第一次执行SELECT语句，执行成功。执行这个语句之后执行其他操作，报错。下次再执行同一个SELECT的时候，报异常connection closed. 一下是第一次执行后再执行的同一个SELECT时，报的异常
select CONTRACT_NO from USER_CONTRACT where USER_ID=? and CONTRACT_ID=?

Cause: java.sql.SQLException: connection closed

; uncategorized SQLException; SQL state [null]; error code [0]; connection closed; nested exception is java.sql.SQLException: connection closed
at org.springframework.jdbc.support.AbstractFallbackSQLExceptionTranslator.translate(AbstractFallbackSQLExceptionTranslator.java:89)
at org.springframework.jdbc.support.AbstractFallbackSQLExceptionTranslator.translate(AbstractFallbackSQLExceptionTranslator.java:81)
at org.springframework.jdbc.support.AbstractFallbackSQLExceptionTranslator.translate(AbstractFallbackSQLExceptionTranslator.java:81)
at org.mybatis.spring.MyBatisExceptionTranslator.translateExceptionIfPossible(MyBatisExceptionTranslator.java:88)
at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:440)
at com.sun.proxy.$Proxy48.selectOne(Unknown Source)
at org.mybatis.spring.SqlSessionTemplate.selectOne(SqlSessionTemplate.java:159)
at org.apache.ibatis.binding.MapperMethod.execute(MapperMethod.java:87)
at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:93)
at com.sun.proxy.$Proxy61.queryContractNoById(Unknown Source)
at cfca.trustsign.ucprocessor.dao.UserContractDAO.queryContractNoById(UserContractDAO.java:168)
at cfca.trustsign.ucprocessor.service.UserContractService.lambda$null$74(UserContractService.java:1075)
at cfca.trustsign.ucprocessor.service.UserContractService$$Lambda$457/1006911611.accept(Unknown Source)
at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175)
at java.util.Iterator.forEachRemaining(Iterator.java:116)
at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:512)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502)
at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)
at cfca.trustsign.ucprocessor.service.UserContractService.lambda$checkNoIsNull$75(UserContractService.java:1069)
at cfca.trustsign.ucprocessor.service.UserContractService$$Lambda$455/94124101.accept(Unknown Source)
at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175)
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:512)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502)
at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)
at cfca.trustsign.ucprocessor.service.UserContractService.checkNoIsNull(UserContractService.java:1069)
at cfca.trustsign.ucprocessor.service.UserContractService$$FastClassBySpringCGLIB$$987c5e6f.invoke()
at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:218)
at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:749)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163)
at org.springframework.transaction.interceptor.TransactionInterceptor$$Lambda$287/1305961330.proceedWithInvocation(Unknown Source)
at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:295)
at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:98)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186)
at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:688)
at cfca.trustsign.ucprocessor.service.UserContractService$$EnhancerBySpringCGLIB$$4cc193b5.checkNoIsNull()
at cfca.trustsign.manager.service.UserContractQueryService.lambda$checkCnNull$52(UserContractQueryService.java:186)
at cfca.trustsign.manager.service.UserContractQueryService$$Lambda$452/565594602.get(Unknown Source)
at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1582)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.sql.SQLException: connection closed
at com.alibaba.druid.pool.DruidPooledConnection.checkStateInternal(DruidPooledConnection.java:1163)
at com.alibaba.druid.pool.DruidPooledConnection.checkState(DruidPooledConnection.java:1154)
at com.alibaba.druid.pool.DruidPooledConnection.prepareStatement(DruidPooledConnection.java:337)
at sun.reflect.GeneratedMethodAccessor550.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at org.apache.ibatis.logging.jdbc.ConnectionLogger.invoke(ConnectionLogger.java:55)
at com.sun.proxy.$Proxy91.prepareStatement(Unknown Source)
at org.apache.ibatis.executor.statement.PreparedStatementHandler.instantiateStatement(PreparedStatementHandler.java:86)
at org.apache.ibatis.executor.statement.BaseStatementHandler.prepare(BaseStatementHandler.java:88)
at org.apache.ibatis.executor.statement.RoutingStatementHandler.prepare(RoutingStatementHandler.java:59)
at org.apache.ibatis.executor.ReuseExecutor.prepareStatement(ReuseExecutor.java:89)
at org.apache.ibatis.executor.ReuseExecutor.doQuery(ReuseExecutor.java:59)
at org.apache.ibatis.executor.BaseExecutor.queryFromDatabase(BaseExecutor.java:324)
at org.apache.ibatis.executor.BaseExecutor.query(BaseExecutor.java:156)
at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:109)
at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:83)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:147)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:140)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectOne(DefaultSqlSession.java:76)
at sun.reflect.GeneratedMethodAccessor549.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:426)
... 46 common frames omitted