problem during schema drop, statement alter table ACT_RU_VARIABLE
将activiti的databaseSchemaUpdate设置为drop-create抛出DbSqlSession 错误
今天在测试activiti的databaseSchemaUpdate属性的drop-create的时候一直抛出org.activiti.engine.impl.db.DbSqlSession错误信息，开始有点疑惑，折腾了很久才知道，原来是我将activiti数据库中的某些表或者记录删除，造成外键约束混乱，才会抛出这个错误。
错误信息：
7 [main] INFO org.activiti.engine.ProcessEngines - Initializing process engine using configuration 'file:/F:/Project_All/Eclipase_Workspace/Activiti_test01/bin/activiti.cfg.xml'
7 [main] INFO org.activiti.engine.ProcessEngines - initializing process engine for resource file:/F:/Project_All/Eclipase_Workspace/Activiti_test01/bin/activiti.cfg.xml
2015-2-14 9:20:40 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from resource loaded through InputStream
5562 [main] INFO org.activiti.engine.impl.db.DbSqlSession - performing drop on engine with resource org/activiti/db/drop/activiti.mysql.drop.engine.sql
12467 [main] ERROR org.activiti.engine.impl.db.DbSqlSession - problem during schema drop, statement drop index ACT_IDX_VARIABLE_TASK_ID on ACT_RU_VARIABLE
com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Table 'activiti.act_ru_variable' doesn't exist
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:407)
at com.mysql.jdbc.Util.getInstance(Util.java:382)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1052)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3603)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3535)
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1989)
at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2150)
at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2620)
at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2570)
at com.mysql.jdbc.StatementImpl.execute(StatementImpl.java:779)
at com.mysql.jdbc.StatementImpl.execute(StatementImpl.java:622)
at org.activiti.engine.impl.db.DbSqlSession.executeSchemaResource(DbSqlSession.java:930)
at org.activiti.engine.impl.db.DbSqlSession.executeSchemaResource(DbSqlSession.java:881)
at org.activiti.engine.impl.db.DbSqlSession.executeMandatorySchemaResource(DbSqlSession.java:698)
at org.activiti.engine.impl.db.DbSqlSession.dbSchemaDrop(DbSqlSession.java:679)
at org.activiti.engine.impl.db.DbSqlSession.performSchemaOperationsProcessEngineBuild(DbSqlSession.java:1000)
at org.activiti.engine.impl.SchemaOperationsProcessEngineBuild.execute(SchemaOperationsProcessEngineBuild.java:25)
at org.activiti.engine.impl.interceptor.CommandInvoker.execute(CommandInvoker.java:24)
at org.activiti.engine.impl.interceptor.CommandContextInterceptor.execute(CommandContextInterceptor.java:57)
at org.activiti.engine.impl.interceptor.LogInterceptor.execute(LogInterceptor.java:31)
at org.activiti.engine.impl.cfg.CommandExecutorImpl.execute(CommandExecutorImpl.java:40)
at org.activiti.engine.impl.ProcessEngineImpl.<init>(ProcessEngineImpl.java:72)
at org.activiti.engine.impl.cfg.ProcessEngineConfigurationImpl.buildProcessEngine(ProcessEngineConfigurationImpl.java:363)
at org.activiti.engine.ProcessEngines.buildProcessEngine(ProcessEngines.java:194)
at org.activiti.engine.ProcessEngines.initProcessEnginFromResource(ProcessEngines.java:167)
at org.activiti.engine.ProcessEngines.init(ProcessEngines.java:98)
at org.activiti.engine.ProcessEngines.getProcessEngine(ProcessEngines.java:224)
at org.activiti.engine.ProcessEngines.getDefaultProcessEngine(ProcessEngines.java:217)
at com.bug315.ActivitiTest01.main(ActivitiTest01.java:15)
17427 [main] ERROR org.activiti.engine.impl.db.DbSqlSession - problem during schema drop, statement alter table ACT_RU_VARIABLE
drop FOREIGN KEY ACT_FK_VAR_EXE
解决办法：
手动将activiti的所有表删除，然后修改activiti.cfg.xml配置文件中的databaseSchemaUpdate=“drop-create”，最后启动activiti引擎，此时会出现表不存在等错误，因为表已经被你手动删除了。执行完后，再启动activiti引擎，这个问题就不再抛出了。
databaseSchemaUpdate属性的取值：
flase： 默认值。activiti在启动时，会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。
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。
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）。
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）。
