深度剖析SpringBoot框架下关于项目运行时数据库连接失败的可能问题、解决方案

登录失败: ### Error querying database. 
Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Failed to obtain JDBC Connection ### The error may exist in file [D:\develep\projects\Books\target\classes\mapper\UserMapper.xml] ### The error may involve com.example.mapper.UserMapper.selectByUsername ### The error occurred while executing a query ### Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Failed to obtain JDBC Connection

连接失败！ 错误类型: SQLException 错误信息: Access denied for user 'root'@'localhost' (using password: YES)
可能的原因:
MySQL服务未启动
数据库'library'不存在
用户名或密码错误
MySQL端口(3306)被占用或防火墙阻止
MySQL不允许远程连接（虽然这里是localhost） java.sql.SQLException: Access denied for user 'root'@'localhost' (using password: YES) at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:121) at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:114) at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:840) at com.mysql.cj.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:416) at com.mysql.cj.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:237) at com.mysql.cj.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:180) at com.zaxxer.hikari.util.DriverDataSource.getConnection(DriverDataSource.java:144) at com.zaxxer.hikari.pool.PoolBase.newConnection(PoolBase.java:373)

# 这些能被解析为整数
age: 25           → Integer: 25
count: 100        → Integer: 100
# 这些能被解析为浮点数
price: 9.99       → Float: 9.99
rate: 0.5         → Float: 0.5
# 而这则会产生歧义
code: 060711      → 可能被解析为:
                    - 八进制数（YAML 1.1）: 25033
                    - 整数（去掉前导0）: 60711
                    - 字符串（YAML 1.2）: "060711"

# 无论是双引号还是单引号，都能将其解析为字符串:
code: "060711"    → String: "060711"
code: '060711'    → String: "060711"