浅谈SpringBoot的自动配置原理

た入场券 2023-06-29 10:53 9阅读 0赞

SpringBoot的运行原理就是基于SpringBoot的自动配置来实现。我们要想熟练的使用SpringBoot这一框架技术，就必须对其深入学习。

# 一： #

选择SpringBoot的依赖版本：

<parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.0.1.RELEASE</version>
        <relativePath/>
    </parent>

首先，我们先来看一下，当我们启动一个SpringBoot项目时，SpringBoot为我们自动启动了哪些自动配置类。

**操作(以下三种方式任选其一)**：

(1)在application.properties中设置属性：

debug=true

（2）通过cmd命令窗口运行xxx.jar文件

java -jar xxx.jar --debug

（3）在Idea 中设置运行时参数

![20200112144554536.png][]

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**结果**：

\--------------------

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2NDA4MjI5_size_16_color_FFFFFF_t_70 1][]

\--------------------

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--------------------

# 二： #

接下来，我们需要学习一个注解@SpringBootApplication，这个注解是一个组合注解，他的核心功能是由@EnableAutoConfiguration注解提供。

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2NDA4MjI5_size_16_color_FFFFFF_t_70 3][]

好的，然后我们来看一下@EnableAutoConfiguration 的源码：

@Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @AutoConfigurationPackage
    @Import(AutoConfigurationImportSelector.class)
    public @interface EnableAutoConfiguration {
    
    	String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
    
    	Class<?>[] exclude() default {};
    
    	String[] excludeName() default {};
    
    }

兄弟们，快看！！！这其中有一个@Import注解，注意到了吗？？？它的作用可是非常关键的！

它的作用就是将AutoConfigurationImportSelector类导入至Spring IOC容器。于是我们就进去看看，这个类中有一个selectImports方法。具体源码如下：

public String[] selectImports(AnnotationMetadata annotationMetadata) {
            if (!this.isEnabled(annotationMetadata)) {
                return NO_IMPORTS;
            } else {
                AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);
                AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
                List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);
                configurations = this.removeDuplicates(configurations);
                Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);
                this.checkExcludedClasses(configurations, exclusions);
                configurations.removeAll(exclusions);
                configurations = this.filter(configurations, autoConfigurationMetadata);
                this.fireAutoConfigurationImportEvents(configurations, exclusions);
                return StringUtils.toStringArray(configurations);
            }
        }

在这个方法中，调用了AutoConfigurationMetadataLoader类的一个静态方法loadMetadata(ClassLoader classLoader)。

public static AutoConfigurationMetadata loadMetadata(ClassLoader classLoader) {
            return loadMetadata(classLoader, "META-INF/spring-autoconfigure-metadata.properties");
        }

这个方法用来扫描具有"META-INF/spring-autoconfigure-metadata.properties"文件的jar包，而我们的spring-boot-autoconfigure-2.0.1RELEASE.jar里就有spring.factories文件,然后我们将其打开，查看一下此文件具体有哪些自动配置。如图所示：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM2NDA4MjI5_size_16_color_FFFFFF_t_70 4][]

然后我选择一个我们较为熟悉的HttpEncodingAutoConfiguration类

@Configuration //表示这是一个配置类,以前编写的配置文件一样,也可以给容器中添加组件
    @EnableConfigurationProperties({HttpEncodingProperties.class}) //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
    @ConditionalOnWebApplication(
        type = Type.SERVLET
    )
    @ConditionalOnClass({CharacterEncodingFilter.class}) //判断当前项目有没有这个类CharacterEncodingFilter ; SpringMVC中进行乱码解决的过滤器;
    @ConditionalOnProperty(
        prefix = "spring.http.encoding",
        value = {"enabled"},
        matchIfMissing = true
    )//判断配置文件中是否存在某个配置spring.http.encoding.enabled ;如果不存在，判断也是成立的//即使我们配置文件中不配置pring.http.encoding.enabled=true ,也是默认生效的;
    public class HttpEncodingAutoConfiguration {
        private final HttpEncodingProperties properties;
        
        //只有一个有参构造器的情况下,参数的值就会从容器中拿
        public HttpEncodingAutoConfiguration(HttpEncodingProperties properties) {
            this.properties = properties;
        }
    
        @Bean    //给容器中添加一个组件,这个组件的某些值需要从properties中获取
        @ConditionalOnMissingBean      //判断容器没有这个组件?
        public CharacterEncodingFilter characterEncodingFilter() {
            CharacterEncodingFilter filter = new OrderedCharacterEncodingFilter();
            filter.setEncoding(this.properties.getCharset().name());
            filter.setForceRequestEncoding(this.properties.shouldForce(org.springframework.boot.autoconfigure.http.HttpEncodingProperties.Type.REQUEST));
            filter.setForceResponseEncoding(this.properties.shouldForce(org.springframework.boot.autoconfigure.http.HttpEncodingProperties.Type.RESPONSE));
            return filter;
        }
    
        @Bean
        public HttpEncodingAutoConfiguration.LocaleCharsetMappingsCustomizer localeCharsetMappingsCustomizer() {
            return new HttpEncodingAutoConfiguration.LocaleCharsetMappingsCustomizer(this.properties);
        }
    
        private static class LocaleCharsetMappingsCustomizer implements WebServerFactoryCustomizer<ConfigurableServletWebServerFactory>, Ordered {
            private final HttpEncodingProperties properties;
    
            LocaleCharsetMappingsCustomizer(HttpEncodingProperties properties) {
                this.properties = properties;
            }
    
            public void customize(ConfigurableServletWebServerFactory factory) {
                if (this.properties.getMapping() != null) {
                    factory.setLocaleCharsetMappings(this.properties.getMapping());
                }
    
            }
    
            public int getOrder() {
                return 0;
            }
        }
    }

以上源码就是根据当前不同的条件判断,决定这个配置类是否生效?一但这个配置类生效 ;这个配置类就会给容器中添加各种组件;这些组件的属性是从对应的properties类中获取的,这些类里面的每一个属性又是和配置文件绑定的;

所有在配置文件中能配置的属性都是在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，比如：

@ConfigurationProperties(
        prefix = "spring.http.encoding"
    )        //从配置文件中获取指定值和bean的属性进行绑定
    public class HttpEncodingProperties {
        public static final Charset DEFAULT_CHARSET;
        private Charset charset;
        private Boolean force;
        private Boolean forceRequest;
        private Boolean forceResponse;
        private Map<Locale, Charset> mapping;
    
        public HttpEncodingProperties() {
            this.charset = DEFAULT_CHARSET;
        }
    
        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;
        }
    
        public boolean shouldForce(HttpEncodingProperties.Type type) {
            Boolean force = type == HttpEncodingProperties.Type.REQUEST ? this.forceRequest : this.forceResponse;
            if (force == null) {
                force = this.force;
            }
    
            if (force == null) {
                force = type == HttpEncodingProperties.Type.REQUEST;
            }
    
            return force;
        }
    
        static {
            DEFAULT_CHARSET = StandardCharsets.UTF_8;
        }
    
        public static enum Type {
            REQUEST,
            RESPONSE;
    
            private Type() {
            }
        }
    }

**总结：**

**1) SpringBoot启动会加载大量的自动配置类  
2) 我们看我们需要的功能有没有SpringBoot默认写好的自动配置类;  
3) 我们再来看这个自动配置类中到底配置了哪些组件; (只要我们要用的组件有,我们就不需要再来配置了)  
4) 给容器中自动配置类添加组件的时候,会从properties类中获取某些属性。我们就可以在配置文件中指定这些属性的值;  
5)** xxxxAutoConfigurartion **:自动配置类;给容器中添加组件  
   ** xxxxProperties**:封装配置文件中相关属性;**

[20200112144554536.png]: https://img-blog.csdnimg.cn/20200112144554536.png
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