在开发过程中，突然发现自己和周围的小伙伴总有一定的概率连接不上，并且机会很随机，提示如下错误：
Caused by: org.hibernate.tool.schema.spi.SchemaManagementException: Unable to open JDBC connection for schema management target
at org.hibernate.tool.schema.internal.TargetDatabaseImpl.prepare(TargetDatabaseImpl.java:42)
at org.hibernate.tool.schema.internal.SchemaMigratorImpl.doMigration(SchemaMigratorImpl.java:57)
at org.hibernate.tool.hbm2ddl.SchemaUpdate.execute(SchemaUpdate.java:134)
at org.hibernate.tool.hbm2ddl.SchemaUpdate.execute(SchemaUpdate.java:101)
at org.hibernate.internal.SessionFactoryImpl.<init>(SessionFactoryImpl.java:472)
at org.hibernate.boot.internal.SessionFactoryBuilderImpl.build(SessionFactoryBuilderImpl.java:444)
at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.build(EntityManagerFactoryBuilderImpl.java:879)
... 46 common frames omitted
Caused by: java.sql.SQLException: Error preloading the connection pool
at org.apache.commons.dbcp2.BasicDataSource.createDataSource(BasicDataSource.java:2085)
at org.apache.commons.dbcp2.BasicDataSource.getConnection(BasicDataSource.java:1533)
at org.hibernate.engine.jdbc.connections.internal.DatasourceConnectionProviderImpl.getConnection(DatasourceConnectionProviderImpl.java:122)
at org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator$ConnectionProviderJdbcConnectionAccess.obtainConnection(JdbcEnvironmentInitiator.java:180)
at org.hibernate.tool.schema.internal.TargetDatabaseImpl.prepare(TargetDatabaseImpl.java:38)
... 52 common frames omitted
Caused by: java.sql.SQLNonTransientConnectionException: Could not create connection to database server.
at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:550)
at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:537)
at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:527)
at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:512)
at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:480)
at com.mysql.cj.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:1807)
at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:1624)
at com.mysql.cj.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:637)
at com.mysql.cj.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:351)
at com.mysql.cj.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:224)
at org.apache.commons.dbcp2.DriverConnectionFactory.createConnection(DriverConnectionFactory.java:39)
at org.apache.commons.dbcp2.PoolableConnectionFactory.makeObject(PoolableConnectionFactory.java:256)
at org.apache.commons.pool2.impl.GenericObjectPool.create(GenericObjectPool.java:868)
at org.apache.commons.pool2.impl.GenericObjectPool.addObject(GenericObjectPool.java:961)
at org.apache.commons.dbcp2.BasicDataSource.createDataSource(BasicDataSource.java:2081)
... 56 common frames omitted
Caused by: java.lang.ArrayIndexOutOfBoundsException: null用Navicat for MySQL连接数据库，却又一切正常，不合理啊！
“ArrayIndexOutOfBoundsException”，又是一个不正常的现象，连接数据库，跟数组还有关系吗？什么时候数据库连接还需要数组呢？什么时候数组还会越界呢？
继续看异常，还发现了“commons.pool2”的身影，那问题就很好解决了，找到连接池的配置文件，如下：
<bean id="dataSource" destroy-method="close" class="org.apache.commons.dbcp2.BasicDataSource">
<property name="driverClassName" value="${jdbc.driver}"/>
<property name="url" value="${jdbc.url}"/>
<property name="username" value="${jdbc.username}"/>
<property name="password" value="${jdbc.password}"/>
<property name="initialSize" value="50"/>
<property name="maxIdle" value="50"/>
<property name="maxWaitMillis" value="10000"/>
<property name="maxTotal" value="100" />
</bean> 那问题在哪里呢？启动时报错，只可能是initialSize的问（还有createDataSource的明证），将其改为10，提交代码管理库，再次启动，一切正常！
结论
在开发过程中，开发数据库的连接数量是有限的，大家一起申请大量的初始连接池，必然会导致数据库连接不够用。