使用c3p0连接池连接数据库时出现：Connections could not be acquired from the underlying database!

当使用c3p0连接数据库时，配置信息都没有错误，但是出现了如下情况：

下列信息中有一行是java.lang.RuntimeException: java.sql.SQLException: Connections could not be acquired from the underlying database!

也就是连接数据库失败

D:\sofeware\JDK8\JDK8installer\bin\java.exe -ea -Didea.test.cyclic.buffer.size=1048576 "-javaagent:D:\sofeware\IntelliJ IDE\IntelliJ IDEA 2019.3.4\lib\idea_rt.jar=60323:D:\sofeware\IntelliJ IDE\IntelliJ IDEA 2019.3.4\bin" -Dfile.encoding=UTF-8 -classpath "D:\sofeware\IntelliJ IDE\IntelliJ IDEA 2019.3.4\lib\idea_rt.jar;D:\sofeware\IntelliJ IDE\IntelliJ IDEA 2019.3.4\plugins\junit\lib\junit5-rt.jar;D:\sofeware\IntelliJ IDE\IntelliJ IDEA 2019.3.4\plugins\junit\lib\junit-rt.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\charsets.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\deploy.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\ext\access-bridge-64.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\ext\cldrdata.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\ext\dnsns.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\ext\jaccess.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\ext\jfxrt.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\ext\localedata.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\ext\nashorn.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\ext\sunec.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\ext\sunjce_provider.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\ext\sunmscapi.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\ext\sunpkcs11.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\ext\zipfs.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\javaws.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\jce.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\jfr.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\jfxswt.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\jsse.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\management-agent.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\plugin.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\resources.jar;D:\sofeware\JDK8\JDK8installer\jre\lib\rt.jar;D:\sofeware\IntelliJIDEAProjects\day02_eesy_02xmlIoC\target\classes;D:\sofeware\apache_maven_repository\org\springframework\spring-context\5.0.2.RELEASE\spring-context-5.0.2.RELEASE.jar;D:\sofeware\apache_maven_repository\org\springframework\spring-aop\5.0.2.RELEASE\spring-aop-5.0.2.RELEASE.jar;D:\sofeware\apache_maven_repository\org\springframework\spring-beans\5.0.2.RELEASE\spring-beans-5.0.2.RELEASE.jar;D:\sofeware\apache_maven_repository\org\springframework\spring-core\5.0.2.RELEASE\spring-core-5.0.2.RELEASE.jar;D:\sofeware\apache_maven_repository\org\springframework\spring-jcl\5.0.2.RELEASE\spring-jcl-5.0.2.RELEASE.jar;D:\sofeware\apache_maven_repository\org\springframework\spring-expression\5.0.2.RELEASE\spring-expression-5.0.2.RELEASE.jar;D:\sofeware\apache_maven_repository\org\springframework\spring-test\5.0.2.RELEASE\spring-test-5.0.2.RELEASE.jar;D:\sofeware\apache_maven_repository\commons-dbutils\commons-dbutils\1.4\commons-dbutils-1.4.jar;D:\sofeware\apache_maven_repository\mysql\mysql-connector-java\5.1.6\mysql-connector-java-5.1.6.jar;D:\sofeware\apache_maven_repository\c3p0\c3p0\0.9.1.2\c3p0-0.9.1.2.jar;D:\sofeware\apache_maven_repository\junit\junit\4.12\junit-4.12.jar;D:\sofeware\apache_maven_repository\org\hamcrest\hamcrest-core\1.3\hamcrest-core-1.3.jar" com.intellij.rt.junit.JUnitStarter -ideVersion5 -junit4 com.itheima.test.AccountServiceTest,findAllAccountTest
五月 10, 2020 5:06:01 下午 org.springframework.context.support.AbstractApplicationContext prepareRefresh
信息: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@2471cca7: startup date [Sun May 10 17:06:01 CST 2020]; root of context hierarchy
五月 10, 2020 5:06:01 下午 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from class path resource [bean.xml]
五月 10, 2020 5:06:02 下午 com.mchange.v2.log.MLog 
信息: MLog clients using java 1.4+ standard logging.
五月 10, 2020 5:06:04 下午 com.mchange.v2.c3p0.C3P0Registry banner
信息: Initializing c3p0-0.9.1.2 [built 21-May-2007 15:04:56; debug? true; trace: 10]
五月 10, 2020 5:06:04 下午 com.mchange.v2.c3p0.impl.AbstractPoolBackedDataSource getPoolManager
信息: Initializing c3p0 pool... com.mchange.v2.c3p0.ComboPooledDataSource [ acquireIncrement -> 3, acquireRetryAttempts -> 30, acquireRetryDelay -> 1000, autoCommitOnClose -> false, automaticTestTable -> null, breakAfterAcquireFailure -> false, checkoutTimeout -> 0, connectionCustomizerClassName -> null, connectionTesterClassName -> com.mchange.v2.c3p0.impl.DefaultConnectionTester, dataSourceName -> 1hgeby9a91yfh7vijkheip|76a3e297, debugUnreturnedConnectionStackTraces -> false, description -> null, driverClass -> com.mysql.jdbc.Driver, factoryClassLocation -> null, forceIgnoreUnresolvedTransactions -> false, identityToken -> 1hgeby9a91yfh7vijkheip|76a3e297, idleConnectionTestPeriod -> 0, initialPoolSize -> 3, jdbcUrl -> jdbc:mysql://localhost:3306/eesy, maxAdministrativeTaskTime -> 0, maxConnectionAge -> 0, maxIdleTime -> 0, maxIdleTimeExcessConnections -> 0, maxPoolSize -> 15, maxStatements -> 0, maxStatementsPerConnection -> 0, minPoolSize -> 3, numHelperThreads -> 3, numThreadsAwaitingCheckoutDefaultUser -> 0, preferredTestQuery -> null, properties -> {user=******, password=******}, propertyCycle -> 0, testConnectionOnCheckin -> false, testConnectionOnCheckout -> false, unreturnedConnectionTimeout -> 0, usesTraditionalReflectiveProxies -> false ]
五月 10, 2020 5:06:34 下午 com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask run
警告: com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@45f64cf6 -- Acquisition Attempt Failed!!! Clearing pending acquires. While trying to acquire a needed new resource, we failed to succeed more than the maximum number of allowed acquisition attempts (30). Last acquisition attempt exception: 
java.sql.SQLException: Unknown system variable 'tx_isolation'
	at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1055)
	at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:956)
	at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3491)
	at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3423)
	at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1936)
	at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2060)
	at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2536)
	at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2465)
	at com.mysql.jdbc.StatementImpl.executeQuery(StatementImpl.java:1383)
	at com.mysql.jdbc.ConnectionImpl.getTransactionIsolation(ConnectionImpl.java:3117)
	at com.mchange.v2.c3p0.impl.NewPooledConnection.(NewPooledConnection.java:107)
	at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:198)
	at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:171)
	at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource(C3P0PooledConnectionPool.java:137)
	at com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1014)
	at com.mchange.v2.resourcepool.BasicResourcePool.access$800(BasicResourcePool.java:32)
	at com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask.run(BasicResourcePool.java:1810)
	at com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547)

五月 10, 2020 5:06:34 下午 com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask run
警告: com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@1ae0017c -- Acquisition Attempt Failed!!! Clearing pending acquires. While trying to acquire a needed new resource, we failed to succeed more than the maximum number of allowed acquisition attempts (30). Last acquisition attempt exception: 
java.sql.SQLException: Unknown system variable 'tx_isolation'
	at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1055)
	at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:956)
	at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3491)
	at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3423)
	at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1936)
	at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2060)
	at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2536)
	at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2465)
	at com.mysql.jdbc.StatementImpl.executeQuery(StatementImpl.java:1383)
	at com.mysql.jdbc.ConnectionImpl.getTransactionIsolation(ConnectionImpl.java:3117)
	at com.mchange.v2.c3p0.impl.NewPooledConnection.(NewPooledConnection.java:107)
	at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:198)
	at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:171)
	at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource(C3P0PooledConnectionPool.java:137)
	at com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1014)
	at com.mchange.v2.resourcepool.BasicResourcePool.access$800(BasicResourcePool.java:32)
	at com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask.run(BasicResourcePool.java:1810)
	at com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547)

五月 10, 2020 5:06:34 下午 com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask run
警告: com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@2c13a2f2 -- Acquisition Attempt Failed!!! Clearing pending acquires. While trying to acquire a needed new resource, we failed to succeed more than the maximum number of allowed acquisition attempts (30). Last acquisition attempt exception: 
java.sql.SQLException: Unknown system variable 'tx_isolation'
	at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1055)
	at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:956)
	at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3491)
	at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3423)
	at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1936)
	at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2060)
	at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2536)
	at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2465)
	at com.mysql.jdbc.StatementImpl.executeQuery(StatementImpl.java:1383)
	at com.mysql.jdbc.ConnectionImpl.getTransactionIsolation(ConnectionImpl.java:3117)
	at com.mchange.v2.c3p0.impl.NewPooledConnection.(NewPooledConnection.java:107)
	at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:198)
	at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:171)
	at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource(C3P0PooledConnectionPool.java:137)
	at com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1014)
	at com.mchange.v2.resourcepool.BasicResourcePool.access$800(BasicResourcePool.java:32)
	at com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask.run(BasicResourcePool.java:1810)
	at com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547)


java.lang.RuntimeException: java.sql.SQLException: Connections could not be acquired from the underlying database!

	at com.itheima.dao.impl.AccountDaoImpl.findAllAccount(AccountDaoImpl.java:24)
	at com.itheima.service.impl.AccountServiceImpl.findAllAccount(AccountServiceImpl.java:14)
	at com.itheima.test.AccountServiceTest.findAllAccountTest(AccountServiceTest.java:25)
	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.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
	at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
	at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230)
	at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58)
Caused by: java.sql.SQLException: Connections could not be acquired from the underlying database!
	at com.mchange.v2.sql.SqlUtils.toSQLException(SqlUtils.java:106)
	at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:529)
	at com.mchange.v2.c3p0.impl.AbstractPoolBackedDataSource.getConnection(AbstractPoolBackedDataSource.java:128)
	at org.apache.commons.dbutils.AbstractQueryRunner.prepareConnection(AbstractQueryRunner.java:156)
	at org.apache.commons.dbutils.QueryRunner.query(QueryRunner.java:303)
	at com.itheima.dao.impl.AccountDaoImpl.findAllAccount(AccountDaoImpl.java:22)
	... 24 more
Caused by: com.mchange.v2.resourcepool.CannotAcquireResourceException: A ResourcePool could not acquire a resource from its primary factory or source.
	at com.mchange.v2.resourcepool.BasicResourcePool.awaitAvailable(BasicResourcePool.java:1319)
	at com.mchange.v2.resourcepool.BasicResourcePool.prelimCheckoutResource(BasicResourcePool.java:557)
	at com.mchange.v2.resourcepool.BasicResourcePool.checkoutResource(BasicResourcePool.java:477)
	at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:525)
	... 28 more


Process finished with exit code -1

我当时使用的是5.1.6版本的

    mysql
    mysql-connector-java
    5.1.6

当把数据库驱动的版本改为5.1.44时，问题就解决了