SpringBoot的运行原理之自动配置

SpringBoot的核心就是自动配置，自动配置又是基于条件判断来配置Bean。关于自动配置的源码在spring-boot-autoconfigure-2.0.3.RELEASE.jar，在这里看：

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查看配置报告
在配置文件中加入如下配置，查看当前已开启和未开启的自动配置报告：

#查看当前项目中已启用和未启用的自动配置的报告
debug: true

启动项目

已开启的配置：

未开启的配置：

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SpringBoot运行原理
先看@SpringBootApplication

• @SpringBootConfiguration：标记当前类为配置类
• @EnableAutoConfiguration：开启自动配置
• @ComponentScan：扫描主类所在的同级包以及下级包里的Bean

关键是@EnableAutoConfiguration

@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 {};
}

关键是@Import(AutoConfigurationImportSelector.class)导入的配置功能，
AutoConfigurationImportSelector中的方法getCandidateConfigurations，得到待配置的class的类名集合

/**
     * Return the auto-configuration class names that should be considered. By default
     * this method will load candidates using {@link SpringFactoriesLoader} with
     * {@link #getSpringFactoriesLoaderFactoryClass()}.
     * @param metadata the source metadata
     * @param attributes the {@link #getAttributes(AnnotationMetadata) annotation
     * attributes}
     * @return a list of candidate configurations
     */
    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;
    }

通过SpringFactoriesLoader.loadFactoryNames()方法扫描META-INF/spring.factories文件

这文件列出了哪些需要自动配置。
随便打开一个配置类，例如：

都能看到各种各样的条件判断注解，满足条件时就加载这个Bean。
此类的条件注解是：@ConditionalOnProperty

• @ConditionalOnBean：当容器里有指定Bean的条件下
• @ConditionalOnClass：当类路径下有指定的类的条件下
• @ConditionalOnExpression：基于SpEL表达式作为判断条件
• @ConditionalOnJava：基于JVM版本作为判断条件
• @ConditionalOnJndi：在JNDI存在的条件下查找指定的位置
• @ConditionalOnMissingBean：当容器里没有指定Bean的情况下
• @ConditionalOnMissingClass：当容器里没有指定类的情况下
• @ConditionalOnWebApplication：当前项目时Web项目的条件下
• @ConditionalOnNotWebApplication：当前项目不是Web项目的条件下
• @ConditionalOnProperty：指定的属性是否有指定的值
• @ConditionalOnResource：类路径是否有指定的值
• @ConditionalOnOnSingleCandidate：当指定Bean在容器中只有一个，或者有多个但是指定首选的Bean

这些注解都组合了@Conditional注解，只是使用了不同的条件。

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通过实例，http编码的默认配置来讲解配置流程
双击shift搜索HttpEncodingProperties，这个类型安全的参数绑定的类就为自动配置http默认编码时提供配置参数。

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

    public static final Charset DEFAULT_CHARSET = StandardCharsets.UTF_8;

    /**
     * Charset of HTTP requests and responses. Added to the "Content-Type" header if not
     * set explicitly.
     */
    private Charset charset = DEFAULT_CHARSET;//默认编码方式UTF-8

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

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

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

    /**
     * Locale in which to encode mapping.
     */
    private Map<Locale, Charset> mapping;
    省略set/get
}

• @ConfigurationProperties(prefix = “spring.http.encoding”)：application配置文件配置时前缀是”spring.http.encoding”

双击shift搜索HttpEncodingAutoConfiguration http编码自动配置类，只截取部分代码

package org.springframework.boot.autoconfigure.web.servlet;

@Configuration
@EnableConfigurationProperties(HttpEncodingProperties.class)
@ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.SERVLET)
@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//设置Bean   CharacterEncodingFilter 
    @ConditionalOnMissingBean
    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;
    }
}

• @Configuration：标明为配置类
• @EnableConfigurationProperties(HttpEncodingProperties.class)声明开启属性注入
• @ConditionalOnClass(CharacterEncodingFilter.class)当CharacterEncodingFilter在类路径的条件下
• @ConditionalOnProperty(prefix = “spring.http.encoding”, value = “enabled”, matchIfMissing = true)当spring.http.encoding=enabled的情况下，如果没有设置则默认为true，即条件符合
• @ConditionalOnMissingBean当容器中没有这个Bean时新建Bean

SpringBoot的核心，自动配置的流程大概就是这个样子，下一篇演示创建自己的starter pom。
SpringBoot核心自动配置之创建自己的starter pom maven依赖包

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