ConditionalOnClass，为什么JVM在运行时允许类不在类路径中

Question

通过学习使用spring boot，了解了spring boot ConditionalOnClass的用法和背后的逻辑，我的问题是：

1. 为什么“由于这个注解是通过加载类字节码来解析的，所以在这里指定最终可能不在类路径上的类是安全的”。
2. JVM 规范在哪里与此相关？

https://github.com/spring-projects/spring-boot/blob/master/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/condition/ConditionalOnClass.java#L39

/**
 * The classes that must be present. Since this annotation is parsed by loading class
 * bytecode, it is safe to specify classes here that may ultimately not be on the
 * classpath, only if this annotation is directly on the affected component and
 * <b>not</b> if this annotation is used as a composed, meta-annotation. In order to
 * use this annotation as a meta-annotation, only use the {@link #name} attribute.
 * @return the classes that must be present
 */
Class<?>[] value() default {};

Answer 1

因为 Spring 捕获 ClassNotFoundException。 检查源代码 - https://github.com/spring-projects/spring-boot/blob/d3c34ee3d1bfd3db4a98678c524e145ef9bca51c/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/condition/OnClassCondition.java#L218