最近在学习activiti，各种坑不断，导入了基本的流程包：

<dependency>
            <groupId>org.activiti</groupId>
            <artifactId>activiti-spring-boot-starter</artifactId>
            <version>7.1.0.M4</version>
            <exclusions>
                <exclusion>
                    <groupId>org.mybatis</groupId>
                    <artifactId>mybatis</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
        <dependency>
            <groupId>org.activiti.dependencies</groupId>
            <artifactId>activiti-dependencies</artifactId>
            <version>7.1.0.M4</version>
            <type>pom</type>
        </dependency>
        <!-- 生成流程图 -->
        <dependency>
            <groupId>org.activiti</groupId>
            <artifactId>activiti-image-generator</artifactId>
            <version>7.1.0.M4</version>
        </dependency>

运行起来后，又报：Error creating bean with name 'processEngine': FactoryBean threw exception on object creation;

完整错误：

Error creating bean with name 'processEngine': FactoryBean threw exception on object creation; nested exception is org.activiti.engine.ActivitiException: couldn't check if tables are already present using metadata: null
    at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:798)
    at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:539)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1338)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1177)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:557)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:517)
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:323)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:321)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
    at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1108)
    at org.springframework.boot.actuate.endpoint.annotation.EndpointDiscoverer.createEndpointBean(EndpointDiscoverer.java:143)
    at org.springframework.boot.actuate.endpoint.annotation.EndpointDiscoverer.createEndpointBeans(EndpointDiscoverer.java:133)
    at org.springframework.boot.actuate.endpoint.annotation.EndpointDiscoverer.discoverEndpoints(EndpointDiscoverer.java:122)
    at org.springframework.boot.actuate.endpoint.annotation.EndpointDiscoverer.getEndpoints(EndpointDiscoverer.java:116)
    at org.springframework.boot.actuate.autoconfigure.endpoint.web.ServletEndpointManagementContextConfiguration$WebMvcServletEndpointManagementContextConfiguration.servletEndpointRegistrar(ServletEndpointManagementContextConfiguration.java:65)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:154)
    ... 56 common frames omitted
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'processEngine': FactoryBean threw exception on object creation; nested exception is org.activiti.engine.ActivitiException: couldn't check if tables are already present using metadata: null
    at org.springframework.beans.factory.support.FactoryBeanRegistrySupport.doGetObjectFromFactoryBean(FactoryBeanRegistrySupport.java:178)
    at org.springframework.beans.factory.support.FactoryBeanRegistrySupport.getObjectFromFactoryBean(FactoryBeanRegistrySupport.java:101)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getObjectForBeanInstance(AbstractBeanFactory.java:1818)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.getObjectForBeanInstance(AbstractAutowireCapableBeanFactory.java:1266)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)
    at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1287)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1207)
    at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:885)
    at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:789)
    ... 76 common frames omitted
Caused by: org.activiti.engine.ActivitiException: couldn't check if tables are already present using metadata: null
    at org.activiti.engine.impl.db.DbSqlSession.isTablePresent(DbSqlSession.java:1187)
    at org.activiti.engine.impl.db.DbSqlSession.isEngineTablePresent(DbSqlSession.java:1128)
    at org.activiti.engine.impl.db.DbSqlSession.dbSchemaUpdate(DbSqlSession.java:1050)
    at org.activiti.engine.impl.db.DbSqlSession.performSchemaOperationsProcessEngineBuild(DbSqlSession.java:1501)
    at org.activiti.engine.impl.SchemaOperationsProcessEngineBuild.execute(SchemaOperationsProcessEngineBuild.java:28)
    at org.activiti.engine.impl.interceptor.CommandInvoker$1.run(CommandInvoker.java:37)
    at org.activiti.engine.impl.interceptor.CommandInvoker.executeOperation(CommandInvoker.java:78)
    at org.activiti.engine.impl.interceptor.CommandInvoker.executeOperations(CommandInvoker.java:57)
    at org.activiti.engine.impl.interceptor.CommandInvoker.execute(CommandInvoker.java:42)
    at org.activiti.engine.impl.interceptor.TransactionContextInterceptor.execute(TransactionContextInterceptor.java:48)
    at org.activiti.engine.impl.interceptor.CommandContextInterceptor.execute(CommandContextInterceptor.java:59)
    at org.activiti.spring.SpringTransactionInterceptor$1.doInTransaction(SpringTransactionInterceptor.java:47)
    at org.springframework.transaction.support.TransactionTemplate.execute(TransactionTemplate.java:140)
    at org.activiti.spring.SpringTransactionInterceptor.execute(SpringTransactionInterceptor.java:45)
    at org.activiti.engine.impl.interceptor.LogInterceptor.execute(LogInterceptor.java:29)
    at org.activiti.engine.impl.cfg.CommandExecutorImpl.execute(CommandExecutorImpl.java:44)
    at org.activiti.engine.impl.ProcessEngineImpl.<init>(ProcessEngineImpl.java:68)
    at org.activiti.engine.impl.cfg.ProcessEngineConfigurationImpl.buildProcessEngine(ProcessEngineConfigurationImpl.java:848)
    at org.activiti.spring.SpringProcessEngineConfiguration.buildProcessEngine(SpringProcessEngineConfiguration.java:79)
    at org.activiti.spring.ProcessEngineFactoryBean.getObject(ProcessEngineFactoryBean.java:57)
    at org.activiti.spring.ProcessEngineFactoryBean.getObject(ProcessEngineFactoryBean.java:32)
    at org.springframework.beans.factory.support.FactoryBeanRegistrySupport.doGetObjectFromFactoryBean(FactoryBeanRegistrySupport.java:171)
    ... 86 common frames omitted
Caused by: java.lang.NullPointerException: null
    at org.apache.shardingsphere.shardingjdbc.jdbc.core.datasource.metadata.ResultSetReturnedDatabaseMetaData.getActualTableNamePattern(ResultSetReturnedDatabaseMetaData.java:265)
    at org.apache.shardingsphere.shardingjdbc.jdbc.core.datasource.metadata.ResultSetReturnedDatabaseMetaData.getTables(ResultSetReturnedDatabaseMetaData.java:94)
    at org.activiti.engine.impl.db.DbSqlSession.isTablePresent(DbSqlSession.java:1173)
    ... 107 common frames omitted

配置文件：

  activiti:
    #数据库类型
    database-type: postgres
    # 1.false：默认值。activiti在启动时，对比数据库表中保存的版本，如果没有表或者版本不匹配，将抛出异常。
    # 2.true：activiti会对数据库中所有表进行更新操作。如果表不存在，则自动创建
    # 3.create-drop：在activiti启动时创建表，在关闭时删除表(必须手动关闭引擎，才能删除表)
    # 4.drop-create：在activiti启动时删除原来的旧表，然后在创建新表（不需要手动关闭引擎）
    # 检测历史表是否存在 activiti7 默认没有开启数据库历史记录  启动数据库历史记录
    #database-schema-update: true 自动生成activiti的表 
    database-schema-update: true
    db-history-used: true
    # 记录历史等级 可配置的历史级别有 none、activity、audit、full
    # none：不保存任何的历史数据，因此，在流程执行过程中，这是最高效的。
    # activity：级别高于none，保存流程实列与流程行为，其他数据不保存。
    # audit：除了activity级别会保存的数据外，还会保存全部的流程任务及其属性。audit为history的默认值。
    # full：保存历史数据的最高级别，除了会保存audit级别的数据外，还会保存其他全部流程相关的细节数据，包括一些流程参数等。
    history-level: FULL
    # 校验流程文件，默认校验resource下processes文件夹里的流程文件。
    # 自动部署验证设置:true-开启（默认）、false-关闭
    #check-process-definitions: true
    # 流程定义文件存放目录，默认路径为 classpath:/processes/
    process-definition-location-prefix: classpath:/bpmn/

一开始以为是数据库配置的问题，但是其他微服务都正常的，而且他报的错误的是表相关的，而我库内有没有新生成的表，就很奇怪了，然后想到一个属性：

 database-schema-update: true，如果为true，则为更新，但是我的表不存在自然就报错了，我就给true改成了database-schema-update: drop-create属性，结果启动成功了。

启动后，我又给属性改为true，则还是报错。我理解为某个属性可能不存在导致。改为false则正常启动。