构造一个 ContinueProcessOperation 实例，并放入了 operations 队列中，
@Override
public void planContinueProcessOperation(ExecutionEntity execution) {
planOperation(new ContinueProcessOperation(commandContext, execution));
}
@Override
public void planOperation(Runnable operation) {
// 将 operation 任务放到 operations 队列中，等待定时任务从队列中取出任务并执行
operations.add(operation);
if (operation instanceof AbstractOperation) {
ExecutionEntity execution = ((AbstractOperation) operation).getExecution();
if (execution != null) {
// 这里添加的内容，在 org/activiti/engine/impl/interceptor/CommandInvoker的execute(CommandConfig, Command<T>)
// 中有使用，目的是告诉 CommandInvoker 有待执行的 AbstractOperation
commandContext.addInvolvedExecution(execution);
}
}
logger.debug("Operation {} added to agenda", operation.getClass());
}ContinueProcessOperation 类定义如下：
public abstract class AbstractOperation implements Runnable {
// 省略部分代码
}
public class ContinueProcessOperation extends AbstractOperation {
// 省略部分代码
}
执行 ContinueProcessOperation
由异步任务执行器 org/activiti/engine/impl/asyncexecutor/DefaultAsyncJobExecutor 中的 AcquireAsyncJobsDueRunnable 调用 DefaultActivitiEngineAgenda 的 getNextOperation 来取出 operations 队列中的 AbstractOperation 任务，然后由 CommandInvoker 来执行（异步任务执行器 DefaultAsyncJobExecutor 是在我们调用 ProcessEngines.getDefaultProcessEngine() 创建 ProcessEngine 阶段完成的初始化）。
CommandInvoker 中执行 AbstractOperation 任务的代码是下面这一段
protected void executeOperations(final CommandContext commandContext) {
// 任务不是空的
while (!commandContext.getAgenda().isEmpty()) {
// 取出任务
Runnable runnable = commandContext.getAgenda().getNextOperation();
// 执行任务
executeOperation(runnable);
}
}
public void executeOperation(Runnable runnable) {
// ContinueProcessOperation 继承 AbstractOperation，所以这个if条件的值是true，进入此if内部逻辑
// 除了 ContinueProcessOperation 外， 其它继承 AbstractOperation 的Operation类都能进入这个if逻辑
if (runnable instanceof AbstractOperation) {
AbstractOperation operation = (AbstractOperation) runnable;
// Execute the operation if the operation has no execution (i.e. it's an operation not working on a process instance)
// or the operation has an execution and it is not ended
if (operation.getExecution() == null || !operation.getExecution().isEnded()) {
if (logger.isDebugEnabled()) {
logger.debug("Executing operation {} ", operation.getClass());
}
// 直接调用 ContinueProcessOperation（Runnable）的run方法，
// 会不会开启异步线程？不会，直接调用run方法，就是调用一个普通的方法
runnable.run();
}
} else {
// 直接调用 Runnable 的run方法，会不会开启异步线程？不会，直接调用run方法，就是调用一个普通的方法
runnable.run();
}
}执行 ContinueProcessOperation 的 run() 方法后，来到了 ContinueProcessOperation 中
public class ContinueProcessOperation extends AbstractOperation {
// 省略部分代码
/**
* 当 ContinueProcessOperation 操作被执行时，以 run 方法作为入口
*/
@Override
public void run() {
// 第一次执行这个逻辑，取出来的 currentFlowElement 是一个 StartEvent，StartEvent 是 FlowNode 的子类
FlowElement currentFlowElement = getCurrentFlowElement(execution);
if (currentFlowElement instanceof FlowNode) {
// 继续执行 FlowNode 类型的类，StartEvent会走这个方法
continueThroughFlowNode((FlowNode) currentFlowElement);
} else if (currentFlowElement instanceof SequenceFlow) {
// 继续执行 SequenceFlow 类型的类
continueThroughSequenceFlow((SequenceFlow) currentFlowElement);
} else {
throw new ActivitiException("Programmatic error: no current flow element found or invalid type: " + currentFlowElement + ". Halting.");
}
}
protected void continueThroughFlowNode(FlowNode flowNode) {
// Check if it's the initial flow element. If so, we must fire the execution listeners for the process too
if (flowNode.getIncomingFlows() != null
&& flowNode.getIncomingFlows().size() == 0
&& flowNode.getSubProcess() == null) {
executeProcessStartExecutionListeners();
}
// For a subprocess, a new child execution is created that will visit the steps of the subprocess
// The original execution that arrived here will wait until the subprocess is finished
// and will then be used to continue the process instance.
if (flowNode instanceof SubProcess) {
createChildExecutionForSubProcess((SubProcess) flowNode);
}
if (flowNode instanceof Activity && ((Activity) flowNode).hasMultiInstanceLoopCharacteristics()) {
// the multi instance execution will look at async
executeMultiInstanceSynchronous(flowNode);
} else if (forceSynchronousOperation || !flowNode.isAsynchronous()) {
// 同步执行，StartEvent走的是这里
executeSynchronous(flowNode);
} else {
// 异步执行
executeAsynchronous(flowNode);
}
}
/**
* 同步执行
* @param flowNode
*/
protected void executeSynchronous(FlowNode flowNode) {
// 调用HistoryManager记录流程活动开始了，这里并不是单单指StartEvent活动，而是所有活动
commandContext.getHistoryManager().recordActivityStart(execution);
// Execution listener: event 'start'
// 执行 StartEvent 上的监听器，发布 Start 的 Event 事件
if (CollectionUtil.isNotEmpty(flowNode.getExecutionListeners())) {
executeExecutionListeners(flowNode, ExecutionListener.EVENTNAME_START);
}
// Execute any boundary events, sub process boundary events will be executed from the activity behavior
if (!inCompensation && flowNode instanceof Activity) { // Only activities can have boundary events
// 获取节点上的边界事件，如果有，就执行
List<BoundaryEvent> boundaryEvents = ((Activity) flowNode).getBoundaryEvents();
if (CollectionUtil.isNotEmpty(boundaryEvents)) {
executeBoundaryEvents(boundaryEvents, execution);
}
}
// Execute actual behavior
// 取出节点上的行为
ActivityBehavior activityBehavior = (ActivityBehavior) flowNode.getBehavior();
// 当 activityBehavior 不为空，走此方法，此方法后续也会调用 planTakeOutgoingSequenceFlowsOperation
// activityBehavior 表示一个节点上拥有的行为
// StartEvent 节点的行为是 NoneStartEventActivityBehavior，没有做其它业务，仅仅是过度
// UserTask 节点的行为是 UserTaskActivityBehavior，这个行为会把任务写入到数据库后，等待用户完成任务，流程才会继续走下去
if (activityBehavior != null) {
// 执行节点上的行为，StartEvent后走的是这里，
executeActivityBehavior(activityBehavior, flowNode);
} else {
logger.debug("No activityBehavior on activity '{}' with execution {}", flowNode.getId(), execution.getId());
// StartEvent后不走这里，计划执行 TakeOutgoingSequenceFlows 操作，这个操作是一个连线行为，第一步先找出当前节点的出口，第二步从出口走到下一个节点。
Context.getAgenda().planTakeOutgoingSequenceFlowsOperation(execution, true);
}
}
/**
* 执行节点上的行为
* @param activityBehavior
* @param flowNode
*/
protected void executeActivityBehavior(ActivityBehavior activityBehavior,
FlowNode flowNode) {
logger.debug("Executing activityBehavior {} on activity '{}' with execution {}",
activityBehavior.getClass(),
flowNode.getId(),
execution.getId());
if (Context.getProcessEngineConfiguration() != null && Context.getProcessEngineConfiguration().getEventDispatcher().isEnabled()) {
Context.getProcessEngineConfiguration().getEventDispatcher().dispatchEvent(
ActivitiEventBuilder.createActivityEvent(ActivitiEventType.ACTIVITY_STARTED,
flowNode.getId(),
flowNode.getName(),
execution.getId(),
execution.getProcessInstanceId(),
execution.getProcessDefinitionId(),
flowNode));
}
try {
// 这方法里面后续会执行 planTakeOutgoingSequenceFlowsOperation
// 执行 StartEvent 的 NoneStartEventActivityBehavior 行为
activityBehavior.execute(execution);
} catch (RuntimeException e) {
if (LogMDC.isMDCEnabled()) {
LogMDC.putMDCExecution(execution);
}
throw e;
}
}
}
执行 NoneStartEventActivityBehavior
因为 NoneStartEventActivityBehavior 是空实现，但它继承自 FlowNodeActivityBehavior ，拥有了父类的能力，所以 activityBehavior.execute(execution) 实际执行的是父类FlowNodeActivityBehavior 的 execute(DelegateExecution) 方法。
public class NoneStartEventActivityBehavior extends FlowNodeActivityBehavior {
private static final long serialVersionUID = 1L;
// Nothing to see here.
// The default behaviour of the BpmnActivity is exactly what
// a none start event should be doing.
}在 execute(DelegateExecution) 方法中调用了 leave(execution) 方法，详情见代码
public abstract class FlowNodeActivityBehavior implements TriggerableActivityBehavior {
private static final long serialVersionUID = 1L;
protected BpmnActivityBehavior bpmnActivityBehavior = new BpmnActivityBehavior();
/**
* Default behaviour: just leave the activity with no extra functionality.
* NoneStartEventActivityBehavior 自己没有实现 execute 方法，所以会调用父类
* FlowNodeActivityBehavior 的 execute 方法
*/
public void execute(DelegateExecution execution) {
leave(execution);
}
/**
* Default way of leaving a BPMN 2.0 activity: evaluate the conditions on the outgoing sequence flow and take those that evaluate to true.
* 离开 BPMN 2.0 activity的默认方式：评估 Outgoing sequence 流上的条件，并选取那些评估为 true 的条件
*/
public void leave(DelegateExecution execution) {
// 走到 bpmnActivityBehavior 中的 performDefaultOutgoingBehavior 方法
bpmnActivityBehavior.performDefaultOutgoingBehavior((ExecutionEntity) execution);
}
}BpmnActivityBehavior 的 performDefaultOutgoingBehavior 内部调用 Context.getAgenda().planTakeOutgoingSequenceFlowsOperation(ExecutionEntity, boolean) 方法，这里的 Context.getAgenda() 获取到的 Agenda 和上面的 Agenda 是同一个，取到 Agenda后，调用了它的 planTakeOutgoingSequenceFlowsOperation(ExecutionEntity, boolean) 方法，此方法将产生连线行为，将 StartEvent 根据 Process 流程模型的定义连接到下一个节点。
public class BpmnActivityBehavior implements Serializable {
private static final long serialVersionUID = 1L;
/**
* Performs the default outgoing BPMN 2.0 behavior, which is having parallel paths of executions for the outgoing sequence flow.
* <p>
* More precisely: every sequence flow that has a condition which evaluates to true (or which doesn't have a condition), is selected for continuation of the process instance. If multiple sequencer
* flow are selected, multiple, parallel paths of executions are created.
*/
public void performDefaultOutgoingBehavior(ExecutionEntity activityExecution) {
performOutgoingBehavior(activityExecution,
true,
false);
}
/**
* Actual implementation of leaving an activity.
* @param execution The current execution context
* @param checkConditions Whether or not to check conditions before determining whether or not to take a transition.
* @param throwExceptionIfExecutionStuck If true, an {@link ActivitiException} will be thrown in case no transition could be found to leave the activity.
* 最后在这个方法里调用了 planTakeOutgoingSequenceFlowsOperation 方法，用于走出当前就节点，走到下一个节点
*/
protected void performOutgoingBehavior(ExecutionEntity execution,
boolean checkConditions,
boolean throwExceptionIfExecutionStuck) {
Context.getAgenda().planTakeOutgoingSequenceFlowsOperation(execution,
true);
}
}TakeOutgoingSequenceFlowsOperation 操作
构造 TakeOutgoingSequenceFlowsOperation
DefaultActivitiEngineAgenda 的 planTakeOutgoingSequenceFlowsOperation 方法如下，将 ExecutionEntity 封装成 TakeOutgoingSequenceFlowsOperation，再放入 operations 队列中。
public class DefaultActivitiEngineAgenda implements ActivitiEngineAgenda {
@Override
public void planTakeOutgoingSequenceFlowsOperation(ExecutionEntity execution, boolean evaluateConditions) {
planOperation(new TakeOutgoingSequenceFlowsOperation(commandContext, execution, evaluateConditions));
}
}执行 TakeOutgoingSequenceFlowsOperation
TakeOutgoingSequenceFlowsOperation 也和 ContinueProcessOperation 一样是 AbstractionOperation 的子类，同样也是由定时任务通过 CommandInvoker 来调用 TakeOutgoingSequenceFlowsOperation 中的 run() 方法。
public class TakeOutgoingSequenceFlowsOperation extends AbstractOperation {
// 省略部分代码
@Override
public void run() {
FlowElement currentFlowElement = getCurrentFlowElement(execution);
// Compensation check
if ((currentFlowElement instanceof Activity)
&& (((Activity) currentFlowElement)).isForCompensation()) {
cleanupCompensation();
return;
}
// When leaving the current activity, we need to delete any related execution (eg active boundary events)
// 清除当前节点的一些附属信息
cleanupExecutions(currentFlowElement);
// 当前 currentFlowElement 是一个 StartEvent，StartEvent 是 FlowNode 的子类
if (currentFlowElement instanceof FlowNode) {
// 处理 FlowNode 流节点
handleFlowNode((FlowNode) currentFlowElement);
} else if (currentFlowElement instanceof SequenceFlow) {
// 处理 SequenceFlow 系列流
handleSequenceFlow();
}
}
/**
* uai 处理流节点
* @param flowNode
*/
protected void handleFlowNode(FlowNode flowNode) {
// 记录 StartEvent 活动结束
handleActivityEnd(flowNode);
if (flowNode.getParentContainer() != null
&& flowNode.getParentContainer() instanceof AdhocSubProcess) {
handleAdhocSubProcess(flowNode);
} else {
// StartEvent走这里， 离开流节点
leaveFlowNode(flowNode);
}
}
/**
* 离开流节点
* @param flowNode
*/
protected void leaveFlowNode(FlowNode flowNode) {
// 省略部分代码
String defaultSequenceFlowId = null;
if (flowNode instanceof Activity) {
defaultSequenceFlowId = ((Activity) flowNode).getDefaultFlow();
} else if (flowNode instanceof Gateway) {
defaultSequenceFlowId = ((Gateway) flowNode).getDefaultFlow();
}
// Determine which sequence flows can be used for leaving
// 声明 List<SequenceFlow> 实例，用来存储符合外出流条件的 SequenceFlow
List<SequenceFlow> outgoingSequenceFlows = new ArrayList<SequenceFlow>();
// 获取 StartEvent 节点可以外出的顺序流，并放到外出顺序流集合 outgoingSequenceFlows 中
for (SequenceFlow sequenceFlow : flowNode.getOutgoingFlows()) {
String skipExpressionString = sequenceFlow.getSkipExpression();
// 通过 isSkipExpressionEnabled 判断 execution 是否开启了跳过表达式
if (!SkipExpressionUtil.isSkipExpressionEnabled(execution, skipExpressionString)) {
if (!evaluateConditions || (evaluateConditions && ConditionUtil.hasTrueCondition(sequenceFlow, execution)
&& (defaultSequenceFlowId == null || !defaultSequenceFlowId.equals(sequenceFlow.getId())))) {
// 符合外出流条件的放入到 outgoingSequenceFlows
outgoingSequenceFlows.add(sequenceFlow);
}
} else if (flowNode.getOutgoingFlows().size() == 1 || SkipExpressionUtil.shouldSkipFlowElement(commandContext,
execution,
skipExpressionString)) {
// The 'skip' for a sequence flow means that we skip the condition, not the sequence flow.
// 符合外出流条件的放入到 outgoingSequenceFlows
outgoingSequenceFlows.add(sequenceFlow);
}
}
// Check if there is a default sequence flow
if (outgoingSequenceFlows.size() == 0 && evaluateConditions) { // The elements that set this to false also have no support for default sequence flow
if (defaultSequenceFlowId != null) {
for (SequenceFlow sequenceFlow : flowNode.getOutgoingFlows()) {
if (defaultSequenceFlowId.equals(sequenceFlow.getId())) {
// 符合外出流条件的放入到 outgoingSequenceFlows
outgoingSequenceFlows.add(sequenceFlow);
break;
}
}
}
}
// No outgoing found. Ending the execution
// 如果没有当前节点没有可以外出的顺序流，则流程到当前节点后，设置为结束。StartEvent不走这里
if (outgoingSequenceFlows.size() == 0) {
if (flowNode.getOutgoingFlows() == null || flowNode.getOutgoingFlows().size() == 0) {
logger.debug("No outgoing sequence flow found for flow node '{}'.", flowNode.getId());
// 当前节点没有可以外出的顺序流，走向结束节点
Context.getAgenda().planEndExecutionOperation(execution);
} else {
throw new ActivitiException("No outgoing sequence flow of element '" + flowNode.getId() + "' could be selected for continuing the process");
}
} else {
// Leave, and reuse the incoming sequence flow, make executions for all the others (if applicable)
ExecutionEntityManager executionEntityManager = commandContext.getExecutionEntityManager();
List<ExecutionEntity> outgoingExecutions = new ArrayList<ExecutionEntity>(flowNode.getOutgoingFlows().size());
// 当前节点有多个可以外出的出口顺序流，取出第一个
SequenceFlow sequenceFlow = outgoingSequenceFlows.get(0);
// Reuse existing one
/**
* uai
* 给已经存在的execution(ExecutionEntity)实例更新状态，表示流程走到了 sequenceFlow 这个节点.
* Process流程模型可以理解为一条高速公路，execution(ExecutionEntity)理解为一辆车，高速公路上有多个服务区，每个服务区是一个 sequenceFlow。
* 现有一辆货车，我们规定它每走到一个服务区时车辆要加油或者司机要休息一下，在加油或者休息时司机需要把当前所在的服务区(sequenceFlow)告诉他的老板，让
* 老板知道货运到哪里了(也就是在整个审批流程中，setCurrentFlowElement这一操作，让我们知道流程走到哪里了)
*/
// 将 execution 中的 CurrentFlowElement 从之前的 StartEvent 更新为 SequenceFlow
execution.setCurrentFlowElement(sequenceFlow);
execution.setActive(true);
// 将从 StartEvent 中出发的 execution 实例放入到 outgoingExecutions 链表中
outgoingExecutions.add((ExecutionEntity) execution);
// Executions for all the other one
// 有多条出口，我的审批流程图中，StartEvent 只有一条外出流，所以不进入这个 if 语句
if (outgoingSequenceFlows.size() > 1) {
for (int i = 1; i < outgoingSequenceFlows.size(); i++) {
// 如果当前 execution 有 parent，则大家共用这个 parent，
// 如果当前 execution 没有 parent，则用当前 execution 作为 parent，
ExecutionEntity parent = execution.getParentId() != null ? execution.getParent() : execution;
// 根据 父ExecutionEntity 信息，创建 子ExecutionEntity 实例，相当于高速公里有分叉路
ExecutionEntity outgoingExecutionEntity = commandContext.getExecutionEntityManager().createChildExecution(parent);
SequenceFlow outgoingSequenceFlow = outgoingSequenceFlows.get(i);
// 给 子ExecutionEntity 实例设置当前节点为 outgoingSequenceFlow
outgoingExecutionEntity.setCurrentFlowElement(outgoingSequenceFlow);
// 存储到数据库
executionEntityManager.insert(outgoingExecutionEntity);
outgoingExecutions.add(outgoingExecutionEntity);
}
}
// Leave (only done when all executions have been made, since some queries depend on this)
// 把 outgoingExecution 放入到 Agenda 的队列中，计划离开当前节点，前往下一个节点
// 这里可能有多个节点，所以通过遍历走所有的外出流，当前的审批流程图中，只有一条外出流，所以 outgoingExecutions 的大小是1
for (ExecutionEntity outgoingExecution : outgoingExecutions) {
// 准备执行 ContinueProcessOperation 操作
Context.getAgenda().planContinueProcessOperation(outgoingExecution);
}
}
}
}上面这个代码，最重要的是从 StartEvent 节点的信息中找到了外出流 (SequenceFlow)，随即调用 execution 的 setCurrentFlowElement 把活跃节点从之前的 StartEvent 更新为 SequenceFlow，这个操作表示流程从 StartEvent 走到了 SequenceFlow 这里。后续将从 SequenceFlow 继续走到下个节点。
分析上面代码发现流程最终又走到了 Context.getAgenda().planContinueProcessOperation(ExecutionEntity) 方法去执行 ContinueProcessOperation 中的业务，是不是重复执行了？
表面上看是重复执行了，但实际上有一些微妙的变化，这个变化是：传给 planContinueProcessOperation 方法的 execution 实例，它的 currentFlowElement 从 StartEvent 变成了 SequenceFlow。这个变化就会使得流程进入到 ContinueProcessOperation 的 else if 语句中，而不是第一次刚进来的 if 语句。
public class ContinueProcessOperation extends AbstractOperation {
// 省略部分代码
/**
* 当 ContinueProcessOperation 操作被执行时，以 run 方法作为入口
*/
@Override
public void run() {
// 第一次执行这个逻辑，取出来的 currentFlowElement 是一个 StartEvent，StartEvent 是 FlowNode 的子类
FlowElement currentFlowElement = getCurrentFlowElement(execution);
if (currentFlowElement instanceof FlowNode) {
// 继续执行 FlowNode 类型的类，StartEvent会走这个方法
continueThroughFlowNode((FlowNode) currentFlowElement);
} else if (currentFlowElement instanceof SequenceFlow) {
// 继续执行 SequenceFlow 类型的类
continueThroughSequenceFlow((SequenceFlow) currentFlowElement);
} else {
throw new ActivitiException("Programmatic error: no current flow element found or invalid type: " + currentFlowElement + ". Halting.");
}
}
}ContinueProcessOperation 操作(第二次)
构造 ContinueProcessOperation
忽略阐述此重复内容。
执行 ContinueProcessOperation
接上面的内容，流程进入到 ContinueProcessOperation 的 else if 语句中，而不是第一次刚进来的 if 语句。
else if 语句中调用的方法是 continueThroughSequenceFlow(SequenceFlow)。
/**
* 执行 SequenceFlow 逻辑
* @param sequenceFlow
*/
protected void continueThroughSequenceFlow(SequenceFlow sequenceFlow) {
// Execution listener. Sequenceflow only 'take' makes sense ... but we've supported all three since the beginning
// 执行 SequenceFlow 上的监听器
if (CollectionUtil.isNotEmpty(sequenceFlow.getExecutionListeners())) {
executeExecutionListeners(sequenceFlow,
ExecutionListener.EVENTNAME_START);
executeExecutionListeners(sequenceFlow,
ExecutionListener.EVENTNAME_TAKE);
executeExecutionListeners(sequenceFlow,
ExecutionListener.EVENTNAME_END);
}
// Firing event that transition is being taken
if (Context.getProcessEngineConfiguration() != null && Context.getProcessEngineConfiguration().getEventDispatcher().isEnabled()) {
FlowElement sourceFlowElement = sequenceFlow.getSourceFlowElement();
FlowElement targetFlowElement = sequenceFlow.getTargetFlowElement();
Context.getProcessEngineConfiguration().getEventDispatcher().dispatchEvent(
ActivitiEventBuilder.createSequenceFlowTakenEvent(
(ExecutionEntity) execution,
ActivitiEventType.SEQUENCEFLOW_TAKEN,
sequenceFlow.getId(),
sourceFlowElement != null ? sourceFlowElement.getId() : null,
sourceFlowElement != null ? (String) sourceFlowElement.getName() : null,
sourceFlowElement != null ? sourceFlowElement.getClass().getName() : null,
sourceFlowElement != null ? ((FlowNode) sourceFlowElement).getBehavior() : null,
targetFlowElement != null ? targetFlowElement.getId() : null,
targetFlowElement != null ? targetFlowElement.getName() : null,
targetFlowElement != null ? targetFlowElement.getClass().getName() : null,
targetFlowElement != null ? ((FlowNode) targetFlowElement).getBehavior() : null));
}
// 获取 sequenceFlow 的下一个节点
FlowElement targetFlowElement = sequenceFlow.getTargetFlowElement();
// 更新 execution 当前所处的节点，前面更新过两次，第一次设置为 StartEvent，第二次设置为 SequenceFlow，当前第三次更新为审批流程图中的UserTask
execution.setCurrentFlowElement(targetFlowElement);
logger.debug("Sequence flow '{}' encountered. Continuing process by following it using execution {}",
sequenceFlow.getId(),
execution.getId());
// 继续前往下个节点，在这里又执行了一次 planContinueProcessOperation
Context.getAgenda().planContinueProcessOperation(execution);
}上面代码中，最后一行又是 Context.getAgenda().planContinueProcessOperation(ExecutionEntity) 方法，此方法去执行 ContinueProcessOperation 中的业务。这一次传给 planContinueProcessOperation 方法的 execution 实例，它的 currentFlowElement 从 SequenceFlow 变成了 UserTask。
ContinueProcessOperation 操作(第三次)
构造 ContinueProcessOperation
忽略阐述此重复内容。
执行 ContinueProcessOperation
UserTask 和 StartEvent 一样，它也是 FlowNode 的子类，因此在 ContinueProcessOperation 的 run 方法中，流程进入到 if 语句 中。
public class ContinueProcessOperation extends AbstractOperation {
// 省略部分代码
/**
* 当 ContinueProcessOperation 操作被执行时，以 run 方法作为入口
*/
@Override
public void run() {
// 第一次执行这个逻辑，取出来的 currentFlowElement 是一个 StartEvent，StartEvent 是 FlowNode 的子类
FlowElement currentFlowElement = getCurrentFlowElement(execution);
if (currentFlowElement instanceof FlowNode) {
// 继续执行 FlowNode 类型的类，StartEvent会走这个方法
continueThroughFlowNode((FlowNode) currentFlowElement);
} else if (currentFlowElement instanceof SequenceFlow) {
// 继续执行 SequenceFlow 类型的类
continueThroughSequenceFlow((SequenceFlow) currentFlowElement);
} else {
throw new ActivitiException("Programmatic error: no current flow element found or invalid type: " + currentFlowElement + ". Halting.");
}
}
}进入后，大致逻辑和 StartEvent 相似，但不同的点在于 UserTask 上的节点行为是 UserTaskActivityBehavior，此行为不像 StartEvent 的 NoneStartEventActivityBehavior 行为，UserTaskActivityBehavior 它定义了这个 UserTask 上的具体行为。在经过这个行为后，流程会暂时停在 UserTask 节点上，等待用户审批完成才会继续走下去。UserTask 节点的具体的行为见下一篇文章03 节点行为。下面这个代码和前面是重复的，为方便观察，所以粘贴一份。
/**
* 同步执行
* @param flowNode
*/
protected void executeSynchronous(FlowNode flowNode) {
// 调用HistoryManager记录流程活动开始了，这里并不是单单指StartEvent活动，而是所有活动
commandContext.getHistoryManager().recordActivityStart(execution);
// Execution listener: event 'start'
// 执行 StartEvent 上的监听器，发布 Start 的 Event 事件
if (CollectionUtil.isNotEmpty(flowNode.getExecutionListeners())) {
executeExecutionListeners(flowNode, ExecutionListener.EVENTNAME_START);
}
// Execute any boundary events, sub process boundary events will be executed from the activity behavior
if (!inCompensation && flowNode instanceof Activity) { // Only activities can have boundary events
// 获取节点上的边界事件
List<BoundaryEvent> boundaryEvents = ((Activity) flowNode).getBoundaryEvents();
if (CollectionUtil.isNotEmpty(boundaryEvents)) {
executeBoundaryEvents(boundaryEvents, execution);
}
}
// Execute actual behavior
// 执行实际的行为，UserTask对应的behavior是UserTaskActivityBehavior，这个UserTaskActivityBehavior里面会把task写入到act_ru_task
ActivityBehavior activityBehavior = (ActivityBehavior) flowNode.getBehavior();
// 当 activityBehavior 不为空，走此方法，此方法后续也会调用 planTakeOutgoingSequenceFlowsOperation
// activityBehavior 表示一个节点上拥有的行为
// StartEvent 节点的行为是 NoneStartEventActivityBehavior，没有做其它业务，仅仅是过度
// UserTask 节点的行为是 UserTaskActivityBehavior，这个行为会把任务写入到数据库后，等待用户完成任务，流程才会继续走下去
if (activityBehavior != null) {
executeActivityBehavior(activityBehavior, flowNode);
} else {
logger.debug("No activityBehavior on activity '{}' with execution {}", flowNode.getId(), execution.getId());
// 计划执行 TakeOutgoingSequenceFlows 操作，这个操作是一个连线行为，第一步先找出当前节点的出口，第二步从出口走到下一个节点。
Context.getAgenda().planTakeOutgoingSequenceFlowsOperation(execution, true);
}
}总结
本章节内容有点绕也有点复杂，下面画个简图辅助理解。图中的 ExecutionEntity 是 ProcessInstance 接口的实现类，所以 ExecutionEntity 本身就是 ProcessInstance 的一个实例，这个实例叫 流程实例。一个流程实例至少有一个或多个 ExecutionEntity 实例，流程实例本身称之为 Parent ExecutionEntity，而流程实例下的一个或多个 ExecutionEntity 称之为 Sub ExecutionEntity， 它们之间属于父子关系。像本章开头的审批流程图，因没有并行分支，所以只会产生一个 Sub ExecutionEntity。
总结流程流转要点：
- 启动流程实例：创建 ExecutionEntity 的实例，并将此实例的 CurrentFlowElement 指向 StartEvent。
- 流程流转到SequenceFlow：ExecutionEntity 流转到 SequenceFlow，此时将 CurrentFlowElement 指向 SequenceFlow。
- 流程流转到UserTask：ExecutionEntity 流转到 UserTask，此时将 CurrentFlowElement 指向 UserTask。
- 以此类推，当 ExecutionEntity 流转到 EndEvent 时，将 CurrentFlowElement 指向 EndEvent，标记此流程实例结束。
流程流转对应的示意图如下：
