详解SpringBoot中@ConditionalOnClass注解的使用

@Configuration
@ConditionalOnClass(value = {Client.class})
public class MyAutoConfig {
    public MyAutoConfig(){
        System.out.println("constructor MyAutoConfig");
    }
}

org.springframework.boot.autoconfigure.data.elasticsearch.Client

@Configuration
@ConditionalOnClass(name = {"com.my.template.config.ClassA"})
public class MyAutoConfig {
    public MyAutoConfig(){
        System.out.println("constructor MyAutoConfig");
    }
}

constructor MyAutoConfig
constructor MyAutoConfig2
constructor classA
2022-07-30 17:18:54.113 

@Configuration
@ConditionalOnClass(name = {"com.my.template.config.ClassD"})
public class MyAutoConfig {
    public MyAutoConfig(){
        System.out.println("constructor MyAutoConfig");
    }
}

constructor MyAutoConfig2
constructor classA
2022-07-30 21:43:30.550  INFO 13116 --- [  

@Override
	public final boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        //获得该注解标准的类或方法
		String classORMethodName = getClassOrMethodName(metadata);
		try {
            //模板方法，该实现在OnClassCodition类中
			ConditionOutcome outcome = getMatchOutcome(context, metadata);
			loGoutcome(classOrMethodName, outcome);
			recordEvaluation(context, classOrMethodName, outcome);
            //返回是否符合条件
			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);
		}
	}

@Override
	public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
		ClassLoader classLoader = context.getClassLoader();
		ConditionMessage matchMessage = ConditionMessage.empty();
         //获得@ConditionalOnClass注解中配置的value和name属性的值
		List<String> onClasses = getCandidates(metadata, ConditionalOnClass.class);
		if (onClasses != null) {
            //判断@ConditionOnClass注解配置的类是否都可以加载到，如有加载不到的则放到missing中
			List<String> missing = filter(onClasses, ClassNameFilter.MISSING, classLoader);
			if (!missing.isEmpty()) {
                //有加载不到的，则返回ConditionOutcome对下，其中属性match为false
				return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnClass.class)
						.didNotFind("required class", "required classes").items(Style.QUOTE, missing));
			}
			matchMessage = matchMessage.andCondition(ConditionalOnClass.class)
					.found("required class", "required classes")
					.items(Style.QUOTE, filter(onClasses, ClassNameFilter.PRESENT, classLoader));
		}
        //@ConditionalOnMissinGClass的处理逻辑
		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));
		}
        //返回ConditionOutCome对下，其match属性为true
		return ConditionOutcome.match(matchMessage);
	}

protected final List<String> filter(Collection<String> classNames, ClassNameFilter classNameFilter,
			ClassLoader classLoader) {
		if (CollectionUtils.isEmpty(classNames)) {
			return Collections.emptyList();
		}
		List<String> matches = new ArrayList<>(classNames.size());
		for (String candidate : classNames) {
            //循环调用matches方法
			if (classNameFilter.matches(candidate, classLoader)) {
				matches.add(candidate);
			}
		}
		return matches;
	}

static boolean isPresent(String className, ClassLoader classLoader) {
            if (classLoader == null) {
                classLoader = ClassUtils.getDefaultClassLoader();
            }

            try {
                //具体实现逻辑
                FilteringSpringBootCondition.resolve(className, classLoader);
                return true;
            } catch (Throwable var3) {
                return false;
            }
        }

protected static Class<?> resolve(String className, ClassLoader classLoader) throws ClassNotFoundException {
        return classLoader != null ? Class.forName(className, false, classLoader) : Class.forName(className);
    }