springboot情操陶冶-@Conditional和@AutoConfigureAfter注解解析
承接前文springboot情操陶冶-@Configuration注解解析，本文将在前文的基础上阐述
@AutoConfigureAfter和@Conditional注解的作用与解析
1.@Conditional
根据单词来理解，其就是条件的意思。在分析之前我们可以看下其内部源码
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Conditional {
/**
* All {@link Condition}s that must {@linkplain Condition#matches match}
* in order for the component to be registered.
*/
Class<? extends Condition>[] value();
}
其作用于类、方法上，且指定的value值必须是org.springframework.context.annotation.Condition的实现类，供条件判断。
以此为基础而扩展的注解还有@ConditionalBean、@ConditionalOnWebApplication、@ConditionalOnClass、@ConditionalOnMissingBean等等。
@Conditional注解被解析入口
那么我们肯定想知道，其中的注解是如何被解析的呢。其实在前文中的ConfigurationClassParser类中，在执行真正的doProcessConfigurationClass()方法前，会执行如下的代码
protected void processConfigurationClass(ConfigurationClass configClass) throws IOException {
// 条件判断，满足则直接返回，不进行后续的解析
if (this.conditionEvaluator.shouldSkip(configClass.getMetadata(), ConfigurationPhase.PARSE_CONFIGURATION)) {
return;
}
....
// Recursively process the configuration class and its superclass hierarchy.
SourceClass sourceClass = asSourceClass(configClass);
do {
sourceClass = doProcessConfigurationClass(configClass, sourceClass);
}
while (sourceClass != null);
this.configurationClasses.put(configClass, configClass);
}
也就是会执行上述的ConditionEvaluator#shouldSkip()方法，只有条件不满足后才会继续往下执行真正的@Configuration注解解析。
ConditionEvaluator#shouldSkip()
废话不多说，直接上源码
// metadata为被注解的类元素，返回值为true表明条件满足应该被忽略
public boolean shouldSkip(@Nullable AnnotatedTypeMetadata metadata, @Nullable ConfigurationPhase phase) {
// 1.判断类是否含有@Conditional注解，否则直接返回
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);
}
// 2.获取类上所有含有@Conditional注解的value集合(其会递归找寻注解的注解)
List<Condition> conditions = new ArrayList<>();
for (String[] conditionClasses : getConditionClasses(metadata)) {
for (String conditionClass : conditionClasses) {
Condition condition = getCondition(conditionClass, this.context.getClassLoader());
conditions.add(condition);
}
}
// 3.根据Order来进行排序
AnnotationAwareOrderComparator.sort(conditions);
// 4.对集合内的condition统一调用matches()方法,一旦遇到条件判断不满足的则返回true对此注解类元素进行忽略
for (Condition condition : conditions) {
ConfigurationPhase requiredPhase = null;
if (condition instanceof ConfigurationCondition) {
requiredPhase = ((ConfigurationCondition) condition).getConfigurationPhase();
}
if ((requiredPhase == null || requiredPhase == phase) && !condition.matches(this.context, metadata)) {
return true;
}
}
return false;
}
具体的代码解释已经按照注释给出了，其实也很简单，读者稍微阅读就能明白了。另外额外的注解比如@ConditionalOnMissingBean等读者可自行去阅读代码分析，笔者此处就不展开了
2.@AutoConfigureAfter
与@AutoConfigureBefore类同，代表的含义就是自动注入在什么类加载前或者之后。先来看下其内部源码
@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.TYPE })
@Documented
public @interface AutoConfigureAfter {
/**
* The auto-configure classes that should have already been applied.
* @return the classes
*/
Class<?>[] value() default {};
/**
* The names of the auto-configure classes that should have already been applied.
* @return the class names
* @since 1.2.2
*/
String[] name() default {};
}
只作用于类上，内部属性name表明beanDefinition的类名；内部属性value表明beanDefinition的类。
那么其是如何被解析的呢，也是基于前文的ConfigurationClassParser#parse()方法，具体如下
public void parse(Set<BeanDefinitionHolder> configCandidates) {
this.deferredImportSelectors = new LinkedList<>();
// 解析@Configuration注解
....
// 解析DeferredImportSelector接口类，表面上也就是延迟解析的意思
processDeferredImportSelectors();
}
笔者此处只关注processDeferredImportSelectors()方法，通过此方法便可察觉到@AutoConfigureAfter等注解的蛛丝马迹
ConfigurationClassParser#processDeferredImportSelectors()
直接阅读源码
private void processDeferredImportSelectors() {
// 1.通过processImport()方法得到DeferredImportSelector接口集合，无则直接返回
List<DeferredImportSelectorHolder> deferredImports = this.deferredImportSelectors;
this.deferredImportSelectors = null;
if (deferredImports == null) {
return;
}
// 2.排序
deferredImports.sort(DEFERRED_IMPORT_COMPARATOR);
// 3.遍历DeferredImportSelector接口集合，获取Group集合类，默认为DefaultDeferredImportSelectorGroup
Map<Object, DeferredImportSelectorGrouping> groupings = new LinkedHashMap<>();
Map<AnnotationMetadata, ConfigurationClass> configurationClasses = new HashMap<>();
for (DeferredImportSelectorHolder deferredImport : deferredImports) {
// notice this........
Class<? extends Group> group = deferredImport.getImportSelector().getImportGroup();
DeferredImportSelectorGrouping grouping = groupings.computeIfAbsent(
(group != null ? group : deferredImport),
key -> new DeferredImportSelectorGrouping(createGroup(group)));
grouping.add(deferredImport);
configurationClasses.put(deferredImport.getConfigurationClass().getMetadata(),
deferredImport.getConfigurationClass());
}
// 4. 遍历Group集合，作用也是调用processImport()方法用于解析@Import
for (DeferredImportSelectorGrouping grouping : groupings.values()) {
grouping.getImports().forEach(entry -> {
ConfigurationClass configurationClass = configurationClasses.get(entry.getMetadata());
try {
processImports(configurationClass, asSourceClass(configurationClass),
asSourceClasses(entry.getImportClassName()), false);
}
catch (BeanDefinitionStoreException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanDefinitionStoreException(
"Failed to process import candidates for configuration class [" +
configurationClass.getMetadata().getClassName() + "]", ex);
}
});
}
}
笔者和读者此处只需要关注deferredImport.getImportSelector().getImportGroup()这个方法即可，此处以AutoConfigurationImportSelector.class为例
AutoConfigurationImportSelector
首先看下其getImportGroup()方法
public Class<? extends Group> getImportGroup() {
return AutoConfigurationGroup.class;
}
再观察下AutoConfigurationGroup此类的selectImports()方法
public Iterable<Entry> selectImports() {
return sortAutoConfigurations().stream()
.map((importClassName) -> new Entry(this.entries.get(importClassName),
importClassName))
.collect(Collectors.toList());
}
关键点来了，就在sortAutoConfigurations()方法,其会通过AutoConfigurationSorter类来对导入的class类进行排序，至于如何排序我们继续往下看
AutoConfigurationSorter
排序方法getInPriorityOrder()，我们看下源码
public List<String> getInPriorityOrder(Collection<String> classNames) {
AutoConfigurationClasses classes = new AutoConfigurationClasses(
this.metadataReaderFactory, this.autoConfigurationMetadata, classNames);
List<String> orderedClassNames = new ArrayList<>(classNames);
// Initially sort alphabetically.首先根据ASCII来进行排序
Collections.sort(orderedClassNames);
// Then sort by order，再根据Order来进行排序
orderedClassNames.sort((o1, o2) -> {
int i1 = classes.get(o1).getOrder();
int i2 = classes.get(o2).getOrder();
return Integer.compare(i1, i2);
});
// Then respect @AutoConfigureBefore @AutoConfigureAfter
orderedClassNames = sortByAnnotation(classes, orderedClassNames);
return orderedClassNames;
}
可以得出，最关键的排序来自sortByAnnotation()方法，具体就不看了，无非是根据before/after，来对importClassName进行排序得出一个有序的集合。
1.最后再回到ConfigurationClassParser#processDeferredImportSelectors()方法的最后一段，其会对上述的有序的集合遍历操作
processImports()方法，如果对应的class类不存在则会报错，也就满足了AutoConfigureBefore/AutoConfigureAfter的含义。2.上述的@AutoConfigureAfter注解解析只作用于META-INF\spring.factories文件中EnableAutoConfiguration属性对应的class类集合。
（v2.0版本以下支持用户使用该注解直接应用自定义类；v2.0版本以上，如果用户也使用了该注解，也需要在META-INF\spring.factories配置相应的EnableAutoConfiguration属性）
小结
针对@Conditional和@AutoConfigureAfter的具体解析可见上文，本文也是对前文的补充。希望读者在阅读此文的同时务必阅读前文方可理解上述的代码含义。同时因为这两个注解具有条件性，所以springboot多用此两注解来相互搭配构建不同条件的依赖部署，对去配置化起到了很大的作用。以WebMvcAutoConfiguration类作为结尾
@Configuration
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass({ Servlet.class, DispatcherServlet.class, WebMvcConfigurer.class })
@ConditionalOnMissingBean(WebMvcConfigurationSupport.class)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10)
@AutoConfigureAfter({ DispatcherServletAutoConfiguration.class,
ValidationAutoConfiguration.class })
public class WebMvcAutoConfiguration {
}
