Mastering MethodReplacer in Spring Boot 3: From Basics to Advanced Overrides

1. Introduction

MethodReplacer was an early Spring mechanism for method injection, allowing replacement of a bean's method but required cumbersome configuration and only supported full replacement. With Spring AOP, @Aspect provides a more flexible way to add before, after, and around advice, and is now the standard approach.

2. Practical Example

2.1 Business class

@Service
public class UserService {
    public User query(Long id) {
        return new User(id, "默认用户", 33);
    }
}

2.2 Implement MethodReplacer

@Component
public class ReplaceUserService implements MethodReplacer {
    @Override
    public Object reimplement(Object obj, Method method, Object[] args) throws Throwable {
        return new User((Long) args[0], "替换后的名称", 33);
    }
}

2.3 Custom ReplaceOverride

public class UserReplaceOverride extends ReplaceOverride {
    public UserReplaceOverride(String methodName, String methodReplacerBeanName) {
        super(methodName, methodReplacerBeanName);
    }
}

2.4 Register BeanDefinition Override

@Component
public class ReplaceBeanFactoryPostProcessor implements BeanFactoryPostProcessor {
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        BeanDefinition beanDefinition = beanFactory.getBeanDefinition("userService");
        if (beanDefinition instanceof GenericBeanDefinition beanDef) {
            beanDef.getMethodOverrides().addOverride(
                new UserReplaceOverride("query", "replaceUserService"));
        }
    }
}

2.5 Test Controller

@RestController
@RequestMapping("/users")
public class UserController {
    private final UserService userService;
    public UserController(UserService userService) {
        this.userService = userService;
    }
    @GetMapping("{id}")
    public User query(@PathVariable Long id) {
        return this.userService.query(id);
    }
}

Running the endpoint returns a User object whose name is "替换后的名称", confirming that ReplaceUserService successfully replaced the original method.

2.6 Extension – Conditional Override

By extending ReplaceOverride and overriding isOverloaded(), the replacement can be toggled via a configuration property, e.g., pack.replace.enabled=true.

public class UserReplaceOverride extends ReplaceOverride {
    private Environment environment;
    public UserReplaceOverride(Environment environment, String methodName, String methodReplacerBeanName) {
        super(methodName, methodReplacerBeanName);
        this.environment = environment;
    }
    @Override
    protected boolean isOverloaded() {
        return "true".equals(this.environment.getProperty("pack.replace.enabled"));
    }
}

In practice, using @Aspect is recommended over MethodReplacer for better readability and maintainability.