[Java知识库]spring-boot自动配置原理

Spring-boot自动配置原理

• 启动类SpringAutoconfigurationStudyApplication
• spring-boot注解@SpringBootApplication

1.@SpringBootApplication

进入@SpringBootApplication注解内部

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(
    excludeFilters = {@Filter(
    type = FilterType.CUSTOM,
    classes = {TypeExcludeFilter.class}
), @Filter(
    type = FilterType.CUSTOM,
    classes = {AutoConfigurationExcludeFilter.class}
)}
)
public @interface SpringBootApplication {
}

@SpringBootConfiguration表名这是一个spring boot 应用，可以被@Configuration替换

/**
 * Indicates that a class provides Spring Boot application
 * {@link Configuration @Configuration}. Can be used as an alternative to the Spring's
 * standard {@code @Configuration} annotation so that configuration can be found
 * automatically (for example in tests).
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
@Indexed
public @interface SpringBootConfiguration {
}

2.@EnableAutoConfiguration

其中@EnableAutoConfiguration为自动配置的核心内容

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

@AutoConfigurationPackage注入启动类所在的包或者自定义的包到容器中

/**
 * Registers packages with {@link AutoConfigurationPackages}. When no {@link #basePackages
 * base packages} or {@link #basePackageClasses base package classes} are specified, the
 * package of the annotated class is registered.
 *
 * @author Phillip Webb
 * @since 1.3.0
 * @see AutoConfigurationPackages
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {
  	String[] basePackages() default {};
  	Class<?>[] basePackageClasses() default {};
}

进入Registrar.class可知，注册了当前启动类所在包到容器中

2.1@AutoConfigurationImportSelector

@Import(AutoConfigurationImportSelector.class)将配置类导入容器当中

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

进入AutoConfigurationImportSelector.class中，

	protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
		if (!isEnabled(annotationMetadata)) {
			return EMPTY_ENTRY;
		}
		AnnotationAttributes attributes = getAttributes(annotationMetadata);
    
		List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
    
		configurations = removeDuplicates(configurations);
		Set<String> exclusions = getExclusions(annotationMetadata, attributes);
		checkExcludedClasses(configurations, exclusions);
		configurations.removeAll(exclusions);
		configurations = getConfigurationClassFilter().filter(configurations);
		fireAutoConfigurationImportEvents(configurations, exclusions);
		return new AutoConfigurationEntry(configurations, exclusions);
	}

其中getCandidateConfigurations方法用于获取配置类名称，点进去查看

		List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);


	/**
	 * Return the auto-configuration class names that should be considered. By default
	 * this method will load candidates using {@link SpringFactoriesLoader} with
	 * {@link #getSpringFactoriesLoaderFactoryClass()}.
	 * @param metadata the source metadata
	 * @param attributes the {@link #getAttributes(AnnotationMetadata) annotation
	 * attributes}
	 * @return a list of candidate configurations
	 */
	protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
    //加载所有的META-INF/spring.factories下键值对的value
		List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
				getBeanClassLoader());
		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;
	}

	public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
		ClassLoader classLoaderToUse = classLoader;
		if (classLoaderToUse == null) {
			classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
		}
		String factoryTypeName = factoryType.getName();
    //加载配置类名
		return loadSpringFactories(classLoaderToUse).getOrDefault(factoryTypeName, Collections.emptyList());
	}

	private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader) {
		Map<String, List<String>> result = cache.get(classLoader);
		if (result != null) {
			return result;
		}

		result = new HashMap<>();
		try {
      //FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
      //类加载器读取spring.factories下的内容
			Enumeration<URL> urls = classLoader.getResources(FACTORIES_RESOURCE_LOCATION);
			while (urls.hasMoreElements()) {
				URL url = urls.nextElement();
				UrlResource resource = new UrlResource(url);
				Properties properties = PropertiesLoaderUtils.loadProperties(resource);
				for (Map.Entry<?, ?> entry : properties.entrySet()) {
					String factoryTypeName = ((String) entry.getKey()).trim();
					String[] factoryImplementationNames =
							StringUtils.commaDelimitedListToStringArray((String) entry.getValue());
					for (String factoryImplementationName : factoryImplementationNames) {
            //将解析出来的名字加入result的value当中
						result.computeIfAbsent(factoryTypeName, key -> new ArrayList<>())
								.add(factoryImplementationName.trim());
					}
				}
			}
		//······省略部分代码······

		return result;
	}

从下图可以看出我们自定义的starter下的META-INF/spring.factories已经被扫描到

自定义的配置类被加入result中

修改了包名的样子

3.自定义starter

3.1.项目包结构

--src
	-main
		-java
			-org
				-test
					-CountProperties
					-CountServer
					-CountServerAutoConfiguration
		-resources
			-META-INF
				-spring.factories

maven pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<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>org.test</groupId>
    <artifactId>demo-autoconfiguration</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-autoconfigure</artifactId>
            <version>2.6.4</version>
        </dependency>

    </dependencies>
</project>

CountProperties

@ConfigurationProperties(prefix = "count")
public class CountProperties {
    private Integer number;

    private String type = "订单项";

    public Integer getNumber() {
        return number;
    }

    public void setNumber(Integer number) {
        this.number = number;
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }
}

CountServer

public class CountServer {

    @Autowired
    private CountProperties countProperties;

    public Integer getItemNum(){
        return countProperties.getNumber();
    }

}

CountServerAutoConfiguration

/**
 * 下方一个条件不满足则不会配置
 * @date 2022/3/29
 */
@Configuration  //标记为配置类
@ConditionalOnClass(CountServer.class)  //当该类存在时才会进行配置
@EnableConfigurationProperties(CountProperties.class)   //开启属性类配置
@ConditionalOnProperty(prefix = "count.server",name = "enable",havingValue = "true",matchIfMissing = true)
public class CountServerAutoConfiguration {

    @Bean
    @ConditionalOnMissingBean(CountServer.class)    //CountServer这个bean对象在容器中不存在才会注入
    public CountServer countServer(){
        return new CountServer();
    }

}

spring.factories

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.test.CountServerAutoConfiguration

3.2加入本地仓库

 mvn clean install

3.3其他spring项目

创建一个项目，依赖之前的项目

        <dependency>
            <groupId>org.test</groupId>
            <artifactId>demo-autoconfiguration</artifactId>
            <version>1.0-SNAPSHOT</version>
        </dependency>

在项目中引入代码

@RestController
public class CountWeb {

    @Resource
    private CountServer countServer;

    @GetMapping("/item")
    public Integer getCount(){
        return countServer.getItemNum();
    }
}

在application.properties添加如下内容

count.number=10
#count.server.enable=false

启动成功，说明bean注入容器成功

浏览器访问http://localhost:8080/item

返回10，说明配置成功。

综上，自动配置主要靠@EnableAutoConfiguration然后解析出spring.factories下配置的配置类，spring在幕后将其注入factory当中。

参考：
1.https://blog.csdn.net/zjcjava/article/details/84028222