最近搭建了一个Spring MVC项目，配置完数据库连接，运行时一直报错：
警告: com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask@1316264 -- 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: com.microsoft.sqlserver.jdbc.SQLServerException: 用户 'wenjianbao' 登录失败。 at com.microsoft.sqlserver.jdbc.SQLServerException.makeFromDatabaseError(SQLServerException.java:196) at com.microsoft.sqlserver.jdbc.TDSTokenHandler.onEOF(tdsparser.java:246) at com.microsoft.sqlserver.jdbc.TDSParser.parse(tdsparser.java:83) at com.microsoft.sqlserver.jdbc.SQLServerConnection.sendLogon(SQLServerConnection.java:2532) at com.microsoft.sqlserver.jdbc.SQLServerConnection.logon(SQLServerConnection.java:1929) at com.microsoft.sqlserver.jdbc.SQLServerConnection.access$000(SQLServerConnection.java:41) at com.microsoft.sqlserver.jdbc.SQLServerConnection$LogonCommand.doExecute(SQLServerConnection.java:1917) at com.microsoft.sqlserver.jdbc.TDSCommand.execute(IOBuffer.java:4026) at com.microsoft.sqlserver.jdbc.SQLServerConnection.executeCommand(SQLServerConnection.java:1416) at com.microsoft.sqlserver.jdbc.SQLServerConnection.connectHelper(SQLServerConnection.java:1061) at com.microsoft.sqlserver.jdbc.SQLServerConnection.login(SQLServerConnection.java:833) at com.microsoft.sqlserver.jdbc.SQLServerConnection.connect(SQLServerConnection.java:716) at com.microsoft.sqlserver.jdbc.SQLServerDriver.connect(SQLServerDriver.java:841) at com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:134)
数据库配置如下：
(1) 文件 spring-mybatis.xml
<context:property-placeholder location="classpath:/db.properties"/>
<bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource" destroy-method="close"
p:driverClass="${driver}"
p:jdbcUrl="${url}"
p:user="${username}"
p:password="${password}"
p:idleConnectionTestPeriod="${idleConnectionTestPeriod}"
p:maxIdleTime="${maxIdleTime}"
p:acquireIncrement="${acquireIncrement}"
p:initialPoolSize="${initialPoolSize}"
p:maxPoolSize="${maxPoolSize}"
p:minPoolSize="${minPoolSize}"
p:autoCommitOnClose="${autoCommitOnClose}"
p:checkoutTimeout="${checkoutTimeout}"
p:acquireRetryAttempts="${acquireRetryAttempts}"
p:preferredTestQuery="SELECT 1"
p:maxConnectionAge="3000"/>
(2) 文件 db.properties
driver=com.mysql.cj.jdbc.Driver url=jdbc:mysql://localhost:3306/java_web?serverTimezone=Asia/Shanghai&useUnicode=true&characterEncoding=utf8&useSSL=false username=root password=123456 idleConnectionTestPeriod=60 maxIdleTime=240 acquireIncrement=5 initialPoolSize=10 maxPoolSize=30 minPoolSize=10 autoCommitOnClose=false checkoutTimeout=1000 acquireRetryAttempts=2
检查了半天，读取配置没问题，数据库配置也没问题。最后看到一篇博客说是${}取值的问题，觉得可能是这个问题，于是把数据库配置改成如下：
(1) 文件 spring-mybatis.xml
<context:property-placeholder location="classpath:/db.properties"/>
<bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource" destroy-method="close"
p:driverClass="${db_driver}"
p:jdbcUrl="${db_url}"
p:user="${db_username}"
p:password="${db_password}"
p:idleConnectionTestPeriod="${db_idleConnectionTestPeriod}"
p:maxIdleTime="${db_maxIdleTime}"
p:acquireIncrement="${db_acquireIncrement}"
p:initialPoolSize="${db_initialPoolSize}"
p:maxPoolSize="${db_maxPoolSize}"
p:minPoolSize="${db_minPoolSize}"
p:autoCommitOnClose="${db_autoCommitOnClose}"
p:checkoutTimeout="${db_checkoutTimeout}"
p:acquireRetryAttempts="${db_acquireRetryAttempts}"
p:preferredTestQuery="SELECT 1"
p:maxConnectionAge="3000"/>
(2) 文件 db.properties
db_driver=com.mysql.cj.jdbc.Driver db_url=jdbc:mysql://localhost:3306/java_web?serverTimezone=Asia/Shanghai&useUnicode=true&characterEncoding=utf8&useSSL=false db_username=root db_password=123456 db_idleConnectionTestPeriod=60 db_maxIdleTime=240 db_acquireIncrement=5 db_initialPoolSize=10 db_maxPoolSize=30 db_minPoolSize=10 db_autoCommitOnClose=false db_checkoutTimeout=1000 db_acquireRetryAttempts=2
然后，数据连接成功了。
问题原因：
变量名有冲突，重名了，加个前置（如：db_）保证其唯一性 即可！
参考：https://blog.csdn.net/qq_21134557/article/details/81004205
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