怎样排查 Cannot determine embedded database driver class for database type NONE

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 NONE

Action:

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，就能够找到解决办法。

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

搜索下这个，就能够在spring的官方网站上找到相关的信息：https://docs.spring.io/spring-boot/docs/current/reference/html/using-boot-auto-configuration.html

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，就能够知道有几种打开这个的方式：

在args里增长--debug
在application.properties里增长debug=true
经过-Ddebug=true
增长debug开关以后的信息
增长debug开关以后，能够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 exception

org.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;
    }

能够看出来是没有找到 DataSource 的driver class，而后抛出了 DataSourceBeanCreationException。

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

可是应用本身的代码里并无使用DataSource，哪里致使spring boot要建立一个DataSource对象？哪里致使spring boot要建立DataSource
从异常栈上，能够找到DataSourceConfiguration$Tomcat 这个类，那么查找下它的引用，能够发现它是被

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？从代码上能够看到@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，能够看到是尝试查找dataSourceClass，若是找到，条件就成立。那么debug下，能够发现查找到的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依赖树能够看到，依赖是来自：spring-boot-starter-jdbc。因此是应用依赖了spring-boot-starter-jdbc，可是并无配置DataSource引发的问题。

问题解决办法
有两种：

没有使用到DataSource，则能够把spring-boot-starter-jdbc的依赖去掉，这样就不会触发spring boot相关的代码
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：

在main函数上配置exclude

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

在application.properties里配置：

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

总结
应用没有使用到DataSource，可是在pom.xml里引入了spring-boot-starter-jdbc
spring-boot-starter-jdbc带入了tomcat-jdbc，它里面有org.apache.tomcat.jdbc.pool.DataSource
spring boot里的PooledDataSourceConfiguration，判断classpath下面有DataSource的实现类，尝试去建立DataSource bean
在初始化DataSourceProperties时，尝试经过jdbc的url来探测driver class
由于应用并无配置url，因此最终在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，能够按异常栈，一层层排查Configuration是怎么引入的，再排查Condition具体的判断代码。