聊聊Spring的bean覆盖（存在同名name/id问题），介绍Spring名称生成策略接口BeanNameGenerator【享学Spring】

public interface SingletonBeanRegistry {
	...
	// ===注意它没有remove方法====
	void registerSingleton(String beanName, Object singletonObject);
	Object getSingleton(String beanName);
	boolean containsSingleton(String beanName);
	int getSingletonCount();
	...
}

public interface BeanDefinitionRegistry extends AliasRegistry {
	...
	void registerBeanDefinition(String beanName, BeanDefinition beanDefinition) throws BeanDefinitionStoreException;
	// 请注意：SingletonBeanRegistry 可没有移除方法~
	void removeBeanDefinition(String beanName) throws NoSuchBeanDefinitionException;
	BeanDefinition getBeanDefinition(String beanName) throws NoSuchBeanDefinitionException;
	boolean containsBeanDefinition(String beanName);
	int getBeanDefinitionCount();
	...
}

// @since 2.0.3  是2.0后出来的
public interface BeanNameGenerator {
	// 入参竟然有两个  definition和bean定义注册器
	String generateBeanName(BeanDefinition definition, BeanDefinitionRegistry registry);
}

// @since 2.0.3
public class DefaultBeanNameGenerator implements BeanNameGenerator {

	@Override
	public String generateBeanName(BeanDefinition definition, BeanDefinitionRegistry registry) {
		// isInnerBean 如果是内部类表示true，这个工具类也能处理
		return BeanDefinitionReaderUtils.generateBeanName(definition, registry);
	}

}

public abstract class BeanDefinitionReaderUtils {

	// unique, "#1", "#2" etc will be appended, until the name becomes 
	public static final String GENERATED_BEAN_NAME_SEPARATOR = BeanFactoryUtils.GENERATED_BEAN_NAME_SEPARATOR;

	// isInnerBean：是为了区分内部bean(innerBean）和顶级bean（top-level bean).
	public static String generateBeanName(BeanDefinition definition, BeanDefinitionRegistry registry, boolean isInnerBean) throws BeanDefinitionStoreException {

		// 拿到Bean定义信息里面的BeanClassName全类名
		// 注意这个不是必须的，因为如果是继承关系，配上父类的依旧行了
		String generatedBeanName = definition.getBeanClassName();
		if (generatedBeanName == null) {
				
			// 若没有配置本类全类名，去拿到父类的全类名+$child"俩表示自己
			if (definition.getParentName() != null) {
				generatedBeanName = definition.getParentName() + "$child";
			}
			// 工厂Bean的  就用方法的名字+"$created"
			else if (definition.getFactoryBeanName() != null) {
				generatedBeanName = definition.getFactoryBeanName() + "$created";
			}
		}
		// 若一个都没找到，抛错~
		if (!StringUtils.hasText(generatedBeanName)) {
			throw new BeanDefinitionStoreException("Unnamed bean definition specifies neither " + "'class' nor 'parent' nor 'factory-bean' - can't generate bean name");
		}

		//isInnerBean=true表示你是内部类的话，名字又增加了如下变化
		String id = generatedBeanName;
		if (isInnerBean) {
			// Inner bean: generate identity hashcode suffix.
			id = generatedBeanName + GENERATED_BEAN_NAME_SEPARATOR + ObjectUtils.getIdentityHexString(definition);
		}
	
		// 如果不是内部类（绝大多数情况下都如此）
		// 此方法注意：一定能够保证到你的BeanName是唯一的~~~~
		else {
			// Top-level bean: use plain class name with unique suffix if necessary.
			// Top-level表示最外层的Bean，也就是说非内部类  这里生成绝对唯一的BeanName~~~~
			return uniqueBeanName(generatedBeanName, registry);
		}
		return id;
	}
	public static String uniqueBeanName(String beanName, BeanDefinitionRegistry registry) {
		String id = beanName;
		int counter = -1;

		// Increase counter until the id is unique.
		while (counter == -1 || registry.containsBeanDefinition(id)) {
			counter++;
			id = beanName + GENERATED_BEAN_NAME_SEPARATOR + counter;
		}
		return id;
	}

}

// @since 11.12.2003
public abstract class AbstractBeanDefinitionReader implements BeanDefinitionReader, EnvironmentCapable {

	private BeanNameGenerator beanNameGenerator = new DefaultBeanNameGenerator();
	public void setBeanNameGenerator(@Nullable BeanNameGenerator beanNameGenerator) {
		this.beanNameGenerator = (beanNameGenerator != null ? beanNameGenerator : new DefaultBeanNameGenerator());
	}	
}

//  @since 2.5 它的出现是伴随着@Component出现
public class AnnotationBeanNameGenerator implements BeanNameGenerator {

	// 支持的最基本的注解（包含其派生注解）
	private static final String COMPONENT_ANNOTATION_CLASSNAME = "org.springframework.stereotype.Component";

	@Override
	public String generateBeanName(BeanDefinition definition, BeanDefinitionRegistry registry) {
		
		// //判断是否是否是AnnotatedBeanDefinition的子类， AnnotatedBeanDefinition是BeanDefinition的一个子类
		// 显然这个生成器只为AnnotatedBeanDefinition它来自动生成名称
		if (definition instanceof AnnotatedBeanDefinition) {
	
			// determineBeanNameFromAnnotation这个方法简而言之，就是看你的注解有没有标注value值，若指定了就以指定的为准
			// 支持的所有注解：上面已经说明了~~~
			// 此处若配置了多个注解且都指定了value值，但发现value值有不同的，就抛出异常了~~~~~
			String beanName = determineBeanNameFromAnnotation((AnnotatedBeanDefinition) definition);
			if (StringUtils.hasText(beanName)) {
				// Explicit bean name found.
				return beanName;
			}
		}
		// Fallback: generate a unique default bean name.
		// 若没指定，此处叫交给生成器来生成吧~~~
		return buildDefaultBeanName(definition, registry);
	}

	// 它的方法是protected 由此可见若我们想自定义生成器的话  可以继承它  然后复写
	protected String buildDefaultBeanName(BeanDefinition definition, BeanDefinitionRegistry registry) {
		return buildDefaultBeanName(definition);
	}
	// 这里是先拿到ClassUtils.getShortName 短名称
	protected String buildDefaultBeanName(BeanDefinition definition) {
		String beanClassName = definition.getBeanClassName();
		Assert.state(beanClassName != null, "No bean class name set");
		String shortClassName = ClassUtils.getShortName(beanClassName);
	
		// 调用java.beans.Introspector的方法  首字母小写
		return Introspector.decapitalize(shortClassName);
	}

}

public class AnnotatedBeanDefinitionReader {
	...
	private BeanNameGenerator beanNameGenerator = new AnnotationBeanNameGenerator();
	...
}

public class ClassPathBeanDefinitionScanner extends ClassPathScanningCandidateComponentProvider {
	private BeanNameGenerator beanNameGenerator = new AnnotationBeanNameGenerator();
}

public class ConfigurationClassPostProcessor implements BeanDefinitionRegistryPostProcessor, PriorityOrdered, ResourceLoaderAware, BeanClassLoaderAware, EnvironmentAware {
	private BeanNameGenerator componentScanBeanNameGenerator = new AnnotationBeanNameGenerator();
}

@Configuration
public class RootConfig {

    @Bean("personBean")
    public Person person() {
        return new Person("personBean", 18);
    }
    @Bean("personBean")
    public Person person2() {
        return new Person("personBean----2222", 18);
    }
}

    public static void main(String[] args) throws Exception {
        ApplicationContext context = new AnnotationConfigApplicationContext(RootConfig.class);

        Object personBean = context.getBean("personBean");
        System.out.println(personBean);

    }

// 结果打印：Person{name='personBean', age=18}

@Configuration
public class RootConfig {
	
    @Bean("personBean")
    public Person person2() {
        return new Person("personBean----2222", 18);
    }
	
	// 注意：此处我使用的类型是Child，但是beanName没变
    @Bean("personBean")
    public Child person() {
        return new Child();
    }

}
// 结果打印：Person{name='personBean----2222', age=18}

@Configuration
public class RootConfig {

    @Bean("personBean")
    public Person person() {
        return new Person("RootConfig----Bean", 18);
    }
}

@Configuration
public class TempConfig {

    @Bean("personBean")
    public Person person() {
        return new Person("TempConfig----Bean", 18);
    }
}

    public static void main(String[] args) throws Exception {
    	// 注意此处传值的顺序是先rootConfig  在tempConfig
        ApplicationContext context = new AnnotationConfigApplicationContext(RootConfig.class, TempConfig.class);

        Object personBean = context.getBean("personBean");
        System.out.println(personBean);

    }

// 结果打印：Person{name='TempConfig----Bean', age=18}

// 结果打印：Person{name='RootConfig----Bean', age=18}

@ComponentScan( ... )
@Configuration
public class RootConfig {

    @Bean("personBean")
    public Person person() {
        return new Person("RootConfig----Bean", 18);
    }
}

@Service("personBean")
public class B implements BInterface { ... }

    public static void main(String[] args) throws Exception {
        ApplicationContext context = new AnnotationConfigApplicationContext(RootConfig.class);

        Object personBean = context.getBean("personBean");
        System.out.println(personBean);
    }

// 打印结果：Person{name='RootConfig----Bean', age=18}

@Configuration
public class RootConfig {
    @Bean
    public Person person() {
        Person person = new Person();
        person.setName("parent application");
        return person;
    }
}

@Configuration
@EnableWebMvc
public class WebMvcConfig extends WebMvcConfigurerAdapter {
    @Bean
    public Person person() {
        Person person = new Person();
        person.setName("child application");
        return person;
    }
}

@Service
public class HelloServiceImpl implements HelloService {

    @Autowired
    private Person person;
    @Override
    public Person getPerson() {
        return person;
    }
}

@RestController
@RequestMapping
public class HelloController {
    @Autowired
    private ApplicationContext applicationContext;
    @Autowired
    private HelloService helloService;
    @Autowired
    private Person person;
    
    @GetMapping("/getPerson")
    public Object getPerson() {
        System.out.println(person);
        System.out.println(helloService.getPerson());

        //从父子容器中获取
        System.out.println(applicationContext.getBean(Person.class));
        System.out.println(applicationContext.getParent().getBean(Person.class));
        return "hello world";
    }
}

Person(name=child application, age=null, child=null)
Person(name=parent application, age=null, child=null)
Person(name=child application, age=null, child=null)
Person(name=parent application, age=null, child=null)

String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);

public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {
	@Override
	public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition) throws BeanDefinitionStoreException {
		...
		// 先拿出旧的（可能有  可能木有）  显然此处只讨论不为null的情况~
		BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
		if (existingDefinition != null) {
			
			// allowBeanDefinitionOverriding属性值默认是true  后面会通过修改此属性值来达到fastfail的效果~
			if (!isAllowBeanDefinitionOverriding()) {
				throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
			}
			// =======到此处，说明Spring是允许同名的Bean覆盖的~========
			// =============下面的elseif  全部是记录日志===============

			// int ROLE_APPLICATION = 0;
			// int ROLE_SUPPORT = 1;
			// int ROLE_INFRASTRUCTURE = 2;
			// 简单一句话  新的Role比老的大  那就输入日志(因为0一般才是我们自定义的)  如果Spring内建的覆盖了我们的，那起步警告一下吗
			else if (existingDefinition.getRole() < beanDefinition.getRole()) {
				if (logger.isInfoEnabled()) {
					logger.info("Overriding user-defined bean definition for bean '" + beanName +
							"' with a framework-generated bean definition: replacing [" +
							existingDefinition + "] with [" + beanDefinition + "]");
				}
			}
			// 这里是debug信息：就是打印一下说“我被覆盖了”
			else if (!beanDefinition.equals(existingDefinition)) {
				if (logger.isDebugEnabled()) {
					logger.debug("Overriding bean definition for bean '" + beanName +
							"' with a different definition: replacing [" + existingDefinition +
							"] with [" + beanDefinition + "]");
				}
			}
			else {
				if (logger.isTraceEnabled()) {
					logger.trace("Overriding bean definition for bean '" + beanName +
							"' with an equivalent definition: replacing [" + existingDefinition +
							"] with [" + beanDefinition + "]");
				}
			}
			// ===============================

			// 最终执行覆盖~~~~~~
			this.beanDefinitionMap.put(beanName, beanDefinition);
		}
		...
	}
}

@Configuration
public class RootConfig {
}

    public static void main(String[] args) throws Exception {
        ApplicationContext context = new AnnotationConfigApplicationContext(RootConfig.class);
        System.out.println(context.getBean("systemProperties"));
    }
// 打印结果：{java.runtime.name=Java(TM) SE Runtime Environment, sun.boot.library.path=E:...

@Configuration
public class RootConfig {

    @Bean("systemProperties")
    public Person person() {
        return new Person("RootConfig----Bean", 18);
    }

}

Person{name='RootConfig----Bean', age=18}

public class MyApplicationContextInitializer implements ApplicationContextInitializer {

    @Override
    public void initialize(ConfigurableApplicationContext applicationContext) {
        if (applicationContext instanceof AbstractRefreshableApplicationContext) {
            ((AbstractRefreshableApplicationContext) applicationContext).setAllowBeanDefinitionOverriding(false);
        } else if (applicationContext instanceof GenericApplicationContext) {
            ((GenericApplicationContext) applicationContext).setAllowBeanDefinitionOverriding(false);
        }
    }
}

public class MyWebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {
    @Override
    protected ApplicationContextInitializer<?>[] getServletApplicationContextInitializers() {
        return new ApplicationContextInitializer[]{new MyApplicationContextInitializer()};
    }

    @Override
    protected ApplicationContextInitializer<?>[] getRootApplicationContextInitializers() {
        return new ApplicationContextInitializer[]{new MyApplicationContextInitializer()};
    }	
}

<context-param>
        <param-name>contextInitializerClasses</param-name>
        <param-value>MyApplicationContextInitializer的全类名</param-value>
 </context-param>

// @since 17.02.2003
public class ContextLoader {
	public static final String CONTEXT_INITIALIZER_CLASSES_PARAM = "contextInitializerClasses";
	public static final String GLOBAL_INITIALIZER_CLASSES_PARAM = "globalInitializerClasses";
}

@Configuration
public class RootConfig {

    @Bean("personBean")
    public Person person() {
        return new Person("RootConfig----Bean", 18);
    }
    @Bean("personBean")
    public Person person2() {
        return new Person("RootConfig----Bean2", 18);
    }

}

org.springframework.beans.factory.support.BeanDefinitionOverrideException: Invalid bean definition with name 'personBean' defined in com.config.RootConfig: Cannot register bean definition [Root bean: class [null]; scope=; abstract=false; lazyInit=false; autowireMode=3; dependencyCheck=0; autowireCandidate=true; primary=false; factoryBeanName=rootConfig; factoryMethodName=person; initMethodName=null; destroyMethodName=(inferred); defined in com.config.RootConfig] for bean 'personBean': There is already [Generic bean: class [com.fsx.dependency.B]; scope=singleton; abstract=false; lazyInit=false; autowireMode=0; dependencyCheck=0; autowireCandidate=true; primary=false; factoryBeanName=null; factoryMethodName=null; initMethodName=null; destroyMethodName=null; defined in file [E:\dev_env\apache-tomcat-9.0.17\webapps\demo_war_war\WEB-INF\classes\com\fsx\dependency\B.class]] bound.
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.registerBeanDefinition(DefaultListableBeanFactory.java:897)
	at org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader.loadBeanDefinitionsForBeanMethod(ConfigurationClassBeanDefinitionReader.java:274)
	at org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader.loadBeanDefinitionsForConfigurationClass(ConfigurationClassBeanDefinitionReader.java:141)