• Spring--getBean()与@Autowired的对比

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

    简介

    说明

    本文介绍getBean()与@Autowired的对比。

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

    项@AutowiredgetBean()
    是否触发依赖注入

    是。

    如果注入的对象还未注册到容器，则会先注册它。

    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。

    单例与多例

    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。

    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。

    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

    1. package com.example.controller;
    2.  
    3. import com.example.service.UserService;
    4. import org.springframework.beans.factory.annotation.Autowired;
    5. import org.springframework.web.bind.annotation.GetMapping;
    6. import org.springframework.web.bind.annotation.RequestMapping;
    7. import org.springframework.web.bind.annotation.RestController;
    8.  
    9. @RestController
    10. @RequestMapping("/hello")
    11. public class HelloController {
    12.     @Autowired
    13.     private UserService userService;
    14.  
    15.     @GetMapping("/test1")
    16.     public String test1() {
    17.         return userService.toString();
    18.     }
    19.  
    20.     @GetMapping("/test2")
    21.     public String test2() {
    22.         return userService.toString();
    23.     }
    24. }
    1. package com.example.service;
    2.  
    3. import org.springframework.stereotype.Component;
    4.  
    5. @Component
    6. public class UserService {
    7. }

    测试（都是单例的）

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

    将UserService改为多例

    1. package com.example.service;
    2.  
    3. import org.springframework.beans.factory.config.ConfigurableBeanFactory;
    4. import org.springframework.context.annotation.Scope;
    5. import org.springframework.stereotype.Component;
    6.  
    7. @Component
    8. @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
    9. public class UserService {
    10. }

    测试（还是单例的）

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

    ApplicationContext实例

    1. package com.example.controller;
    2.  
    3. import com.example.service.UserService;
    4. import org.springframework.beans.BeansException;
    5. import org.springframework.context.ApplicationContext;
    6. import org.springframework.context.ApplicationContextAware;
    7. import org.springframework.web.bind.annotation.GetMapping;
    8. import org.springframework.web.bind.annotation.RestController;
    9.  
    10. @RestController
    11. public class HelloController implements ApplicationContextAware {
    12.     private ApplicationContext applicationContext;
    13.  
    14.     @GetMapping("/test1")
    15.     public String test1() {
    16.         UserService userService = applicationContext.getBean(UserService.class);
    17.         return userService.toString();
    18.     }
    19.  
    20.     @GetMapping("/test2")
    21.     public String test2() {
    22.         UserService userService = applicationContext.getBean(UserService.class);
    23.         return userService.toString();
    24.     }
    25.  
    26.     @Override
    27.     public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
    28.         this.applicationContext = applicationContext;
    29.     }
    30. }
    1. package com.example.service;
    2.  
    3. import org.springframework.stereotype.Component;
    4.  
    5. @Component
    6. public class UserService {
    7. }

    测试（都是单例的）

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

    1. package com.example.service;
    2.  
    3. import org.springframework.beans.factory.config.ConfigurableBeanFactory;
    4. import org.springframework.context.annotation.Scope;
    5. import org.springframework.stereotype.Component;
    6.  
    7. @Component
    8. @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
    9. public class UserService {
    10. }

    将UserService改为多例

    1. package com.example.service;
    2.  
    3. import org.springframework.beans.factory.config.ConfigurableBeanFactory;
    4. import org.springframework.context.annotation.Scope;
    5. import org.springframework.stereotype.Component;
    6.  
    7. @Component
    8. @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
    9. public class UserService {
    10. }

    测试（是多例了）

    http://localhost:8080/test1    结果：com.example.service.UserService@393b1c
    http://localhost:8080/test2     结果：com.example.service.UserService@23c2e5c2
    再次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

    1. package com.example.controller;
    2.  
    3. import org.springframework.beans.factory.annotation.Autowired;
    4. import org.springframework.context.ApplicationEventPublisher;
    5. import org.springframework.web.bind.annotation.GetMapping;
    6. import org.springframework.web.bind.annotation.RestController;
    7.  
    8. @RestController
    9. public class HelloController {
    10.     @Autowired
    11.     private ApplicationEventPublisher applicationEventPublisher;
    12.  
    13.     @GetMapping("/test1")
    14.     public String test1() {
    15.         System.out.println(applicationEventPublisher);
    16.         return "test1 success";
    17.     }
    18. }

    访问：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

    1. package com.example.controller;
    2.  
    3. import org.springframework.beans.BeansException;
    4. import org.springframework.context.ApplicationContext;
    5. import org.springframework.context.ApplicationContextAware;
    6. import org.springframework.context.ApplicationEventPublisher;
    7. import org.springframework.web.bind.annotation.GetMapping;
    8. import org.springframework.web.bind.annotation.RestController;
    9.  
    10. @RestController
    11. public class HelloController implements ApplicationContextAware {
    12.     private ApplicationEventPublisher applicationEventPublisher;
    13.  
    14.     @GetMapping("/test1")
    15.     public String test1() {
    16.         System.out.println(applicationEventPublisher);
    17.         return "test1 success";
    18.     }
    19.  
    20.     @Override
    21.     public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
    22.         applicationEventPublisher = applicationContext.getBean(ApplicationEventPublisher.class);
    23.     }
    24. }

    启动时就报错（找不到这个bean）

    1. Description:
    2.  
    3. A component required a bean of type 'org.springframework.context.ApplicationEventPublisher' that could not be found.
    4.  
    5.  
    6. Action:
    7.  
    8. 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, 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

    1. for (Map.Entry, Object> classObjectEntry : this.resolvableDependencies.entrySet()) {
    2.     Class autowiringType = classObjectEntry.getKey();
    3.     if (autowiringType.isAssignableFrom(requiredType)) {
    4.         Object autowiringValue = classObjectEntry.getValue();
    5.         autowiringValue = AutowireUtils.resolveAutowiringValue(autowiringValue, requiredType);
    6.         if (requiredType.isInstance(autowiringValue)) {
    7.             result.put(ObjectUtils.identityToString(autowiringValue), autowiringValue);
    8.             break;
    9.         }
    10.     }
    11. }

    编写程序复现

    1. package com.example.tmp;
    2.  
    3. import org.springframework.beans.BeansException;
    4. import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
    5. import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
    6. import org.springframework.stereotype.Component;
    7.  
    8. @Component
    9. public class MyBeanFactoryPostProcessor implements BeanFactoryPostProcessor {
    10.     @Override
    11.     public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
    12.         beanFactory.registerResolvableDependency(MyBean.class, new MyBean());
    13.     }
    14. }
    1. package com.example.tmp;
    2.  
    3. public class MyBean {
    4. }

    使用@Autowired注入

    1. package com.example.controller;
    2.  
    3. import com.example.tmp.MyBean;
    4. import org.springframework.beans.factory.annotation.Autowired;
    5. import org.springframework.web.bind.annotation.GetMapping;
    6. import org.springframework.web.bind.annotation.RestController;
    7.  
    8. @RestController
    9. public class HelloController {
    10.     @Autowired
    11.     private MyBean myBean;
    12.  
    13.     @GetMapping("/test1")
    14.     public String test1() {
    15.         System.out.println(myBean);
    16.         return "test1 success";
    17.     }
    18.  
    19. }

    访问：http://localhost:8080/test1

    后台结果（成功注入）

    com.example.tmp.MyBean@f08fdce

    使用ApplicationContext#getBean() 

    1. package com.example.controller;
    2.  
    3. import com.example.tmp.MyBean;
    4. import org.springframework.beans.BeansException;
    5. import org.springframework.context.ApplicationContext;
    6. import org.springframework.context.ApplicationContextAware;
    7. import org.springframework.web.bind.annotation.GetMapping;
    8. import org.springframework.web.bind.annotation.RestController;
    9.  
    10. @RestController
    11. public class HelloController implements ApplicationContextAware {
    12.     private MyBean myBean;
    13.  
    14.     @GetMapping("/test1")
    15.     public String test1() {
    16.         System.out.println(myBean);
    17.         return "test1 success";
    18.     }
    19.  
    20.     @Override
    21.     public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
    22.         myBean = applicationContext.getBean(MyBean.class);
    23.     }
    24. }

    启动即报错（无法获得bean）

    1. Description:
    2.  
    3. A component required a bean of type 'com.example.tmp.MyBean' that could not be found.
    4.  
    5.  
    6. Action:
    7.  
    8. 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 requiredType) //DefaultListableBeanFactory.java

    下边的程序都是DefaultListableBeanFactory中的

    getBean(Class 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：

    1. package com.example.tmp;
    2.  
    3. import org.springframework.stereotype.Component;
    4.  
    5. @Component
    6. public class MyBean {
    7. }
    1. package com.example.controller;
    2.  
    3. import com.example.tmp.MyBean;
    4. import org.springframework.beans.BeansException;
    5. import org.springframework.context.ApplicationContext;
    6. import org.springframework.context.ApplicationContextAware;
    7. import org.springframework.web.bind.annotation.GetMapping;
    8. import org.springframework.web.bind.annotation.RestController;
    9.  
    10. @RestController
    11. public class HelloController implements ApplicationContextAware {
    12.     private MyBean myBean;
    13.  
    14.     @GetMapping("/test1")
    15.     public String test1() {
    16.         System.out.println(myBean);
    17.         return "test1 success";
    18.     }
    19.  
    20.     @Override
    21.     public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
    22.         myBean = applicationContext.getBean(MyBean.class);
    23.     }
    24. }

    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

    1. ​public String[] getBeanNamesForType(@Nullable Class type, boolean includeNonSingletons, boolean allowEagerInit) {
    2.     if (!isConfigurationFrozen() || type == null || !allowEagerInit) {
    3.         return doGetBeanNamesForType(ResolvableType.forRawClass(type), includeNonSingletons, allowEagerInit);
    4.     }
    5.     // includeNonSingletons为true，会使用this.allBeanNamesByType
    6.     //此时，cache一项都没有
    7.     Map, String[]> cache =
    8.             (includeNonSingletons ? this.allBeanNamesByType : this.singletonBeanNamesByType);
    9.     // type：“class com.example.tmp.MyBean”，此步resolvedBeanNames为null
    10.     String[] resolvedBeanNames = cache.get(type);
    11.     // 此判断为false，不会进入
    12.     if (resolvedBeanNames != null) {
    13.         return resolvedBeanNames;
    14.     }
    15.     //通过type获得bean的名字。结果为：myBean
    16.     resolvedBeanNames = doGetBeanNamesForType(ResolvableType.forRawClass(type), includeNonSingletons, true);
    17.     if (ClassUtils.isCacheSafe(type, getBeanClassLoader())) {
    18.         cache.put(type, resolvedBeanNames);
    19.     }
    20.     return resolvedBeanNames;
    21. }

    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 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 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();

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  • 原文地址：https://blog.csdn.net/feiying0canglang/article/details/126244735