c3p0连接数据库时报错，连接不上Communications link failure

具体错误如下：

 [2017-08-10 15:14:40|WARN |com.mchange.v2.resourcepool.BasicResourcePool] - com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@6cde6d8b -- 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 (10). Last acquisition attempt exception:   com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failureThe last packet successfully received from the server was 15 milliseconds ago.  The last packet sent successfully to the server was 15 milliseconds ago.    at sun.reflect.GeneratedConstructorAccessor53.newInstance(Unknown Source)    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)    at com.mysql.jdbc.Util.handleNewInstance(Util.java:404)    at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:988)    at com.mysql.jdbc.ExportControlled.transformSocketToSSLSocket(ExportControlled.java:164)    at com.mysql.jdbc.MysqlIO.negotiateSSLConnection(MysqlIO.java:4894)    at com.mysql.jdbc.MysqlIO.proceedHandshakeWithPluggableAuthentication(MysqlIO.java:1661)    at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:1228)    at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2253)    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2284)    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2083)    at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:806)    at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:47)    at sun.reflect.GeneratedConstructorAccessor50.newInstance(Unknown Source)    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)    at com.mysql.jdbc.Util.handleNewInstance(Util.java:404)    at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:410)    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:328)    at com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:134)    at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:182)    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)

博主出现这个错误的原因是因为以前用低版本的jdbc连接高版本的数据库时，加上过这样一句useSSL=true，现在数据库版本改变了，然后就报错了，找了很久。最后发现去掉这个就可以了。
连接url变为：db.url=jdbc:mysql://localhost:3306/xxx?characterEncoding=utf8