写在前面

这个demo来说明怎么一步步排查一个常见的spring boot AutoConfiguration的错误。

调试排查 Cannot determine embedded database driver class for database type NONE 的错误

把工程导入IDE里，直接启动应用，抛出来的异常信息是：

Error starting ApplicationContext. To display the auto-configuration report re-run your application with 'debug' enabled.2017-11-29 14:26:34.478 ERROR 29736 --- [           main] o.s.b.d.LoggingFailureAnalysisReporter   :***************************APPLICATION FAILED TO START***************************Description:Cannot determine embedded database driver class for database type NONEAction:If you want an embedded database please put a supported one on the classpath. If you have database settings to be loaded from a particular profile you may need to active it (no profiles are currently active).

其实这时有两个思路，直接google搜索Cannot determine embedded database driver class for database type NONE，就可以找到解决办法。

第二种方式，仔细查看日志内容，可以发现有To display the auto-configuration report re-run your application with 'debug' enabled.。

搜索下这个，就可以在spring的官方网站上找到相关的信息：

就是用户只要配置了debug这个开关，就会把auto-configuration 相关的信息打印出来。

熟悉spring的环境变量注入的话，就可以知道有几种打开这个的方式：

• 在args里增加--debug
• 在application.properties里增加debug=true
• 通过-Ddebug=true

增加debug开关之后的信息

增加debug开关之后，可以看到打印出了错误堆栈：

2017-11-29 14:33:08.776 DEBUG 29907 --- [           main] o.s.b.d.LoggingFailureAnalysisReporter   : Application failed to start due to an exceptionorg.springframework.boot.autoconfigure.jdbc.DataSourceProperties$DataSourceBeanCreationException: Cannot determine embedded database driver class for database type NONE. If you want an embedded database please put a supported one on the classpath. If you have database settings to be loaded from a particular profile you may need to active it (no profiles are currently active).    at org.springframework.boot.autoconfigure.jdbc.DataSourceProperties.determineDriverClassName(DataSourceProperties.java:245) ~[spring-boot-autoconfigure-1.4.7.RELEASE.jar:1.4.7.RELEASE]    at org.springframework.boot.autoconfigure.jdbc.DataSourceProperties.initializeDataSourceBuilder(DataSourceProperties.java:182) ~[spring-boot-autoconfigure-1.4.7.RELEASE.jar:1.4.7.RELEASE]    at org.springframework.boot.autoconfigure.jdbc.DataSourceConfiguration.createDataSource(DataSourceConfiguration.java:42) ~[spring-boot-autoconfigure-1.4.7.RELEASE.jar:1.4.7.RELEASE]    at org.springframework.boot.autoconfigure.jdbc.DataSourceConfiguration$Tomcat.dataSource(DataSourceConfiguration.java:53) ~[spring-boot-autoconfigure-1.4.7.RELEASE.jar:1.4.7.RELEASE]    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_112]

抛出异常的代码是：

/**     * Determine the driver to use based on this configuration and the environment.     * @return the driver to use     * @since 1.4.0     */    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)) {            throw new DataSourceBeanCreationException(this.embeddedDatabaseConnection,                    this.environment, "driver class");        }        return driverClassName;    }

可以看出来是没有找到 DataSource 的driver class，然后抛出了 DataSourceBeanCreationException。

那么一种解决办法是，在maven依赖里加入一些 DataSource driver class。

但是应用自己的代码里并没有使用DataSource，哪里导致spring boot要创建一个DataSource对象？

哪里导致spring boot要创建DataSource

从异常栈上，可以找到DataSourceConfiguration$Tomcat 这个类，那么查找下它的引用，可以发现它是被org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration.PooledDataSourceConfiguration import引入的。

@Configuration    @Conditional(PooledDataSourceCondition.class)    @ConditionalOnMissingBean({ DataSource.class, XADataSource.class })    @Import({ DataSourceConfiguration.Tomcat.class, DataSourceConfiguration.Hikari.class,            DataSourceConfiguration.Dbcp.class, DataSourceConfiguration.Dbcp2.class,            DataSourceConfiguration.Generic.class })    protected static class PooledDataSourceConfiguration {
         }

那么 PooledDataSourceConfiguration 是怎么生效的呢？从代码上可以看到@Conditional(PooledDataSourceCondition.class)。

那么再看PooledDataSourceCondition的具体实现：

/**     * {@link AnyNestedCondition} that checks that either {@code spring.datasource.type}     * is set or {@link PooledDataSourceAvailableCondition} applies.     */    static class PooledDataSourceCondition extends AnyNestedCondition {        PooledDataSourceCondition() {            super(ConfigurationPhase.PARSE_CONFIGURATION);        }        @ConditionalOnProperty(prefix = "spring.datasource", name = "type")        static class ExplicitType {        }        @Conditional(PooledDataSourceAvailableCondition.class)        static class PooledDataSourceAvailable {        }    }

PooledDataSourceCondition引入了@Conditional(PooledDataSourceAvailableCondition.class) ：

/**     * {@link Condition} to test if a supported connection pool is available.     */    static class PooledDataSourceAvailableCondition extends SpringBootCondition {        @Override        public ConditionOutcome getMatchOutcome(ConditionContext context,                AnnotatedTypeMetadata metadata) {            ConditionMessage.Builder message = ConditionMessage                    .forCondition("PooledDataSource");            if (getDataSourceClassLoader(context) != null) {                return ConditionOutcome                        .match(message.foundExactly("supported DataSource"));            }            return ConditionOutcome                    .noMatch(message.didNotFind("supported DataSource").atAll());        }        /**         * Returns the class loader for the {@link DataSource} class. Used to ensure that         * the driver class can actually be loaded by the data source.         * @param context the condition context         * @return the class loader         */        private ClassLoader getDataSourceClassLoader(ConditionContext context) {            Class
      dataSourceClass = new DataSourceBuilder(context.getClassLoader())                    .findType();            return (dataSourceClass == null ? null : dataSourceClass.getClassLoader());        }    }

从代码里，可以看到是尝试查找dataSourceClass，如果找到，条件就成立。那么debug下，可以发现查找到的dataSourceClass是：org.apache.tomcat.jdbc.pool.DataSource 。

那么再看下org.apache.tomcat.jdbc.pool.DataSource这个类是从哪里来的呢？

从maven依赖树可以看到，依赖是来自：spring-boot-starter-jdbc。所以是应用依赖了spring-boot-starter-jdbc，但是并没有配置DataSource引起的问题。

问题解决办法

有两种：

1. 没有使用到DataSource，则可以把spring-boot-starter-jdbc的依赖去掉，这样就不会触发spring boot相关的代码
2. 把spring boot自动初始化DataSource相关的代码禁止掉

禁止的办法有两种：

1. 在main函数上配置exclude

   @SpringBootApplication(exclude = { DataSourceAutoConfiguration.class, DataSourceTransactionManagerAutoConfiguration.class })

2. 在application.properties里配置：

   spring.autoconfigure.exclude=org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration

总结

1. 应用没有使用到DataSource，但是在pom.xml里引入了spring-boot-starter-jdbc
2. spring-boot-starter-jdbc带入了tomcat-jdbc，它里面有org.apache.tomcat.jdbc.pool.DataSource
3. spring boot里的PooledDataSourceConfiguration，判断classpath下面有DataSource的实现类，尝试去创建DataSource bean
4. 在初始化DataSourceProperties时，尝试通过jdbc的url来探测driver class
5. 因为应用并没有配置url，所以最终在DataSourceProperties.determineDriverClassName()里抛出Cannot determine embedded database driver class for database type NONE

最后：

• 排查spring boot的AutoConfiguration问题时，可以按异常栈，一层层排查Configuration是怎么引入的，再排查Condition具体的判断代码。