@ConditionalOnClass注解

KafkaAutoConfiguration

@ConditionalOnClass(KafkaTemplate.class)

/**
 * {@link Conditional} that only matches when the specified classes are on the classpath.
 *
 * @author Phillip Webb
 */
@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnClassCondition.class)
public @interface ConditionalOnClass {

   /**
    * 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 {};

   /**
    * The classes names that must be present.
    * @return the class names that must be present.
    */
   String[] name() default {};

}