SpringBoot自动装配流程源码分析

SpringBoot

传统方式的SSM框架因为需要配置大量文件而被开发人员诟病重复性工作，所以SpringBoot的出现在减少开发人员做大量重复性配置的工作，使得开发人员能够快速的开始项目开发、更加专注于业务代码的编写。但SpringBoot跟SSM有什么框架不同呢？为什么SpringBoot可以自动装配呢？SpringBoot自动装配是如何实现的呢？

SpringBoot入口

写过SpringBoot应用的开发者都知道，SpringBoot应用的启动类是被@SpringBootApplication注解的类，如下我写的一个demo.后续的自动装配源码也是从这个demo入口进入的。

package com.chandler.auto_configure.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DemoApplication {

public static void main(String[] args) {
SpringApplication.run(DemoApplication.class, args);
}

}

1.主入口只有一个main方法，SpringApplication.run()方法。
2.@SpringBootApplication.

SpringBootApplication注解

package org.springframework.boot.autoconfigure;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.boot.SpringBootConfiguration;
import org.springframework.boot.context.TypeExcludeFilter;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.FilterType;
import org.springframework.context.annotation.ComponentScan.Filter;
import org.springframework.core.annotation.AliasFor;

@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 {
@AliasFor(
annotation = EnableAutoConfiguration.class
)
Class<?>[] exclude() default {};

@AliasFor(
annotation = EnableAutoConfiguration.class
)
String[] excludeName() default {};

@AliasFor(
annotation = ComponentScan.class,
attribute = "basePackages"
)
String[] scanBasePackages() default {};

@AliasFor(
annotation = ComponentScan.class,
attribute = "basePackageClasses"
)
Class<?>[] scanBasePackageClasses() default {};

@AliasFor(
annotation = Configuration.class
)
boolean proxyBeanMethods() default true;
}

SpringBootApplication是一个组合注解，@SpringBootConfiguration、@EnableAutoConfiguratio、@ComponentScan都在包含咋该注解中。

SpringBootConfiguration注解

package org.springframework.boot;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.annotation.AliasFor;

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {
@AliasFor(
annotation = Configuration.class
)
boolean proxyBeanMethods() default true;
}

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {
@AliasFor(
annotation = Component.class
)
String value() default "";

boolean proxyBeanMethods() default true;
}

EnableAutoConfigureation注解

该注解开启了自动装配功能，我们进入看看具体是如何实现的。

@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 {};
}

/**
* Indicates that the package containing the annotated class should be registered with
* {@link AutoConfigurationPackages}.
*
* @author Phillip Webb
* @since 1.3.0
* @see AutoConfigurationPackages
*/
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {

}

EnableAutoConfigureation注解包含了@AutoConfigurationPackage，@AutoConfigurationPackage中在容器中@Import了一个AutoConfigurationPackages.Registrar.class，我们继续看看这个Registrar.class。

/**
* {@link ImportBeanDefinitionRegistrar} to store the base package from the importing
* configuration.
*/
static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

@Override
public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
register(registry, new PackageImport(metadata).getPackageName());
}

@Override
public Set<Object> determineImports(AnnotationMetadata metadata) {
return Collections.singleton(new PackageImport(metadata));
}

}

Registrar.获取扫描的包路径。获取的就是被@SpringBootApplication注解类所在包以及子包。所以包名一定要注意，在主配置类（@SpringBootApplication注解的类）。现在以及知道从哪里去加载了，但还有一个问题就是加载谁？？EnableAutoConfigureation注解中还@Import了一个AutoConfigurationImportSelector.class。

有了这些自动装配的功能，就省去了配置的重复性工作，但这些配置又是从哪里读取得呢？默认情况下都会有一套默认配置。这些配置是Spring-boot-autoconfiguration中META-INF下的spring.factories.

/**
* {@link EnableAutoConfiguration Auto-configuration} for {@link EnableWebMvc Web MVC}.
*
* @author Phillip Webb
* @author Dave Syer
* @author Andy Wilkinson
* @author Sébastien Deleuze
* @author Eddú Meléndez
* @author Stephane Nicoll
* @author Kristine Jetzke
* @author Bruce Brouwer
* @author Artsiom Yudovin
* @since 2.0.0
*/
@Configuration(proxyBeanMethods = false)
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass({ Servlet.class, DispatcherServlet.class, WebMvcConfigurer.class })
@ConditionalOnMissingBean(WebMvcConfigurationSupport.class)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10)
@AutoConfigureAfter({ DispatcherServletAutoConfiguration.class, TaskExecutionAutoConfiguration.class,
ValidationAutoConfiguration.class })
public class WebMvcAutoConfiguration {

public static final String DEFAULT_PREFIX = "";

public static final String DEFAULT_SUFFIX = "";

private static final String[] SERVLET_LOCATIONS = { "/" };

@Bean
@ConditionalOnMissingBean(HiddenHttpMethodFilter.class)
@ConditionalOnProperty(prefix = "spring.mvc.hiddenmethod.filter", name = "enabled", matchIfMissing = false)
public OrderedHiddenHttpMethodFilter hiddenHttpMethodFilter() {
return new OrderedHiddenHttpMethodFilter();
}

@Bean
@ConditionalOnMissingBean(FormContentFilter.class)
@ConditionalOnProperty(prefix = "spring.mvc.formcontent.filter", name = "enabled", matchIfMissing = true)
public OrderedFormContentFilter formContentFilter() {
return new OrderedFormContentFilter();
}

static String[] getResourceLocations(String[] staticLocations) {
String[] locations = new String[staticLocations.length + SERVLET_LOCATIONS.length];
System.arraycopy(staticLocations, 0, locations, 0, staticLocations.length);
System.arraycopy(SERVLET_LOCATIONS, 0, locations, staticLocations.length, SERVLET_LOCATIONS.length);
return locations;
}

// Defined as a nested config to ensure WebMvcConfigurer is not read when not
// on the classpath
@Configuration(proxyBeanMethods = false)
@Import(EnableWebMvcConfiguration.class)
@EnableConfigurationProperties({ WebMvcProperties.class, ResourceProperties.class })
@Order(0)
public static class WebMvcAutoConfigurationAdapter implements WebMvcConfigurer {

private static final Log logger = LogFactory.getLog(WebMvcConfigurer.class);

private final ResourceProperties resourceProperties;

private final WebMvcProperties mvcProperties;

private final ListableBeanFactory beanFactory;

private final ObjectProvider<HttpMessageConverters> messageConvertersProvider;

final ResourceHandlerRegistrationCustomizer resourceHandlerRegistrationCustomizer;

public WebMvcAutoConfigurationAdapter(ResourceProperties resourceProperties, WebMvcProperties mvcProperties,
ListableBeanFactory beanFactory, ObjectProvider<HttpMessageConverters> messageConvertersProvider,
ObjectProvider<ResourceHandlerRegistrationCustomizer> resourceHandlerRegistrationCustomizerProvider) {
this.resourceProperties = resourceProperties;
this.mvcProperties = mvcProperties;
this.beanFactory = beanFactory;
this.messageConvertersProvider = messageConvertersProvider;
this.resourceHandlerRegistrationCustomizer = resourceHandlerRegistrationCustomizerProvider.getIfAvailable();
}

@Override
public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
this.messageConvertersProvider
.ifAvailable((customConverters) -> converters.addAll(customConverters.getConverters()));
}

@Override
public void configureAsyncSupport(AsyncSupportConfigurer configurer) {
if (this.beanFactory.containsBean(TaskExecutionAutoConfiguration.APPLICATION_TASK_EXECUTOR_BEAN_NAME)) {
Object taskExecutor = this.beanFactory
.getBean(TaskExecutionAutoConfiguration.APPLICATION_TASK_EXECUTOR_BEAN_NAME);
if (taskExecutor instanceof AsyncTaskExecutor) {
configurer.setTaskExecutor(((AsyncTaskExecutor) taskExecutor));
}
}
Duration timeout = this.mvcProperties.getAsync().getRequestTimeout();
if (timeout != null) {
configurer.setDefaultTimeout(timeout.toMillis());
}
}

@Override
public void configurePathMatch(PathMatchConfigurer configurer) {
configurer.setUseSuffixPatternMatch(this.mvcProperties.getPathmatch().isUseSuffixPattern());
configurer.setUseRegisteredSuffixPatternMatch(
this.mvcProperties.getPathmatch().isUseRegisteredSuffixPattern());
}

@Override
public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
WebMvcProperties.Contentnegotiation contentnegotiation = this.mvcProperties.getContentnegotiation();
configurer.favorPathExtension(contentnegotiation.isFavorPathExtension());
configurer.favorParameter(contentnegotiation.isFavorParameter());
if (contentnegotiation.getParameterName() != null) {
configurer.parameterName(contentnegotiation.getParameterName());
}
Map<String, MediaType> mediaTypes = this.mvcProperties.getContentnegotiation().getMediaTypes();
mediaTypes.forEach(configurer::mediaType);
}

@Bean
@ConditionalOnMissingBean
public InternalResourceViewResolver defaultViewResolver() {
InternalResourceViewResolver resolver = new InternalResourceViewResolver();
resolver.setPrefix(this.mvcProperties.getView().getPrefix());
resolver.setSuffix(this.mvcProperties.getView().getSuffix());
return resolver;
}

@Bean
@ConditionalOnBean(View.class)
@ConditionalOnMissingBean
public BeanNameViewResolver beanNameViewResolver() {
BeanNameViewResolver resolver = new BeanNameViewResolver();
resolver.setOrder(Ordered.LOWEST_PRECEDENCE - 10);
return resolver;
}

@Bean
@ConditionalOnBean(ViewResolver.class)
@ConditionalOnMissingBean(name = "viewResolver", value = ContentNegotiatingViewResolver.class)
public ContentNegotiatingViewResolver viewResolver(BeanFactory beanFactory) {
ContentNegotiatingViewResolver resolver = new ContentNegotiatingViewResolver();
resolver.setContentNegotiationManager(beanFactory.getBean(ContentNegotiationManager.class));
// ContentNegotiatingViewResolver uses all the other view resolvers to locate
// a view so it should have a high precedence
resolver.setOrder(Ordered.HIGHEST_PRECEDENCE);
return resolver;
}

@Bean
@ConditionalOnMissingBean
@ConditionalOnProperty(prefix = "spring.mvc", name = "locale")
public LocaleResolver localeResolver() {
if (this.mvcProperties.getLocaleResolver() == WebMvcProperties.LocaleResolver.FIXED) {
return new FixedLocaleResolver(this.mvcProperties.getLocale());
}
AcceptHeaderLocaleResolver localeResolver = new AcceptHeaderLocaleResolver();
localeResolver.setDefaultLocale(this.mvcProperties.getLocale());
return localeResolver;
}

@Override
public MessageCodesResolver getMessageCodesResolver() {
if (this.mvcProperties.getMessageCodesResolverFormat() != null) {
DefaultMessageCodesResolver resolver = new DefaultMessageCodesResolver();
resolver.setMessageCodeFormatter(this.mvcProperties.getMessageCodesResolverFormat());
return resolver;
}
return null;
}

@Override
public void addFormatters(FormatterRegistry registry) {
ApplicationConversionService.addBeans(registry, this.beanFactory);
}

@Override
public void addResourceHandlers(ResourceHandlerRegistry registry) {
if (!this.resourceProperties.isAddMappings()) {
logger.debug("Default resource handling disabled");
return;
}
Duration cachePeriod = this.resourceProperties.getCache().getPeriod();
CacheControl cacheControl = this.resourceProperties.getCache().getCachecontrol().toHttpCacheControl();
if (!registry.hasMappingForPattern("/webjars/**")) {
customizeResourceHandlerRegistration(registry.addResourceHandler("/webjars/**")
.addResourceLocations("classpath:/META-INF/resources/webjars/")
.setCachePeriod(getSeconds(cachePeriod)).setCacheControl(cacheControl));
}
String staticPathPattern = this.mvcProperties.getStaticPathPattern();
if (!registry.hasMappingForPattern(staticPathPattern)) {
customizeResourceHandlerRegistration(registry.addResourceHandler(staticPathPattern)
.addResourceLocations(getResourceLocations(this.resourceProperties.getStaticLocations()))
.setCachePeriod(getSeconds(cachePeriod)).setCacheControl(cacheControl));
}
}

private Integer getSeconds(Duration cachePeriod) {
return (cachePeriod != null) ? (int) cachePeriod.getSeconds() : null;
}

private void customizeResourceHandlerRegistration(ResourceHandlerRegistration registration) {
if (this.resourceHandlerRegistrationCustomizer != null) {
this.resourceHandlerRegistrationCustomizer.customize(registration);
}
}

@Bean
@ConditionalOnMissingBean({ RequestContextListener.class, RequestContextFilter.class })
@ConditionalOnMissingFilterBean(RequestContextFilter.class)
public static RequestContextFilter requestContextFilter() {
return new OrderedRequestContextFilter();
}

}

/**
* Configuration equivalent to {@code @EnableWebMvc}.
*/
@Configuration(proxyBeanMethods = false)
public static class EnableWebMvcConfiguration extends DelegatingWebMvcConfiguration implements ResourceLoaderAware {

private final ResourceProperties resourceProperties;

private final WebMvcProperties mvcProperties;

private final ListableBeanFactory beanFactory;

private final WebMvcRegistrations mvcRegistrations;

private ResourceLoader resourceLoader;

public EnableWebMvcConfiguration(ResourceProperties resourceProperties,
ObjectProvider<WebMvcProperties> mvcPropertiesProvider,
ObjectProvider<WebMvcRegistrations> mvcRegistrationsProvider, ListableBeanFactory beanFactory) {
this.resourceProperties = resourceProperties;
this.mvcProperties = mvcPropertiesProvider.getIfAvailable();
this.mvcRegistrations = mvcRegistrationsProvider.getIfUnique();
this.beanFactory = beanFactory;
}

@Bean
@Override
public RequestMappingHandlerAdapter requestMappingHandlerAdapter(
@Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager,
@Qualifier("mvcConversionService") FormattingConversionService conversionService,
@Qualifier("mvcValidator") Validator validator) {
RequestMappingHandlerAdapter adapter = super.requestMappingHandlerAdapter(contentNegotiationManager,
conversionService, validator);
adapter.setIgnoreDefaultModelOnRedirect(
this.mvcProperties == null || this.mvcProperties.isIgnoreDefaultModelOnRedirect());
return adapter;
}

@Override
protected RequestMappingHandlerAdapter createRequestMappingHandlerAdapter() {
if (this.mvcRegistrations != null && this.mvcRegistrations.getRequestMappingHandlerAdapter() != null) {
return this.mvcRegistrations.getRequestMappingHandlerAdapter();
}
return super.createRequestMappingHandlerAdapter();
}

@Bean
@Primary
@Override
public RequestMappingHandlerMapping requestMappingHandlerMapping(
@Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager,
@Qualifier("mvcConversionService") FormattingConversionService conversionService,
@Qualifier("mvcResourceUrlProvider") ResourceUrlProvider resourceUrlProvider) {
// Must be @Primary for MvcUriComponentsBuilder to work
return super.requestMappingHandlerMapping(contentNegotiationManager, conversionService,
resourceUrlProvider);
}

@Bean
public WelcomePageHandlerMapping welcomePageHandlerMapping(ApplicationContext applicationContext,
FormattingConversionService mvcConversionService, ResourceUrlProvider mvcResourceUrlProvider) {
WelcomePageHandlerMapping welcomePageHandlerMapping = new WelcomePageHandlerMapping(
new TemplateAvailabilityProviders(applicationContext), applicationContext, getWelcomePage(),
this.mvcProperties.getStaticPathPattern());
welcomePageHandlerMapping.setInterceptors(getInterceptors(mvcConversionService, mvcResourceUrlProvider));
return welcomePageHandlerMapping;
}

private Optional<Resource> getWelcomePage() {
String[] locations = getResourceLocations(this.resourceProperties.getStaticLocations());
return Arrays.stream(locations).map(this::getIndexHtml).filter(this::isReadable).findFirst();
}

private Resource getIndexHtml(String location) {
return this.resourceLoader.getResource(location + "index.html");
}

private boolean isReadable(Resource resource) {
try {
return resource.exists() && (resource.getURL() != null);
}
catch (Exception ex) {
return false;
}
}

@Bean
@Override
public FormattingConversionService mvcConversionService() {
WebConversionService conversionService = new WebConversionService(this.mvcProperties.getDateFormat());
addFormatters(conversionService);
return conversionService;
}

@Bean
@Override
public Validator mvcValidator() {
if (!ClassUtils.isPresent("javax.validation.Validator", getClass().getClassLoader())) {
return super.mvcValidator();
}
return ValidatorAdapter.get(getApplicationContext(), getValidator());
}

@Override
protected RequestMappingHandlerMapping createRequestMappingHandlerMapping() {
if (this.mvcRegistrations != null && this.mvcRegistrations.getRequestMappingHandlerMapping() != null) {
return this.mvcRegistrations.getRequestMappingHandlerMapping();
}
return super.createRequestMappingHandlerMapping();
}

@Override
protected ConfigurableWebBindingInitializer getConfigurableWebBindingInitializer(
FormattingConversionService mvcConversionService, Validator mvcValidator) {
try {
return this.beanFactory.getBean(ConfigurableWebBindingInitializer.class);
}
catch (NoSuchBeanDefinitionException ex) {
return super.getConfigurableWebBindingInitializer(mvcConversionService, mvcValidator);
}
}

@Override
protected ExceptionHandlerExceptionResolver createExceptionHandlerExceptionResolver() {
if (this.mvcRegistrations != null && this.mvcRegistrations.getExceptionHandlerExceptionResolver() != null) {
return this.mvcRegistrations.getExceptionHandlerExceptionResolver();
}
return super.createExceptionHandlerExceptionResolver();
}

@Override
protected void extendHandlerExceptionResolvers(List<HandlerExceptionResolver> exceptionResolvers) {
super.extendHandlerExceptionResolvers(exceptionResolvers);
if (this.mvcProperties.isLogResolvedException()) {
for (HandlerExceptionResolver resolver : exceptionResolvers) {
if (resolver instanceof AbstractHandlerExceptionResolver) {
((AbstractHandlerExceptionResolver) resolver).setWarnLogCategory(resolver.getClass().getName());
}
}
}
}

@Bean
@Override
public ContentNegotiationManager mvcContentNegotiationManager() {
ContentNegotiationManager manager = super.mvcContentNegotiationManager();
List<ContentNegotiationStrategy> strategies = manager.getStrategies();
ListIterator<ContentNegotiationStrategy> iterator = strategies.listIterator();
while (iterator.hasNext()) {
ContentNegotiationStrategy strategy = iterator.next();
if (strategy instanceof PathExtensionContentNegotiationStrategy) {
iterator.set(new OptionalPathExtensionContentNegotiationStrategy(strategy));
}
}
return manager;
}

@Override
public void setResourceLoader(ResourceLoader resourceLoader) {
this.resourceLoader = resourceLoader;
}

}

@Configuration(proxyBeanMethods = false)
@ConditionalOnEnabledResourceChain
static class ResourceChainCustomizerConfiguration {

@Bean
ResourceChainResourceHandlerRegistrationCustomizer resourceHandlerRegistrationCustomizer() {
return new ResourceChainResourceHandlerRegistrationCustomizer();
}

}

interface ResourceHandlerRegistrationCustomizer {

void customize(ResourceHandlerRegistration registration);

}

static class ResourceChainResourceHandlerRegistrationCustomizer implements ResourceHandlerRegistrationCustomizer {

@Autowired
private ResourceProperties resourceProperties = new ResourceProperties();

@Override
public void customize(ResourceHandlerRegistration registration) {
ResourceProperties.Chain properties = this.resourceProperties.getChain();
configureResourceChain(properties, registration.resourceChain(properties.isCache()));
}

private void configureResourceChain(ResourceProperties.Chain properties, ResourceChainRegistration chain) {
Strategy strategy = properties.getStrategy();
if (properties.isCompressed()) {
chain.addResolver(new EncodedResourceResolver());
}
if (strategy.getFixed().isEnabled() || strategy.getContent().isEnabled()) {
chain.addResolver(getVersionResourceResolver(strategy));
}
if (properties.isHtmlApplicationCache()) {
chain.addTransformer(new AppCacheManifestTransformer());
}
}

private ResourceResolver getVersionResourceResolver(ResourceProperties.Strategy properties) {
VersionResourceResolver resolver = new VersionResourceResolver();
if (properties.getFixed().isEnabled()) {
String version = properties.getFixed().getVersion();
String[] paths = properties.getFixed().getPaths();
resolver.addFixedVersionStrategy(version, paths);
}
if (properties.getContent().isEnabled()) {
String[] paths = properties.getContent().getPaths();
resolver.addContentVersionStrategy(paths);
}
return resolver;
}

}

/**
* Decorator to make {@link PathExtensionContentNegotiationStrategy} optional
* depending on a request attribute.
*/
static class OptionalPathExtensionContentNegotiationStrategy implements ContentNegotiationStrategy {

private static final String SKIP_ATTRIBUTE = PathExtensionContentNegotiationStrategy.class.getName() + ".SKIP";

private final ContentNegotiationStrategy delegate;

OptionalPathExtensionContentNegotiationStrategy(ContentNegotiationStrategy delegate) {
this.delegate = delegate;
}

@Override
public List<MediaType> resolveMediaTypes(NativeWebRequest webRequest)
throws HttpMediaTypeNotAcceptableException {
Object skip = webRequest.getAttribute(SKIP_ATTRIBUTE, RequestAttributes.SCOPE_REQUEST);
if (skip != null && Boolean.parseBoolean(skip.toString())) {
return MEDIA_TYPE_ALL_LIST;
}
return this.delegate.resolveMediaTypes(webRequest);
}

}

}

这样是不是就很多配置都不需要我们自行配置了，SPringboot已经给我们配置好了。