在使用SpringBoot時,啟動類會使用@SpringBootApplication這個注解(如下):
@SpringBootApplication //是SpringBoot的主配置類
public class MySpringBootApplication {
public static void main(String[] args) {
SpringApplication.run(MySpringBootApplication.class, args);
}
}
點選這個注解源碼發現有三個注解@SpringBootConfiguration @EnableAutoConfiguration @ComponentScan,(代碼如下所示),我們可以認為@SpringBootApplication可被@SpringBootConfiguration @EnableAutoConfiguration @ComponentScan三個注解代替,即啟動類上可以标注這三個注解(不是說其他的注解沒用,隻是相對這三個對springboot而言作用很小)。
@Target(ElementType.TYPE) //設定目前注解可以标記在哪
@Retention(RetentionPolicy.RUNTIME) //當注解标注的類編譯以什麼方式保留(會被jvm加載)
@Documented //java doc 會生成注解資訊
@Inherited //是否會被繼承
@SpringBootConfiguration //表示這是一個Spring Boot的配置類
@EnableAutoConfiguration //SpringBoot開啟自動配置,會幫我們自動去加載自動配置類
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
//掃描包,如果沒有basePackage指定spring底層會自動掃描目前配置類所有在的包
public @interface SpringBootApplication {
@AliasFor(annotation = EnableAutoConfiguration.class)
Class<?>[] exclude() default {};
而這三個注解跟自動配置有關系的就是@EnableAutoConfiguration了,從名字上也可以看出,點進去看看源碼如下:
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage//将目前配置類所在包儲存在BasePackages的Bean中,供Spring内部使用
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
Class<?>[] exclude() default {};
String[] excludeName() default {};
}
可以看到上面的源代碼,有@Import(AutoConfigurationImportSelector.class),而這個注解就是關鍵,導入了AutoConfigurationImportSelector類,這個類實作了DeferredImportSelector接口,并且重寫了getImportGroup和selectImports方法。
實作了DeferredImportSelector接口的類被用上@Import後,會在spring加載完bean後處理,假如有重寫getImportGroup就會使用此函數中的group類,調用他的process方法,再調用這個group類的selectImports方法,如果沒有group類會直接調用AutoConfigurationImportSelector的getImportGroup方法(這是spring源碼時執行邏輯),Group類的源碼如下:
@Override
public Class<? extends Group> getImportGroup() {
return AutoConfigurationGroup.class;
}
@Override
public void process(AnnotationMetadata annotationMetadata, DeferredImportSelector deferredImportSelector) {
Assert.state(deferredImportSelector instanceof AutoConfigurationImportSelector,
() -> String.format("Only %s implementations are supported, got %s",
AutoConfigurationImportSelector.class.getSimpleName(),
deferredImportSelector.getClass().getName()));
AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector)
.getAutoConfigurationEntry(annotationMetadata);//會加載配置檔案的自動配置類
this.autoConfigurationEntries.add(autoConfigurationEntry);//在selectImports中會使用
for (String importClassName : autoConfigurationEntry.getConfigurations()) {
this.entries.putIfAbsent(importClassName, annotationMetadata);//在selectImports中排序用
}
}
@Override
public Iterable<Entry> selectImports() {
if (this.autoConfigurationEntries.isEmpty()) {
return Collections.emptyList();
}
//從自動配置類的集合中擷取Exclusions和Configurations
Set<String> allExclusions = this.autoConfigurationEntries.stream()
.map(AutoConfigurationEntry::getExclusions).flatMap(Collection::stream).collect(Collectors.toSet());
Set<String> processedConfigurations = this.autoConfigurationEntries.stream()
.map(AutoConfigurationEntry::getConfigurations).flatMap(Collection::stream)
.collect(Collectors.toCollection(LinkedHashSet::new));
processedConfigurations.removeAll(allExclusions);
return sortAutoConfigurations(processedConfigurations, getAutoConfigurationMetadata()).stream()
.map((importClassName) -> new Entry(this.entries.get(importClassName), importClassName))
.collect(Collectors.toList());//為自動配置類排序
}
下面分析下process函數中調用的getAutoConfigurationEntry方法,源碼如下:
protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return EMPTY_ENTRY;
}
//擷取注解屬性資訊
AnnotationAttributes attributes = getAttributes(annotationMetadata);
// 從META‐INF/spring.factories中獲得所有的自動配置類(包含所有jar及項目自己寫的META‐INF/spring.factories)
List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
//使用LinkedHashSet排重
configurations = removeDuplicates(configurations);
//根據EnableAutoConfiguration注解中屬性,擷取不需要自動裝配的類名單
Set<String> exclusions = getExclusions(annotationMetadata, attributes);
// 根據:@EnableAutoConfiguration.exclude
// @EnableAutoConfiguration.excludeName
// spring.autoconfigure.exclude 進行排除
checkExcludedClasses(configurations, exclusions);
configurations.removeAll(exclusions);
//通過讀取spring.factories中的OnBeanCondition\OnClassCondition\OnWebApplicationCondition條件注解進行過濾
configurations = getConfigurationClassFilter().filter(configurations);
//添加監聽器事件,分别把候選的配置名單,和排除的配置名單傳進去做擴充
fireAutoConfigurationImportEvents(configurations, exclusions);
return new AutoConfigurationEntry(configurations, exclusions);
}
自動配置META‐INF/spring.factories格式如下:
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.baomidou.mybatisplus.autoconfigure.IdentifierGeneratorAutoConfiguration,\
com.baomidou.mybatisplus.autoconfigure.MybatisPlusLanguageDriverAutoConfiguration,\
com.baomidou.mybatisplus.autoconfigure.MybatisPlusAutoConfiguration
@EnableAutoConfiguration注解通過@SpringBootApplication被間接的标記在了Spring Boot的啟動類上。在MySpringBootApplication.run(...)的内部就會解析@Import注解,執行group的process方法,找到所有JavaConfig自動配置類的全限定名對應的class,然後将所有自動配置類加載到Spring容器中。
springboot自動配置流程圖