SpringBoot的核心就是自动配置，自动配置是基于条件判断配置Bean
自动配置的源码在spring-boot-autoconfigure-2.2.13.RELEASE
SpringBoot运行原理
先看@SpringBootApplication
//
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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.Configuration;
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
@ComponentScan(
excludeFilters = {@Filter(
type = FilterType.CUSTOM,
classes = {TypeExcludeFilter.class}
), @Filter(
type = FilterType.CUSTOM,
classes = {AutoConfigurationExcludeFilter.class}
)}
)
public @interface SpringBootApplication {
@AliasFor(
annotation = EnableAutoConfiguration.class
)
Class[] exclude() default {};
@AliasFor(
annotation = EnableAutoConfiguration.class
)
String[] excludeName() default {};
@AliasFor(
annotation = ComponentScan.class,
attribute = "basePackages"
)
String[] scanbasePackages() default {};
@AliasFor(
annotation = ComponentScan.class,
attribute = "basePackageClasses"
)
Class[] scanbasePackageClasses() default {};
@AliasFor(
annotation = Configuration.class
)
boolean proxyBeanMethods() default true;
}
主要关注的几个注解如下
@SpringBootConfiguration：标记当前类为配置类
@EnableAutoConfiguration：开启自动配置
@ComponentScan：扫描主类所在的同级包以及下级包里的Bean
关键是@EnableAutoConfiguration
//
// 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.context.annotation.import;
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@documented
@Inherited
@AutoConfigurationPackage
@import({AutoConfigurationimportSelector.class})
public @interface EnableAutoConfiguration {
String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
Class[] exclude() default {};
String[] excludeName() default {};
}
最关键的要属@import(EnableAutoConfigurationimportSelector.class)，借助EnableAutoConfigurationimportSelector，@EnableAutoConfiguration可以帮助SpringBoot应用将所有符合条件的@Configuration配置都加载到当前SpringBoot创建并使用的IoC容器:通过@import(AutoConfigurationimportSelector.class)导入的配置功能，
AutoConfigurationimportSelector中的方法getCandidateConfigurations，得到待配置的class的类名集合,这个集合就是所有需要进行自动配置的类，而是否配置的关键在于meta-INF/spring.factories文件中是否存在该配置信息
protected ListgetCandidateConfigurations(Annotationmetadata metadata, AnnotationAttributes attributes) { List 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; }
打开，如下图可以看到所有需要配置的类全路径都在文件中，每行一个配置，多个类名逗号分隔,而表示忽略换行
样例讲解
以SpringApplicationAdminJmxAutoConfiguration类来看其主要构成部分
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//
package org.springframework.boot.autoconfigure.admin;
import java.util.Iterator;
import javax.management.MalformedObjectNameException;
import org.springframework.beans.factory.ObjectProvider;
import org.springframework.boot.admin.SpringApplicationAdminMXBeanRegistrar;
import org.springframework.boot.autoconfigure.AutoConfigureAfter;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.env.Environment;
import org.springframework.jmx.export.MBeanExporter;
@Configuration(
proxyBeanMethods = false
)
@AutoConfigureAfter({JmxAutoConfiguration.class})
@ConditionalOnProperty(
prefix = "spring.application.admin",
value = {"enabled"},
havingValue = "true",
matchIfMissing = false
)
public class SpringApplicationAdminJmxAutoConfiguration {
private static final String JMX_NAME_PROPERTY = "spring.application.admin.jmx-name";
private static final String DEFAULT_JMX_NAME = "org.springframework.boot:type=Admin,name=SpringApplication";
public SpringApplicationAdminJmxAutoConfiguration() {
}
@Bean
@ConditionalOnMissingBean
public SpringApplicationAdminMXBeanRegistrar springApplicationAdminRegistrar(ObjectProvider mbeanExporters, Environment environment) throws MalformedObjectNameException {
String jmxName = environment.getProperty("spring.application.admin.jmx-name", "org.springframework.boot:type=Admin,name=SpringApplication");
if (mbeanExporters != null) {
Iterator var4 = mbeanExporters.iterator();
while(var4.hasNext()) {
MBeanExporter mbeanExporter = (MBeanExporter)var4.next();
mbeanExporter.addExcludedBean(jmxName);
}
}
return new SpringApplicationAdminMXBeanRegistrar(jmxName);
}
}
都能看到各种各样的条件判断注解，满足条件时就加载这个Bean并实例化
手动实现自动配置的demo
项目目录
1.XiaoServer文件
package com.xiao.configure;
public class XiaoServer {
private String name;
public String sayServerName(){
return "I'm " + name + "! ";
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
2.XiaoServerProperties文件
package com.xiao.configure;
import org.springframework.boot.context.properties.ConfigurationProperties;
@ConfigurationProperties(prefix = "xiao")
public class XiaoServerProperties {
private static final String NAME = "xiao_server0";
private String name = NAME;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
3.XiaoServiceAutoConfiguration文件
package com.xiao.configure;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
@Configuration
@EnableConfigurationProperties(XiaoServerProperties.class)
@ConditionalOnClass(XiaoServer.class)//判断XiaoServer这个类在类路径中是否存在
@ConditionalOnProperty(prefix = "xiao",value = "enabled",matchIfMissing = true)
public class XiaoServiceAutoConfiguration {
@Autowired
private XiaoServerProperties mistraServiceProperties;
@Bean(name = "xiaoServer")
@ConditionalOnMissingBean(XiaoServer.class)//当容器中没有这个Bean时(XiaoServer)就自动配置这个Bean，Bean的参数来自于XiaoServerProperties
public XiaoServer mistraService(){
XiaoServer mistraService = new XiaoServer();
mistraService.setName(mistraServiceProperties.getName());
return mistraService;
}
}
4.SpringbootAutoconfigureApplication文件
package com.xiao;
import com.xiao.configure.XiaoServer;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import java.util.Date;
@SpringBootApplication
@RestController
public class SpringbootAutoconfigureApplication {
@Autowired
private XiaoServer xiaoService;
public static void main(String[] args) {
SpringApplication.run(SpringbootAutoconfigureApplication.class, args);
}
@RequestMapping("/")
public Object index(){
return "helll demo "+xiaoService.getName()+" "+new Date();
}
}
5.spring.factories
# Auto Configure org.springframework.boot.autoconfigure.EnableAutoConfiguration= com.xiao.configure.XiaoServiceAutoConfiguration
6.application.properties
xiao.name=test service
7.pom.xml
4.0.0 com.xiao springboot-autoconfigure0.0.1-SNAPSHOT springboot-autoconfigure Demo project for Spring Boot 1.8 org.springframework.boot spring-boot-starter2.5.0 org.springframework.boot spring-boot-starter-web2.5.0 org.springframework.boot spring-boot-starter-test2.2.13.RELEASE test org.springframework.boot spring-boot-maven-plugin
运行结果：
总结：
一、什么是springBoot自动配置
SSM项目创建一次，需要写大量的配置文件，而springboot只需要写少量的配置，其他的都交给自动配置，按条件创建bean
二、springboot自动配置是怎么运行的
- 当SpringBoot项目启动时，会先去扫描所有jar包下的meta-INF/spring.factories配置文件
2.通过读取spring.factories配置文件得到所有自动配置类的全限定类名，并将这些自动配置类全部注入到IOC容器
3.那么多类是全部都注入进去？当然不是，在这些配置类中都有条件注解，这些条件注解会去判断是否要导入
4.经过以上步骤，就实现了自动配置的过程，当然详细的实现细节肯定不会这么简单
