Springboot自动装配以及启动原理解析
一、引子
在传统的Spring项目中，需要引入大量的配置来载入相关的功能模块，实现导入相关功能作用，使项目创建和引入新功能时做了大量重复繁杂的工作。Springboot项目出现，使项目中只需引入相关功能模块的依赖包，即可使用程序相关功能。
二、自动配置类
Springboot中把相关功能的Bean放到了一个配置类中，分门别类，针对不同的场景，定义了不同的自动配置类，如：
- ServletWebServerFactoryAutoConfiguration：配置了Servlet Web场景中所需要的一些Bean
- TransactionAutoConfiguration：配置了事务场景中所需要的一些Bean
- AopAutoConfiguration：配置了AOP场景中所需要的一些Bean
- RabbitAutoConfiguration：配置了Rabbitmq场景中所需要的一些Bean 等等
使用这种结构后，SpringBoot就能让程序员更为方便的来控制某个Bean或某些Bean要不要生效，如
果某个自动配置类不生效，那该配置类中所定义的Bean则都不会生效。
三、条件注解
1 自定义条件注解 @Conditional
SpringBoot中众多的条件注解，都是基于Spring中的@Conditional来实现的。
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Conditional {
/**
* All {@link Condition} classes that must {@linkplain Condition#matches match}
* in order for the component to be registered.
*/
Class<? extends Condition>[] value();
}
使用自定义条件注解
public class SonnyCondition implements Condition {
@Override
public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
Map<String, Object> annotationAttributes = metadata.getAnnotationAttributes(ConditionalOnClass.class.getName());
String className = (String) annotationAttributes.get("value");
try {
context.getClassLoader().loadClass(className);
return true;
} catch (ClassNotFoundException e) {
return false;
}
}
}
matches方法中来定义条件逻辑：
- ConditionContext：表示条件上下文，可以通过ConditionContext获取到当前的类加载器、BeanFactory、
Environment环境变量对象 - AnnotatedTypeMetadata：表示当前正在进行条件判断的Bean所对应的类信息，或方法信息（比如@Bean定义
的一个Bean），可以通过AnnotatedTypeMetadata获取到当前类或方法相关的信息，从而就可以拿到条件注解的
信息，当然如果一个Bean上使用了多个条件注解，那么在解析过程中都可以获取到，同时也能获取Bean上定义
的其他注解信息
2 ConditionalOnClass
@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnClassCondition.class)
public @interface ConditionalOnClass {
/**
* The classes that must be present. Since this annotation is parsed by loading class
* bytecode, it is safe to specify classes here that may ultimately not be on the
* classpath, only if this annotation is directly on the affected component and
* not if this annotation is used as a composed, meta-annotation. In order to
* use this annotation as a meta-annotation, only use the {@link #name} attribute.
* @return the classes that must be present
*/
Class<?>[] value() default {};
/**
* The classes names that must be present.
* @return the class names that must be present.
*/
String[] name() default {};
}
主要作用注解 @Conditional(OnClassCondition.class) ，使用了 OnClassCondition 类，接下来我们看这个类做了什么。
@Order(Ordered.HIGHEST_PRECEDENCE)
class OnClassCondition extends FilteringSpringBootCondition {
@Override
protected final ConditionOutcome[] getOutcomes(String[] autoConfigurationClasses,
AutoConfigurationMetadata autoConfigurationMetadata) {
// Split the work and perform half in a background thread if more than one
// processor is available. Using a single additional thread seems to offer the
// best performance. More threads make things worse.
if (autoConfigurationClasses.length > 1 && Runtime.getRuntime().availableProcessors() > 1) {
return resolveOutcomesThreaded(autoConfigurationClasses, autoConfigurationMetadata);
}
else {
OutcomesResolver outcomesResolver = new StandardOutcomesResolver(autoConfigurationClasses, 0,
autoConfigurationClasses.length, autoConfigurationMetadata, getBeanClassLoader());
return outcomesResolver.resolveOutcomes();
}
}
private ConditionOutcome[] resolveOutcomesThreaded(String[] autoConfigurationClasses,
AutoConfigurationMetadata autoConfigurationMetadata) {
// 分两半，开启两个线程进行匹配
int split = autoConfigurationClasses.length / 2;
OutcomesResolver firstHalfResolver = createOutcomesResolver(autoConfigurationClasses, 0, split,
autoConfigurationMetadata);
OutcomesResolver secondHalfResolver = new StandardOutcomesResolver(autoConfigurationClasses, split,
autoConfigurationClasses.length, autoConfigurationMetadata, getBeanClassLoader());
ConditionOutcome[] secondHalf = secondHalfResolver.resolveOutcomes();
ConditionOutcome[] firstHalf = firstHalfResolver.resolveOutcomes();
ConditionOutcome[] outcomes = new ConditionOutcome[autoConfigurationClasses.length];
System.arraycopy(firstHalf, 0, outcomes, 0, firstHalf.length);
System.arraycopy(secondHalf, 0, outcomes, split, secondHalf.length);
return outcomes;
}
private OutcomesResolver createOutcomesResolver(String[] autoConfigurationClasses, int start, int end,
AutoConfigurationMetadata autoConfigurationMetadata) {
OutcomesResolver outcomesResolver = new StandardOutcomesResolver(autoConfigurationClasses, start, end,
autoConfigurationMetadata, getBeanClassLoader());
try {
return new ThreadedOutcomesResolver(outcomesResolver);
}
catch (AccessControlException ex) {
return outcomesResolver;
}
}
@Override
public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
ClassLoader classLoader = context.getClassLoader();
ConditionMessage matchMessage = ConditionMessage.empty();
// 拿到ConditionalOnClass注解中的value值，也就是要判断是否存在的类名
List<String> onClasses = getCandidates(metadata, ConditionalOnClass.class);
if (onClasses != null) {
// 判断onClasses中不存在的类
List<String> missing = filter(onClasses, ClassNameFilter.MISSING, classLoader);
// 如果有缺失的类，那就表示不匹配
if (!missing.isEmpty()) {
return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnClass.class)
.didNotFind("required class", "required classes").items(Style.QUOTE, missing));
}
// 否则就表示匹配
matchMessage = matchMessage.andCondition(ConditionalOnClass.class)
.found("required class", "required classes")
.items(Style.QUOTE, filter(onClasses, ClassNameFilter.PRESENT, classLoader));
}
// 和上面类似，只不过是判断onMissingClasses是不是全部缺失，如果是则表示匹配
List<String> onMissingClasses = getCandidates(metadata, ConditionalOnMissingClass.class);
if (onMissingClasses != null) {
List<String> present = filter(onMissingClasses, ClassNameFilter.PRESENT, classLoader);
if (!present.isEmpty()) {
return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnMissingClass.class)
.found("unwanted class", "unwanted classes").items(Style.QUOTE, present));
}
matchMessage = matchMessage.andCondition(ConditionalOnMissingClass.class)
.didNotFind("unwanted class", "unwanted classes")
.items(Style.QUOTE, filter(onMissingClasses, ClassNameFilter.MISSING, classLoader));
}
return ConditionOutcome.match(matchMessage);
}
private List<String> getCandidates(AnnotatedTypeMetadata metadata, Class<?> annotationType) {
MultiValueMap<String, Object> attributes = metadata.getAllAnnotationAttributes(annotationType.getName(), true);
if (attributes == null) {
return null;
}
List<String> candidates = new ArrayList<>();
addAll(candidates, attributes.get("value"));
addAll(candidates, attributes.get("name"));
return candidates;
}
private void addAll(List<String> list, List<Object> itemsToAdd) {
if (itemsToAdd != null) {
for (Object item : itemsToAdd) {
Collections.addAll(list, (String[]) item);
}
}
}
private interface OutcomesResolver {
ConditionOutcome[] resolveOutcomes();
}
private static final class ThreadedOutcomesResolver implements OutcomesResolver {
private final Thread thread;
private volatile ConditionOutcome[] outcomes;
private ThreadedOutcomesResolver(OutcomesResolver outcomesResolver) {
this.thread = new Thread(() -> this.outcomes = outcomesResolver.resolveOutcomes());
this.thread.start();
}
@Override
public ConditionOutcome[] resolveOutcomes() {
try {
this.thread.join();
}
catch (InterruptedException ex) {
Thread.currentThread().interrupt();
}
return this.outcomes;
}
}
private static final class StandardOutcomesResolver implements OutcomesResolver {
private final String[] autoConfigurationClasses;
private final int start;
private final int end;
private final AutoConfigurationMetadata autoConfigurationMetadata;
private final ClassLoader beanClassLoader;
private StandardOutcomesResolver(String[] autoConfigurationClasses, int start, int end,
AutoConfigurationMetadata autoConfigurationMetadata, ClassLoader beanClassLoader) {
this.autoConfigurationClasses = autoConfigurationClasses;
this.start = start;
this.end = end;
this.autoConfigurationMetadata = autoConfigurationMetadata;
this.beanClassLoader = beanClassLoader;
}
@Override
public ConditionOutcome[] resolveOutcomes() {
return getOutcomes(this.autoConfigurationClasses, this.start, this.end, this.autoConfigurationMetadata);
}
private ConditionOutcome[] getOutcomes(String[] autoConfigurationClasses, int start, int end,
AutoConfigurationMetadata autoConfigurationMetadata) {
// 记录每个自动配置的匹配结果
ConditionOutcome[] outcomes = new ConditionOutcome[end - start];
// 遍历每个自动配置进行匹配
for (int i = start; i < end; i++) {
String autoConfigurationClass = autoConfigurationClasses[i];
if (autoConfigurationClass != null) {
// 从autoConfigurationMetadata中获取当前自动配置的ConditionalOnClass的属性，拿到的就是当前自动配置所需要的类
String candidates = autoConfigurationMetadata.get(autoConfigurationClass, "ConditionalOnClass");
if (candidates != null) {
// 判断需要的类存不存在
outcomes[i - start] = getOutcome(candidates);
}
}
}
return outcomes;
}
private ConditionOutcome getOutcome(String candidates) {
try {
if (!candidates.contains(",")) {
return getOutcome(candidates, this.beanClassLoader);
}
for (String candidate : StringUtils.commaDelimitedListToStringArray(candidates)) {
ConditionOutcome outcome = getOutcome(candidate, this.beanClassLoader);
if (outcome != null) {
return outcome;
}
}
}
catch (Exception ex) {
// We'll get another chance later
}
return null;
}
private ConditionOutcome getOutcome(String className, ClassLoader classLoader) {
if (ClassNameFilter.MISSING.matches(className, classLoader)) {
return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnClass.class)
.didNotFind("required class").items(Style.QUOTE, className));
}
return null;
}
}
}
主要匹配的方法是 getMatchOutcome()
- 拿到使用 ConditionalOnClass 的所有类，存入 onClasses 集合；
- onClasses 集合不为空情况下，找到 onClasses 中不存在的类集合 missing，missing有记录表示不匹配，否则表示匹配；
- 查找使用 ConditionalOnMissingClass 的所有类，存入 onMissingClasses 集合；
- onMissingClasses集合不为空，找到 onMissingClasses 中存在的类集合 present，present有记录则表示不匹配，否则表示匹配。
3 ConditionalOnBean
@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnBeanCondition.class)
public @interface ConditionalOnBean {
/**
* The class types of beans that should be checked. The condition matches when beans
* of all classes specified are contained in the {@link BeanFactory}.
* @return the class types of beans to check
*/
Class<?>[] value() default {};
/**
* The class type names of beans that should be checked. The condition matches when
* beans of all classes specified are contained in the {@link BeanFactory}.
* @return the class type names of beans to check
*/
String[] type() default {};
/**
* The annotation type decorating a bean that should be checked. The condition matches
* when all of the annotations specified are defined on beans in the
* {@link BeanFactory}.
* @return the class-level annotation types to check
*/
Class<? extends Annotation>[] annotation() default {};
/**
* The names of beans to check. The condition matches when all of the bean names
* specified are contained in the {@link BeanFactory}.
* @return the names of beans to check
*/
String[] name() default {};
/**
* Strategy to decide if the application context hierarchy (parent contexts) should be
* considered.
* @return the search strategy
*/
SearchStrategy search() default SearchStrategy.ALL;
/**
* Additional classes that may contain the specified bean types within their generic
* parameters. For example, an annotation declaring {@code value=Name.class} and
* {@code parameterizedContainer=NameRegistration.class} would detect both
* {@code Name} and {@code NameRegistration}.
* @return the container types
* @since 2.1.0
*/
Class<?>[] parameterizedContainer() default {};
}
主要作用注解 @Conditional(OnBeanCondition.class) ，使用了 OnClassCondition 类，接下来我们看这个类做了什么。
@Order(Ordered.LOWEST_PRECEDENCE)
class OnBeanCondition extends FilteringSpringBootCondition implements ConfigurationCondition {
@Override
public ConfigurationPhase getConfigurationPhase() {
return ConfigurationPhase.REGISTER_BEAN;
}
@Override
protected final ConditionOutcome[] getOutcomes(String[] autoConfigurationClasses,
AutoConfigurationMetadata autoConfigurationMetadata) {
ConditionOutcome[] outcomes = new ConditionOutcome[autoConfigurationClasses.length];
// 遍历处理每个自动配置类
for (int i = 0; i < outcomes.length; i++) {
String autoConfigurationClass = autoConfigurationClasses[i];
if (autoConfigurationClass != null) {
// 当前自动配置中@ConditionalOnBean所依赖的类
Set<String> onBeanTypes = autoConfigurationMetadata.getSet(autoConfigurationClass, "ConditionalOnBean");
// 如果onBeanTypes都存在，则返回null
outcomes[i] = getOutcome(onBeanTypes, ConditionalOnBean.class);
if (outcomes[i] == null) {
// 继续判断@ConditionalOnSingleCandidate所依赖的类
Set<String> onSingleCandidateTypes = autoConfigurationMetadata.getSet(autoConfigurationClass,
"ConditionalOnSingleCandidate");
outcomes[i] = getOutcome(onSingleCandidateTypes, ConditionalOnSingleCandidate.class);
}
}
}
return outcomes;
}
private ConditionOutcome getOutcome(Set<String> requiredBeanTypes, Class<? extends Annotation> annotation) {
List<String> missing = filter(requiredBeanTypes, ClassNameFilter.MISSING, getBeanClassLoader());
// 有缺失的，则不会null
if (!missing.isEmpty()) {
ConditionMessage message = ConditionMessage.forCondition(annotation)
.didNotFind("required type", "required types").items(Style.QUOTE, missing);
return ConditionOutcome.noMatch(message);
}
// 没有缺失的，requiredBeanTypes类都存在则返回null
return null;
}
@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);
// Bean不存在
if (!matchResult.isAllMatched()) {
return ConditionOutcome.noMatch(spec.message().didNotFind("any beans").atAll());
}
// Bean存在
Set<String> allBeans = matchResult.getNamesOfAllMatches();
// 如果只有一个
if (allBeans.size() == 1) {
matchMessage = spec.message(matchMessage).found("a single bean").items(Style.QUOTE, allBeans);
}
else {
// 如果有多个
List<String> primaryBeans = getPrimaryBeans(context.getBeanFactory(), allBeans,
spec.getStrategy() == SearchStrategy.ALL);
// 没有主Bean，那就不匹配
if (primaryBeans.isEmpty()) {
return ConditionOutcome.noMatch(
spec.message().didNotFind("a primary bean from beans").items(Style.QUOTE, allBeans));
}
// 有多个主Bean，那就不匹配
if (primaryBeans.size() > 1) {
return ConditionOutcome
.noMatch(spec.message().found("multiple primary beans").items(Style.QUOTE, primaryBeans));
}
// 只有一个主Bean
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);
//有任意一个Bean存在，那就条件不匹配
if (matchResult.isAnyMatched()) {
String reason = createOnMissingBeanNoMatchReason(matchResult);
return ConditionOutcome.noMatch(spec.message().because(reason));
}
// 都不存在在，则匹配
matchMessage = spec.message(matchMessage).didNotFind("any beans").atAll();
}
return ConditionOutcome.match(matchMessage);
}
protected final MatchResult getMatchingBeans(ConditionContext context, Spec<?> spec) {
ClassLoader classLoader = context.getClassLoader();
// 获取BeanFactory
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()) {
// 获取指定类型的beanName
Collection<String> typeMatches = getBeanNamesForType(classLoader, considerHierarchy, beanFactory, type,
parameterizedContainers);
// 如果beansIgnoredByType包含了当前类型，则把当前类型从typeMatches移除掉
Iterator<String> iterator = typeMatches.iterator();
while (iterator.hasNext()) {
String match = iterator.next();
if (beansIgnoredByType.contains(match) || ScopedProxyUtils.isScopedTarget(match)) {
iterator.remove();
}
}
// 该类型没有匹配的Bean
if (typeMatches.isEmpty()) {
result.recordUnmatchedType(type);
}
else {
// 记录该类型所匹配的beanName结合
result.recordMatchedType(type, typeMatches);
}
}
for (String annotation : spec.getAnnotations()) {
// 获取加了当前注解的beanNames
Set<String> annotationMatches = getBeanNamesForAnnotation(classLoader, beanFactory, annotation,
considerHierarchy);
// 把beansIgnoredByType中的beanNames移除掉
annotationMatches.removeAll(beansIgnoredByType);
if (annotationMatches.isEmpty()) {
result.recordUnmatchedAnnotation(annotation);
}
else {
result.recordMatchedAnnotation(annotation, annotationMatches);
}
}
for (String beanName : spec.getNames()) {
// 当前beanName是否存在对应的bean
if (!beansIgnoredByType.contains(beanName) && containsBean(beanFactory, beanName, considerHierarchy)) {
result.recordMatchedName(beanName);
}
else {
result.recordUnmatchedName(beanName);
}
}
return result;
}
private Set<String> getNamesOfBeansIgnoredByType(ClassLoader classLoader, ListableBeanFactory beanFactory,
boolean considerHierarchy, Set<String> ignoredTypes, Set<Class<?>> parameterizedContainers) {
Set<String> result = null;
for (String ignoredType : ignoredTypes) {
Collection<String> ignoredNames = getBeanNamesForType(classLoader, considerHierarchy, beanFactory,
ignoredType, parameterizedContainers);
result = addAll(result, ignoredNames);
}
return (result != null) ? result : Collections.emptySet();
}
private Set<String> getBeanNamesForType(ClassLoader classLoader, boolean considerHierarchy,
ListableBeanFactory beanFactory, String type, Set<Class<?>> parameterizedContainers) throws LinkageError {
try {
return getBeanNamesForType(beanFactory, considerHierarchy, resolve(type, classLoader),
parameterizedContainers);
}
catch (ClassNotFoundException | NoClassDefFoundError ex) {
return Collections.emptySet();
}
}
private Set<String> getBeanNamesForType(ListableBeanFactory beanFactory, boolean considerHierarchy, Class<?> type,
Set<Class<?>> parameterizedContainers) {
Set<String> result = collectBeanNamesForType(beanFactory, considerHierarchy, type, parameterizedContainers,
null);
return (result != null) ? result : Collections.emptySet();
}
private Set<String> collectBeanNamesForType(ListableBeanFactory beanFactory, boolean considerHierarchy,
Class<?> type, Set<Class<?>> parameterizedContainers, Set<String> result) {
result = addAll(result, beanFactory.getBeanNamesForType(type, true, false));
for (Class<?> container : parameterizedContainers) {
ResolvableType generic = ResolvableType.forClassWithGenerics(container, type);
result = addAll(result, beanFactory.getBeanNamesForType(generic, true, false));
}
if (considerHierarchy && beanFactory instanceof HierarchicalBeanFactory) {
BeanFactory parent = ((HierarchicalBeanFactory) beanFactory).getParentBeanFactory();
if (parent instanceof ListableBeanFactory) {
result = collectBeanNamesForType((ListableBeanFactory) parent, considerHierarchy, type,
parameterizedContainers, result);
}
}
return result;
}
private Set<String> getBeanNamesForAnnotation(ClassLoader classLoader, ConfigurableListableBeanFactory beanFactory,
String type, boolean considerHierarchy) throws LinkageError {
Set<String> result = null;
try {
result = collectBeanNamesForAnnotation(beanFactory, resolveAnnotationType(classLoader, type),
considerHierarchy, result);
}
catch (ClassNotFoundException ex) {
// Continue
}
return (result != null) ? result : Collections.emptySet();
}
@SuppressWarnings("unchecked")
private Class<? extends Annotation> resolveAnnotationType(ClassLoader classLoader, String type)
throws ClassNotFoundException {
return (Class<? extends Annotation>) resolve(type, classLoader);
}
private Set<String> collectBeanNamesForAnnotation(ListableBeanFactory beanFactory,
Class<? extends Annotation> annotationType, boolean considerHierarchy, Set<String> result) {
result = addAll(result, beanFactory.getBeanNamesForAnnotation(annotationType));
if (considerHierarchy) {
BeanFactory parent = ((HierarchicalBeanFactory) beanFactory).getParentBeanFactory();
if (parent instanceof ListableBeanFactory) {
result = collectBeanNamesForAnnotation((ListableBeanFactory) parent, annotationType, considerHierarchy,
result);
}
}
return result;
}
private boolean containsBean(ConfigurableListableBeanFactory beanFactory, String beanName,
boolean considerHierarchy) {
if (considerHierarchy) {
return beanFactory.containsBean(beanName);
}
return beanFactory.containsLocalBean(beanName);
}
private String createOnBeanNoMatchReason(MatchResult matchResult) {
StringBuilder reason = new StringBuilder();
appendMessageForNoMatches(reason, matchResult.getUnmatchedAnnotations(), "annotated with");
appendMessageForNoMatches(reason, matchResult.getUnmatchedTypes(), "of type");
appendMessageForNoMatches(reason, matchResult.getUnmatchedNames(), "named");
return reason.toString();
}
private void appendMessageForNoMatches(StringBuilder reason, Collection<String> unmatched, String description) {
if (!unmatched.isEmpty()) {
if (reason.length() > 0) {
reason.append(" and ");
}
reason.append("did not find any beans ");
reason.append(description);
reason.append(" ");
reason.append(StringUtils.collectionToDelimitedString(unmatched, ", "));
}
}
private String createOnMissingBeanNoMatchReason(MatchResult matchResult) {
StringBuilder reason = new StringBuilder();
appendMessageForMatches(reason, matchResult.getMatchedAnnotations(), "annotated with");
appendMessageForMatches(reason, matchResult.getMatchedTypes(), "of type");
if (!matchResult.getMatchedNames().isEmpty()) {
if (reason.length() > 0) {
reason.append(" and ");
}
reason.append("found beans named ");
reason.append(StringUtils.collectionToDelimitedString(matchResult.getMatchedNames(), ", "));
}
return reason.toString();
}
private void appendMessageForMatches(StringBuilder reason, Map<String, Collection<String>> matches,
String description) {
if (!matches.isEmpty()) {
matches.forEach((key, value) -> {
if (reason.length() > 0) {
reason.append(" and ");
}
reason.append("found beans ");
reason.append(description);
reason.append(" '");
reason.append(key);
reason.append("' ");
reason.append(StringUtils.collectionToDelimitedString(value, ", "));
});
}
}
private List<String> getPrimaryBeans(ConfigurableListableBeanFactory beanFactory, Set<String> beanNames,
boolean considerHierarchy) {
List<String> primaryBeans = new ArrayList<>();
for (String beanName : beanNames) {
BeanDefinition beanDefinition = findBeanDefinition(beanFactory, beanName, considerHierarchy);
if (beanDefinition != null && beanDefinition.isPrimary()) {
primaryBeans.add(beanName);
}
}
return primaryBeans;
}
private BeanDefinition findBeanDefinition(ConfigurableListableBeanFactory beanFactory, String beanName,
boolean considerHierarchy) {
if (beanFactory.containsBeanDefinition(beanName)) {
return beanFactory.getBeanDefinition(beanName);
}
if (considerHierarchy && beanFactory.getParentBeanFactory() instanceof ConfigurableListableBeanFactory) {
return findBeanDefinition(((ConfigurableListableBeanFactory) beanFactory.getParentBeanFactory()), beanName,
considerHierarchy);
}
return null;
}
private static Set<String> addAll(Set<String> result, Collection<String> additional) {
if (CollectionUtils.isEmpty(additional)) {
return result;
}
result = (result != null) ? result : new LinkedHashSet<>();
result.addAll(additional);
return result;
}
private static Set<String> addAll(Set<String> result, String[] additional) {
if (ObjectUtils.isEmpty(additional)) {
return result;
}
result = (result != null) ? result : new LinkedHashSet<>();
Collections.addAll(result, additional);
return result;
}
/**
* A search specification extracted from the underlying annotation.
*/
private static class Spec<A extends Annotation> {
private final ClassLoader classLoader;
private final Class<? extends Annotation> annotationType;
private final Set<String> names;
private final Set<String> types;
private final Set<String> annotations;
private final Set<String> ignoredTypes;
private final Set<Class<?>> parameterizedContainers;
private final SearchStrategy strategy;
Spec(ConditionContext context, AnnotatedTypeMetadata metadata, MergedAnnotations annotations,
Class<A> annotationType) {
MultiValueMap<String, Object> attributes = annotations.stream(annotationType)
.filter(MergedAnnotationPredicates.unique(MergedAnnotation::getMetaTypes))
.collect(MergedAnnotationCollectors.toMultiValueMap(Adapt.CLASS_TO_STRING));
MergedAnnotation<A> annotation = annotations.get(annotationType);
this.classLoader = context.getClassLoader();
this.annotationType = annotationType;
this.names = extract(attributes, "name");
this.annotations = extract(attributes, "annotation");
this.ignoredTypes = extract(attributes, "ignored", "ignoredType");
this.parameterizedContainers = resolveWhenPossible(extract(attributes, "parameterizedContainer"));
this.strategy = annotation.getValue("search", SearchStrategy.class).orElse(null);
Set<String> types = extractTypes(attributes);
BeanTypeDeductionException deductionException = null;
if (types.isEmpty() && this.names.isEmpty()) {
try {
types = deducedBeanType(context, metadata);
}
catch (BeanTypeDeductionException ex) {
deductionException = ex;
}
}
this.types = types;
validate(deductionException);
}
protected Set<String> extractTypes(MultiValueMap<String, Object> attributes) {
return extract(attributes, "value", "type");
}
private Set<String> extract(MultiValueMap<String, Object> attributes, String... attributeNames) {
if (attributes.isEmpty()) {
return Collections.emptySet();
}
Set<String> result = new LinkedHashSet<>();
for (String attributeName : attributeNames) {
List<Object> values = attributes.getOrDefault(attributeName, Collections.emptyList());
for (Object value : values) {
if (value instanceof String[]) {
merge(result, (String[]) value);
}
else if (value instanceof String) {
merge(result, (String) value);
}
}
}
return result.isEmpty() ? Collections.emptySet() : result;
}
private void merge(Set<String> result, String... additional) {
Collections.addAll(result, additional);
}
private Set<Class<?>> resolveWhenPossible(Set<String> classNames) {
if (classNames.isEmpty()) {
return Collections.emptySet();
}
Set<Class<?>> resolved = new LinkedHashSet<>(classNames.size());
for (String className : classNames) {
try {
resolved.add(resolve(className, this.classLoader));
}
catch (ClassNotFoundException | NoClassDefFoundError ex) {
}
}
return resolved;
}
protected void validate(BeanTypeDeductionException ex) {
if (!hasAtLeastOneElement(this.types, this.names, this.annotations)) {
String message = getAnnotationName() + " did not specify a bean using type, name or annotation";
if (ex == null) {
throw new IllegalStateException(message);
}
throw new IllegalStateException(message + " and the attempt to deduce the bean's type failed", ex);
}
}
private boolean hasAtLeastOneElement(Set<?>... sets) {
for (Set<?> set : sets) {
if (!set.isEmpty()) {
return true;
}
}
return false;
}
protected final String getAnnotationName() {
return "@" + ClassUtils.getShortName(this.annotationType);
}
private Set<String> deducedBeanType(ConditionContext context, AnnotatedTypeMetadata metadata) {
if (metadata instanceof MethodMetadata && metadata.isAnnotated(Bean.class.getName())) {
return deducedBeanTypeForBeanMethod(context, (MethodMetadata) metadata);
}
return Collections.emptySet();
}
private Set<String> deducedBeanTypeForBeanMethod(ConditionContext context, MethodMetadata metadata) {
try {
Class<?> returnType = getReturnType(context, metadata);
return Collections.singleton(returnType.getName());
}
catch (Throwable ex) {
throw new BeanTypeDeductionException(metadata.getDeclaringClassName(), metadata.getMethodName(), ex);
}
}
private Class<?> getReturnType(ConditionContext context, MethodMetadata metadata)
throws ClassNotFoundException, LinkageError {
// Safe to load at this point since we are in the REGISTER_BEAN phase
ClassLoader classLoader = context.getClassLoader();
Class<?> returnType = resolve(metadata.getReturnTypeName(), classLoader);
if (isParameterizedContainer(returnType)) {
returnType = getReturnTypeGeneric(metadata, classLoader);
}
return returnType;
}
private boolean isParameterizedContainer(Class<?> type) {
for (Class<?> parameterizedContainer : this.parameterizedContainers) {
if (parameterizedContainer.isAssignableFrom(type)) {
return true;
}
}
return false;
}
private Class<?> getReturnTypeGeneric(MethodMetadata metadata, ClassLoader classLoader)
throws ClassNotFoundException, LinkageError {
Class<?> declaringClass = resolve(metadata.getDeclaringClassName(), classLoader);
Method beanMethod = findBeanMethod(declaringClass, metadata.getMethodName());
return ResolvableType.forMethodReturnType(beanMethod).resolveGeneric();
}
private Method findBeanMethod(Class<?> declaringClass, String methodName) {
Method method = ReflectionUtils.findMethod(declaringClass, methodName);
if (isBeanMethod(method)) {
return method;
}
Method[] candidates = ReflectionUtils.getAllDeclaredMethods(declaringClass);
for (Method candidate : candidates) {
if (candidate.getName().equals(methodName) && isBeanMethod(candidate)) {
return candidate;
}
}
throw new IllegalStateException("Unable to find bean method " + methodName);
}
private boolean isBeanMethod(Method method) {
return method != null && MergedAnnotations.from(method, MergedAnnotations.SearchStrategy.TYPE_HIERARCHY)
.isPresent(Bean.class);
}
private SearchStrategy getStrategy() {
return (this.strategy != null) ? this.strategy : SearchStrategy.ALL;
}
Set<String> getNames() {
return this.names;
}
Set<String> getTypes() {
return this.types;
}
Set<String> getAnnotations() {
return this.annotations;
}
Set<String> getIgnoredTypes() {
return this.ignoredTypes;
}
Set<Class<?>> getParameterizedContainers() {
return this.parameterizedContainers;
}
ConditionMessage.Builder message() {
return ConditionMessage.forCondition(this.annotationType, this);
}
ConditionMessage.Builder message(ConditionMessage message) {
return message.andCondition(this.annotationType, this);
}
@Override
public String toString() {
boolean hasNames = !this.names.isEmpty();
boolean hasTypes = !this.types.isEmpty();
boolean hasIgnoredTypes = !this.ignoredTypes.isEmpty();
StringBuilder string = new StringBuilder();
string.append("(");
if (hasNames) {
string.append("names: ");
string.append(StringUtils.collectionToCommaDelimitedString(this.names));
string.append(hasTypes ? " " : "; ");
}
if (hasTypes) {
string.append("types: ");
string.append(StringUtils.collectionToCommaDelimitedString(this.types));
string.append(hasIgnoredTypes ? " " : "; ");
}
if (hasIgnoredTypes) {
string.append("ignored: ");
string.append(StringUtils.collectionToCommaDelimitedString(this.ignoredTypes));
string.append("; ");
}
string.append("SearchStrategy: ");
string.append(this.strategy.toString().toLowerCase(Locale.ENGLISH));
string.append(")");
return string.toString();
}
}
/**
* Specialized {@link Spec specification} for
* {@link ConditionalOnSingleCandidate @ConditionalOnSingleCandidate}.
*/
private static class SingleCandidateSpec extends Spec<ConditionalOnSingleCandidate> {
private static final Collection<String> FILTERED_TYPES = Arrays.asList("", Object.class.getName());
SingleCandidateSpec(ConditionContext context, AnnotatedTypeMetadata metadata, MergedAnnotations annotations) {
super(context, metadata, annotations, ConditionalOnSingleCandidate.class);
}
@Override
protected Set<String> extractTypes(MultiValueMap<String, Object> attributes) {
Set<String> types = super.extractTypes(attributes);
types.removeAll(FILTERED_TYPES);
return types;
}
@Override
protected void validate(BeanTypeDeductionException ex) {
Assert.isTrue(getTypes().size() == 1,
() -> getAnnotationName() + " annotations must specify only one type (got "
+ StringUtils.collectionToCommaDelimitedString(getTypes()) + ")");
}
}
/**
* Results collected during the condition evaluation.
*/
private static final class MatchResult {
private final Map<String, Collection<String>> matchedAnnotations = new HashMap<>();
private final List<String> matchedNames = new ArrayList<>();
private final Map<String, Collection<String>> matchedTypes = new HashMap<>();
private final List<String> unmatchedAnnotations = new ArrayList<>();
private final List<String> unmatchedNames = new ArrayList<>();
private final List<String> unmatchedTypes = new ArrayList<>();
private final Set<String> namesOfAllMatches = new HashSet<>();
private void recordMatchedName(String name) {
this.matchedNames.add(name);
this.namesOfAllMatches.add(name);
}
private void recordUnmatchedName(String name) {
this.unmatchedNames.add(name);
}
private void recordMatchedAnnotation(String annotation, Collection<String> matchingNames) {
this.matchedAnnotations.put(annotation, matchingNames);
this.namesOfAllMatches.addAll(matchingNames);
}
private void recordUnmatchedAnnotation(String annotation) {
this.unmatchedAnnotations.add(annotation);
}
private void recordMatchedType(String type, Collection<String> matchingNames) {
this.matchedTypes.put(type, matchingNames);
this.namesOfAllMatches.addAll(matchingNames);
}
private void recordUnmatchedType(String type) {
this.unmatchedTypes.add(type);
}
boolean isAllMatched() {
return this.unmatchedAnnotations.isEmpty() && this.unmatchedNames.isEmpty()
&& this.unmatchedTypes.isEmpty();
}
boolean isAnyMatched() {
return (!this.matchedAnnotations.isEmpty()) || (!this.matchedNames.isEmpty())
|| (!this.matchedTypes.isEmpty());
}
Map<String, Collection<String>> getMatchedAnnotations() {
return this.matchedAnnotations;
}
List<String> getMatchedNames() {
return this.matchedNames;
}
Map<String, Collection<String>> getMatchedTypes() {
return this.matchedTypes;
}
List<String> getUnmatchedAnnotations() {
return this.unmatchedAnnotations;
}
List<String> getUnmatchedNames() {
return this.unmatchedNames;
}
List<String> getUnmatchedTypes() {
return this.unmatchedTypes;
}
Set<String> getNamesOfAllMatches() {
return this.namesOfAllMatches;
}
}
/**
* Exception thrown when the bean type cannot be deduced.
*/
static final class BeanTypeDeductionException extends RuntimeException {
private BeanTypeDeductionException(String className, String beanMethodName, Throwable cause) {
super("Failed to deduce bean type for " + className + "." + beanMethodName, cause);
}
}
}
此条件注解解析含 ConditionalOnBean 的类，逻辑和 OnClassCondition 类似。
四、Starter机制
Starter就是一个Maven依赖，当我们在项目的pom.xml文件中添加某个Starter
依赖时，其实就是简单的添加了很多其他的依赖，比如：
- spring-boot-starter-web：引入了spring-boot-starter、spring-boot-starter-json、spring-boot-starter-tomcat等和
Web开发相关的依赖包。 - spring-boot-starter-tomcat：引入了tomcat-embed-core、tomcat-embed-el、tomcat-embed-websocket等和
Tomcat相关的依赖包。
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
引入此starter的pom包，Springboot会使用条件注解自动将tomcat的依赖加到项目中。
@Configuration(proxyBeanMethods = false)
@ConditionalOnMissingBean(ReactiveWebServerFactory.class)
@ConditionalOnClass({ org.apache.catalina.startup.Tomcat.class })
static class EmbeddedTomcat {
@Bean
TomcatReactiveWebServerFactory tomcatReactiveWebServerFactory(
ObjectProvider<TomcatConnectorCustomizer> connectorCustomizers,
ObjectProvider<TomcatContextCustomizer> contextCustomizers,
ObjectProvider<TomcatProtocolHandlerCustomizer<?>> protocolHandlerCustomizers) {
TomcatReactiveWebServerFactory factory = new TomcatReactiveWebServerFactory();
factory.getTomcatConnectorCustomizers()
.addAll(connectorCustomizers.orderedStream().collect(Collectors.toList()));
factory.getTomcatContextCustomizers()
.addAll(contextCustomizers.orderedStream().collect(Collectors.toList()));
factory.getTomcatProtocolHandlerCustomizers()
.addAll(protocolHandlerCustomizers.orderedStream().collect(Collectors.toList()));
return factory;
}
}
通过 @ConditionalOnClass 条件注解，发现 org.apache.catalina.startup.Tomcat.class 类存在，则引入 EmbeddedTomcat 类。
五、自动配置开启原理
@SpringBootApplication
public class MyApplication {
public static void main(String[] args) {
SpringApplication.run(MyApplication.class, args);
}
}
Springboot项目中通常使用 SpringBootApplication 注解，此注解是合成注解，里面包含启动所需的配置类。
@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) })
// AutoConfigurationExcludeFilter的作用是扫描到的配置类名字如果在自动配置类名集合中，就不解析
public @interface SpringBootApplication {
...
}
可以发现这个注解上有另外三个注解：
- @SpringBootConfiguration
- @EnableAutoConfiguration
- @ComponentScan
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
@Indexed
public @interface SpringBootConfiguration {
...
}
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
/**
* Environment property that can be used to override when auto-configuration is
* enabled.
*/
String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
/**
* Exclude specific auto-configuration classes such that they will never be applied.
* @return the classes to exclude
*/
Class<?>[] exclude() default {};
/**
* Exclude specific auto-configuration class names such that they will never be
* applied.
* @return the class names to exclude
* @since 1.3.0
*/
String[] excludeName() default {};
}
EnableAutoConfiguration 核心是@Import(AutoConfigurationImportSelector.class)，而AutoConfigurationImportSelector实现了DeferredImportSelector这个接口，Spring容器在启动时，会在解析完其他所有程序员定义的配置类之后，来调用AutoConfigurationImportSelector中的selectImports方法，然后把该方法返回的类名对应的类作为配置类进行解析。
该方法会利用SpringFactoriesLoader找到所有的META-INF/spring.factories文件中key为
EnableAutoConfiguration.class的value值，也就是众多自动配置类的类名。
拿到这些类名后会进行去重，去重的代码为：
new ArrayList<>(new LinkedHashSet<>(list))
去重完之后，就会看是否存在某些自动配置类需要排除，我们可以通过@EnableAutoConfiguration
注解的exclude属性，或者spring.autoconfigure.exclude配置来指定一些自动配置类的名字，然后把
它们从自动配置类集合中排除掉。
然后会继续利用ConfigurationClassFilter对自动配置类进行进一步筛选，ConfigurationClassFilter会利用AutoConfigurationMetadata进行筛选，而AutoConfigurationMetadata对象对应的是"METAINF/spring-autoconfigure-metadata.properties"文件中的内容，这是一种加快SpringBoot启动速度的机制，默认是开启了的（不过要通过maven或gradle的方式引入springboot的依赖来使用才能看到效果，因为这个文件的内容是在SpringBoot源码工程编译的时候自动生成出来的。
此配置文件可手动创建：自动配置类名.条件注解=条件
有了这个文件的内容，SpringBoot会在通过spring.facotries文件找到所有的自动配置类后，会把这个
文件中的内容读出来，然后利用AutoConfigurationImportFilter对所有的自动配置类进行条件匹配，
这里的条件判断，只会判断所需要的类是否存在，如果需要的类，或者需要的Bean对应的类，都不存
在，那么肯定不符合条件了，对于像@ConditionalOnMissingBean这样的条件，在这一步是不会去
判断的，最后条件匹配成功的自动配置类就会记录下来，并最终返回给Spring容器，继续进行其他条
件的匹配。
所以通过这个机制，使得Spring并不需要解析所有的自动配置类，从而提高了效率。
六、Spring Boot整合Tomcat底层原理
项目添加的starter为：spring-boot-starter-web，那么我们启动项目时，SpringBoot就会自动启动一个Tomcat。
在spring-boot-starter-web这个starter中，其实简介的引入了spring-bootstarter-tomcat这个starter，这个spring-boot-starter-tomcat又引入了tomcat-embed-core依赖，所以只要我们项目中依赖了spring-boot-starter-web就相当于依赖了Tomcat。
@Configuration(proxyBeanMethods = false)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@ConditionalOnClass(ServletRequest.class)
@ConditionalOnWebApplication(type = Type.SERVLET)
@EnableConfigurationProperties(ServerProperties.class)
@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,
ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,
ServletWebServerFactoryConfiguration.EmbeddedJetty.class,
ServletWebServerFactoryConfiguration.EmbeddedUndertow.class })
public class ServletWebServerFactoryAutoConfiguration {
...
}
这个自动配置类所需要的条件：
- @ConditionalOnClass(ServletRequest.class)：表示项目依赖中要有ServletRequest类（server api）
- @ConditionalOnWebApplication(type = Type.SERVLET)：表示项目应用类型得是SpringMVC（讲启动过程的时
候就知道如何判断一个SpringBoot应用的类型了）
在上面提到的spring-boot-starter-web中，其实还间接的引入了spring-web、spring-webmvc等依赖，这就使得第二个条件满足，而对于第一个条件的ServletRequest类，虽然它是Servlet规范中的类，但是在我们所依赖的tomcat-embed-core这个jar包中是存在这个类的，这是因为Tomcat在自己的源码中把Servlet规范中的一些代码也包含进去了，比如：
这就使得ServletWebServerFactoryAutoConfiguration这个自动配置的两个条件都符合，那么Spring就能去解析它，一解析它就发现这个自动配置类Import进来了三个类：
- ServletWebServerFactoryConfiguration.EmbeddedTomcat.class
- ServletWebServerFactoryConfiguration.EmbeddedJetty.class
- ServletWebServerFactoryConfiguration.EmbeddedUndertow.class
Import进来的这三个类应该是差不多，我们看EmbeddedTomcat这个类：
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
static class EmbeddedTomcat {
...
}
可以发现这个类是一个配置类，所以Spring也会来解析它，不过它也有两个条件：
- @ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })：项目依赖中要有Servlet.class、 Tomcat.class、UpgradeProtocol.class这三个类，这个条件比较容易理解，项目依赖中有Tomcat的类，那这个条件就符合。
- @ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)，项目中没有ServletWebServerFactory类型的Bean，因为这个配置类的内部就是定义了一个
TomcatServletWebServerFactory类型的Bean，TomcatServletWebServerFactory实现了
ServletWebServerFactory接口，所以这个条件注解的意思就是，如果程序员自己没有定义
ServletWebServerFactory类型的Bean，那么就符合条件，不然，如果程序员自己定义了
ServletWebServerFactory类型的Bean，那么条件就不符合，也就导致SpringBoot给我们定义的
TomcatServletWebServerFactory这个Bean就不会生效，最终生效的就是程序员自己定义的。
所以，通常只要我们项目依赖中有Tomcat依赖，那就符合条件，那最终Spring容器中就会有
TomcatServletWebServerFactory这个Bean。
对于另外的EmbeddedJetty和EmbeddedUndertow，也差不多，都是判断项目依赖中是否有Jetty和Undertow的依赖，如果有，那么对应在Spring容器中就会存在JettyServletWebServerFactory类型的Bean、或者存在UndertowServletWebServerFactory类型的Bean。
总结一下：
- 有Tomcat依赖，就有TomcatServletWebServerFactory这个Bean
- 有Jetty依赖，就有JettyServletWebServerFactory这个Bean
- 有Undertow依赖，就有UndertowServletWebServerFactory这个Bean
ServletWebServerFactory其实就是用来获得WebServer对象的，而WebServer拥有启动、停止、获取端口等方法，那么很自然，我们就发现WebServer其实指的就是Tomcat、Jetty、Undertow，而TomcatServletWebServerFactory就是用来生成Tomcat所对应的WebServer对象，具体一点就是TomcatWebServer对象，并且在生成TomcatWebServer对象时会把Tomcat给启动起来，在源码中，调用TomcatServletWebServerFactory对象的getWebServer()方法时就会启动Tomcat。
再看TomcatServletWebServerFactory这个Bean的定义：
@Bean
TomcatServletWebServerFactory tomcatServletWebServerFactory(
ObjectProvider<TomcatConnectorCustomizer> connectorCustomizers,
ObjectProvider<TomcatContextCustomizer> contextCustomizers,
ObjectProvider<TomcatProtocolHandlerCustomizer<?>> protocolHandlerCustomizers) {
TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();
// orderedStream()调用时会去Spring容器中找到TomcatConnectorCustomizer类型的Bean，默认是没有的，程序员可以自己定义
factory.getTomcatConnectorCustomizers()
.addAll(connectorCustomizers.orderedStream().collect(Collectors.toList()));
factory.getTomcatContextCustomizers()
.addAll(contextCustomizers.orderedStream().collect(Collectors.toList()));
factory.getTomcatProtocolHandlerCustomizers()
.addAll(protocolHandlerCustomizers.orderedStream().collect(Collectors.toList()));
return factory;
}
要构造这个Bean，Spring会从Spring容器中获取到TomcatConnectorCustomizer、TomcatContextCustomizer、TomcatProtocolHandlerCustomizer这三个类型的Bean，然后把它们添加到TomcatServletWebServerFactory对象中去，很明显这三种Bean是用来配置Tomcat的，比如：
- TomcatConnectorCustomizer：是用来配置Tomcat中的Connector组件的
- TomcatContextCustomizer：是用来配置Tomcat中的Context组件的
- TomcatProtocolHandlerCustomizer：是用来配置Tomcat中的ProtocolHandler组件的
也就是我们可以通过定义TomcatConnectorCustomizer类型的Bean，来对Tomcat进行配置，比如：
@SpringBootApplication
public class MyApplication {
@Bean
public TomcatConnectorCustomizer tomcatConnectorCustomizer(){
return new TomcatConnectorCustomizer() {
@Override
public void customize(Connector connector) {
connector.setPort(8888);
}
/*
这样Tomcat就会绑定8888这个端口。有了TomcatServletWebServerFactory这个Bean之后，在SpringBoot的启动过程中，会执行ServletWebServerApplicationContext的onRefresh()方法，而这个方法会调用createWebServer()方法，而这个方法中最为重要的两行代码为：
很明显，getWebServerFactory()负责获取具体的ServletWebServerFactory对象，要么是
TomcatServletWebServerFactory对象，要么是JettyServletWebServerFactory对象，要么是
UndertowServletWebServerFactory对象，注意只能获取到一个，然后调用该对象的getWebServer
方法，启动对应的Tomcat、或者Jetty、或者Undertow。
getWebServerFactory方法中的逻辑比较简单，获取Spring容器中的ServletWebServerFactory类型
的Bean对象，如果没有获取到则抛异常，如果找到多个也抛异常，也就是在Spring容器中只能有一个
ServletWebServerFactory类型的Bean对象。
拿到TomcatServletWebServerFactory对象后，就调用它的getWebServer方法，而在这个方法中就
会生成一个Tomcat对象，并且利用前面的TomcatConnectorCustomizer等等会Tomcat对象进行配
置，最后启动Tomcat。
这样在启动应用时就完成了Tomcat的启动，到此我们通过这个案例也看到了具体的Starter机制、自动
配置的具体使用。
不过额外有一点要提一下，我们前面提到了我们可以利用TomcatConnectorCustomizer对Tomca中
的Connector组件进行配置，我们可能会想到默认情况下，SpringBoot是不是就是提供了一个
*/
};
}
public static void main(String[] args) {
SpringApplication.run(MyApplication.class);
}
}
这样Tomcat就会绑定8888这个端口。
有了TomcatServletWebServerFactory这个Bean之后，在SpringBoot的启动过程中，会执行ServletWebServerApplicationContext的onRefresh()方法，而这个方法会调用createWebServer()方法，而这个方法中最为重要的两行代码为：
ServletWebServerFactory factory = getWebServerFactory();
this.webServer = factory.getWebServer(getSelfInitializer());
getWebServerFactory()负责获取具体的ServletWebServerFactory对象，要么是TomcatServletWebServerFactory对象，要么是JettyServletWebServerFactory对象，要么是UndertowServletWebServerFactory对象，注意只能获取到一个，然后调用该对象的getWebServer方法，启动对应的Tomcat、或者Jetty、或者Undertow。
getWebServerFactory方法中的逻辑比较简单，获取Spring容器中的ServletWebServerFactory类型的Bean对象，如果没有获取到则抛异常，如果找到多个也抛异常，也就是在Spring容器中只能有一个ServletWebServerFactory类型的Bean对象。
拿到TomcatServletWebServerFactory对象后，就调用它的getWebServer方法，而在这个方法中就会生成一个Tomcat对象，并且利用前面的TomcatConnectorCustomizer等等会Tomcat对象进行配置，最后启动Tomcat。
这样在启动应用时就完成了Tomcat的启动，到此我们通过这个案例也看到了具体的Starter机制、自动
配置的具体使用。
在自动配置类ServletWebServerFactoryAutoConfiguration中，会定义一个ServletWebServerFactoryCustomizer类型的Bean:
@Bean
public ServletWebServerFactoryCustomizer servletWebServerFactoryCustomizer(ServerProperties serverProperties,
ObjectProvider<WebListenerRegistrar> webListenerRegistrars,
ObjectProvider<CookieSameSiteSupplier> cookieSameSiteSuppliers) {
return new ServletWebServerFactoryCustomizer(serverProperties,
webListenerRegistrars.orderedStream().collect(Collectors.toList()),
cookieSameSiteSuppliers.orderedStream().collect(Collectors.toList()));
}
这个Bean会接收一个ServerProperties的Bean，ServerProperties的Bean对应的就是properties文
件中前缀为server的配置，我们可以利用ServerProperties对象的getPort方法获取到我们所配置的
server.port的值。
而ServletWebServerFactoryCustomizer是针对一个ServletWebServerFactory的自定义器，也就是用来配TomcatServletWebServerFactory这个Bean的，到时候ServletWebServerFactoryCustomizer就会利用ServerProperties对象来对TomcatServletWebServerFactory对象进行设置。
在ServletWebServerFactoryAutoConfiguration这个自动配置上，除开Import了EmbeddedTomcat、EmbeddedJetty、EmbeddedUndertow这三个配置类，还Import了一个ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class，这个BeanPostProcessorsRegistrar会向Spring容器中注册一个WebServerFactoryCustomizerBeanPostProcessor类型的Bean。
WebServerFactoryCustomizerBeanPostProcessor是一个BeanPosrtProcessor，它专门用来处理类型为WebServerFactory的Bean对象，而我们的TomcatServletWebServerFactory、JettyServletWebServerFactory、UndertowServletWebServerFactory也都实现了这个接口，所以
不管当前项目依赖的情况，只要在Spring在创建比如TomcatServletWebServerFactory这个Bean时WebServerFactoryCustomizerBeanPostProcessor就会对它进行处理，处理的逻辑为：
- 从Spring容器中拿到WebServerFactoryCustomizer类型的Bean，也就是前面说的
ServletWebServerFactoryCustomizer对象 - 然后调用ServletWebServerFactoryCustomizer对象的customize方法，把TomcatServletWebServerFactory对象
传入进去 - customize方法中就会从ServerProperties对象获取各种配置，然后设置给TomcatServletWebServerFactory对象
总结，SpringBoot整合Tomcat的核心原理主要涉及的东西有：
- spring-boot-starter-web：会自动引入Tomcat、SpringMVC的依赖
- ServletWebServerFactoryAutoConfiguration：自动配置类
- ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar：用来注册
WebServerFactoryCustomizerBeanPostProcessor - ServletWebServerFactoryConfiguration.EmbeddedTomcat：配置TomcatServletWebServerFactory
- ServletWebServerFactoryConfiguration.EmbeddedJetty：配置JettyServletWebServerFactory
- ServletWebServerFactoryConfiguration.EmbeddedUndertow：配置UndertowServletWebServerFactory
- ServletWebServerFactoryCustomizer：用来配置ServletWebServerFactory
- WebServerFactoryCustomizerBeanPostProcessor：是一个BeanPostProcessor，利用ServletWebServerFactoryCustomizer来配置ServletWebServerFactory
- ServletWebServerApplicationContext中的onRefresh()方法：负责启动Tomcat