SpringBoot注解@ConditionalOnClass底层源码实现

@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnClassCondition.class)
public @interface ConditionalOnClass {
	Class<?>[] value() default {};
	String[] name() default {};
}

public class ConditionOutcome {
	// 表示条件是否匹配
	private final boolean match;
	// ...
}

// 调用getCandidates方法
List<String> onClasses = getCandidates(metadata, ConditionalOnClass.class);

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;
}

List<String> missing = filter(onClasses, ClassNameFilter.MISSING, classLoader);

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) {
		if (classNameFilter.matches(candidate, classLoader)) {
			matches.add(candidate);
		}
	}
	return matches;
}

protected enum ClassNameFilter {
	// ...
	MISSING {
		@Override
		public boolean matches(String className, ClassLoader classLoader) {
                        // 是否不存在
			return !isPresent(className, classLoader);
		}
	};
	static boolean isPresent(String className, ClassLoader classLoader) {
		if (classLoader == null) {
			classLoader = ClassUtils.getDefaultClassLoader();
		}
		try {
			resolve(className, classLoader);
			return true;
		}
		catch (Throwable ex) {
			return false;
		}
	}
}

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