SpringBoot自动配置原理

SpringBoot自动配置主要通过@EnableAutoConfiguration, @Conditional, @EnableConfigurationProperties或者@ConfigurationProperties 等几个注解来进行自动配置完成的。

@EnableAutoConfiguration开启自动配置，主要作用就是调用Spring-Core包里的loadFactoryNames()，将autoconfig 包里的已经写好的自动配置加载进来。

@Conditional条件注解，通过判断类路径下有没有相应配置的jar包来确定是否加载和自动配置这个类。

@EnableConfigurationProperties的作用就是，给自动配置提供具体的配置参数，只需要写在 application.properties 中，就可以通过映射写入配置类的POJO属性中。

@EnableAutoConfiguration

@Enable*注解并不是SpringBoot新发明的注解，Spring 3框架就引入了这些注释，用这些注释替代XML配置文件。比如：
@EnableTransactionManagement注解，它能够声明事务管理
@EnableWebMvc注解，它能启用Spring MVC
@EnableScheduling注解，它可以初始化一个调度器。

这些注释事实上都是简单的配置，通过@Import注解导入。
从启动类的@SpringBootApplication进入，在里面找到了@EnableAutoConfiguration,

@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 {}

@EnableAutoConfiguration里通过@Import导入了AutoConfigurationImportSelector

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

找到selectImports()方法，他调用了getCandidateConfigurations()方法，在这里，这个方法又调用了Spring Core包中的loadFactoryNames()方法。这个方法的作用是，会查询META-INF/spring.factories文件中包含的JAR文件。

@Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {
    if (!isEnabled(annotationMetadata)) {
        return NO_IMPORTS;
    }
    AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
            .loadMetadata(this.beanClassLoader);
    AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(
            autoConfigurationMetadata, annotationMetadata);
    return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
}

protected AutoConfigurationEntry getAutoConfigurationEntry(
        AutoConfigurationMetadata autoConfigurationMetadata,
        AnnotationMetadata annotationMetadata) {
    if (!isEnabled(annotationMetadata)) {
        return EMPTY_ENTRY;
    }
    AnnotationAttributes attributes = getAttributes(annotationMetadata);
    List<String> configurations = getCandidateConfigurations(annotationMetadata,
            attributes);
    configurations = removeDuplicates(configurations);
    Set<String> exclusions = getExclusions(annotationMetadata, attributes);
    checkExcludedClasses(configurations, exclusions);
    configurations.removeAll(exclusions);
    configurations = filter(configurations, autoConfigurationMetadata);
    fireAutoConfigurationImportEvents(configurations, exclusions);
    return new AutoConfigurationEntry(configurations, exclusions);
}

当找到spring.factories文件后，SpringFactoriesLoader将查询配置文件命名的属性。

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
        AnnotationAttributes attributes) {
    List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
            getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
    Assert.notEmpty(configurations,
            "No auto configuration classes found in META-INF/spring.factories. If you "
                    + "are using a custom packaging, make sure that file is correct.");
    return configurations;
}

public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
    String factoryClassName = factoryClass.getName();
    return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
}
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
    MultiValueMap<String, String> result = cache.get(classLoader);
    if (result != null) {
        return result;
    }
    try {
        Enumeration<URL> urls = (classLoader != null ?
                classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
        result = new LinkedMultiValueMap<>();
        while (urls.hasMoreElements()) {
            URL url = urls.nextElement();
            UrlResource resource = new UrlResource(url);
            Properties properties = PropertiesLoaderUtils.loadProperties(resource);
            for (Map.Entry<?, ?> entry : properties.entrySet()) {
                String factoryClassName = ((String) entry.getKey()).trim();
                for (String factoryName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
                    result.add(factoryClassName, factoryName.trim());
                }
            }
        }
        cache.put(classLoader, result);
        return result;
    }
    catch (IOException ex) {
        throw new IllegalArgumentException("Unable to load factories from location [" +
                FACTORIES_RESOURCE_LOCATION + "]", ex);
    }

public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

org.springframework.boot.autoconfigure的spring.factories

spring.factories文件中，可以看到一系列Spring Boot自动配置的列表

下面我们来看spring.factories文件自动配置Kafka的细节，KafkaAutoConfiguration：

@Configuration
@ConditionalOnClass(KafkaTemplate.class)
@EnableConfigurationProperties(KafkaProperties.class)
@Import({ KafkaAnnotationDrivenConfiguration.class,
        KafkaStreamsAnnotationDrivenConfiguration.class })
public class KafkaAutoConfiguration {

    private final KafkaProperties properties;

    private final RecordMessageConverter messageConverter;

    public KafkaAutoConfiguration(KafkaProperties properties,
            ObjectProvider<RecordMessageConverter> messageConverter) {
        this.properties = properties;
        this.messageConverter = messageConverter.getIfUnique();
    }

    @Bean
    @ConditionalOnMissingBean(KafkaTemplate.class)
    public KafkaTemplate<?, ?> kafkaTemplate(
            ProducerFactory<Object, Object> kafkaProducerFactory,
            ProducerListener<Object, Object> kafkaProducerListener) {
        KafkaTemplate<Object, Object> kafkaTemplate = new KafkaTemplate<>(
                kafkaProducerFactory);
        if (this.messageConverter != null) {
            kafkaTemplate.setMessageConverter(this.messageConverter);
        }
        kafkaTemplate.setProducerListener(kafkaProducerListener);
        kafkaTemplate.setDefaultTopic(this.properties.getTemplate().getDefaultTopic());
        return kafkaTemplate;
    }

    @Bean
    @ConditionalOnMissingBean(ProducerListener.class)
    public ProducerListener<Object, Object> kafkaProducerListener() {
        return new LoggingProducerListener<>();
    }

    @Bean
    @ConditionalOnMissingBean(ConsumerFactory.class)
    public ConsumerFactory<?, ?> kafkaConsumerFactory() {
        return new DefaultKafkaConsumerFactory<>(
                this.properties.buildConsumerProperties());
    }

    @Bean
    @ConditionalOnMissingBean(ProducerFactory.class)
    public ProducerFactory<?, ?> kafkaProducerFactory() {
        DefaultKafkaProducerFactory<?, ?> factory = new DefaultKafkaProducerFactory<>(
                this.properties.buildProducerProperties());
        String transactionIdPrefix = this.properties.getProducer()
                .getTransactionIdPrefix();
        if (transactionIdPrefix != null) {
            factory.setTransactionIdPrefix(transactionIdPrefix);
        }
        return factory;
    }

    @Bean
    @ConditionalOnProperty(name = "spring.kafka.producer.transaction-id-prefix")
    @ConditionalOnMissingBean
    public KafkaTransactionManager<?, ?> kafkaTransactionManager(
            ProducerFactory<?, ?> producerFactory) {
        return new KafkaTransactionManager<>(producerFactory);
    }

    @Bean
    @ConditionalOnProperty(name = "spring.kafka.jaas.enabled")
    @ConditionalOnMissingBean
    public KafkaJaasLoginModuleInitializer kafkaJaasInitializer() throws IOException {
        KafkaJaasLoginModuleInitializer jaas = new KafkaJaasLoginModuleInitializer();
        Jaas jaasProperties = this.properties.getJaas();
        if (jaasProperties.getControlFlag() != null) {
            jaas.setControlFlag(jaasProperties.getControlFlag());
        }
        if (jaasProperties.getLoginModule() != null) {
            jaas.setLoginModule(jaasProperties.getLoginModule());
        }
        jaas.setOptions(jaasProperties.getOptions());
        return jaas;
    }

    @Bean
    @ConditionalOnMissingBean
    public KafkaAdmin kafkaAdmin() {
        KafkaAdmin kafkaAdmin = new KafkaAdmin(this.properties.buildAdminProperties());
        kafkaAdmin.setFatalIfBrokerNotAvailable(this.properties.getAdmin().isFailFast());
        return kafkaAdmin;
    }

}

这个类进行了简单的Spring配置，声明了Kafka所需典型Bean，和其它很多类一样，重度依赖于Spring Boot注释：
1）@ConditionOnClass激活一个配置，当类路径中存在这个类时才会配置该类
2）@EnableConfigurationProperties自动映射一个POJO到Spring Boot配置文件（默认是application.properties文件）的属性集。
3）@ConditionalOnMissingBean启用一个Bean定义，但必须是这个Bean之前未定义过才有效。
还可以使用@ AutoConfigureBefore注释、@AutoConfigureAfter注释来定义这些配置类的载入顺序。

着重了解@Conditional注释，Spring 4框架的新特性
此注释使得只有在特定条件满足时才启用一些配置。SrpingBoot的AutoConfig大量使用了@Conditional，它会根据运行环境来动态注入Bean。这里介绍一些@Conditional的使用和原理，并自定义@Conditional来自定义功能。

@Conditional是SpringFramework的功能，SpringBoot在它的基础上定义了
@ConditionalOnClass，@ConditionalOnProperty等一系列的注解来实现更丰富的内容。
具体几个@Conditon*注解的含义
@ConditionalOnBean
仅仅在当前上下文中存在某个对象时，才会实例化一个Bean

@ConditionalOnClass
某个class位于类路径上，才会实例化一个Bean)，该注解的参数对应的类必须存在，否则不解析该注解修饰的配置类

@ConditionalOnExpression
当表达式为true的时候，才会实例化一个Bean

@ConditionalOnMissingBean
仅仅在当前上下文中不存在某个对象时，才会实例化一个Bean，该注解表示，如果存在它修饰的类的bean，则不需要再创建这个bean，可以给该注解传入参数例如@ConditionOnMissingBean(name = "example")，这个表示如果name为“example”的bean存在，这该注解修饰的代码块不执行

@ConditionalOnMissingClass
某个class类路径上不存在的时候，才会实例化一个Bean

@ConditionalOnNotWebApplication
不是web应用时，才会执行

Properties系列注释

@EnableConfigurationProperties
@ConfigurationProperties(prefix = "may")

在需要注入配置的类上加上这个注解，prefix的意思是，以该前缀打头的配置

如果不用系统初始的application.properties配置类，而是使用自己的如winner.properties，可以如下配置

/**
 * @PropertySource 只能加载.properties文件
 * @author winner_0715
 */
@Configuration
@PropertySource("classpath:winner.properties")
@ConfigurationProperties(prefix = "winner")
public class WinnerConfig {
    private String name;
    private String email;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }
}

最后注意在spring Boot入口类加上@EnableConfigurationProperties

@ConfigurationProperties(prefix = "spring.kafka")
public class KafkaProperties {

    /**
     * Comma-delimited list of host:port pairs to use for establishing the initial
     * connections to the Kafka cluster. Applies to all components unless overridden.
     */
    private List<String> bootstrapServers = new ArrayList<>(
            Collections.singletonList("localhost:9092"));

    /**
     * ID to pass to the server when making requests. Used for server-side logging.
     */
    private String clientId;

    /**
     * Additional properties, common to producers and consumers, used to configure the
     * client.
     */
    private final Map<String, String> properties = new HashMap<>();

    private final Consumer consumer = new Consumer();

Ref:

https://www.cnblogs.com/leihuazhe/p/7743479.html