一、问题

在启动springboot项目中遇到如下问题：

Description:
Failed to configure a DataSource: ‘url’ attribute is not specified and no embedded datasource could be configured.
Reason: Failed to determine a suitable driver class

Action:
Consider the following:
If you want an embedded database (H2, HSQL or Derby), please put it on the classpath.
If you have database settings to be loaded from a particular profile you may need to activate it (no profiles are currently active).

这个问题其实是url和数据源配置错误问题，果断查看配置文件和依赖。

另外，本项目用的是mybatisPlus+MySQL，使用默认连接池hakis

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二、原代码展示

1、配置文件

server:
  port: 9999
spring:
  redis:
    host: 127.0.0.1
    port: 6379
  datasource:
    driver-class-name: com.mysql.cj.jdbc.Driver
    url: jdbc:mysql://localhost:3306/lkd_user?characterEncoding=utf-8&serverTimezone=UTC
    username: root
    password: root

经过校对，没有问题

2、依赖

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <packaging>pom</packaging>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.1.6.RELEASE</version>
    </parent>
    <groupId>com.zb</groupId>
    <artifactId>mysec</artifactId>
    <version>1.0-SNAPSHOT</version>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.10</version>
            <scope>provided</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-security</artifactId>
        </dependency>
        <!--redis依赖-->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-redis</artifactId>
        </dependency>
        <!--fastjson依赖-->
        <dependency>
            <groupId>com.alibaba</groupId>
            <artifactId>fastjson</artifactId>
            <version>1.2.33</version>
        </dependency>
        <!--jwt依赖-->
        <dependency>
            <groupId>io.jsonwebtoken</groupId>
            <artifactId>jjwt</artifactId>
            <version>0.9.0</version>
        </dependency>
        <dependency>
            <groupId>com.baomidou</groupId>
            <artifactId>mybatis-plus-boot-starter</artifactId>
            <version>3.4.3</version>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
        </dependency>
    </dependencies>
</project>

经过校对，暂时没有发现错误

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三、发现

发现没有存在明显的配置文件和依赖的问题，于是又看了下控制台打印日志，真正错误其实在上面，日志级别为warn。

2022-08-21 19:10:31.248 WARN 39656 — [ main] ConfigServletWebServerApplicationContext :
Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name ‘userDetailServiceImpl’: Unsatisfied dependency expressed through field ‘tbUserMapper’; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name ‘tbUserMapper’ defined in file [D:\maven\spring\springcloud\mysec\target\classes\com\zb\mapper\TbUserMapper.class]: Unsatisfied dependency expressed through bean property ‘sqlSessionFactory’; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name ‘sqlSessionFactory’ defined in class path resource [com/baomidou/mybatisplus/autoconfigure/MybatisPlusAutoConfiguration.class]: Unsatisfied dependency expressed through method ‘sqlSessionFactory’ parameter 0; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name ‘dataSource’ defined in class path resource [org/springframework/boot/autoconfigure/jdbc/DataSourceConfiguration$Hikari.class]:Beaninstantiationviafactorymethodfailed;nestedexceptionisorg.springframework.beans.BeanInstantiationException:Failedtoinstantiate[com.zaxxer.hikari.HikariDataSource]:Factorymetho{d}^{\prime }dataSourc{e}^{\prime }threwexception;nestedexceptionisorg.springframework.boot.autoconfigure.jdbc.DataSourceProperties$DataSourceBeanCreationException: Failed to determine a suitable driver class

这里打印了bean的依赖问题，如下：

userDetailServiceImpl——依赖——》tbUserMapperl——依赖——》 sqlSessionFactory——依赖——》MybatisPlusAutoConfiguration ——依赖——》dataSource——依赖——》jdbc.DataSourceProperties

这里最终原因是：Failed to determine a suitable driver class，翻译下就是：无法确定合适的驱动程序类。错误主要跟DataSource有关。

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四、找原因

1、查看DataSourceConfiguration类，看看导入的实际DataSource配置。

abstract class DataSourceConfiguration {
	//创建DataBase
	@SuppressWarnings("unchecked")
	protected static <T> T createDataSource(DataSourceProperties properties, Class<? extends DataSource> type) {
		return (T) properties.initializeDataSourceBuilder().type(type).build();
	}
	
	/**
	 * Hikari DataSource configuration.
	 */
	@Configuration
	@ConditionalOnClass(HikariDataSource.class)
	@ConditionalOnMissingBean(DataSource.class)
	@ConditionalOnProperty(name = "spring.datasource.type", havingValue = "com.zaxxer.hikari.HikariDataSource",
			matchIfMissing = true)
	static class Hikari {

		@Bean
		@ConfigurationProperties(prefix = "spring.datasource.hikari")
		public HikariDataSource dataSource(DataSourceProperties properties) {
			HikariDataSource dataSource = createDataSource(properties, HikariDataSource.class);
			if (StringUtils.hasText(properties.getName())) {
				dataSource.setPoolName(properties.getName());
			}
			return dataSource;
		}
	//省略其他Database配置（tomcat 、dbcp2、generic）
	}
}

这里断点，走的是HikariDataSource的dataSource()工厂方法，然后调用createDataSource()。

评估表达式(ALT+F8)计算createDataSource()，正好是导致异常的最终原因：

org.springframework.boot.autoconfigure.jdbc.DataSourceProperties$DataSourceBeanCreationException: Failed to determine a suitable driver class

（断点的时候已经发现jdbc爆红，没有成功导入）

继续往下走，进入到DataSourceProperties类initializeDataSourceBuilder()

public DataSourceBuilder<?> initializeDataSourceBuilder() {
	return DataSourceBuilder.create(getClassLoader()).type(getType()).driverClassName(determineDriverClassName())
			.url(determineUrl()).username(determineUsername()).password(determinePassword());
}

这里采用建造者模式链式调用，方法依次为创建——》类型——》驱动类名——》URL——》用户名——》密码。

评估表达式(ALT+F8)递进式计算：链式调用的返回结果。发现链式调用到driverClassName(determineDriverClassName())报错，并且后面也的determine()全部报错。

（这里已经发现问题，猜测配置文件声明的DriverClassName、url、username、password没有被识别到）

继续往下走，进入到determineDriverClassName()方法

public String determineDriverClassName() {
		if (StringUtils.hasText(this.driverClassName)) {
			Assert.state(driverClassIsLoadable(), () -> "Cannot load driver class: " + this.driverClassName);
			return this.driverClassName;
		}
		String driverClassName = null;
		if (StringUtils.hasText(this.url)) {
			driverClassName = DatabaseDriver.fromJdbcUrl(this.url).getDriverClassName();
		}
		if (!StringUtils.hasText(driverClassName)) {
			driverClassName = this.embeddedDatabaseConnection.getDriverClassName();
		}
		if (!StringUtils.hasText(driverClassName)) {
			//这里正好是控制台日志warn信息的最后一行提示
			throw new DataSourceBeanCreationException("Failed to determine a suitable driver class", this,
					this.embeddedDatabaseConnection);
		}
		return driverClassName;
	}

运行到这已经结束，错误信息中嵌套的最后异常就是出自这里

DataSourceBeanCreationException: Failed to determine a suitable driver class

断点中发现，该方法的类DataSourceProperties中的字段driverClassName、url、username、password全部为NULL。

于是又打开了其他运行正常的项目，断点上述各个位置，重新debug

首先进入到DataSourceConfiguration类的dataSource方法

可以看到，这个时候已经完成配置项的读取，往下走到DataSourceProperties类的initializeDataSourceBuilder()方法

这里并没有引发异常，同时发现determineXXX()方法都能得到对应的值。

到这，可以确定是配置信息没有成功被读取到，也就是YAML配置失效。

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五、解决方案

１、检查YAML文件位置、是否存在语法错误（四个配置项都没有成功读取，可以排除）

２、查看POM文件是否存在配置错误

最后发现问题是POM打包方式错误，多写了一行

    <modelVersion>4.0.0</modelVersion>
<!--    <packaging>pom</packaging>-->
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.1.6.RELEASE</version>
    </parent>

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六、建议

1、不用看mybatis的mapper接口和映射文件、实体类。

项目启动时没有查数据库，也就没有调用mapper层，自然不会引发这个错误。
就算报错上提示信息也很明确，网上很多说要检查这个，感觉没有必要。

2、看yml/properties配置文件中跟url相关的，是否存在错误。

配置写多了，基本不会出现语法或声明错误，而且有代码提示

3、多断点调试，没必要重复性检查代码是否写错，结果半天也找不出来。