Enable auto-configuration of the Spring Application Context, attempting to guess and
configure beans that you are likely to need. Auto-configuration classes are usually
applied based on your classpath and what beans you have defined. For example, if you
have {@code tomcat-embedded.jar} on your classpath you are likely to want a
{@link TomcatServletWebServerFactory} (unless you have defined your own
{@link ServletWebServerFactory} bean).
-- Spring的自动配置上下文，会大致预估你需要使用的Bean，其依据就是项目中classpath中的jar、配置文件+我们自己定义的Bean(借助于上面SpringFramework的方式之一)，举例：比如引入了tomcat-embedded.jar，那么自动配置认为你大概率会使用TomcatServletWebServerFactory，于是就帮你初始化，除非你自己定义了一个自定义Bean:ServletWebServerFactory,即：自定义的优先级>系统默认配置，同级覆盖
    
When using {@link SpringBootApplication @SpringBootApplication}, the auto-configuration
of the context is automatically enabled and adding this annotation has therefore no
additional effect.
-- 当使用@SpringBootApplication注解时，自动配置注解再加到主启动类上不会有什么用，也是因为@SpringBootApplication注解是组合注解，其内部已经包含了自动配置的注解
    
Auto-configuration tries to be as intelligent as possible and will back-away as you
define more of your own configuration. You can always manually {@link #exclude()} any
configuration that you never want to apply (use {@link #excludeName()} if you don't
have access to them). You can also exclude them via the
{@code spring.autoconfigure.exclude} property. Auto-configuration is always applied
after user-defined beans have been registered.
 -- 自动化配置会尽可能地智能化，当你的配置和自动化配置一致时，自动化配置自动就会失效。即用户配置>自动化配置权限。对于你不需要自动化配置的，可以通过exclude进行手动排除(就像上面2.3小结我们举例的
@ComponentScan(excludeFilters = {@ComponentScan.Filter(type = FilterType.ASSIGNABLE_TYPE, classes = {TestBean.class})}))，当然你也可以在配置文件中进行排除：spring.autoconfigure.exclude。
  最后说了下，自动化配置的永远在用户定义的Bean注册之后进行。
    
The package of the class that is annotated with {@code @EnableAutoConfiguration},
usually via {@code @SpringBootApplication}, has specific significance and is often used
as a 'default'. For example, it will be used when scanning for {@code @Entity} classes.
It is generally recommended that you place {@code @EnableAutoConfiguration} (if you're
not using {@code @SpringBootApplication}) in a root package so that all sub-packages
and classes can be searched.
-- 就是说@SpringBootApplication,@EnableAutoConfiguration标注的类是具备意义的，一般会被系统认作默认配置启动类，因此建议标注有 @SpringBootApplication注解的文件最好放在根包中，这样所有的子包才能被扫描到。
    
Auto-configuration classes are regular Spring {@link Configuration @Configuration}
beans. They are located using the {@link SpringFactoriesLoader} mechanism (keyed
against this class). Generally auto-configuration beans are
{@link Conditional @Conditional} beans (most often using
{@link ConditionalOnClass @ConditionalOnClass} and
{@link ConditionalOnMissingBean @ConditionalOnMissingBean} annotations).
    
-- 被@Configuration标注的Spring的Bean就被认为是一个配置类，等同于ApplicationContext.xml，采用SpringFactoriesLoader机制，自动装配Bean常用以下注解实现：@Conditional，@ConditionalOnClass，@ConditionalOnMissingBean

PackageImport(AnnotationMetadata metadata) {
    this.packageName = ClassUtils.getPackageName(metadata.getClassName());
}

/**
 * Register bean definitions as necessary based on the given annotation metadata of
 * the importing {@code @Configuration} class.
 * <p>Note that {@link BeanDefinitionRegistryPostProcessor} types may <em>not</em> be
 * registered here, due to lifecycle constraints related to {@code @Configuration}
 * class processing.
 * <p>The default implementation is empty.
 * @param importingClassMetadata annotation metadata of the importing class
 * @param registry current bean definition registry
 */
default void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
}

private static String[] addBasePackages(ConstructorArgumentValues constructorArguments, String[] packageNames) {
    String[] existing = (String[]) constructorArguments.getIndexedArgumentValue(0, String[].class).getValue();
    Set<String> merged = new LinkedHashSet<>();
    // 添加现有全部
    merged.addAll(Arrays.asList(existing));
    // 添加此次
    merged.addAll(Arrays.asList(packageNames));
    // 返回全部
    return StringUtils.toStringArray(merged);
}

protected boolean isEnabled(AnnotationMetadata metadata) {
    if (getClass() == AutoConfigurationImportSelector.class) {
        return getEnvironment().getProperty(EnableAutoConfiguration.ENABLED_OVERRIDE_PROPERTY, Boolean.class, true);
    }
    return true;
}

static AutoConfigurationMetadata loadMetadata(ClassLoader classLoader, String path) {
    try {
        Enumeration<URL> urls = (classLoader != null) ? classLoader.getResources(path)
            : ClassLoader.getSystemResources(path);
        Properties properties = new Properties();
        while (urls.hasMoreElements()) {
            properties.putAll(PropertiesLoaderUtils.loadProperties(new UrlResource(urls.nextElement())));
        }
        return loadMetadata(properties);
    }
    catch (IOException ex) {
        throw new IllegalArgumentException("Unable to load @ConditionalOnClass location [" + path + "]", ex);
    }
}

protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,
      AnnotationMetadata annotationMetadata) {
    if (!isEnabled(annotationMetadata)) {
        return EMPTY_ENTRY;
    }
    // 1.获取所有的attributes
    AnnotationAttributes attributes = getAttributes(annotationMetadata);
    
    // 2.SPI机制加载自动配置类configurations
    List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
   
    // 3.步骤2结果去重
    configurations = removeDuplicates(configurations);
    
     // 4.基于1获取全部去重的Exclusion【过滤限制的配置项】
    Set<String> exclusions = getExclusions(annotationMetadata, attributes);
    
    /**
     * 5.检查是否有无效Exclusion，有则抛出IllegalStateException
     * The following classes could not be excluded because they are not auto-configuration classes
   * 不能排除下列类，因为它们不是自动配置类
     */
    checkExcludedClasses(configurations, exclusions);
    
    // 6.清空configurations中的exclusions
    configurations.removeAll(exclusions);
   
     // 7.重新生成configurations，前面4.2的继承体系在这里发挥作用
    configurations = filter(configurations, autoConfigurationMetadata);
    fireAutoConfigurationImportEvents(configurations, exclusions);
    return new AutoConfigurationEntry(configurations, exclusions);
}

<!-- redis template definition -->
<bean id="redisTemplate" class="org.springframework.data.redis.core.RedisTemplate">
  <property name="connectionFactory" ref="jedisConnectionFactory" />
  <property name="keySerializer">
    <bean class="org.springframework.data.redis.serializer.StringRedisSerializer" />
  </property>
  <property name="valueSerializer">
    <bean class="org.springframework.data.redis.serializer.JdkSerializationRedisSerializer" />
  </property>
  <property name="hashKeySerializer">
    <bean class="org.springframework.data.redis.serializer.StringRedisSerializer" />
  </property>
  <property name="hashValueSerializer">
    <bean class="org.springframework.data.redis.serializer.JdkSerializationRedisSerializer" />
  </property>
  <!--开启事务  -->  
  <property name="enableTransactionSupport" value="true"></property>  
</bean>

@Autowired 
public RedisTemplate redisTemplate;