Spring Boot -- 启动源码分析

1 启动代码

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

通过以Java Application 的方式就能运行Spring Boot项目(web项目需要引入对应的starter,和普通的Java项目)

2 SpringApplication 源码分析

进入SpringApplication 的源码,分析Spring Boot的启动过程

2.1 SpringApplication 类的几个重要属性

/** * The class name of application context that will be used by default for non-web * environments. */
public static final String DEFAULT_CONTEXT_CLASS = "org.springframework.context."
            + "annotation.AnnotationConfigApplicationContext";

public static final String DEFAULT_WEB_CONTEXT_CLASS = "org.springframework.boot."
            + "web.servlet.context.AnnotationConfigServletWebServerApplicationContext";

说明: DEFAULT_CONTEXT_CLASS 属性值 AnnotationConfigApplicationContext
DEFAULT_WEB_CONTEXT_CLASS 属性AnnotationConfigServletWebServerApplicationContext 从这两个Spring Context可以看出来都是Annotation类型的,这也符合Spring Boot设计的初衷 用Annotation代替XML的配置,然后通过starter来实现功能的模块化和自动注入。

2.2 SpringApplication.run代码

//常用的Spring boot启动调用方法
public static ConfigurableApplicationContext run(Class<?> primarySource,String... args) {
        return run(new Class<?>[] { primarySource }, args);
}
//上面调用的是这个
public static ConfigurableApplicationContext run(Class<?> primarySource,
            String... args) {
        return run(new Class<?>[] { primarySource }, args);
}
//上面调用的这个
public static ConfigurableApplicationContext run(Class<?>[] primarySources,
            String[] args) {
        return new SpringApplication(primarySources).run(args);
}

通过上面三个方法可以看到最后是调用了

new SpringApplication(primarySources).run(args)

就来看一下SpringApplication的构造方法

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

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
        //设置resourceLoader -- 这个地方为NULL
        this.resourceLoader = resourceLoader;
        Assert.notNull(primarySources, "PrimarySources must not be null");
        this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
        //判断Application类型
        this.webApplicationType = deduceWebApplicationType();
        //Context初始化
        setInitializers((Collection) getSpringFactoriesInstances(
                ApplicationContextInitializer.class));
            //设置监听器
        setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
        //设置主类
        this.mainApplicationClass = deduceMainApplicationClass();
}

这里主要关注下两个地方:Context初始化 和 监听器的设置,这两个调用的都是getSpringFactoriesInstances 方法。

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();
        //获取Factory名称
        Set<String> names = new LinkedHashSet<>(
                SpringFactoriesLoader.loadFactoryNames(type, classLoader));
        //实例化Factory
        List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
                classLoader, args, names);
        //加载顺序排序
        AnnotationAwareOrderComparator.sort(instances);
        return instances;
}

这里我们重点关注下SpringFactoriesLoader.loadFactoryNames(type, classLoader)这个方法。

public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
        String factoryClassName = factoryClass.getName();
        return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
    }

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()) {
                    List<String> factoryClassNames = Arrays.asList(
                            StringUtils.commaDelimitedListToStringArray((String) entry.getValue()));
                    result.addAll((String) entry.getKey(), factoryClassNames);
                }
            }
            cache.put(classLoader, result);
            return result;
        }
        catch (IOException ex) {
            throw new IllegalArgumentException("Unable to load factories from location [" +
                    FACTORIES_RESOURCE_LOCATION + "]", ex);
        }
    }

说明:这里主要关注一个就是META-INF/spring.factories这个路径。加载的Spring Factory都配置在这里。
《Spring Boot -- 启动源码分析》
到这里SpringApplication的自身初始化完成。

2.3 SpringApplication.run执行

SpringApplication实例化完成后调用run方法来执行。下面关注下run方法

    public ConfigurableApplicationContext run(String... args) {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        ConfigurableApplicationContext context = null;
        Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
        configureHeadlessProperty();
        //获取SpringApplicationRunListener--包括SpringBoot的和开发自定义的
        SpringApplicationRunListeners listeners = getRunListeners(args);
        //启动监听
        listeners.starting();
        try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(
                    args);
            ConfigurableEnvironment environment = prepareEnvironment(listeners,
                    applicationArguments);
            configureIgnoreBeanInfo(environment);
            //打印Banner --控制台的默认的Spring Boot
            Banner printedBanner = printBanner(environment);
            //创建Context
            context = createApplicationContext();
            exceptionReporters = getSpringFactoriesInstances(
                    SpringBootExceptionReporter.class,
                    new Class[] { ConfigurableApplicationContext.class }, context);
            //处理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);
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, exceptionReporters, listeners);
            throw new IllegalStateException(ex);
        }

        try {
            //启动监听器
            listeners.running(context);
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, exceptionReporters, null);
            throw new IllegalStateException(ex);
        }
        return context;
    }

到这里为止就已经启动了。

2.4 prepareContext 方法

在SpringApplication调用中有一个prepareContext 方法,下面来重点关注下这个方法的作用。

private void prepareContext(ConfigurableApplicationContext context,
            ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
            ApplicationArguments applicationArguments, Banner printedBanner) {
        //设置context的Environment
        context.setEnvironment(environment);
        postProcessApplicationContext(context);
        //应用ApplicationContextInitializer
        applyInitializers(context);
        //设置监听器的context
        listeners.contextPrepared(context);
        if (this.logStartupInfo) {
            logStartupInfo(context.getParent() == null);
            logStartupProfileInfo(context);
        }

        // Add boot specific singleton beans
        ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
        beanFactory.registerSingleton("springApplicationArguments", applicationArguments);
        if (printedBanner != null) {
            beanFactory.registerSingleton("springBootBanner", printedBanner);
        }
        if (beanFactory instanceof DefaultListableBeanFactory) {
            ((DefaultListableBeanFactory) beanFactory)
                    .setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
        }
        // Load the sources
        Set<Object> sources = getAllSources();
        Assert.notEmpty(sources, "Sources must not be empty");
        load(context, sources.toArray(new Object[0]));
        listeners.contextLoaded(context);
    }
    原文作者:Spring Boot
    原文地址: https://blog.csdn.net/ljbmxsm/article/details/81609267
    本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系博主进行删除。
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