SpringBoot源码学习系列之自动配置原理简介

SpringBoot源码学习系列之自动配置原理简介

ok，本博客尝试跟一下Springboot的自动配置源码，做一下笔记记录，自动配置是Springboot的一个很关键的特性，也容易被忽略的属性，因为这个属性被包括在@SpringBootApplication注解里，所以不去跟一下源码都不知道还有这个属性，ps：本博客源码基于SpringBoot1.5.7版本

@SpringBootApplication

ok，跟一下@SpringBootApplication，发现@SpringBootApplication其实是一个复合的注解，由很多注解构成，@EnableAutoConfiguration其实只是其一部分，@EnableAutoConfiguration就是开启自动配置的注解

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package org.springframework.boot.autoconfigure;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.boot.SpringBootConfiguration;
import org.springframework.boot.context.TypeExcludeFilter;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.FilterType;
import org.springframework.context.annotation.ComponentScan.Filter;
import org.springframework.core.annotation.AliasFor;

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration 
@EnableAutoConfiguration //自动配置注解
@ComponentScan(
    excludeFilters = {@Filter(
    type = FilterType.CUSTOM,
    classes = {TypeExcludeFilter.class}
), @Filter(
    type = FilterType.CUSTOM,
    classes = {AutoConfigurationExcludeFilter.class}
)}
)
public @interface SpringBootApplication {
	....
    Class<?>[] scanBasePackageClasses() default {};
}

@EnableAutoConfiguration

点进@EnableAutoConfiguration，比较重要的列出来：

• @AutoConfigurationPackage
• @Import({EnableAutoConfigurationImportSelector.class})

package org.springframework.boot.autoconfigure;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.context.annotation.Import;

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({EnableAutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    Class<?>[] exclude() default {};

    String[] excludeName() default {};
}

@AutoConfigurationPackage

先看@AutoConfigurationPackage源码，这个注解是开启自动配置包的，关注点在@Import({Registrar.class})，核心在Registrar类

备注：@import注解是Spring的底层注解，作用是导入一个组件到容器里

package org.springframework.boot.autoconfigure;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.boot.autoconfigure.AutoConfigurationPackages.Registrar;
import org.springframework.context.annotation.Import;

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

Registrar 类是AutoConfigurationPackages类的静态内部类，

 static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {
        Registrar() {
        }

        public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
            AutoConfigurationPackages.register(registry, (new AutoConfigurationPackages.PackageImport(metadata)).getPackageName());
        }

        public Set<Object> determineImports(AnnotationMetadata metadata) {
            return Collections.singleton(new AutoConfigurationPackages.PackageImport(metadata));
        }
    }

看一下(new AutoConfigurationPackages.PackageImport(metadata)).getPackageName()获取的是什么，用idea的工具，计算表达式，可以看到其实获取的是SpringBoot启动类上面的包名

AnnotationMetadata：SpringBoot注解元数据

所以，@AutoConfigurationPackage开启后，就可以将主配置类(@SpringBootApplication)所在包及其子包里面的所有组件都扫描到Spring容器里

public static void register(BeanDefinitionRegistry registry, String... packageNames) {
        if (registry.containsBeanDefinition(BEAN)) {
            BeanDefinition beanDefinition = registry.getBeanDefinition(BEAN);
            ConstructorArgumentValues constructorArguments = beanDefinition.getConstructorArgumentValues();
            constructorArguments.addIndexedArgumentValue(0, addBasePackages(constructorArguments, packageNames));
        } else {// Spring容器里没有找到对应组件
        	/* 将组件注册到Spring容器里 */
          	GenericBeanDefinition beanDefinition = new GenericBeanDefinition();                  beanDefinition.setBeanClass(AutoConfigurationPackages.BasePackages.class);
beanDefinition.getConstructorArgumentValues().addIndexedArgumentValue(0, packageNames);
            beanDefinition.setRole(2);
            registry.registerBeanDefinition(BEAN, beanDefinition);
        }

    }

ok，跟了源码，当然只是简单跟一下，没有特别细的跟，这个过程可以看出这里的组件扫描只是扫描主配置类(@SpringBootApplication)所在包及其子包里面的所有组件，所以，写了例子验证一下：
在Application类包外写个Controller测试类

@RestController
public class TestController {

    @GetMapping("/hello")
    public String hello(){
        return "hello world!";
    }
}

启动项目，进行访问，发现都是404找不到这个接口，再将这个Controller放在包里面，才可以扫描到

@Import({EnableAutoConfigurationImportSelector.class})

package org.springframework.boot.autoconfigure;

import org.springframework.core.type.AnnotationMetadata;

/** @deprecated */
@Deprecated
public class EnableAutoConfigurationImportSelector extends AutoConfigurationImportSelector {
    public EnableAutoConfigurationImportSelector() {
    }

    protected boolean isEnabled(AnnotationMetadata metadata) {
        return this.getClass().equals(EnableAutoConfigurationImportSelector.class) ? (Boolean)this.getEnvironment().getProperty("spring.boot.enableautoconfiguration", Boolean.class, true) : true;
    }
}

主要看一下其基类AutoConfigurationImportSelector代码，看一下selectImport方法：

/**装载很多自动配置类，以全类名的方式返回一个字符数组**/
public String[] selectImports(AnnotationMetadata annotationMetadata) {
        if (!this.isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        } else {
            try {
            	//获取自动配置的元数据
                AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);
                //装载配置属性
                AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
                //通过类加载器读取所有的配置类全类名
                List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);
                configurations = this.removeDuplicates(configurations);
                configurations = this.sort(configurations, autoConfigurationMetadata);
                Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);
                this.checkExcludedClasses(configurations, exclusions);
                configurations.removeAll(exclusions);
                configurations = this.filter(configurations, autoConfigurationMetadata);
                this.fireAutoConfigurationImportEvents(configurations, exclusions);
                return (String[])configurations.toArray(new String[configurations.size()]);
            } catch (IOException var6) {
                throw new IllegalStateException(var6);
            }
        }
    }

• AutoConfigurationMetadata信息，获取整个JavaEE体系的一些配置类，当然是Springboot集成的，比如有WebMvcAutoConfiguration自动配置类
  

跟一下getCandidateConfigurations方法，SpringFactoriesLoader是Spring-code工程的工厂加载类，使用SpringFactoriesLoader，需要在模块的META-INF/spring.factories文件自己配置属性，这个Properties格式的文件中的key是接口、注解、或抽象类的全名，value是以逗号 “ , “ 分隔的实现类，使用SpringFactoriesLoader可以实现将相应的实现类注入Spirng容器

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
        List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.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;
    }

loadFactoryNames方法代码，

public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
	/* 将spring.factories的类都装载到Spring容器*/
     public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
        String factoryClassName = factoryClass.getName();

        try {
        //将META-INF/spring.factories文件里配置的属性都装载到Enumeration数据结构里
            Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
            ArrayList result = new ArrayList();
			//遍历获取属性，然后再获取对应的配置类全类名
            while(urls.hasMoreElements()) {
                URL url = (URL)urls.nextElement();
                Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
                String factoryClassNames = properties.getProperty(factoryClassName);
                result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
            }

            return result;
        } catch (IOException var8) {
            throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() + "] factories from location [" + "META-INF/spring.factories" + "]", var8);
        }
    }

然后META-INF/spring.factories文件放在哪？很显然是放在Springboot的自动配置模块里，如图：

所以，@Import({EnableAutoConfigurationImportSelector.class})开启之后，主要是EnableAutoConfigurationImportSelector这个类的作用就是在SpringBoot启动时候将从SpringBoot自动配置工程的META-INF/spring.factories文件中获取指定的值，经过SpringFactoriesLoader加载之后将很多自动配置类加载到Spring容器，所以我们不需要配置，mvc等等默认配置就已经随着SpringBoot启动而自动生效

ok，Springboot的自动配置类都在这个包里，源码很多，所以本博客只是简单跟一下源码