怎樣排查 Cannot determine embedded database driver class for database type NONE

調試排查 Cannot determine embedded database driver class for database type NONE 的錯誤
把工程導入IDE裏，直接啓動應用，拋出來的異常信息是：

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).

其實這時有兩個思路，直接google搜索Cannot determine embedded database driver class for database type NONE，就能夠找到解決辦法。

第二種方式，仔細查看日誌內容，能夠發現有To display the auto-configuration report re-run your application with 'debug' enabled.。

搜索下這個，就能夠在spring的官方網站上找到相關的信息：https://docs.spring.io/spring-boot/docs/current/reference/html/using-boot-auto-configuration.html

就是用戶只要配置了debug這個開關，就會把auto-configuration 相關的信息打印出來。

熟悉spring的環境變量注入的話，就能夠知道有幾種打開這個的方式：

在args裏增長--debug
在application.properties裏增長debug=true
經過-Ddebug=true
增長debug開關以後的信息
增長debug開關以後，能夠看到打印出了錯誤堆棧：

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]

拋出異常的代碼是：    

/**
     * 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;
    }

能夠看出來是沒有找到 DataSource 的driver class，而後拋出了 DataSourceBeanCreationException。

那麼一種解決辦法是，在maven依賴里加入一些 DataSource driver class。

可是應用本身的代碼裏並無使用DataSource，哪裏致使spring boot要建立一個DataSource對象？哪裏致使spring boot要建立DataSource
從異常棧上，能夠找到DataSourceConfiguration$Tomcat 這個類，那麼查找下它的引用，能夠發現它是被

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 {

    }

那麼 PooledDataSourceConfiguration 是怎麼生效的呢？從代碼上能夠看到@Conditional(PooledDataSourceCondition.class)。

那麼再看PooledDataSourceCondition的具體實現：  

 /**
     * {@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());
        }

    }

從代碼裏，能夠看到是嘗試查找dataSourceClass，若是找到，條件就成立。那麼debug下，能夠發現查找到的dataSourceClass是：org.apache.tomcat.jdbc.pool.DataSource 。

那麼再看下org.apache.tomcat.jdbc.pool.DataSource這個類是從哪裏來的呢？

從maven依賴樹能夠看到，依賴是來自：spring-boot-starter-jdbc。因此是應用依賴了spring-boot-starter-jdbc，可是並無配置DataSource引發的問題。

問題解決辦法
有兩種：

沒有使用到DataSource，則能夠把spring-boot-starter-jdbc的依賴去掉，這樣就不會觸發spring boot相關的代碼
把spring boot自動初始化DataSource相關的代碼禁止掉
禁止的辦法有兩種：

在main函數上配置exclude

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

在application.properties裏配置：

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

總結
應用沒有使用到DataSource，可是在pom.xml裏引入了spring-boot-starter-jdbc
spring-boot-starter-jdbc帶入了tomcat-jdbc，它裏面有org.apache.tomcat.jdbc.pool.DataSource
spring boot裏的PooledDataSourceConfiguration，判斷classpath下面有DataSource的實現類，嘗試去建立DataSource bean
在初始化DataSourceProperties時，嘗試經過jdbc的url來探測driver class
由於應用並無配置url，因此最終在DataSourceProperties.determineDriverClassName()裏拋出Cannot determine embedded database driver class for database type NONE
最後：

排查spring boot的AutoConfiguration問題時，能夠按異常棧，一層層排查Configuration是怎麼引入的，再排查Condition具體的判斷代碼。