前言：
Spring4推出了@Conditional注解，方便程序根据当前环境或者容器情况来动态注入bean，对@Conditional注解不熟悉的朋友可移步至 Spring @Conditional注解 详细讲解及示例 这篇博客进行学习。
继@Conditional注解后，又基于此注解推出了很多派生注解，比如@ConditionalOnBean、@ConditionalOnMissingBean、@ConditionalOnExpression、@ConditionalOnClass......动态注入bean变得更方便了。本篇将讲解@ConditionalOnBean注解。
配置类中有两个Computer类的bean，一个是笔记本电脑，一个是备用电脑。如果当前容器中已经有电脑bean了，就不注入备用电脑，如果没有，则注入备用电脑，这里需要使用到@ConditionalOnMissingBean。
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@Configuration
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public
class BeanConfig {
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@Bean(name =
"notebookPC")
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public Computer computer1(){
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return
new Computer(
"笔记本电脑");
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}
-
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@ConditionalOnMissingBean(Computer.class)
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@Bean(
"reservePC")
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public Computer computer2(){
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return
new Computer(
"备用电脑");
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}
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}
这个注解就实现了功能，这个@ConditionalOnMissingBean为我们做了什么呢？我们来一探究竟.。
一探究竟：
首先，来看@ConditionalOnMissingBean的声明：
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//可以标注在类和方法上
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@Target({ElementType.TYPE, ElementType.METHOD})
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@Retention(RetentionPolicy.RUNTIME)
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@Documented
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//使用了@Conditional注解，条件类是OnBeanCondition
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@Conditional({OnBeanCondition.class})
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public
@interface ConditionalOnMissingBean {
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Class<?>[] value()
default {};
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String[] type()
default {};
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Class<?>[] ignored()
default {};
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String[] ignoredType()
default {};
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Class<? extends Annotation>[] annotation()
default {};
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String[] name()
default {};
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SearchStrategy search() default SearchStrategy.ALL;
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}
这时候，我们就看到了我们熟悉的@Conditional注解，OnBeanCondition作为条件类。
OnBeanCondition类的声明：
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//定义带注释的组件的排序顺序，2147483647即为默认值
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@Order(
2147483647)
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class OnBeanCondition extends SpringBootCondition implements ConfigurationCondition {
它继承了SpringBootCondition类，OnBeanCondition类中没有matches方法，而SpringBootCondition类中有实现matches方法。OnBeanCondition还实现了ConfigurationCondition，ConfigurationCondition接口不熟悉的读者可以到Spring ConfigurationCondition接口详解 了解接口。OnBeanCondition类重写了getConfigurationPhase()方法，表示在注册bean的时候注解生效：
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public ConfigurationPhase getConfigurationPhase() {
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return ConfigurationPhase.REGISTER_BEAN;
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}
就从matches方法开始：
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//SpringBootCondition类中的matches方法
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public final boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
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//获取当前的类名或者方法名（由标注的位置决定）
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String classOrMethodName = getClassOrMethodName(metadata);
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try {
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//关键代码：这里就会判断出结果
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ConditionOutcome outcome =
this.getMatchOutcome(context, metadata);
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//存入日志
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this.logOutcome(classOrMethodName, outcome);
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//存入记录
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this.recordEvaluation(context, classOrMethodName, outcome);
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//最后返回ConditionOutcome的isMatch就是返回boolean类型结果
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return outcome.isMatch();
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}
catch (NoClassDefFoundError var5) {
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throw
new IllegalStateException(
"Could not evaluate condition on " + classOrMethodName +
" due to " + var5.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)", var5);
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}
catch (RuntimeException var6) {
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throw
new IllegalStateException(
"Error processing condition on " +
this.getName(metadata), var6);
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}
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}
关键代码在OnBeanCondition的getMatchOutcome方法上：
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/**
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* 获得判断结果的方法，ConditionOutcome类中存着boolean类型的结果
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*/
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public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
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//返回一个新的ConditionMessage
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ConditionMessage matchMessage = ConditionMessage.empty();
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OnBeanCondition.BeanSearchSpec spec;
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List matching;
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//这是metadata会调用isAnnotated方法判断当前标注的注解是不是ConditionalOnMissingBean
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//其实@ConditionalOnBean、@ConditionalOnMissingBean和@ConditionalOnSingleCandidate都是使用这个条件类，所以这里做判断
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if (metadata.isAnnotated(ConditionalOnBean.class.getName())) {
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spec =
new OnBeanCondition.BeanSearchSpec(context, metadata, ConditionalOnBean.class);
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matching =
this.getMatchingBeans(context, spec);
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if (matching.isEmpty()) {
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return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnBean.class,
new Object[]{spec}).didNotFind(
"any beans").atAll());
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}
-
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matchMessage = matchMessage.andCondition(ConditionalOnBean.class,
new Object[]{spec}).found(
"bean",
"beans").items(Style.QUOTE, matching);
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}
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if (metadata.isAnnotated(ConditionalOnSingleCandidate.class.getName())) {
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OnBeanCondition.BeanSearchSpec spec =
new OnBeanCondition.SingleCandidateBeanSearchSpec(context, metadata, ConditionalOnSingleCandidate.class);
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matching =
this.getMatchingBeans(context, spec);
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if (matching.isEmpty()) {
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return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnSingleCandidate.class,
new Object[]{spec}).didNotFind(
"any beans").atAll());
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}
-
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if (!
this.hasSingleAutowireCandidate(context.getBeanFactory(), matching, spec.getStrategy() == SearchStrategy.ALL)) {
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return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnSingleCandidate.class,
new Object[]{spec}).didNotFind(
"a primary bean from beans").items(Style.QUOTE, matching));
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}
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matchMessage = matchMessage.andCondition(ConditionalOnSingleCandidate.class,
new Object[]{spec}).found(
"a primary bean from beans").items(Style.QUOTE, matching);
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}
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//如果当前注入的bean是@ConditionalOnMissingBean
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if (metadata.isAnnotated(ConditionalOnMissingBean.class.getName())) {
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//返回一个spec（说明），这里的spec规定了搜索的内容，比如搜索策略、需要搜索的类名......
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spec =
new OnBeanCondition.BeanSearchSpec(context, metadata, ConditionalOnMissingBean.class);
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//主要的搜索实现在这个方法里，最后返回一个list
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matching =
this.getMatchingBeans(context, spec);
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//判断搜索出来的结果
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if (!matching.isEmpty()) {
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return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnMissingBean.class,
new Object[]{spec}).found(
"bean",
"beans").items(Style.QUOTE, matching));
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}
-
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matchMessage = matchMessage.andCondition(ConditionalOnMissingBean.class,
new Object[]{spec}).didNotFind(
"any beans").atAll();
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}
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return ConditionOutcome.match(matchMessage);
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}
spec = new OnBeanCondition.BeanSearchSpec(context, metadata, ConditionalOnBean.class);
这句中，相当于从内部类中将标注@ConditionalOnMissingBean注解时的属性都取出来：
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BeanSearchSpec(ConditionContext context, AnnotatedTypeMetadata metadata, Class<?> annotationType) {
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this.annotationType = annotationType;
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MultiValueMap<String, Object> attributes = metadata.getAllAnnotationAttributes(annotationType.getName(),
true);
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//将attributes这个map中的数据放到对应的list成员变量中
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this.collect(attributes,
"name",
this.names);
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this.collect(attributes,
"value",
this.types);
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this.collect(attributes,
"type",
this.types);
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this.collect(attributes,
"annotation",
this.annotations);
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this.collect(attributes,
"ignored",
this.ignoredTypes);
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this.collect(attributes,
"ignoredType",
this.ignoredTypes);
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this.strategy = (SearchStrategy)metadata.getAnnotationAttributes(annotationType.getName()).get(
"search");
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OnBeanCondition.BeanTypeDeductionException deductionException =
null;
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try {
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if (
this.types.isEmpty() &&
this.names.isEmpty()) {
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this.addDeducedBeanType(context, metadata,
this.types);
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}
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}
catch (OnBeanCondition.BeanTypeDeductionException var7) {
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deductionException = var7;
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}
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this.validate(deductionException);
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}
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//验证的方法
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protected void validate(OnBeanCondition.BeanTypeDeductionException ex) {
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if (!
this.hasAtLeastOne(
this.types,
this.names,
this.annotations)) {
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String message =
this.annotationName() +
" did not specify a bean using type, name or annotation";
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if (ex ==
null) {
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throw
new IllegalStateException(message);
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}
else {
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throw
new IllegalStateException(message +
" and the attempt to deduce the bean's type failed", ex);
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}
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}
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}
看一下OnBeanCondition类中的getMatchingBeans方法，里面有用到搜索策略，详见搜索策略介绍
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private List<String> getMatchingBeans(ConditionContext context, OnBeanCondition.BeanSearchSpec beans) {
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//获得当前bean工厂
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ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
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//判断当前的搜索策略是否是PARENTS或者ANCESTORS，默认是ALL
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if (beans.getStrategy() == SearchStrategy.PARENTS || beans.getStrategy() == SearchStrategy.ANCESTORS) {
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BeanFactory parent = beanFactory.getParentBeanFactory();
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Assert.isInstanceOf(ConfigurableListableBeanFactory.class, parent,
"Unable to use SearchStrategy.PARENTS");
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//如果是PARENTS或者ANCESTORS，当前bean工厂就用父工厂
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beanFactory = (ConfigurableListableBeanFactory)parent;
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}
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if (beanFactory ==
null) {
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return Collections.emptyList();
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}
else {
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List<String> beanNames =
new ArrayList();
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//如果当前搜索策略等于CURRENT，为true
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boolean considerHierarchy = beans.getStrategy() != SearchStrategy.CURRENT;
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//这里的type就是需要查找的bean的类型
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//下面，会从属性中找bean
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Iterator var6 = beans.getTypes().iterator();
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String beanName;
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while(var6.hasNext()) {
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beanName = (String)var6.next();
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//如果找到了类型，接下来就是根据类型找bean的实例名，找示例名的方法在下方，实际上就是一个getNamesForType
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beanNames.addAll(
this.getBeanNamesForType(beanFactory, beanName, context.getClassLoader(), considerHierarchy));
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}
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var6 = beans.getIgnoredTypes().iterator();
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while(var6.hasNext()) {
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beanName = (String)var6.next();
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beanNames.removeAll(
this.getBeanNamesForType(beanFactory, beanName, context.getClassLoader(), considerHierarchy));
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}
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var6 = beans.getAnnotations().iterator();
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while(var6.hasNext()) {
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beanName = (String)var6.next();
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beanNames.addAll(Arrays.asList(
this.getBeanNamesForAnnotation(beanFactory, beanName, context.getClassLoader(), considerHierarchy)));
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}
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var6 = beans.getNames().iterator();
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while(var6.hasNext()) {
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beanName = (String)var6.next();
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if (
this.containsBean(beanFactory, beanName, considerHierarchy)) {
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beanNames.add(beanName);
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}
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}
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//将存放bean实例名的list返回
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return beanNames;
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}
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}
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//根据类型获取bean的name
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private Collection<String> getBeanNamesForType(ListableBeanFactory beanFactory, String type, ClassLoader classLoader, boolean considerHierarchy) throws LinkageError {
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try {
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Set<String> result =
new LinkedHashSet();
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this.collectBeanNamesForType(result, beanFactory, ClassUtils.forName(type, classLoader), considerHierarchy);
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return result;
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}
catch (ClassNotFoundException var6) {
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return Collections.emptySet();
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}
catch (NoClassDefFoundError var7) {
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return Collections.emptySet();
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}
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}
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private void collectBeanNamesForType(Set<String> result, ListableBeanFactory beanFactory, Class<?> type, boolean considerHierarchy) {
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result.addAll(BeanTypeRegistry.get(beanFactory).getNamesForType(type));
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if (considerHierarchy && beanFactory
instanceof HierarchicalBeanFactory) {
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BeanFactory parent = ((HierarchicalBeanFactory)beanFactory).getParentBeanFactory();
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if (parent
instanceof ListableBeanFactory) {
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this.collectBeanNamesForType(result, (ListableBeanFactory)parent, type, considerHierarchy);
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}
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}
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}
找完bean了之后，回到刚才的代码里：
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//如果当前注入的bean是@ConditionalOnMissingBean
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if (metadata.isAnnotated(ConditionalOnMissingBean.class.getName())) {
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//返回一个spec（说明），这里的spec规定了搜索的内容，比如搜索策略、需要搜索的类名......
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spec =
new OnBeanCondition.BeanSearchSpec(context, metadata, ConditionalOnMissingBean.class);
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matching =
this.getMatchingBeans(context, spec);
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if (!matching.isEmpty()) {
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return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnMissingBean.class,
new Object[]{spec}).found(
"bean",
"beans").items(Style.QUOTE, matching));
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}
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matchMessage = matchMessage.andCondition(ConditionalOnMissingBean.class,
new Object[]{spec}).didNotFind(
"any beans").atAll();
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}
如果第5行返回的list不是空的，就会返回ConditionOutcome对象noMatch方法，表示不匹配。ConditionOutcome类用于存放过滤结果，只有两个变量：
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/**
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* 过滤结果类
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*/
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public
class ConditionOutcome {
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/**
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* 匹配结果 true or false
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*/
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private
final
boolean match;
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/**
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* 匹配结果信息
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*/
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private
final ConditionMessage message;
两者区别：
@ConditionOnBean在判断list的时候，如果list没有值，返回false，否则返回true
@ConditionOnMissingBean在判断list的时候，如果list没有值，返回true，否则返回false，其他逻辑都一样
例子：
- @ConditionalOnBean(javax.sql.DataSource.class)
Spring容器或者所有父容器中需要存在至少一个javax.sql.DataSource类的实例