关于注解@SpringBootApplication的一些认识

找到主配置类，点进@SpringBootApplication注解

下面是@SpringBootApplication注解的主要配置：

@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(
    excludeFilters = {@Filter(
    type = FilterType.CUSTOM,
    classes = {TypeExcludeFilter.class}
), @Filter(
    type = FilterType.CUSTOM,
    classes = {AutoConfigurationExcludeFilter.class}
)}
)
public @interface SpringBootApplication {

重点看看@EnableAutoConfiguration

点进@EnableAutoConfiguration

下面是@EnableAutoConfiguration注解的主要配置：

@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

@AutoConfigurationPackage在Hello World中已经说过，让我们点进AutoConfigurationImportSelector类

点进AutoConfigurationImportSelector类

• 在AutoConfigurationImportSelector类中有selectImports方法

@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());
}

• 该方法中调用了getAutoConfigurationEntry方法，点进getAutoConfigurationEntry方法

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);
}

• 发现configurations集合，里面调用了getCandidateConfigurations方法，再点进getCandidateConfigurations方法

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;
}

发现调用了SpringFactoriesLoader.loadFactoryNames方法

点进loadFactoryNames方法

发现如下代码：

public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
        String factoryTypeName = factoryType.getName();
        return (List)loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
    }

    private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
        MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
        if (result != null) {
            return result;
        } else {
            try {
                Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
                LinkedMultiValueMap result = new LinkedMultiValueMap();

                ...

                cache.put(classLoader, result);
                return result;
            } catch (IOException var13) {
                throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);
            }
        }
    }

可以看到，Spring Boot会扫描所有jar包类路径下META-INF/spring.factories里的内容，并包装成properties对象

那么factoryTypeName又是什么呢，其实就是传进来的Class<?> factoryType

查看传入的Class<?> factoryType

• Class<?> factoryType是由前面的SpringFactoriesLoader.loadFactoryNames传入的

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;
}

• 点进getSpringFactoriesLoaderFactoryClass()

protected Class<?> getSpringFactoriesLoaderFactoryClass() {
	return EnableAutoConfiguration.class;
}

可以看到，最终会返回EnableAutoConfiguration.class类名对应的值，即把这些添加到容器中

打开一个jar包，看看什么样

点开一个jar包，打开META-INF/spring.factories

# 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.CloudServiceConnectorsAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
...

终于找到了EnableAutoConfiguration对应的值，这样每一个xxxAutoConfiguration就被加载进来了

接下来，让我们以HttpEncodingAutoConfiguration为例，看看我们在application.properties中究竟能配置哪些属性呢

点开HttpEncodingAutoConfiguration类

@Configuration(proxyBeanMethods = false)
@EnableConfigurationProperties(HttpProperties.class)
@ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.SERVLET)
@ConditionalOnClass(CharacterEncodingFilter.class)
@ConditionalOnProperty(prefix = "spring.http.encoding", value = "enabled", matchIfMissing = true)
public class HttpEncodingAutoConfiguration {

发现了EnableConfigurationProperties注解，点进HttpProperties去看看

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

现在你应该知道如何配置application.properties文件了吧