原创 标题:  SpringBoot运行源代码分析

我们知道SpringBoot是基于“约定优于配置”,也知道可以根据starters自动加载和配置相应的服务,那么SpringBoot底层是怎么实现这些操作呢?这篇文章带大家通过源码分析,了解SpringBoot运行原理。

SpringApplication的拆解

通常创建SpringBoot项目之后,默认的启动代码只有一行,通过默认的配置基本上可以完成大多数的功能,但如果需要对启动流程的扩展,就需要对SpringBoot的启动方法进行拆解。其实这个操作在《SpringBoot基础之banner玩法解析》这篇文章中已经实践过。通过扩展来设置banner相关操作。我们还可以通过其他set操作来设置其他扩展。

@SpringBootApplication
public class SpringLearnApplication {

    public static void main(String[] args) {
        SpringApplication app = new SpringApplication(SpringLearnApplication.class);
        app.setXXX(...);// 自定义扩展
        app.run(args);
    }
}

SpringBoot实例化

先来看看SpringBoot实例化时都做了些什么:

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
        this.resourceLoader = resourceLoader;
        Assert.notNull(primarySources, "PrimarySources must not be null");
        this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
        // 1.判断应用的类型
        this.webApplicationType = WebApplicationType.deduceFromClasspath();
        // 2.加载扫描到的Initializer
        setInitializers((Collection) getSpringFactoriesInstances(
                ApplicationContextInitializer.class));
        // 3.设置**ApplicationListener
        setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
        // 4.推断并设置main方法的定义类
        this.mainApplicationClass = deduceMainApplicationClass();
    }

通过代码中的注释,可以了解到SpringApplication初始化可分为四部:

  • 判断应用的类型;
  • 加载扫描到的Initializer;
  • 加载扫描到的Listener;
  • 推断并设置main方法的定义类。

下面针对四个步骤再看一下相应的源代码实现。

判断应用的类型

此功能在枚举类WebApplicationType中实现,根据INDICATOR_CLASSES的类型来判断当前web应用是什么类型的,分别有:REACTIVE(reactive web应用)、SERVLET(基于servlet的web应用)、NONE(不启动web应用)。

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;
    }

加载扫描到的Initializer

使用SpringFactoriesLoader扫描加载classpath下的META-INF/spring.factories文件中所有可用的Initializer。

private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
        MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
        if (result != null) {
            return result;
        } else {
            try {
                Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
                LinkedMultiValueMap result = new LinkedMultiValueMap();

                while(urls.hasMoreElements()) {
                    URL url = (URL)urls.nextElement();
                    UrlResource resource = new UrlResource(url);
                    Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                    Iterator var6 = properties.entrySet().iterator();

                    while(var6.hasNext()) {
                        Entry<?, ?> entry = (Entry)var6.next();
                        String factoryClassName = ((String)entry.getKey()).trim();
                        String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());
                        int var10 = var9.length;

                        for(int var11 = 0; var11 < var10; ++var11) {
                            String factoryName = var9[var11];
                            result.add(factoryClassName, factoryName.trim());
                        }
                    }
                }

                cache.put(classLoader, result);
                return result;
            } catch (IOException var13) {
                throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);
            }
        }
    }

其中spring.factories默认在spring-boot-autoconfigure包下,文件的内容格式为:

# 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,\
# 省略其他的

加载扫描到的ApplicationListener

同样是使用SpringFactoriesLoader扫描加载classpath下的META-INF/spring.factories文件中所有可用的Listener。对应文件类的Listener为:

# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener
# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer

# Auto Configuration Import Listeners
org.springframework.boot.autoconfigure.AutoConfigurationImportListener=\
org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener

推断并设置main方法的定义类

private Class<?> deduceMainApplicationClass() {
        try {
            StackTraceElement[] stackTrace = new RuntimeException().getStackTrace();
            for (StackTraceElement stackTraceElement : stackTrace) {
                if ("main".equals(stackTraceElement.getMethodName())) {
                    return Class.forName(stackTraceElement.getClassName());
                }
            }
        }
        catch (ClassNotFoundException ex) {
            // Swallow and continue
        }
        return null;
    }

run方法的基本操作

当实例化对象之后,会调用run方法。下面通过源代码来了解一下run方法都做了些什么。

public ConfigurableApplicationContext run(String... args) {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        ConfigurableApplicationContext context = null;
        Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
        configureHeadlessProperty();
        // 创建SpringApplicationRunListener
        SpringApplicationRunListeners listeners = getRunListeners(args);
        // 遍历并调用SpringApplicationRunListener的starting方法
        listeners.starting();
        try {
            // 创建参数配置
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(
                    args);
            // 环境参数准备
            ConfigurableEnvironment environment = prepareEnvironment(listeners,
                    applicationArguments);
            configureIgnoreBeanInfo(environment);
            // 打印banner
            Banner printedBanner = printBanner(environment);
            // 创建ApplicationContext
            context = createApplicationContext();
            // 获得SpringFactories实例
            exceptionReporters = getSpringFactoriesInstances(
                    SpringBootExceptionReporter.class,
                    new Class[] { ConfigurableApplicationContext.class }, context);
            // 设置Environment,加载相关配置
            prepareContext(context, environment, listeners, applicationArguments,
                    printedBanner);
            // 刷新ApplicationContext
            refreshContext(context);
            // 刷新context之后调用此方法
            afterRefresh(context, applicationArguments);
            stopWatch.stop();
            if (this.logStartupInfo) {
                new StartupInfoLogger(this.mainApplicationClass)
                        .logStarted(getApplicationLog(), stopWatch);
            }
            // 刷新context,启动application
            listeners.started(context);
            // 启动程序
            callRunners(context, applicationArguments);
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, exceptionReporters, listeners);
            throw new IllegalStateException(ex);
        }

        try {
            // 发出running消息,告知程序已启动
            listeners.running(context);
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, exceptionReporters, null);
            throw new IllegalStateException(ex);
        }
        return context;
    }

如果查看当前自动配置

如果想查看当前启动了哪些自动配置有以下几种方式。

(1)运行jar时增加—debug参数。

java -jar xx.jar --debug

(2)在application.properties进行配置。

debug=true

(3)在IDE运行程序中添加VM参数。

-Ddebug

控制台会输出对应日志。已启动的自动配置日志如下:

Positive matches:
-----------------

   CodecsAutoConfiguration matched:
      - @ConditionalOnClass found required class 'org.springframework.http.codec.CodecConfigurer' (OnClassCondition)

   CodecsAutoConfiguration.JacksonCodecConfiguration matched:
      - @ConditionalOnClass found required class 'com.fasterxml.jackson.databind.ObjectMapper' (OnClassCondition)

未启动的自动配置日志为:

Negative matches:
-----------------

   ActiveMQAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required class 'javax.jms.ConnectionFactory' (OnClassCondition)

   AopAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required class 'org.aspectj.lang.annotation.Aspect' (OnClassCondition)

小结

本篇文章我们简单介绍了springboot启动时都做了些什么,后面我们会逐步讲到整个的运作原理。

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