我們在 Spring/ Spring Boot Starter 或者一些框架的源碼裏經常能看到類似如下的註解聲明,可能作用在類上,也可能在某個方法上:
@ConditionalOnProperty(name = "spring.cloud.refresh.enabled", matchIfMissing = true)
@ConditionalOnProperty(prefix = "management.metrics.export.atlas", name = "enabled", havingValue = "true",
matchIfMissing = true)
我們一眼都能看出來,這是來「談條件」的。需要滿足某個屬性存在,或者屬性值是xx這一類的。
對於屬性的匹配,是會在 Environment 裏查找是否包含當前需要的屬性,如果沒指定 havingValue 的話,那需要同時屬性的值不爲「false」這個字符串,其它的東西都視爲true。
今天的這篇做爲鋪墊,先來描述一下註解的工作原理,後面一篇我會寫寫與此有關的一個有趣的案例。
工作原理
濃縮版
在SpringBoot 啓動過程中,會掃描當前依賴裏的 @Configuration,然後遍歷的過程中會判斷其中哪些是要講條件的。對於講條件的這些,會判斷
shouldSkip ,這裏的是否跳過,會根據註解作用在類上,方法上,轉向不同的Metadata,提取對應的實現類,但本質上還是通過 resolver 去Environment 裏找找這個屬性在不在,不在跳過,在的話是否值匹配。從而決定 Confirutaion 是否生效。
源碼版
我們知道 Spring 啓動的過程,也是創建和初始化Bean 的過程,在這個過程中,會先拿到BeanNames,並一個個的去創建和初始化。
此時,對於Configuration,是通過BeanPostProcessor的方式來處理的.
public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
int registryId = System.identityHashCode(registry);
this.registriesPostProcessed.add(registryId);
processConfigBeanDefinitions(registry);// 對,是這裏
}
部分調用棧如下:
java.lang.Thread.State: RUNNABLE
at org.springframework.boot.autoconfigure.condition.OnPropertyCondition.getMatchOutcome(OnPropertyCondition.java:65)
at org.springframework.boot.autoconfigure.condition.SpringBootCondition.matches(SpringBootCondition.java:47)
at org.springframework.context.annotation.ConditionEvaluator.shouldSkip(ConditionEvaluator.java:108)
at org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader.loadBeanDefinitionsForBeanMethod(ConfigurationClassBeanDefinitionReader.java:181)
at org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader.loadBeanDefinitionsForConfigurationClass(ConfigurationClassBeanDefinitionReader.java:142)
at org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader.loadBeanDefinitions(ConfigurationClassBeanDefinitionReader.java:118)
at org.springframework.context.annotation.ConfigurationClassPostProcessor.processConfigBeanDefinitions(ConfigurationClassPostProcessor.java:328)
at org.springframework.context.annotation.ConfigurationClassPostProcessor.postProcessBeanDefinitionRegistry(ConfigurationClassPostProcessor.java:233)
這裏對於 Class 和 Method,都在該方法中,處理入口不一樣,傳入的Meta也有所區別
/**
* Build and validate a configuration model based on the registry of
* {@link Configuration} classes.
*/
public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
List<BeanDefinitionHolder> configCandidates = new ArrayList<>();
String[] candidateNames = registry.getBeanDefinitionNames();
for (String beanName : candidateNames) {
BeanDefinition beanDef = registry.getBeanDefinition(beanName);
if (ConfigurationClassUtils.isFullConfigurationClass(beanDef) ||
ConfigurationClassUtils.isLiteConfigurationClass(beanDef)) {
}
else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) {
configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
}
}
// Return immediately if no @Configuration classes were found
if (configCandidates.isEmpty()) {
return;
}
// Parse each @Configuration class
ConfigurationClassParser parser = new ConfigurationClassParser(
this.metadataReaderFactory, this.problemReporter, this.environment,
this.resourceLoader, this.componentScanBeanNameGenerator, registry);
Set<BeanDefinitionHolder> candidates = new LinkedHashSet<>(configCandidates);
Set<ConfigurationClass> alreadyParsed = new HashSet<>(configCandidates.size());
do {
parser.parse(candidates); // 這裏處理class
parser.validate();
Set<ConfigurationClass> configClasses = new LinkedHashSet<>(parser.getConfigurationClasses());
configClasses.removeAll(alreadyParsed);
// Read the model and create bean definitions based on its content
if (this.reader == null) {
this.reader = new ConfigurationClassBeanDefinitionReader(
registry, this.sourceExtractor, this.resourceLoader, this.environment,
this.importBeanNameGenerator, parser.getImportRegistry());
}
this.reader.loadBeanDefinitions(configClasses); // 這裏處理Method
alreadyParsed.addAll(configClasses);
while (!candidates.isEmpty());
}
裏面的邏輯,則都是在判斷這些Condition 是否match,重點看這一行
condition.matches(this.context, metadata)
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;
}
}
通過觀察 Condition 這個接口你也能發現,和我們上面說的一樣,這裏不同的處理metadata是不同的。
在 SpringBoot 裏,ConditionalOnProperty 的 Condition 實現,運用了一個模板方法模式, SpringBootCondition 做爲模板,再調用各子類的實現方法。
public final boolean matches(ConditionContext context,
AnnotatedTypeMetadata metadata) {
String classOrMethodName = getClassOrMethodName(metadata);
ConditionOutcome outcome = getMatchOutcome(context, metadata);// 這裏交給了抽象方法
recordEvaluation(context, classOrMethodName, outcome);
return outcome.isMatch();
}
來看子類的實現
private ConditionOutcome determineOutcome(AnnotationAttributes annotationAttributes, PropertyResolver resolver) { Spec spec = new Spec(annotationAttributes); List<String> missingProperties = new ArrayList<>(); List<String> nonMatchingProperties = new ArrayList<>(); spec.collectProperties(resolver, missingProperties, nonMatchingProperties); if (!missingProperties.isEmpty()) { return ConditionOutcome.noMatch( ConditionMessage.forCondition(ConditionalOnProperty.class, spec) .didNotFind("property", "properties") .items(Style.QUOTE, missingProperties)); } if (!nonMatchingProperties.isEmpty()) { return ConditionOutcome.noMatch( ConditionMessage.forCondition(ConditionalOnProperty.class, spec) .found("different value in property", "different value in properties") .items(Style.QUOTE, nonMatchingProperties)); } return ConditionOutcome.match(ConditionMessage .forCondition(ConditionalOnProperty.class, spec).because("matched")); }
有了這個判斷,對於 OnClass 之類的,你也能猜個八九不離十。
同樣會有一個子類的實現
只不過判斷的從屬性,換成了在classloader裏查找已加載的類。