# 错误信息：

### Error querying database.  Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Failed to obtain JDBC Connection; nested exception is java.sql.SQLException: Connections could not be acquired from the underlying database!
### The error may exist in com/dao/BookMapper.xml
### The error may involve com.dao.BookMapper.queryAllBook
### The error occurred while executing a query
### Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Failed to obtain JDBC Connection; nested exception is java.sql.SQLException: Connections could not be acquired from the underlying database!
	java.sql.SQLException: Access denied for user 'Charles'@'localhost' (using password: YES)
		at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:129)
		at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:97)
		at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:122)
		at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:835)
		at com.mysql.cj.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:455)
		at com.mysql.cj.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:240)
		at com.mysql.cj.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:207)
		at com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:175)
		at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:220)
		at com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:206)
		at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource(C3P0PooledConnectionPool.java:203)
		at com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1138)
		at com.mchange.v2.resourcepool.BasicResourcePool.doAcquireAndDecrementPendingAcquiresWithinLockOnSuccess(BasicResourcePool.java:1125)
		at com.mchange.v2.resourcepool.BasicResourcePool.access$700(BasicResourcePool.java:44)
		at com.mchange.v2.resourcepool.BasicResourcePool$ScatteredAcquireTask.run(BasicResourcePool.java:1870)
		at com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:696)

原因分析：

1. 查看database.properties文件，但并未出错

driver = com.mysql.cj.jdbc.Driver
url = jdbc:mysql://localhost:3306/ssm?serverTimezone=UTC
username = root
password = admin

2. 使用的数据库是Mysql8.0版本，数据库驱动与数据库连接池的版本也和数据库版本相匹配

<!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.13</version>
        </dependency>

        <!-- 数据库连接池 -->
        <dependency>
            <groupId>com.mchange</groupId>
            <artifactId>c3p0</artifactId>
            <version>0.9.5.2</version>
        </dependency>

3. Project Structure里的 artifacts 也添加了相关依赖
   

4. spring-dao 配置文件里也并未写错，所有的引用路径都没错

	<context:property-placeholder location="classpath:database.properties"/>

    <bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource">
        <!-- 配置连接池属性 -->
        <property name="driverClass" value="${driver}"/>
        <property name="jdbcUrl" value="${url}"/>
        <property name="user" value="${username}"/>
        <property name="password" value="${password}"/>

        <!-- c3p0连接池的私有属性 -->
        <property name="maxPoolSize" value="30"/>
        <property name="minPoolSize" value="10"/>

        <!-- 关闭连接后不自动commit -->
        <property name="autoCommitOnClose" value="false"/>
        <!-- 获取连接超时时间 -->
        <property name="checkoutTimeout" value="10000"/>
        <!-- 当获取连接失败重试次数 -->
        <property name="acquireRetryAttempts" value="2"/>
    </bean>

光标放在value值上也有提示

网上的解决方法都尝试过，并未生效

解决方案：

最后偶然尝试将数据库连接池的属性显式赋值，连接成功！