@SpringBootApplication
Spring Boot应用标注 @SpringBootApplication 注解的类说明该类是Spring Boot 的主配置类，需要运行该类的main方法进行启动 Spring Boot 应用
@SpringBootConfiguration
该注解标注表示标注的类是个配置类
@EnableAutoConfiguration
直译：开启自动配置
@AutoConfigurationPackage
将当前配置类所在的包保存在 basePackages 的Bean 中，提供给Spring 使用
@Import(AutoConfigurationPackages.Registrar.class)
注册一个保存当前配置类所在包的Bean
@Import(AutoConfigurationImportSelector.class)
使用@Import注解完成导入AutoConfigurationImportSelector类 的功能。AutoConfigurationImportSelector 实现了DeferredImportSelector类
注：在解析ImportSelector时，所导入的配置类会被直接解析，而DeferredImportSelector导入的配置类会延迟进行解析（延迟在其他配置类都解析完之后）
Spring容器在解析@Import时会去执行DeferredImportSelector 的 selectImports方法:
/**
* Return the {@link AutoConfigurationEntry} based on the {@link AnnotationMetadata}
* of the importing {@link Configuration @Configuration} class.
* @param annotationMetadata the annotation metadata of the configuration class
* @return the auto-configurations that should be imported
*/
protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return EMPTY_ENTRY;
}
AnnotationAttributes attributes = getAttributes(annotationMetadata);
// 从META-INF/spring.factories中获得候选的自动配置类
List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
//去重
configurations = removeDuplicates(configurations);
//根据EnableAutoConfiguration注解中exclude、excludeName属性、
//spring.autoconfigure.exclude 配置的排除项
Set<String> exclusions = getExclusions(annotationMetadata, attributes);
checkExcludedClasses(configurations, exclusions);
configurations.removeAll(exclusions);
// 通过读取spring.factories 中
// OnBeanCondition\OnClassCondition\OnWebApplicationCondition进行过滤
configurations = getConfigurationClassFilter().filter(configurations);
fireAutoConfigurationImportEvents(configurations, exclusions);
return new AutoConfigurationEntry(configurations, exclusions);
}springboot应用中都会引入spring-boot-autoconfigure依赖，spring.factories文件就在该包的META-INF下面。spring.factories文件是Key=Value形式，多个Value时使用“，”逗号进行分割，该文件中定义了关于初始化、监听器、过滤器等信息，而真正使自动配置生效的key是org.springframework.boot.autoconfigure.EnableAutoConfiguration，如下所示：
Spring Boot 提供的自动配置类
https://docs.spring.io/spring-boot/docs/current/reference/html/auto-configuration-classes.html#appendix.auto-configuration-classes
Spring Boot 常用条件注解
ConditionalOnBean：是否存在某个某类或某个名字的Bean
ConditionalOnMissingBean：是否缺失某个某类或某个名字的Bean
ConditionalOnSingleCandidate：是否符合指定类型的Bean只有一个
ConditionalOnClass：是否存在某个类
ConditionalOnMissingClass：是否缺失某个类
ConditionalOnExpression：指定的表达式返回的是true还是false
ConditionalOnJava：判断Java版本
ConditionalOnWebApplication：当前应用是不是一个Web应用
ConditionalOnNotWebApplication：当前应用不是一个Web应用
ConditionalOnProperty：Environment中是否存在某个属性
我们也可以使用@Conditional注解进行自定义条件注解
条件注解可以写在类和方法上，如果某个@xxxCondition条件注解写在自动配置类上，那该自动配置类会不会生效就要看当前条件是否符合条件，或者条件注解写在某个@Bean修饰的方法上，那么Bean生不生效也要看当前的条件是否符条件。
Spring容器在解析某个自动配置类时，会先判断该自动配置类上是否有条件注解，如果有，则进一步判断条件注解所指定的条件当前情况是否满足，如果满足，则继续解析该配置类，如果不满足则不进行解析该配置类，于是配置类所定义的Bean也都得不到解析，那么在Spring容器中就不会存在该Bean。
同理，Spring在解析某个@Bean的方法时，也会先判断方法上是否有条件注解，然后进行解析，如果不满足条件，则该Bean也不会生效。
Spring Boot提供的自动配置，实际上就是Spring Boot源码中预先写好准备了一些常用的配置类，预先定义好了一些Bean，在用Spring Boot时，这些配置类就已经在我们项目的依赖中了，而这些自动配置类或自动配置Bean是否生效，就需要看具体指定的条件是否满足。
下面代码就是根据 @Conditional 确定是否应跳过忽略
/**
* Determine if an item should be skipped based on {@code @Conditional} annotations.
* @param metadata the meta data
* @param phase the phase of the call
* @return if the item should be skipped
*/
public boolean shouldSkip(@Nullable AnnotatedTypeMetadata metadata, @Nullable ConfigurationPhase phase) {
if (metadata == null || !metadata.isAnnotated(Conditional.class.getName())) {
return false;
}
if (phase == null) {
if (metadata instanceof AnnotationMetadata &&
ConfigurationClassUtils.isConfigurationCandidate((AnnotationMetadata) metadata)) {
return shouldSkip(metadata, ConfigurationPhase.PARSE_CONFIGURATION);
}
return shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN);
}
List<Condition> conditions = new ArrayList<>();
for (String[] conditionClasses : getConditionClasses(metadata)) {
for (String conditionClass : conditionClasses) {
Condition condition = getCondition(conditionClass, this.context.getClassLoader());
conditions.add(condition);
}
}
AnnotationAwareOrderComparator.sort(conditions);
for (Condition condition : conditions) {
ConfigurationPhase requiredPhase = null;
if (condition instanceof ConfigurationCondition) {
requiredPhase = ((ConfigurationCondition) condition).getConfigurationPhase();
}
if ((requiredPhase == null || requiredPhase == phase) &&
// 重点判断
!condition.matches(this.context, metadata)) {
return true;
}
}
return false;
}以@ConditionalOnBean底层工作原理为示例
OnBeanCondition类继承了FilteringSpringBootCondition，FilteringSpringBootCondition类又继承SpringBootCondition，而SpringBootCondition实现了Condition接口，matches()方法也是在
SpringBootCondition这个类中实现的：
public abstract class SpringBootCondition implements Condition {
private final Log logger = LogFactory.getLog(getClass());
@Override
public final boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
// 获取当前解析的类名或方法名
String classOrMethodName = getClassOrMethodName(metadata);
try {
// 进行具体的条件匹配，ConditionOutcome表示匹配结果
ConditionOutcome outcome = getMatchOutcome(context, metadata);
//日志记录匹配结果
logOutcome(classOrMethodName, outcome);
recordEvaluation(context, classOrMethodName, outcome);
// 返回匹配结果 true/false
return outcome.isMatch();
}
catch (NoClassDefFoundError ex) {
throw new IllegalStateException("Could not evaluate condition on " + classOrMethodName + " due to "
+ ex.getMessage() + " not found. Make sure your own configuration does not rely on "
+ "that class. This can also happen if you are "
+ "@ComponentScanning a springframework package (e.g. if you "
+ "put a @ComponentScan in the default package by mistake)", ex);
}
catch (RuntimeException ex) {
throw new IllegalStateException("Error processing condition on " + getName(metadata), ex);
}
}
//....
public abstract ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata);
} 具体的条件匹配逻辑在getMatchOutcome方法中实现的，而SpringBootCondition类中的
getMatchOutcome方法是一个抽象方法，具体的实现逻辑就在子类OnBeanCondition：
@Override
public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
ConditionMessage matchMessage = ConditionMessage.empty();
MergedAnnotations annotations = metadata.getAnnotations();
// 如果存在ConditionalOnBean注解
if (annotations.isPresent(ConditionalOnBean.class)) {
Spec<ConditionalOnBean> spec = new Spec<>(context, metadata, annotations, ConditionalOnBean.class);
MatchResult matchResult = getMatchingBeans(context, spec);
// 如果某个Bean不存在
if (!matchResult.isAllMatched()) {
String reason = createOnBeanNoMatchReason(matchResult);
return ConditionOutcome.noMatch(spec.message().because(reason));
}
// 所有Bean都存在
matchMessage = spec.message(matchMessage)
.found("bean", "beans")
.items(Style.QUOTE, matchResult.getNamesOfAllMatches());
}
// 如果存在ConditionalOnSingleCandidate注解
if (metadata.isAnnotated(ConditionalOnSingleCandidate.class.getName())) {
Spec<ConditionalOnSingleCandidate> spec = new SingleCandidateSpec(context, metadata, annotations);
MatchResult matchResult = getMatchingBeans(context, spec);
if (!matchResult.isAllMatched()) {
return ConditionOutcome.noMatch(spec.message().didNotFind("any beans").atAll());
}
Set<String> allBeans = matchResult.getNamesOfAllMatches();
if (allBeans.size() == 1) {
matchMessage = spec.message(matchMessage).found("a single bean").items(Style.QUOTE, allBeans);
}
else {
List<String> primaryBeans = getPrimaryBeans(context.getBeanFactory(), allBeans,
spec.getStrategy() == SearchStrategy.ALL);
if (primaryBeans.isEmpty()) {
return ConditionOutcome
.noMatch(spec.message().didNotFind("a primary bean from beans").items(Style.QUOTE, allBeans));
}
if (primaryBeans.size() > 1) {
return ConditionOutcome
.noMatch(spec.message().found("multiple primary beans").items(Style.QUOTE, primaryBeans));
}
matchMessage = spec.message(matchMessage)
.found("a single primary bean '" + primaryBeans.get(0) + "' from beans")
.items(Style.QUOTE, allBeans);
}
}
// 存在ConditionalOnMissingBean注解
if (metadata.isAnnotated(ConditionalOnMissingBean.class.getName())) {
Spec<ConditionalOnMissingBean> spec = new Spec<>(context, metadata, annotations,
ConditionalOnMissingBean.class);
MatchResult matchResult = getMatchingBeans(context, spec);
if (matchResult.isAnyMatched()) {
String reason = createOnMissingBeanNoMatchReason(matchResult);
return ConditionOutcome.noMatch(spec.message().because(reason));
}
matchMessage = spec.message(matchMessage).didNotFind("any beans").atAll();
}
return ConditionOutcome.match(matchMessage);
}
protected final MatchResult getMatchingBeans(ConditionContext context, Spec<?> spec) {
ClassLoader classLoader = context.getClassLoader();
ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
boolean considerHierarchy = spec.getStrategy() != SearchStrategy.CURRENT;
Set<Class<?>> parameterizedContainers = spec.getParameterizedContainers();
if (spec.getStrategy() == SearchStrategy.ANCESTORS) {
BeanFactory parent = beanFactory.getParentBeanFactory();
Assert.isInstanceOf(ConfigurableListableBeanFactory.class, parent,
"Unable to use SearchStrategy.ANCESTORS");
beanFactory = (ConfigurableListableBeanFactory) parent;
}
MatchResult result = new MatchResult();
Set<String> beansIgnoredByType = getNamesOfBeansIgnoredByType(classLoader, beanFactory, considerHierarchy,
spec.getIgnoredTypes(), parameterizedContainers);
for (String type : spec.getTypes()) {
Collection<String> typeMatches = getBeanNamesForType(classLoader, considerHierarchy, beanFactory, type,
parameterizedContainers);
typeMatches
.removeIf((match) -> beansIgnoredByType.contains(match) || ScopedProxyUtils.isScopedTarget(match));
if (typeMatches.isEmpty()) {
result.recordUnmatchedType(type);
}
else {
result.recordMatchedType(type, typeMatches);
}
}
for (String annotation : spec.getAnnotations()) {
Set<String> annotationMatches = getBeanNamesForAnnotation(classLoader, beanFactory, annotation,
considerHierarchy);
annotationMatches.removeAll(beansIgnoredByType);
if (annotationMatches.isEmpty()) {
result.recordUnmatchedAnnotation(annotation);
}
else {
result.recordMatchedAnnotation(annotation, annotationMatches);
}
}
for (String beanName : spec.getNames()) {
if (!beansIgnoredByType.contains(beanName) && containsBean(beanFactory, beanName, considerHierarchy)) {
result.recordMatchedName(beanName);
}
else {
result.recordUnmatchedName(beanName);
}
}
return result;
}getMatchingBeans方法中会利用BeanFactory去获取指定类型的Bean，如果没有指定类型的Bean，则会将该类型记录在MatchResult对象的unmatchedTypes集合中，如果有该类型的Bean，则会把该Bean的beanName记录在MatchResult对象的matchedNames集合中，所以MatchResult对象中记录了哪些类没有对应的Bean，哪些类有对应的Bean。
大概流程如下：
Spring在解析某个配置类，或某个Bean定义时如果发现它们上面用到了条件注解，就会取出所有的条件注解，并生成对应的条件对象，比如OnBeanCondition对象；
依次调用条件对象的matches方法，进行条件匹配，看是否符合条件
条件匹配逻辑中，会拿到@ConditionalOnBean等条件注解的信息，判断哪些Bean存在
然后利用BeanFactory来进行判断
最后只有所有条件注解的条件都匹配，那么当前Bean定义才算符合条件生效
自定义Spring Boot Starter
SpringBoot 最强大的功能就是把我们常用的业务场景抽取成了一个个starter（场景启动器），我们通过引入SpringBoot 提供的这些场景启动器，再进行少量的配置就能使用相应的功能。但是，SpringBoot 不能囊括我们所有的业务使用场景，往往我们需要自定义starter，来简化我们对springboot的使用。
下面是自定义starter的示例代码地址：
lp-springboot-start: 自定义Spring Boot Start
