Spring--getBean()与@Autowired的对比

短命女 2023-09-29 16:55 78阅读 0赞

原文网址：[Spring--getBean()与@Autowired的对比\_IT利刃出鞘的博客-CSDN博客][Spring--getBean_Autowired_IT_-CSDN]

## 简介 ##

**说明**

本文介绍getBean()与@Autowired的对比。

**ApplicationContext\#getBean()与@Autowired的简要对比**

<table> 
 <tbody> 
 <tr> 
 <td>项</td> 
 <td>@Autowired</td> 
 <td>getBean()</td> 
 </tr> 
 <tr> 
 <td>是否触发依赖注入</td> 
 <td> 是。 如果注入的对象还未注册到容器，则会先注册它。 </td> 
 <td> 否。 如果注入的对象还未注册到容器，不会去注册它，只会获得一个null。 </td> 
 </tr> 
 <tr> 
 <td>单例与多例</td> 
 <td> 默认是单例。 就算是将bean加注解改为多例，此时注入仍为单例。 </td> 
 <td> 默认是单例。 但可将bean加注解改为多例，此时getBean()获取即为多例。 </td> 
 </tr> 
 </tbody> 
</table>

## 单例与多例 ##

注意：这里可能是因为@Controller注入比较特殊，只有这时将bean加注解改为多例，此时注入仍为单例。不过还需要验证。

### **@Autowired实例** ###

package com.example.controller;
    
    import com.example.service.UserService;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.web.bind.annotation.GetMapping;
    import org.springframework.web.bind.annotation.RequestMapping;
    import org.springframework.web.bind.annotation.RestController;
    
    @RestController
    @RequestMapping("/hello")
    public class HelloController {
        @Autowired
        private UserService userService;
    
        @GetMapping("/test1")
        public String test1() {
            return userService.toString();
        }
    
        @GetMapping("/test2")
        public String test2() {
            return userService.toString();
        }
    }

package com.example.service;
    
    import org.springframework.stereotype.Component;
    
    @Component
    public class UserService {
    }

**测试**（都是单例的）

[http://localhost:8080/test1][http_localhost_8080_test1] 结果：com.example.service.UserService@27a70937  
[http://localhost:8080/test2][http_localhost_8080_test2] 结果：com.example.service.UserService@27a70937  
再次[http://localhost:8080/test2][http_localhost_8080_test2] 结果：com.example.service.UserService@27a70937

**将UserService改为多例**

package com.example.service;
    
    import org.springframework.beans.factory.config.ConfigurableBeanFactory;
    import org.springframework.context.annotation.Scope;
    import org.springframework.stereotype.Component;
    
    @Component
    @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
    public class UserService {
    }

**测试**（还是单例的）

[http://localhost:8080/test1][http_localhost_8080_test1] 结果：com.example.service.UserService@12d4c297  
[http://localhost:8080/test2][http_localhost_8080_test2] 结果：com.example.service.UserService@12d4c297  
再次[http://localhost:8080/test2][http_localhost_8080_test2] 结果：com.example.service.UserService@12d4c297

### **ApplicationContext实例** ###

package com.example.controller;
    
    import com.example.service.UserService;
    import org.springframework.beans.BeansException;
    import org.springframework.context.ApplicationContext;
    import org.springframework.context.ApplicationContextAware;
    import org.springframework.web.bind.annotation.GetMapping;
    import org.springframework.web.bind.annotation.RestController;
    
    @RestController
    public class HelloController implements ApplicationContextAware {
        private ApplicationContext applicationContext;
    
        @GetMapping("/test1")
        public String test1() {
            UserService userService = applicationContext.getBean(UserService.class);
            return userService.toString();
        }
    
        @GetMapping("/test2")
        public String test2() {
            UserService userService = applicationContext.getBean(UserService.class);
            return userService.toString();
        }
    
        @Override
        public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
            this.applicationContext = applicationContext;
        }
    }

package com.example.service;
    
    import org.springframework.stereotype.Component;
    
    @Component
    public class UserService {
    }

**测试**（都是单例的）

**将UserService改为多例**

**测试（是多例了）**

[http://localhost:8080/test1][http_localhost_8080_test1] 结果：com.example.service.UserService@393b1c  
[http://localhost:8080/test2][http_localhost_8080_test2] 结果：com.example.service.UserService@23c2e5c2  
再次[http://localhost:8080/test2][http_localhost_8080_test2] 结果：com.example.service.UserService@5e743c55

## 触发依赖注入 ##

**结论**

@Autowired可以触发依赖注入，ApplicationContext.getBean()不能触发依赖注入。

也就是说：如果要获取的那个Bean还不存在，@Autowired方式可以去转而实例化那个对象，从而能获取Bean；ApplicationContext.getBean()不能转而去实例化对象，只能获得null。

### **问题复现** ###

目标：在Controller中注入ApplicationEventPublisher（Spring的事件发布器）。

本处使用@Autowired是可以获取的，但使用ApplicationContext.getBean()就不可以。

**@Autowired注入的方式**

package com.example.controller;
    
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.context.ApplicationEventPublisher;
    import org.springframework.web.bind.annotation.GetMapping;
    import org.springframework.web.bind.annotation.RestController;
    
    @RestController
    public class HelloController {
        @Autowired
        private ApplicationEventPublisher applicationEventPublisher;
    
        @GetMapping("/test1")
        public String test1() {
            System.out.println(applicationEventPublisher);
            return "test1 success";
        }
    }

访问：[http://localhost:8080/test1][http_localhost_8080_test1]

结果（成功注入）

org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext@5bf0fe62, started on Thu Mar 18 19:10:33 CST 2021

**ApplicationContext.getBean()方式**

package com.example.controller;
    
    import org.springframework.beans.BeansException;
    import org.springframework.context.ApplicationContext;
    import org.springframework.context.ApplicationContextAware;
    import org.springframework.context.ApplicationEventPublisher;
    import org.springframework.web.bind.annotation.GetMapping;
    import org.springframework.web.bind.annotation.RestController;
    
    @RestController
    public class HelloController implements ApplicationContextAware {
        private ApplicationEventPublisher applicationEventPublisher;
    
        @GetMapping("/test1")
        public String test1() {
            System.out.println(applicationEventPublisher);
            return "test1 success";
        }
    
        @Override
        public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
            applicationEventPublisher = applicationContext.getBean(ApplicationEventPublisher.class);
        }
    }

启动时就报错（找不到这个bean）

Description:
    
    A component required a bean of type 'org.springframework.context.ApplicationEventPublisher' that could not be found.
    
    
    Action:
    
    Consider defining a bean of type 'org.springframework.context.ApplicationEventPublisher' in your configuration.

### 原因初探 ###

refresh()// AbstractApplicationContext 
prepareBeanFactory //AbstractApplicationContext 
beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory); 
beanFactory.registerResolvableDependency(ResourceLoader.class, this); 
beanFactory.registerResolvableDependency(**ApplicationEventPublisher**.class, this); 
beanFactory.registerResolvableDependency(ApplicationContext.class, this); 
// DefaultListableBeanFactory类（实现了ConfigurableListableBeanFactory接口） 
registerResolvableDependency(Class<?> dependencyType, @Nullable Object autowiredValue) 
resolvableDependencies.put(dependencyType, autowiredValue); //DefaultListableBeanFactory类 
 
private final Map<Class<?>, Object> resolvableDependencies = new ConcurrentHashMap<>(16);

一些特殊实例对象是存放在DefaultListableBeanFactory\#resolvableDependencies变量中的，在容器启动时，如果发现需要注入这些特定的实例对象，就直接在该变量中获取。通过BeanFactory\#getBean方法不会去DefaultListableBeanFactory\#resolvableDependencies取，所以取不到它。

处理注入的代码：findAutowireCandidates //DefaultListableBeanFactory.java

for (Map.Entry<Class<?>, Object> classObjectEntry : this.resolvableDependencies.entrySet()) {
 Class<?> autowiringType = classObjectEntry.getKey();
 if (autowiringType.isAssignableFrom(requiredType)) {
 Object autowiringValue = classObjectEntry.getValue();
 autowiringValue = AutowireUtils.resolveAutowiringValue(autowiringValue, requiredType);
 if (requiredType.isInstance(autowiringValue)) {
 result.put(ObjectUtils.identityToString(autowiringValue), autowiringValue);
 break;
 }
 }
 }

**编写程序复现**

package com.example.tmp;
    
    import org.springframework.beans.BeansException;
    import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
    import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
    import org.springframework.stereotype.Component;
    
    @Component
    public class MyBeanFactoryPostProcessor implements BeanFactoryPostProcessor {
        @Override
        public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
            beanFactory.registerResolvableDependency(MyBean.class, new MyBean());
        }
    }

package com.example.tmp;
    
    public class MyBean {
    }

**使用@Autowired注入**

package com.example.controller;
    
    import com.example.tmp.MyBean;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.web.bind.annotation.GetMapping;
    import org.springframework.web.bind.annotation.RestController;
    
    @RestController
    public class HelloController {
        @Autowired
        private MyBean myBean;
    
        @GetMapping("/test1")
        public String test1() {
            System.out.println(myBean);
            return "test1 success";
        }
    
    }

访问：[http://localhost:8080/test1][http_localhost_8080_test1]

后台结果（成功注入）

com.example.tmp.MyBean@f08fdce

**使用ApplicationContext\#getBean()**

package com.example.controller;
    
    import com.example.tmp.MyBean;
    import org.springframework.beans.BeansException;
    import org.springframework.context.ApplicationContext;
    import org.springframework.context.ApplicationContextAware;
    import org.springframework.web.bind.annotation.GetMapping;
    import org.springframework.web.bind.annotation.RestController;
    
    @RestController
    public class HelloController implements ApplicationContextAware {
        private MyBean myBean;
    
        @GetMapping("/test1")
        public String test1() {
            System.out.println(myBean);
            return "test1 success";
        }
    
        @Override
        public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
            myBean = applicationContext.getBean(MyBean.class);
        }
    }

启动即报错（无法获得bean）

Description:
    
    A component required a bean of type 'com.example.tmp.MyBean' that could not be found.
    
    
    Action:
    
    Consider defining a bean of type 'com.example.tmp.MyBean' in your configuration.

### 源码追踪 ###

**追踪1：追踪“编写程序复现”的getBean**

getBean(MyBean.class) 
getBeanFactory().getBean(requiredType) //AbstractApplicationContext.class 
getBean(Class<T> requiredType) //DefaultListableBeanFactory.java

下边的程序都是DefaultListableBeanFactory中的

getBean(Class<T> requiredType) 
getBean(requiredType, (Object\[\]) null) 
resolveBean(ResolvableType.forRawClass(requiredType), args, false) 
resolveNamedBean(requiredType, args, nonUniqueAsNull) 
getBeanNamesForType(requiredType); 
getBeanNamesForType(type, true, true) 
getBeanNamesForType(resolved, includeNonSingletons, allowEagerInit) 
resolvedBeanNames = doGetBeanNamesForType(ResolvableType.forRawClass(type), includeNonSingletons, true);

**追踪2：正常的bean通过ApplicationContext.getBean()获取**

把上边的程序稍微改造：

package com.example.tmp;
    
    import org.springframework.stereotype.Component;
    
    @Component
    public class MyBean {
    }

追踪它，跟上边一样的，也是追踪到了DefaultListableBeanFactory\#getBeanNamesForType(resolved, includeNonSingletons, allowEagerInit)，但这是数据有所变化

public String[] getBeanNamesForType(@Nullable Class<?> type, boolean includeNonSingletons, boolean allowEagerInit) {
 if (!isConfigurationFrozen() || type == null || !allowEagerInit) {
 return doGetBeanNamesForType(ResolvableType.forRawClass(type), includeNonSingletons, allowEagerInit);
 }
 // includeNonSingletons为true，会使用this.allBeanNamesByType
 //此时，cache一项都没有
 Map<Class<?>, String[]> cache =
 (includeNonSingletons ? this.allBeanNamesByType : this.singletonBeanNamesByType);
 // type：“class com.example.tmp.MyBean”，此步resolvedBeanNames为null
 String[] resolvedBeanNames = cache.get(type);
 // 此判断为false，不会进入
 if (resolvedBeanNames != null) {
 return resolvedBeanNames;
 }
 //通过type获得bean的名字。结果为：myBean
 resolvedBeanNames = doGetBeanNamesForType(ResolvableType.forRawClass(type), includeNonSingletons, true);
 if (ClassUtils.isCacheSafe(type, getBeanClassLoader())) {
 cache.put(type, resolvedBeanNames);
 }
 return resolvedBeanNames;
 }

与“**追踪1：追踪“编写程序复现”的getBean**”的区别在于：doGetBeanNamesForType获取到了bean的名字（也就是说，找到了bean）

**总体流程**

getBean(Class<T> requiredType) 
getBean(requiredType, (Object\[\]) null) 
resolveBean(ResolvableType.forRawClass(requiredType), args, false) 
// namedBean有两个属性：beanName(String类型)，beanInstance(泛型，实例对象) 
NamedBeanHolder<T> namedBean = resolveNamedBean(requiredType, args, nonUniqueAsNull) 
String\[\] candidateNames = getBeanNamesForType(requiredType); 
getBeanNamesForType(type, true, true) 
getBeanNamesForType(resolved, includeNonSingletons, allowEagerInit) 
resolvedBeanNames = **doGetBeanNamesForType**(ResolvableType.forRawClass(type), includeNonSingletons, true); 
// 调用到AbstractBeanFactory\#getBean 
return new NamedBeanHolder<>(beanName, (T) getBean(beanName, requiredType.toClass(), args)); 
// 获得MyBean实例对象 
namedBean.getBeanInstance();

[Spring--getBean_Autowired_IT_-CSDN]: https://knife.blog.csdn.net/article/details/126244735
[http_localhost_8080_test1]: http://localhost:8080/test1
[http_localhost_8080_test2]: http://localhost:8080/test2