深入Spring Boot自动装配

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1、Spring Boot 自动装配原理

依赖 @Enable 模块驱动设计模式,@EnableAutoConfiguration 必然会“@Import”ImportSelector 或 ImportBeanDefinitionRegister 的实现类, 查看源码:

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {......}

其中 AutoConfigurationImportSelector 就是 @EnableAutoConfiguration 所需 ”@Import” 的 DeferredImportSelector 实现类，由于 DeferredImportSelector 作为 ImportSeldector 的子接口，所以组件自动装配逻辑均在 selectImports 方法中.

源码:

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,
		ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
    ......
    @Override
  // 根据 autoConfigurationMetadata 条件执行过滤    
	public String[] selectImports(AnnotationMetadata annotationMetadata) {if (!isEnabled(annotationMetadata)) {return NO_IMPORTS;
		}
    // 加载自动装配的元信息 
		AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
				.loadMetadata(this.beanClassLoader);
		// 获取自动装配的类名集合 
      AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,
				annotationMetadata);
		return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
	}
  }
	protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,
			AnnotationMetadata annotationMetadata) {if (!isEnabled(annotationMetadata)) {return EMPTY_ENTRY;
		}
    // 获取 @EnableAutoConfiguration 标注类的元信息 
		AnnotationAttributes attributes = getAttributes(annotationMetadata);
		// 获取自动装配的候选类名集合 
    List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
    // 移除重复对象 
		configurations = removeDuplicates(configurations);
    // 获取自动装配排除名单 
		Set<String> exclusions = getExclusions(annotationMetadata, attributes);
		checkExcludedClasses(configurations, exclusions);
		configurations.removeAll(exclusions);
    // 根据 autoConfigurationMetadata 条件执行过滤 
		configurations = filter(configurations, autoConfigurationMetadata);
		// 自动装配的导入事件 
    fireAutoConfigurationImportEvents(configurations, exclusions);
		return new AutoConfigurationEntry(configurations, exclusions);
	}

解析:

• AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
  .loadMetadata(this.beanClassLoader); 加载自动装配的元信息
• AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,
  annotationMetadata); 获取自动装配信息
• AnnotationAttributes attributes = getAttributes(annotationMetadata); 获取 @EnableAutoConfiguration 标注类的元信息
• List configurations = getCandidateConfigurations(annotationMetadata, attributes); 获取自动装配的候选类名集合
• configurations = removeDuplicates(configurations); 移除重复对象
• Set exclusions = getExclusions(annotationMetadata, attributes);
  checkExcludedClasses(configurations, exclusions); 获取自动装配黑名单
• configurations = filter(configurations, autoConfigurationMetadata); 根据 autoConfigurationMetadata 条件执行过滤
• fireAutoConfigurationImportEvents(configurations, exclusions); 自动装配的导入事件

1.1、@EnableAutoConfiguration 读取侯选装配组件

源码:

	protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {// 获取自动装配组件 
		List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
		getBeanClassLoader());
    // 如果需要装配的组件为空 
		Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you"
				+ "are using a custom packaging, make sure that file is correct.");
		return configurations;
	}
  // 获取 Spring Factories 加载工厂类 
	protected Class<?> getSpringFactoriesLoaderFactoryClass() {return EnableAutoConfiguration.class;
	}
  // 通过名字讲行加载 
	public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {String factoryTypeName = factoryType.getName();
        return (List)loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
    }
  // 装载 Spring Factories 工厂机制 
	private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {// 从缓存中获取 
        MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
        if (result != null) {return result;
        } else {try {// 搜索指定 Classpath 下所有的 META-INF/spring.factories 资源内容 
                Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
                LinkedMultiValueMap result = new LinkedMultiValueMap();
								// 循环遍历 spring.factories 资源内容 
                while(urls.hasMoreElements()) {URL url = (URL)urls.nextElement();
                    UrlResource resource = new UrlResource(url);
                    // 将资源内容制作为 properties 文件 
                    Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                    Iterator var6 = properties.entrySet().iterator();
										// 合并为一个 key 为接口全类名,value 是实现类全类名列表 
                    while(var6.hasNext()) {Entry<?, ?> entry = (Entry)var6.next();
                        // 接口全类名 
                        String factoryTypeName = ((String)entry.getKey()).trim();
                        String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());
                        int var10 = var9.length;
												
                        for(int var11 = 0; var11 < var10; ++var11) {// 实现类全类名  
                          String factoryImplementationName = var9[var11];
                            result.add(factoryTypeName, factoryImplementationName.trim());
                        }
                    }
                }
								// 放入缓存 
                cache.put(classLoader, result);
                return result;
            } catch (IOException var13) {throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);
            }
        }
    }

1.2、@EnableAutoConfiguration 排除自动装配组件

源码:

	// 获取要排除的自动装配组件 
  protected Set<String> getExclusions(AnnotationMetadata metadata, AnnotationAttributes attributes) {// 用于存储排除名单 
		Set<String> excluded = new LinkedHashSet<>();
    
    // 添加排除类,spring.autoconfigure.exclude
		excluded.addAll(asList(attributes, "exclude"));
		// 添加排除类名,spring.autoconfigure.excludeName            excluded.addAll(Arrays.asList(attributes.getStringArray("excludeName")));
    // 添加所有的自动装配排除集合, 用于后面检查类名集合是否合法 
		excluded.addAll(getExcludeAutoConfigurationsProperty());
		return excluded;
	}

1.3、@EnableAutoConfiguration 过滤自动装配组件

源码:

	 // 获取要过滤的自动装配组件 
   private List<String> filter(List<String> configurations, AutoConfigurationMetadata autoConfigurationMetadata) {long startTime = System.nanoTime();
    // 将自动装配类名转化为字符串数组 
		String[] candidates = StringUtils.toStringArray(configurations);
    boolean[] skip = new boolean[candidates.length];
		boolean skipped = false;
		for (AutoConfigurationImportFilter filter : getAutoConfigurationImportFilters()) {invokeAwareMethods(filter);
      // 查检是否匹配 
			boolean[] match = filter.match(candidates, autoConfigurationMetadata);
			for (int i = 0; i < match.length; i++) {// 如果需要过滤 
        if (!match[i]) {skip[i] = true;
					candidates[i] = null;
					skipped = true;
				}
			}
		}
    // 如果不需要过滤, 直接返回 
		if (!skipped) {return configurations;
		}
		List<String> result = new ArrayList<>(candidates.length);
		// 如果需要过滤, 将过滤后的对象放入 result
    for (int i = 0; i < candidates.length; i++) {if (!skip[i]) {result.add(candidates[i]);
			}
		}
		if (logger.isTraceEnabled()) {int numberFiltered = configurations.size() - result.size();
			logger.trace("Filtered" + numberFiltered + "auto configuration class in"
					+ TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startTime) + "ms");
		}
    // 返回过滤后的类名集合 
		return new ArrayList<>(result);
	}
  // 获取由所有 ImportFilters 集合 
	protected List<AutoConfigurationImportFilter> getAutoConfigurationImportFilters() {return SpringFactoriesLoader.loadFactories(AutoConfigurationImportFilter.class, this.beanClassLoader);
	}

2、自定义 Spring Boot Starter

2.1、新添加 Maven 工程

pom.xml 文件如下:

<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
 
    <groupId>com.tyschool</groupId>
    <artifactId>formatter-spring-boot-starter</artifactId>
    <version>1.0-SNAPSHOT</version>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
            <version>2.2.6.RELEASE</version>
            <optional>true</optional>
        </dependency>
    </dependencies>
 
</project>

2.2、新建格式化接口

public interface Formatter {String format(Object obj);
}

2.3、实现接口

public class DefaultFormatter implements Formatter {public String format(Object obj) {return String.valueOf(obj);
    }
}

2.4、实现 DefaultFormatter 自动装配

@Configuration
public class FormatterAutoConfiguration {@Bean
    public Formatter defaultFormatter(){return new DefaultFormatter();}
}

2.5、在 META-INF/spring.factories 资源声明 FormatterAutoConfiguration

# FormatterAutoConfiguration 自动装配声明
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.tyschool.autoconfigure.config.FormatterAutoConfiguration

2.6、构建 Spring Boot Starter

 mvn -Dmaven.test.skip -U clean install

2.7、添加 formatter-spring-boot-starter 依赖

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
            <version>2.2.6.RELEASE</version>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>com.tyschool</groupId>
            <artifactId>formatter-spring-boot-starter</artifactId>
            <version>1.0-SNAPSHOT</version>
        </dependency>
    </dependencies>

2.8、新建引导类

@SpringBootApplication
public class FormatterBootStrap {public static void main(String[] args) {// 创建 Spring 上下文 
        ConfigurableApplicationContext context = new SpringApplicationBuilder(FormatterBootStrap.class)
                .web(WebApplicationType.NONE)
                .run(args);
        Map<String, Object> map = new HashMap<String, Object>();
        map.put("name","tyschool");
        // 获取 bean
        Formatter formatter = context.getBean(Formatter.class);
        // 格式化数据并输出 
        System.out.println(formatter.format(map));
        context.close();}
}