Spring Boot 原理分析

spring-boot.version：2.4.3.RELEASE

Spring Boot 依赖管理

spring-boot-starter-parent

配置文件管理

<resources>
      <resource>
        <directory>${basedir}/src/main/resources</directory>
        <filtering>true</filtering>
        <includes>
          <include>**/application*.yml</include>
          <include>**/application*.yaml</include>
          <include>**/application*.properties</include>
        </includes>
      </resource>
      <resource>
        <directory>${basedir}/src/main/resources</directory>
        <excludes>
          <exclude>**/application*.yml</exclude>
          <exclude>**/application*.yaml</exclude>
          <exclude>**/application*.properties</exclude>
        </excludes>
      </resource>
    </resources>

spring-boot-dependencies

1.常用技术的版本号管理

<properties>
    <activemq.version>5.15.14</activemq.version> 

2.常用技术的依赖管理

<dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>org.apache.activemq</groupId>
        <artifactId>activemq-amqp</artifactId>
        <version>${activemq.version}</version>
      </dependency>

3.插件管理

<build>
    <pluginManagement>
      <plugins>
        <plugin>
          <groupId>org.codehaus.mojo</groupId>
          <artifactId>build-helper-maven-plugin</artifactId>
          <version>${build-helper-maven-plugin.version}</version>
        </plugin>

spring-boot-starter-web

<dependencies>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter</artifactId>
      <version>2.3.7.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-json</artifactId>
      <version>2.3.7.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-tomcat</artifactId>
      <version>2.3.7.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-web</artifactId>
      <version>5.2.12.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-webmvc</artifactId>
      <version>5.2.12.RELEASE</version>
      <scope>compile</scope>
    </dependency>
  </dependencies>

Spring Boot 自动配置

由@SpringBootApplication配置springboot

源码：

@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

@SpringBootConfiguration

@Configuration
public @interface SpringBootConfiguration {

说明是一个配置类，相当于XML配置文件

@EnableAutoConfiguration

@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

@AutoConfigurationPackage

@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {

static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

        @Override
        public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
            register(registry, new PackageImports(metadata).getPackageNames().toArray(new String[0]));
        }

new PackageImports(metadata).getPackageNames().toArray(new String[0]):获取主程序启动类所在包

AutoConfigurationImportSelector.class中

有个方法getAutoConfigurationEntry（）中

List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);：获取自动配置类

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\

。。。。。。

configurations = getConfigurationClassFilter().filter(configurations);：选出符合当前项目的自动配置类

@ComponentScan

扫描主程序启动类所在包下的组件

Spring Boot 执行流程

SpringApplication.run(Springboot11Application.class, args);

public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
        return run(new Class<?>[] { primarySource }, args);
    }

public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
        return new SpringApplication(primarySources).run(args);
    }

由上面代码看出，程序会创建SpringApplication实例并初始化和调用run方法启动项目

SpringApplication实例并初始化

public SpringApplication(Class<?>... primarySources) {
        this(null, primarySources);
    }

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
        this.resourceLoader = resourceLoader;
        Assert.notNull(primarySources, "PrimarySources must not be null");
        this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
        this.webApplicationType = WebApplicationType.deduceFromClasspath();//判断web应用类型，servlet应用还是reactive应用
        setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));//设置SpringApplication应用的初始化器
        setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));//设置SpringApplication应用的监听器
        this.mainApplicationClass = deduceMainApplicationClass();//推断主程序启动类
    }

static WebApplicationType deduceFromClasspath() {
        if (ClassUtils.isPresent(WEBFLUX_INDICATOR_CLASS, null) && !ClassUtils.isPresent(WEBMVC_INDICATOR_CLASS, null)
                && !ClassUtils.isPresent(JERSEY_INDICATOR_CLASS, null)) {
            return WebApplicationType.REACTIVE;
        }
        for (String className : SERVLET_INDICATOR_CLASSES) {
            if (!ClassUtils.isPresent(className, null)) {
                return WebApplicationType.NONE;
            }
        }
        return WebApplicationType.SERVLET;
    }

run方法

public ConfigurableApplicationContext run(String... args) {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        ConfigurableApplicationContext context = null;
        Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
        configureHeadlessProperty();
        SpringApplicationRunListeners listeners = getRunListeners(args);//获取监听器
        listeners.starting();//启动监听器
        try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
            ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);// Create and configure the environment
            configureIgnoreBeanInfo(environment);//
            Banner printedBanner = printBanner(environment);
            context = createApplicationContext();
            exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
                    new Class[] { ConfigurableApplicationContext.class }, context);
            prepareContext(context, environment, listeners, applicationArguments, printedBanner);
            refreshContext(context);
            afterRefresh(context, applicationArguments);
            stopWatch.stop();
            if (this.logStartupInfo) {
                new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
            }
            listeners.started(context);
            callRunners(context, applicationArguments);//调用自定义的执行器，在启动项目后立即执行一些特定程序
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, exceptionReporters, listeners);
            throw new IllegalStateException(ex);
        }

        try {

/**
     * Called immediately before the run method finishes, when the application context has
     * been refreshed and all {@link CommandLineRunner CommandLineRunners} and
     * {@link ApplicationRunner ApplicationRunners} have been called.
     * @param context the application context.
     * @since 2.0.0
     */
            listeners.running(context);//
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, exceptionReporters, null);
            throw new IllegalStateException(ex);
        }
        return context;
    }