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SpringBoot 常用注解的原理和使用 - twilight0402

 1 year ago
source link: https://www.cnblogs.com/twilight0402/p/16846942.html
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@AutoConfiguration#

读取所有jar包下的 /META-INF/spring.factories 并追加到一个 LinkedMultiValueMap 中。每一个url中记录的文件路径如下：

file:/C:/Users/wangchao/apache-maven-3.5.0/repo/com/baomidou/mybatis-plus-boot-starter/3.5.1/mybatis-plus-boot-starter-3.5.1.jar!/META-INF/spring.factories

按照如下路径查看

// 1. @EnableAutoConfiguration
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
    
}

// 2. AutoConfigurationImportSelector.class#selectImports()
public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,
		ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
	@Override
	public String[] selectImports(AnnotationMetadata annotationMetadata) {
		if (!isEnabled(annotationMetadata)) {
			return NO_IMPORTS;
		}
		AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(annotationMetadata);
		return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
	}
}

// 3. AutoConfigurationImportSelector.class#getAutoConfigurationEntry()
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);
}


// 4. AutoConfigurationImportSelector.class#getCandidateConfigurations()
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
	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;
}

// 5. org.springframework.core.io.support.SpringFactoriesLoader#loadFactoryNames()
public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
	String factoryTypeName = factoryType.getName();
	return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
}

最终使用 loadSpringFactories(@Nullable ClassLoader classLoader) 方法读取所有配置文件。

// 6. org.springframework.core.io.support.SpringFactoriesLoader#loadSpringFactories()
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
		MultiValueMap<String, String> result = cache.get(classLoader);
		if (result != null) {
			return result;
		}

		try {
			Enumeration<URL> urls = (classLoader != null ?
					classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
					ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
			result = new LinkedMultiValueMap<>();
			while (urls.hasMoreElements()) {
			    // 读取所有jar包下的 /META-INF/spring.factories
				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();
					for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
						result.add(factoryTypeName, factoryImplementationName.trim());
					}
				}
			}
			cache.put(classLoader, result);
			return result;
		}
		catch (IOException ex) {
			throw new IllegalArgumentException("Unable to load factories from location [" +
					FACTORIES_RESOURCE_LOCATION + "]", ex);
		}
	}

tomcat的自动配置内置于springboot的autoconfiguration中。参考tomcat的自动配置 https://www.cnblogs.com/zhaokejin/p/15626392.html

mybatis-plus的配置没有被springboot包括。因此mybatis-stater中包含一个包mybatis-spring-boot-autoconfigure，这其中配置了需要自动配置的类。

因此我们也可以在自己的项目下新建 /META-INF/spring.factories ，并配置自动配置类。

@Import#

@Import 用于导入配置类或需要前置加载的类。被导入的类会注册为Bean，可直接作为Bean被引用。它的 value 属性可以支持三种类型：

  • 被 @Configuration 修饰的配置类、或普通类（4.2版本之后可以）。
  • ImportSelector 接口的实现。
  • ImportBeanDefinitionRegistrar 接口的实现。

@Import 的配置

@Configuration
@Import(value = {TestA.class, TestB.class, MyImportSelector.class, MyImportBeanDefinitionRegistrar.class})
public class ConfigurationTest {

}

导入一个普通类

package com.example.ssmpdemo.entity;

public class TestA {
    public void fun(){
        System.out.println("testA");
    }
}

导入一个配置类

package com.example.ssmpdemo.entity;

import org.springframework.context.annotation.Configuration;

@Configuration
public class TestB {
    public void fun(){
        System.out.println("testB");
    }
}

通过实现 ImportSelector 接口

import org.springframework.context.annotation.ImportSelector;
import org.springframework.core.type.AnnotationMetadata;

public class MyImportSelector implements ImportSelector {
    @Override
    public String[] selectImports(AnnotationMetadata importingClassMetadata) {
        return new String[]{"com.example.ssmpdemo.entity.TestC"};
    }
}

通过重写 ImportBeanDefinitionRegistrar 的 registerBeanDefinitions 方法。

import com.example.ssmpdemo.entity.TestD;
import org.springframework.beans.factory.support.BeanDefinitionRegistry;
import org.springframework.beans.factory.support.RootBeanDefinition;
import org.springframework.context.annotation.ImportBeanDefinitionRegistrar;
import org.springframework.core.type.AnnotationMetadata;

public class MyImportBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar {

    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
        RootBeanDefinition root = new RootBeanDefinition(TestD.class);
        registry.registerBeanDefinition("testD", root);
    }
}

@ConfigurationProperties#

  • 支持常见的下划线、中划线和驼峰的转换。支持对象引导。比如：user.friend.name 代表的是user对象中的friend对象中的name
  • 需要有set()方法
  • 只添加 @ConfigurationProperties(prefix = "xxx") 并不会生效，需要配合 @Configuration 让容器识别到。
  • @EnableConfigurationProperties(value = ConfigData.class ) 会将value中指定的类注册为Bean，可直接用 @AutoWired 引用。
  1. 定义一个类用来记录所有字段，并使用@ConfigurationProperties(prefix = "xxx")将数据注入到ConfigData中。
package com.example.ssmpdemo.entity;

import lombok.Data;
import org.springframework.boot.context.properties.ConfigurationProperties;

/**
 * 用来记录Configuration的数据
 * @author wangc
 */
@Data
@ConfigurationProperties(value = "spring.datasource.druid")
public class ConfigData {
    private String driverClassName;
    private String url;
    private String username;
    private String password;
}


# 对应的yml文件
spring:
  datasource:
    druid:
      driver-class-name: com.mysql.cj.jdbc.Driver
      url: jdbc:mysql://localhost:5506/ssmpdemo?serverTimezone=UTC
      username: root
      password: xxxx

  1. 使用@EnableConfigurationProperties(JDBCProperties.class) 将 ConfigData 注册为Bean，并提供给ConfigurationTest使用 。可将ConfigData作为参数注入到构造函数和普通函数中。

  2. 可以用以下方式引用被@ConfigurationProperties(value = "spring.datasource.druid")修饰的ConfigData

  • 可以直接把 ConfigData 当成Bean使用
        /**
     * 可直接被注入
     */
    @Autowired
    private ConfigData configData;
  • 可以用构造函数传入进来
    @Data
@Configuration
@EnableConfigurationProperties(value = ConfigData.class )
public class ConfigurationTest {
    private ConfigData configData2;
    /**
     * 作为构造函数的参数注入
     * @param data
     */
    ConfigurationTest(ConfigData data){
        this.configData2 = data;
    }
  • 也可以作为@Bean的方法函数的参数。只有当前类（ConfigurationTest）才可
        /**
     * 直接作为函数的参数
     * @param data
     * @return
     */
    @Bean(name = "configData2")
    HashMap<String, String> getBean(ConfigData data){
        return new HashMap<>(0);
    }
  • 可以省略ConfigData直接将字段注入到返回结果中。
    @Bean
@ConfigurationProperties(value = "spring.datasource.druid")
HashMap<String, String> getBean2(ConfigData data){
    // 会自动为hashMap赋值，或使用set方法为对象赋值
    return new HashMap<>();
}

EnableConfigurationProperties注解的内部如下，它导入了一个实现了 ImportBeanDefinitionRegistrar 接口的类。

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(EnableConfigurationPropertiesRegistrar.class)

class EnableConfigurationPropertiesRegistrar implements ImportBeanDefinitionRegistrar {

	@Override
	public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
		registerInfrastructureBeans(registry);
		ConfigurationPropertiesBeanRegistrar beanRegistrar = new ConfigurationPropertiesBeanRegistrar(registry);
		// 获得@EnableConfigurationProperties的value指向的对象，并注册。
		getTypes(metadata).forEach(beanRegistrar::register);
	}

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