Camunda邮件打印服务示例故障及步骤8执行疑问求助

问题背景 #

你提到的Camunda官方邮件打印服务示例无法正常运行，还遇到了mail变量无法解析的错误，同时有两个疑问需要解答：运行说明第8点的详细解释，以及启动流程前发送邮件的渠道。先来看下你提供的报错日志：

01-Jun-2018 12:06:11.325 SEVERE [http-nio-8080-exec-3] org.camunda.commons.logging.BaseLogger.logError ENGINE-16004 Exception while closing command context: Unknown property used in expression: ${mail.getAttachments().isEmpty()}. Cause: Cannot resolve identifier 'mail' org.camunda.bpm.engine.ProcessEngineException: Unknown property used in expression: ${mail.getAttachments().isEmpty()}. Cause: Cannot resolve identifier 'mail' at org.camunda.bpm.engine.impl.el.JuelExpression.getValue(JuelExpression.java:60) at org.camunda.bpm.engine.impl.el.UelExpressionCondition.evaluate(UelExpressionCondition.java:47) at org.camunda.bpm.engine.impl.el.UelExpressionCondition.evaluate(UelExpressionCondition.java:42) at org.camunda.bpm.engine.impl.bpmn.behavior.ExclusiveGatewayActivityBehavior.doLeave(ExclusiveGatewayActivityBehavior.java:59) at org.camunda.bpm.engine.impl.pvm.runtime.operation.PvmAtomicOperationActivityLeave.execute(PvmAtomicOperationActivityLeave.java:52) at org.camunda.bpm.engine.impl.pvm.runtime.operation.PvmAtomicOperationActivityLeave.execute(PvmAtomicOperationActivityLeave.java:28) at org.camunda.bpm.engine.impl.interceptor.AtomicOperationInvocation.execute(AtomicOperationInvocation.java:91) at org.camunda.bpm.engine.impl.interceptor.CommandInvocationContext.invokeNext(CommandInvocationContext.java:125) at org.camunda.bpm.engine.impl.interceptor.CommandInvocationContext.performNext(CommandInvocationContext.java:104) at org.camunda.bpm.engine.impl.interceptor.CommandInvocationContext.performOperation(CommandInvocationContext.java:79) at org.camunda.bpm.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:618) at org.camunda.bpm.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:594) at org.camunda.bpm.engine.impl.pvm.runtime.PvmExecutionImpl$5.callback(PvmExecutionImpl.java:1897) at org.camunda.bpm.engine.impl.pvm.runtime.PvmExecutionImpl$5.callback(PvmExecutionImpl.java:1894) at org.camunda.bpm.engine.impl.pvm.runtime.PvmExecutionImpl.continueExecutionIfNotCanceled(PvmExecutionImpl.java:1964) at org.camunda.bpm.engine.impl.pvm.runtime.PvmExecutionImpl.dispatchDelayedEventsAndPerformOperation(PvmExecutionImpl.java:1913) at org.camunda.bpm.engine.impl.pvm.runtime.PvmExecutionImpl.dispatchDelayedEventsAndPerformOperation(PvmExecutionImpl.java:1894) at org.camunda.bpm.engine.impl.bpmn.behavior.FlowNodeActivityBehavior.leave(FlowNodeActivityBehavior.java:48) at org.camunda.bpm.engine.impl.bpmn.behavior.FlowNodeActivityBehavior.execute(FlowNodeActivityBehavior.java:40) at org.camunda.bpm.engine.impl.pvm.runtime.operation.PvmAtomicOperationActivityExecute$2.callback(PvmAtomicOperationActivityExecute.java:57) at org.camunda.bpm.engine.impl.pvm.runtime.operation.PvmAtomicOperationActivityExecute$2.callback(PvmAtomicOperationActivityExecute.java:46) at org.camunda.bpm.engine.impl.pvm.runtime.PvmExecutionImpl.continueIfExecutionDoesNotAffectNextOperation(PvmExecutionImpl.java:1958) at org.camunda.bpm.engine.impl.pvm.runtime.operation.PvmAtomicOperationActivityExecute.execute(PvmAtomicOperationActivityExecute.java:38) at org.camunda.bpm.engine.impl.pvm.runtime.operation.PvmAtomicOperationActivityExecute.execute(PvmAtomicOperationActivityExecute.java:27) at org.camunda.bpm.engine.impl.interceptor.AtomicOperationInvocation.execute(AtomicOperationInvocation.java:91) at org.camunda.bpm.engine.impl.interceptor.CommandInvocationContext.invokeNext(CommandInvocationContext.java:125) at org.camunda.bpm.engine.impl.interceptor.CommandInvocationContext.performNext(CommandInvocationContext.java:104) at org.camunda.bpm.engine.impl.interceptor.CommandInvocationContext.performOperation(CommandInvocationContext.java:79) at org.camunda.bpm.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:618) at org.camunda.bpm.engine.impl.persistence.entity.ExecutionEntity.performOperation(ExecutionEntity.java:594) at org.camunda.bpm.engine.impl.pvm.runtime.PvmExecutionImpl$5.callback(PvmExecutionImpl.java:1897) at org.camunda.bpm.engine.impl.pvm.runtime.PvmExecutionImpl$5.callback(PvmExecutionImpl.java:1894) at org.camunda.bpm.engine.impl.pvm.runtime.PvmExecutionImpl.continueExecutionIfNotCanceled(PvmExecutionImpl.java:1964) at org.camunda.bpm.engine.impl.pvm.runtime.PvmExecutionImpl.dispatchDelayedEventsAndPerformOperation(PvmExecutionImpl.java:1913) at org.camunda.bpm.engine.impl.pvm.runtime.PvmExecutionImp

这个错误的核心是流程执行时找不到名为mail的变量，而流程里的排他网关表达式${mail.getAttachments().isEmpty()}依赖这个变量，所以直接抛出了异常。下面针对你的疑问逐一解答：

一、运行说明第8点的详细解释 #

这个示例的核心逻辑是通过接收邮件自动触发流程实例启动，第8点要求“启动流程前发送邮件”的本质是：

• 你配置的Camunda邮件连接器会持续轮询指定的邮箱（IMAP/POP3协议）
• 当你发送一封邮件到这个监听邮箱后，连接器会捕获到这封新邮件，解析邮件的内容、附件、发件人等信息
• 接着连接器会自动创建一个流程实例，并把解析后的邮件对象以mail为变量名注入到流程中，这样流程里的网关才能通过${mail.getAttachments().isEmpty()}判断邮件是否有附件，从而走不同的分支

简单来说，这一步是用邮件作为流程的触发信号，而不是手动去启动流程实例。

二、发送邮件的渠道 #

你需要把邮件发送到你在示例配置文件中指定的监听邮箱，具体步骤：

1. 找到示例中的mail-config.properties文件，里面会有类似如下配置项：

   mail.imap.host=your-imap-server
   mail.imap.port=993
   mail.imap.username=your-monitor-email@xxx.com
   mail.imap.password=your-email-password
   

   这里的mail.imap.username对应的邮箱就是你要发送邮件的目标邮箱。
2. 用任意邮件客户端（比如Outlook、Gmail、Foxmail）或者通过代码发送邮件到这个邮箱，邮件可以带附件或者不带，用来测试流程的分支逻辑。
3. 注意：发送邮件前要确保Camunda邮件服务已经启动，并且连接器处于正常轮询状态，这样才能及时捕获到新邮件并触发流程。

三、错误修复建议 #

针对你遇到的mail变量缺失错误，建议检查以下几点：

• 确认mail-config.properties中的邮箱配置是否正确，是否能正常连接到IMAP/POP3服务器
• 检查邮件连接器的配置，确认是否开启了“邮件触发流程”的功能，并且是否正确将邮件对象注入为mail变量
• 查看流程模型的启动事件，确认是“邮件启动事件”，或者连接器的逻辑是否正确关联到了流程定义

内容的提问来源于stack exchange，提问作者Monu Sharma