3.Bean参数的获取
到此我们已经知道了bean的配置过程，但是还没有看到springboot是如何读取yml或者properites配置文件的的属性来创建数据源的？
在DataSourceAutoConfiguration类里面，我们注意到使用了EnableConfigurationProperties这个注解。

@Configuration
@ConditionalOnClass({ DataSource.class, EmbeddedDatabaseType.class })
@EnableConfigurationProperties(DataSourceProperties.class)
@Import({ DataSourcePoolMetadataProvidersConfiguration.class,
        DataSourceInitializationConfiguration.class })
public class DataSourceAutoConfiguration {

    @Configuration
    @Conditional(EmbeddedDatabaseCondition.class)
    @ConditionalOnMissingBean({ DataSource.class, XADataSource.class })
    @Import(EmbeddedDataSourceConfiguration.class)
    protected static class EmbeddedDatabaseConfiguration {

    }
......

DataSourceProperties中封装了数据源的各个属性，且使用了注解ConfigurationProperties指定了配置文件的前缀。

@ConfigurationProperties(prefix = "spring.datasource")
public class DataSourceProperties implements BeanClassLoaderAware, InitializingBean {

    private ClassLoader classLoader;

    /**
     * Name of the datasource. Default to "testdb" when using an embedded database.
     */
    private String name;

    /**
     * Whether to generate a random datasource name.
     */
    private boolean generateUniqueName;

    /**
     * Fully qualified name of the connection pool implementation to use. By default, it
     * is auto-detected from the classpath.
     */
    private Class<? extends DataSource> type;

    /**
     * Fully qualified name of the JDBC driver. Auto-detected based on the URL by default.
     */
    private String driverClassName;

    /**
     * JDBC URL of the database.
     */
    private String url;

    /**
     * Login username of the database.
     */
    private String username;

    /**
     * Login password of the database.
     */
    private String password;

    /**
     * JNDI location of the datasource. Class, url, username & password are ignored when
     * set.
     */
    private String jndiName;

    /**
     * Initialize the datasource with available DDL and DML scripts.
     */
    private DataSourceInitializationMode initializationMode = DataSourceInitializationMode.EMBEDDED;

    /**
     * Platform to use in the DDL or DML scripts (such as schema-${platform}.sql or
     * data-${platform}.sql).
     */
    private String platform = "all";

    /**
     * Schema (DDL) script resource references.
     */
    private List<String> schema;

    /**
     * Username of the database to execute DDL scripts (if different).
     */
    private String schemaUsername;

    /**
     * Password of the database to execute DDL scripts (if different).
     */
    private String schemaPassword;

    /**
     * Data (DML) script resource references.
     */
    private List<String> data;

    ......

4.Bean的发现
springboot默认扫描启动类所在的包下的主类与子类的所有组件，但并没有包括依赖包的中的类，那么依赖包中的bean是如何被发现和加载的？
我们通常在启动类中加@SpringBootApplication这个注解，点进去看

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
        @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

    /**
     * Exclude specific auto-configuration classes such that they will never be applied.
     * @return the classes to exclude
     */
    @AliasFor(annotation = EnableAutoConfiguration.class)
    Class<?>[] exclude() default {};

    /**
     * Exclude specific auto-configuration class names such that they will never be
     * applied.
     * @return the class names to exclude
     * @since 1.3.0
     */
     ......

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    /**
     * Exclude specific auto-configuration classes such that they will never be applied.
     * @return the classes to exclude
     */
    Class<?>[] exclude() default {};

    /**
     * Exclude specific auto-configuration class names such that they will never be
     * applied.
     * @return the class names to exclude
     * @since 1.3.0
     */
    String[] excludeName() default {};

}

如上源码，@EnableAutoConfiguration注解引入了@AutoConfigurationPackage和@Import这两个注解。@AutoConfigurationPackage的作用就是自动配置的包，@Import导入需要自动配置的组件。

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {

}

~

/**
 * {@link ImportBeanDefinitionRegistrar} to store the base package from the importing
 * configuration.
 */
static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

    @Override
    public void registerBeanDefinitions(AnnotationMetadata metadata,
            BeanDefinitionRegistry registry) {
        register(registry, new PackageImport(metadata).getPackageName());
    }

    @Override
    public Set<Object> determineImports(AnnotationMetadata metadata) {
        return Collections.singleton(new PackageImport(metadata));
    }

}

new AutoConfigurationPackages.PackageImport(metadata)).getPackageName()
new AutoConfigurationPackages.PackageImport(metadata)
这两句代码的作用就是加载启动类所在的包下的主类与子类的所有组件注册到spring容器，这就是前文所说的springboot默认扫描启动类所在的包下的主类与子类的所有组件。
那问题又来了，要搜集并注册到spring容器的那些beans来自哪里？
进入 AutoConfigurationImportSelector类，我们可以发现SpringFactoriesLoader.loadFactoryNames方法调用loadSpringFactories方法从所有的jar包中读取META-INF/spring.factories文件信息。
下面是spring-boot-autoconfigure这个jar中spring.factories文件部分内容，其中有一个key为org.springframework.boot.autoconfigure.EnableAutoConfiguration的值定义了需要自动配置的bean，通过读取这个配置获取一组@Configuration类。

org.springframework.boot.autoconfigure.AutoConfigurationImportListener=org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener

# Auto Configuration Import Filters
org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=org.springframework.boot.autoconfigure.condition.OnClassCondition

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\

每个xxxAutoConfiguration都是一个基于java的bean配置类。实际上，这些xxxAutoConfiguratio不是所有都会被加载，会根据xxxAutoConfiguration上的@ConditionalOnClass等条件判断是否加载;通过反射机制将spring.factories中@Configuration类实例化为对应的java实列。
到此我们已经知道怎么发现要自动配置的bean了，最后一步就是怎么样将这些bean加载到spring容器。

四、总结

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原文地址：https://blog.51cto.com/14442094/2427783