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SpringBoot原理浅析3-自动配置机制

SpringBoot原理浅析3-自动配置机制

作者: Coding小聪 | 来源:发表于2018-06-10 19:30 被阅读73次

前言

在使用Spring或者是Spring MVC时，必须要配置很多的配置项框架才能work，而这些配置项在大部分场景下的取值是相同的。于是SpringBoot提出"约定大于配置"，即给这些配置项合理的默认值，省去开发人员的手动配置工作，当某些配置项预定的默认值无法满足应用需求时，再单独重新制定该配置项的值。这就是SpringBoot的自动配置，对使用者来说方便快捷。如果想要修改某项配置，可以在应用工程中的application.properties或application.yml配置文件中重写该配置值。
如何查看配置文件中都默认支持哪些配置呢？可以去SpringBoot官网查看

自动配置原理

要想使用自动配置功能，必须先告诉SpringBoot框架，给我打开自动配置功能，如何实现呢？可以通过@EnableAutoConfiguration注解启用自动配置功能。一般我们会在程序的启动类上使用@SpringBootApplication注解，该注解是个聚合注解，它里面就包含@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的源码如下

@SuppressWarnings("deprecation")
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(EnableAutoConfigurationImportSelector.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 {};

}

@EnableAutoConfiguration的主要作用是向Spring容器中注入了EnableAutoConfigurationImportSelector对象，主要通过该对象完成自动配置的功能。

我们可以通过@SpringBootApplication(exclude = DataSourceAutoConfiguration.class)关闭特定的自动配置。

Tips：Spring框架当中定义了很多@Enable*注解，除了@EnableAutoConfiguration之外，常见的还有@EnableCaching、@EnableAsync、@EnableAspectJAutoProxy。它们的作用都是在框架中启用某种功能，它们的原理都是通过@Import注解往容器中注入某个Bean对象。

EnableAutoConfigurationImportSelector类的继承关系如下所示

ImportSelector（接口，方法：selectImports()）
    DeferredImportSelector（接口，无方法）
        AutoConfigurationImportSelector
            EnableAutoConfigurationImportSelector

EnableAutoConfigurationImportSelector类中没有selectImports()方法，其selectImports()方法在AutoConfigurationImportSelector类中实现，代码如下：

public String[] selectImports(AnnotationMetadata annotationMetadata) {
    if (!isEnabled(annotationMetadata)) {
        return NO_IMPORTS;
    }
    try {
        AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
                .loadMetadata(this.beanClassLoader);
        AnnotationAttributes attributes = getAttributes(annotationMetadata);
        //获取所有在配置文件中定义的xxxAutoConfiguration类的全路径名
        List<String> configurations = getCandidateConfigurations(annotationMetadata,
                attributes);
        // 去除重复的xxxAutoConfiguration
        configurations = removeDuplicates(configurations);
        // 对xxxAutoConfiguration进行排序
        configurations = sort(configurations, autoConfigurationMetadata);
        Set<String> exclusions = getExclusions(annotationMetadata, attributes);
        checkExcludedClasses(configurations, exclusions);
        configurations.removeAll(exclusions);
        configurations = filter(configurations, autoConfigurationMetadata);
        fireAutoConfigurationImportEvents(configurations, exclusions);
        return configurations.toArray(new String[configurations.size()]);
    }
    catch (IOException ex) {
        throw new IllegalStateException(ex);
    }
}

该方法主要做了以下几件事

  1. 获取所有在配置文件中定义的xxxAutoConfiguration类的全路径名；
  2. 对获取到的AutoConfiguration类名进行去重及排序；
  3. 移除被声明为排除的自动配置类；
  4. 过滤AutoConfiguration，选出当前应用匹配的自动配置类；
  5. 广播AutoConfigurationImportEvent事件。

注意这里只是加载了自动配置的类名，还没有将其注册到Spring容器当中，只有将这些自动配置类注册到Spring容器当中，它们才会生效。

获取所有在配置文件中定义的xxxAutoConfiguration类的全路径名

getCandidateConfigurations()方法会委托给SpringFactoriesLoader.loadFactoryNames()执行，其方法的代码如下

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

public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
    String factoryClassName = factoryClass.getName();
    try {
        Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
        List<String> result = new ArrayList<String>();
        while (urls.hasMoreElements()) {
            URL url = urls.nextElement();
            Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
            String factoryClassNames = properties.getProperty(factoryClassName);
            result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
        }
        return result;
    }
    catch (IOException ex) {
        throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +
                "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
    }
}

urls表示资源加载的路径，它会循环转换成Properties，然后从Properties中查找org.springframework.boot.autoconfigure.EnableAutoConfiguration。我调试的应用urls的取值如下：

jar:file:/D:/app/apache-maven-3.3.9/m2/org/springframework/boot/spring-boot/1.5.4.RELEASE/spring-boot-1.5.4.RELEASE.jar!/META-INF/spring.factories
jar:file:/D:/app/apache-maven-3.3.9/m2/org/springframework/boot/spring-boot-autoconfigure/1.5.4.RELEASE/spring-boot-autoconfigure-1.5.4.RELEASE.jar!/META-INF/spring.factories
jar:file:/D:/app/apache-maven-3.3.9/m2/org/springframework/spring-beans/4.3.9.RELEASE/spring-beans-4.3.9.RELEASE.jar!/META-INF/spring.factories

spring-boot-autoconfigure-1.5.4.RELEASE.jar!/META-INF/spring.factories文件中定义了如下自动配置类的内容。

# 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,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\
org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\
org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\
org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,\
org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastJpaDependencyAutoConfiguration,\
org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration,\
org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,\
org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,\
org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.artemis.ArtemisAutoConfiguration,\
org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\
org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\
org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,\
org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.DeviceResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.DeviceDelegatingViewResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.SitePreferenceAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityFilterAutoConfiguration,\
org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.OAuth2AutoConfiguration,\
org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\
org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\
org.springframework.boot.autoconfigure.social.SocialWebAutoConfiguration,\
org.springframework.boot.autoconfigure.social.FacebookAutoConfiguration,\
org.springframework.boot.autoconfigure.social.LinkedInAutoConfiguration,\
org.springframework.boot.autoconfigure.social.TwitterAutoConfiguration,\
org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\
org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration,\
org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpEncodingAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration,\
org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebClientAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration

截至到selectImports()方法的调用栈如下所示：


对于自动配置机制来说，我们可以简单理解为：从配置文件中获取自动配置类是发生在AbstractApplicationContext.refresh()方法中的invokeBeanFactoryPostProcessors()调用过程中。

到目前为止我们还没有向Spring容器注入加载出来的自动配置类，而注入自动配置类发生在AbstractApplicationContext.refresh()方法中的onRefresh()。
我们在HttpEncodingAutoConfiguration类的构造方法打个断点可以看看其方法调用栈。

常用组件的自动配置分析

在spring-boot-autoconfigure包中，定义了开发过程中常用组件要完成自动化配置代码模块。


常用组件的自动配置实现原理都很类似，下面分析一下HttpEncoding自动化配，首先来看看HttpEncodingAutoConfiguration的代码

@Configuration
//启动指定类的ConfigurationProperties功能；将配置文件中对应的值和HttpEncodingProperties绑定起来；并把HttpEncodingProperties加入到ioc容器中
@EnableConfigurationProperties(HttpEncodingProperties.class) 
@ConditionalOnWebApplication
@ConditionalOnClass(CharacterEncodingFilter.class)
@ConditionalOnProperty(prefix = "spring.http.encoding", value = "enabled", matchIfMissing = true)
public class HttpEncodingAutoConfiguration {

    private final HttpEncodingProperties properties;

    public HttpEncodingAutoConfiguration(HttpEncodingProperties properties) {
        this.properties = properties;
    }

    @Bean
    @ConditionalOnMissingBean(CharacterEncodingFilter.class)
    public CharacterEncodingFilter characterEncodingFilter() {
        CharacterEncodingFilter filter = new OrderedCharacterEncodingFilter();
        filter.setEncoding(this.properties.getCharset().name());
        filter.setForceRequestEncoding(this.properties.shouldForce(Type.REQUEST));
        filter.setForceResponseEncoding(this.properties.shouldForce(Type.RESPONSE));
        return filter;
    }

    @Bean
    public LocaleCharsetMappingsCustomizer localeCharsetMappingsCustomizer() {
        return new LocaleCharsetMappingsCustomizer(this.properties);
    }

    ...

}

@Configuration表明HttpEncodingAutoConfiguration是一个配置类，可以在其中定义Bean对象。
@ConfigurationProperties注解主要用来把properties配置文件转化为bean来使用的，而@EnableConfigurationProperties注解的作用是@ConfigurationProperties注解生效。如果只配置@ConfigurationProperties注解，在IOC容器中是获取不到properties配置文件转化的bean的。

@Conditional扩展注解 作用（判断是否满足当前指定条件）
@ConditionalOnJava 系统的java版本是否符合要求
@ConditionalOnBean 容器中存在指定Bean；
@ConditionalOnMissingBean 容器中不存在指定Bean；
@ConditionalOnExpression 满足SpEL表达式指定
@ConditionalOnClass 系统中有指定的类
@ConditionalOnMissingClass 系统中没有指定的类
@ConditionalOnSingleCandidate 容器中只有一个指定的Bean，或者这个Bean是首选Bean
@ConditionalOnProperty 系统中指定的属性是否有指定的值
@ConditionalOnResource 类路径下是否存在指定资源文件
@ConditionalOnWebApplication 当前是web环境
@ConditionalOnNotWebApplication 当前不是web环境
@ConditionalOnJndi JNDI存在指定项

可以发现HttpEncodingAutoConfiguration中使用了很多@Conditional*注解，它们都是从@Conditional扩展而来，表示只有当条件成立时，才会向Spring容器中添加组件，如果多个注解用在配置类上，则表示只有当所有@Conditional*中的条件都被满足时整个配置类才能生效。常见@Conditional*的含义如下表所示

@Conditional扩展注解 作用（判断是否满足当前指定条件）
@ConditionalOnJava 系统的java版本是否符合要求
@ConditionalOnBean 容器中存在指定Bean；
@ConditionalOnMissingBean 容器中不存在指定Bean；
@ConditionalOnExpression 满足SpEL表达式指定
@ConditionalOnClass 系统中有指定的类
@ConditionalOnMissingClass 系统中没有指定的类
@ConditionalOnSingleCandidate 容器中只有一个指定的Bean，或者这个Bean是首选Bean
@ConditionalOnProperty 系统中指定的属性是否有指定的值
@ConditionalOnResource 类路径下是否存在指定资源文件
@ConditionalOnWebApplication 当前是web环境
@ConditionalOnNotWebApplication 当前不是web环境
@ConditionalOnJndi JNDI存在指定项

看看HttpEncodingProperties的源码

@ConfigurationProperties(prefix = "spring.http.encoding")
public class HttpEncodingProperties {

    public static final Charset DEFAULT_CHARSET = Charset.forName("UTF-8");

    /**
     * Charset of HTTP requests and responses. Added to the "Content-Type" header if not
     * set explicitly.
     */
    private Charset charset = DEFAULT_CHARSET;

    /**
     * Force the encoding to the configured charset on HTTP requests and responses.
     */
    private Boolean force;

    /**
     * Force the encoding to the configured charset on HTTP requests. Defaults to true
     * when "force" has not been specified.
     */
    private Boolean forceRequest;

    /**
     * Force the encoding to the configured charset on HTTP responses.
     */
    private Boolean forceResponse;

    /**
     * Locale to Encoding mapping.
     */
    private Map<Locale, Charset> mapping;

    public Charset getCharset() {
        return this.charset;
    }

    public void setCharset(Charset charset) {
        this.charset = charset;
    }

    public boolean isForce() {
        return Boolean.TRUE.equals(this.force);
    }

    public void setForce(boolean force) {
        this.force = force;
    }

    public boolean isForceRequest() {
        return Boolean.TRUE.equals(this.forceRequest);
    }

    public void setForceRequest(boolean forceRequest) {
        this.forceRequest = forceRequest;
    }

    public boolean isForceResponse() {
        return Boolean.TRUE.equals(this.forceResponse);
    }

    public void setForceResponse(boolean forceResponse) {
        this.forceResponse = forceResponse;
    }

    public Map<Locale, Charset> getMapping() {
        return this.mapping;
    }

    public void setMapping(Map<Locale, Charset> mapping) {
        this.mapping = mapping;
    }

    boolean shouldForce(Type type) {
        Boolean force = (type == Type.REQUEST ? this.forceRequest : this.forceResponse);
        if (force == null) {
            force = this.force;
        }
        if (force == null) {
            force = (type == Type.REQUEST);
        }
        return force;
    }

    enum Type {

        REQUEST, RESPONSE

    }

}

如果要在配置文件中定义HttpEncoding相关的配置，则可以在配置文件中写入spring.http.encoding.x，其中x表示HttpEncodingProperties中的属性。

可以总结出自动配置的大体套路是定义下面两个类：

  1. xxxxAutoConfigurartion：自动配置类，给容器中添加需要的组件；
  2. xxxxProperties:封装配置文件中相关属性，所有在配置文件中可以配置的属性都是在xxxxProperties类中封装者，配置文件能配置什么就可以参照某个功能对应的这个属性类。

自定义自动配置

请参考：初识Spring Boot框架(二)之DIY一个Spring Boot的自动配置

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