在開發過程中，突然發現自己和周圍的小夥伴總有一定的概率連線不上，並且機會很隨機，提示如下錯誤：

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連線資料庫，卻又一切正常，不合理啊！
“ArrayIndexOutOfBoundsException”，又是一個不正常的現象，連線資料庫，跟陣列還有關係嗎？什麼時候資料庫連線還需要陣列呢？什麼時候陣列還會越界呢？

繼續看異常，還發現了“commons.pool2”的身影，那問題就很好解決了，找到連線池的配置檔案，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> 

那問題在哪裡呢？啟動時報錯，只可能是initialSize的問（還有createDataSource的明證），將其改為10，提交程式碼管理庫，再次啟動，一切正常！

結論

在開發過程中，開發資料庫的連線數量是有限的，大家一起申請大量的初始連線池，必然會導致資料庫連線不夠用。