官方介绍:

/** 
 * Returns a specified instance method for a given class.
 * 
 * @param cls The class you want to inspect.
 * @param name The selector of the method you want to retrieve.
 * 
 * @return The method that corresponds to the implementation of the selector specified by 
 *  \e name for the class specified by \e cls, or \c NULL if the specified class or its 
 *  superclasses do not contain an instance method with the specified selector.
 *
 * @note This function searches superclasses for implementations, whereas \c class_copyMethodList does not.
 */
OBJC_EXPORT Method class_getInstanceMethod(Class cls, SEL name)
    __OSX_AVAILABLE_STARTING(__MAC_10_0, __IPHONE_2_0);
 
/** 
 * Returns a pointer to the data structure describing a given class method for a given class.
 * 
 * @param cls A pointer to a class definition. Pass the class that contains the method you want to retrieve.
 * @param name A pointer of type \c SEL. Pass the selector of the method you want to retrieve.
 * 
 * @return A pointer to the \c Method data structure that corresponds to the implementation of the 
 *  selector specified by aSelector for the class specified by aClass, or NULL if the specified 
 *  class or its superclasses do not contain an instance method with the specified selector.
 *
 * @note Note that this function searches superclasses for implementations, 
 *  whereas \c class_copyMethodList does not.
 */
OBJC_EXPORT Method class_getClassMethod(Class cls, SEL name)
    __OSX_AVAILABLE_STARTING(__MAC_10_0, __IPHONE_2_0);

开始实践:

Person类:
Person.h

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface Person : NSObject


@property (nonatomic,assign) NSInteger age;
@property (nonatomic,copy) NSString *name;


+(void)printDZ;
-(void)printDZL;

@end

NS_ASSUME_NONNULL_END

Person.m

#import "Person.h"

@interface Person()

@property(nonatomic,strong) NSString *sex;

@end

@implementation Person



/// class method
+(void)printDZ {
    NSLog(@"this is a class method");
}


/// instacnce method
-(void)printDZL {
    NSLog(@"this is a instance method");
}

@end

测试控制器ViewController:

Person *p1 = [[Person alloc] init];

    
    Method m1 = class_getInstanceMethod([p1 class], @selector(printDZL));
    Method m2 = class_getClassMethod([Person class], @selector(printDZ));
    
    NSLog(@"测试前");
    
    [p1 printDZL];
    [Person printDZ];
    
    method_exchangeImplementations(m1, m2);
    
    NSLog(@"测试后");
    [p1 printDZL];
    [Person printDZ];

输出:

2019-04-19 08:56:57.538723+0800 TestSwizDemo[1268:31931] 测试前
2019-04-19 08:56:57.538881+0800 TestSwizDemo[1268:31931] this is a instance method
2019-04-19 08:56:57.538958+0800 TestSwizDemo[1268:31931] this is a class method
2019-04-19 08:56:57.539793+0800 TestSwizDemo[1268:31931] 测试后
2019-04-19 08:56:57.539907+0800 TestSwizDemo[1268:31931] this is a class method
2019-04-19 08:56:57.540013+0800 TestSwizDemo[1268:31931] this is a instance method

由输出可见，方法体交换了，说明class_getInstanceMethod成功得到了实例方法，class_getClassMethod成功得到了类方法

但是当用class_getInstanceMethod来取类方法，用class_getClassMethod来取实例方法时：

  
    Person * p1 = [[Person alloc] init];
    
    //    Method m1 = class_getInstanceMethod([p1 class], @selector(printDZL));
    //    Method m2 = class_getClassMethod([Person class], @selector(printDZ));
    Method m1 = class_getInstanceMethod([Person class], @selector(printDZ));
    Method m2 = class_getClassMethod([p1 class], @selector(printDZL));
    NSLog(@"测试前：");
    [p1 printDZL];
    [Person printDZ];
    method_exchangeImplementations(m1, m2);
    NSLog(@"测试后：");
    [p1 printDZL];
    [Person printDZ];

输出:

2019-04-19 09:05:34.968824+0800 TestSwizDemo[1350:37896] 测试前
2019-04-19 09:05:34.968940+0800 TestSwizDemo[1350:37896] this is a instance method
2019-04-19 09:05:34.969006+0800 TestSwizDemo[1350:37896] this is a class method
2019-04-19 09:05:34.969090+0800 TestSwizDemo[1350:37896] 测试后
2019-04-19 09:05:34.969158+0800 TestSwizDemo[1350:37896] this is a instance method
2019-04-19 09:05:34.969216+0800 TestSwizDemo[1350:37896] this is a class method

由输出可见，没有发生交换，可知这样是取不到的。

参考链接:
https://blog.csdn.net/baidu_25743639/article/details/51793764