Spring boot启动流程源码解析

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  • 版本:2.0.4
  • 文章中使用/* */注释的方法会做深入分析

正文

    @SpringBootApplication
    public class BootApplication {
        public static void main(String[] args) {
            SpringApplication.run(BootApplication.class, args);
        }
    }

这段代码相信大家都很熟悉,spring boot的启动类,我们就以这段代码作为切入点,来分析spring boot的启动流程:
SpringApplication:

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

    public static ConfigurableApplicationContext run(Class<?>[] primarySources,
            String[] args) {
        /* 构建SpringApplication并运行,创建并且刷新一个新的ApplicationContext */
        return new SpringApplication(primarySources).run(args);
    }

SpringApplication:

    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));
        // 判断是否能够成功加载一些关键的类来确认web应用类型,这个类型后面会用到
        this.webApplicationType = deduceWebApplicationType();
        /* 获取并设置Spring上下文初始化器 */
        setInitializers((Collection) getSpringFactoriesInstances(
                ApplicationContextInitializer.class));
        /* 获取并设置Spring应用监听器 */
        setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
        // 追述到应用主类,也就是main方法所在的类
        this.mainApplicationClass = deduceMainApplicationClass();
    }

这里ApplicationContextInitializer和ApplicationListener我们会在后面用到的时候说明它们的作用。
SpringApplication:

    private <T> Collection<T> getSpringFactoriesInstances(Class<T> type) {
        return getSpringFactoriesInstances(type, new Class<?>[] {});
    }

    private <T> Collection<T> getSpringFactoriesInstances(Class<T> type,
            Class<?>[] parameterTypes, Object... args) {
        ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
        /* 加载工厂名称,Set防止重复 */
        Set<String> names = new LinkedHashSet<>(
                SpringFactoriesLoader.loadFactoryNames(type, classLoader));
        // 创建工厂实例
        List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
                classLoader, args, names);
        // 根据@Order和@Priority进行排序
        AnnotationAwareOrderComparator.sort(instances);
        return instances;
    }

SpringFactoriesLoader:

    public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
        String factoryClassName = factoryClass.getName();
        /* 加载工厂配置,根据传入的factoryClass获取工厂名称集合 */
        return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
    }

SpringFactoriesLoader:

    private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
        MultiValueMap<String, String> result = cache.get(classLoader);
        if (result != null) {
            return result;
        }
        try {
            // 加载资源,路径META-INF/spring.factories
            Enumeration<URL> urls = (classLoader != null ?
                    classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                    ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
            result = new LinkedMultiValueMap<>();
            while (urls.hasMoreElements()) {
                URL url = urls.nextElement();
                UrlResource resource = new UrlResource(url);
                Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                for (Map.Entry<?, ?> entry : properties.entrySet()) {
                    // value用逗号分隔组成集合
                    List<String> factoryClassNames = Arrays.asList(
                            StringUtils.commaDelimitedListToStringArray((String) entry.getValue()));
                    // 添加key和集合的映射
                    result.addAll((String) entry.getKey(), factoryClassNames);
                }
            }
            // 结果缓存
            cache.put(classLoader, result);
            return result;
        }
        catch (IOException ex) {
            throw new IllegalArgumentException("Unable to load factories from location [" +
                    FACTORIES_RESOURCE_LOCATION + "]", ex);
        }
    }

我们来看一下META-INF/spring.factories文件中有什么内容:

    # PropertySource Loaders
    org.springframework.boot.env.PropertySourceLoader=\
    org.springframework.boot.env.PropertiesPropertySourceLoader,\
    org.springframework.boot.env.YamlPropertySourceLoader

    # Run Listeners
    org.springframework.boot.SpringApplicationRunListener=\
    org.springframework.boot.context.event.EventPublishingRunListener

    # Error Reporters
    org.springframework.boot.SpringBootExceptionReporter=\
    org.springframework.boot.diagnostics.FailureAnalyzers

    # Application Context Initializers
    org.springframework.context.ApplicationContextInitializer=\
    org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
    org.springframework.boot.context.ContextIdApplicationContextInitializer,\
    org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
    org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer

    # Application Listeners
    org.springframework.context.ApplicationListener=\
    org.springframework.boot.ClearCachesApplicationListener,\
    org.springframework.boot.builder.ParentContextCloserApplicationListener,\
    org.springframework.boot.context.FileEncodingApplicationListener,\
    org.springframework.boot.context.config.AnsiOutputApplicationListener,\
    org.springframework.boot.context.config.ConfigFileApplicationListener,\
    org.springframework.boot.context.config.DelegatingApplicationListener,\
    org.springframework.boot.context.logging.ClasspathLoggingApplicationListener,\
    org.springframework.boot.context.logging.LoggingApplicationListener,\
    org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener

    # Environment Post Processors
    org.springframework.boot.env.EnvironmentPostProcessor=\
    org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\
    org.springframework.boot.env.SpringApplicationJsonEnvironmentPostProcessor,\
    org.springframework.boot.env.SystemEnvironmentPropertySourceEnvironmentPostProcessor

    # Failure Analyzers
    org.springframework.boot.diagnostics.FailureAnalyzer=\
    org.springframework.boot.diagnostics.analyzer.BeanCurrentlyInCreationFailureAnalyzer,\
    org.springframework.boot.diagnostics.analyzer.BeanNotOfRequiredTypeFailureAnalyzer,\
    org.springframework.boot.diagnostics.analyzer.BindFailureAnalyzer,\
    org.springframework.boot.diagnostics.analyzer.BindValidationFailureAnalyzer,\
    org.springframework.boot.diagnostics.analyzer.UnboundConfigurationPropertyFailureAnalyzer,\
    org.springframework.boot.diagnostics.analyzer.ConnectorStartFailureAnalyzer,\
    org.springframework.boot.diagnostics.analyzer.NoUniqueBeanDefinitionFailureAnalyzer,\
    org.springframework.boot.diagnostics.analyzer.PortInUseFailureAnalyzer,\
    org.springframework.boot.diagnostics.analyzer.ValidationExceptionFailureAnalyzer,\
    org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyNameFailureAnalyzer,\
    org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyValueFailureAnalyzer

    # FailureAnalysisReporters
    org.springframework.boot.diagnostics.FailureAnalysisReporter=\
    org.springframework.boot.diagnostics.LoggingFailureAnalysisReporter

文件中我们看到了ApplicationContextInitializer和ApplicationListener的key value配置,还有一些其他的接口,遇到后我们在进行分析。构建好SpringApplication后,接下来就是运行它的run方法:
SpringApplication:

    public ConfigurableApplicationContext run(String... args) {
        // StopWatch是一个简单的秒表,允许多个任务的计时,暴露每个命名任务的总运行时间和运行时间
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        ConfigurableApplicationContext context = null;
        Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
        configureHeadlessProperty();
        // 获取SpringApplicationRunListener集合,同样是从上面加载的配置中获取
        SpringApplicationRunListeners listeners = getRunListeners(args);
        listeners.starting();
        try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(
                    args);
            /* 准备环境 */
            ConfigurableEnvironment environment = prepareEnvironment(listeners,
                    applicationArguments);
            configureIgnoreBeanInfo(environment);
            // 打印banner,就是我们在控制台看到的那个Spring的logo
            Banner printedBanner = printBanner(environment);
            // 根据不同的webApplicationType返回不同的应用上下文实例
            context = createApplicationContext();
            // 同样从上面加载的配置中获取SpringBootExceptionReporter
            exceptionReporters = getSpringFactoriesInstances(
                    SpringBootExceptionReporter.class,
                    new Class[] { ConfigurableApplicationContext.class }, context);
            /* 准备上下文 */
            prepareContext(context, environment, listeners, applicationArguments,
                    printedBanner);
            /* 刷新上下文 */
            refreshContext(context);
            // 刷新后操作,默认空实现,子类覆盖
            afterRefresh(context, applicationArguments);
            stopWatch.stop();
            if (this.logStartupInfo) {
                new StartupInfoLogger(this.mainApplicationClass)
                        .logStarted(getApplicationLog(), stopWatch);
            }
            listeners.started(context);
            // 调用所有ApplicationRunner和CommandLineRunner的run方法
            callRunners(context, applicationArguments);
        }
        catch (Throwable ex) {
            // 运行失败的异常处理、日志打印和通知
            handleRunFailure(context, ex, exceptionReporters, listeners);
            throw new IllegalStateException(ex);
        }
        try {
            listeners.running(context);
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, exceptionReporters, null);
            throw new IllegalStateException(ex);
        }
        return context;
    }

在这里我们看到很多处SpringApplicationRunListener的相关方法调用,我们来说明一下它的作用,就像它的命名一样,主要是监听SpringApplication的run方法的各个关键步骤,在上面加载的配置中,我们看到了一个实现为EventPublishingRunListener,主要作用是发布应用的启动运行、启动完成等一些关键事件,具体代码有兴趣的同学可以自行查阅。
SpringApplication:

    private ConfigurableEnvironment prepareEnvironment(
            SpringApplicationRunListeners listeners,
            ApplicationArguments applicationArguments) {
        // 创建Environment
        ConfigurableEnvironment environment = getOrCreateEnvironment();
        /* 配置environment */
        configureEnvironment(environment, applicationArguments.getSourceArgs());
        listeners.environmentPrepared(environment);
        // 将environment绑定到SpringApplication
        bindToSpringApplication(environment);
        if (this.webApplicationType == WebApplicationType.NONE) {
            environment = new EnvironmentConverter(getClassLoader())
                    .convertToStandardEnvironmentIfNecessary(environment);
        }
        // 附加一个ConfigurationPropertySource到environment
        ConfigurationPropertySources.attach(environment);
        return environment;
    }

SpringApplication:

    protected void configureEnvironment(ConfigurableEnvironment environment,
            String[] args) {
        // 在此应用程序的环境中添加、删除或重新排序PropertySource
        configurePropertySources(environment, args);
        // 配置此应用程序环境的激活(或默认为激活)配置文件
        // 可以通过spring.profiles.active属性在配置文件处理期间激活其他配置文件
        configureProfiles(environment, args);
    }

SpringApplication:

    private void prepareContext(ConfigurableApplicationContext context,
            ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
            ApplicationArguments applicationArguments, Banner printedBanner) {
        context.setEnvironment(environment);
        // 应用相应的ApplicationContext后置处理,子类可以覆盖,
        // 默认实现为context设置beanNameGenerator和resourceLoader
        postProcessApplicationContext(context);
        // 在context刷新之前应用之前加载的ApplicationContextInitializer
        // 在META-INF/spring.factories中默认配置了4个ApplicationContextInitializer,具体作用可以自行了解一下
        applyInitializers(context);
        listeners.contextPrepared(context);
        // 日志打印
        if (this.logStartupInfo) {
            logStartupInfo(context.getParent() == null);
            logStartupProfileInfo(context);
        }
        // 添加特定的boot单例bean
        context.getBeanFactory().registerSingleton("springApplicationArguments",
                applicationArguments);
        if (printedBanner != null) {
            context.getBeanFactory().registerSingleton("springBootBanner", printedBanner);
        }
        // 获取所有资源,示例中就是获取到我们的启动类
        Set<Object> sources = getAllSources();
        Assert.notEmpty(sources, "Sources must not be empty");
        // 加载资源注册成为Spring的bean
        load(context, sources.toArray(new Object[0]));
        listeners.contextLoaded(context);
    }

SpringApplication:

    private void refreshContext(ConfigurableApplicationContext context) {
        refresh(context);
        if (this.registerShutdownHook) {
            try {
                context.registerShutdownHook();
            }
            catch (AccessControlException ex) {
            }
        }
    }

这里上下文的刷新和注册关闭钩子我们都在之前的Spring源码分析文章中分析过,不再赘述。读者可能会有疑惑,Spring boot不是有非常厉害的自动配置么,文章中并没有看到啊,Spring boot的自动配置我们会用单独的文章来分析,本篇文章我们主要分析整个启动流程的步骤和一些扩展,到这里,整个Spring boot的启动流程就分析完了。


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