SpringBoot启动过程粗略分析

简单的springboot项目启动代码

通过下面的代码，可以启动一个springboot项目

@SpringBootApplication
public class Bootstrap {
    private static final Logger logger = LoggerFactory.getLogger(Bootstrap.class);

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

main方法中，只有一行代码。下面深入来看这行代码做了哪些事情

SpringApplication

来看下它的这个静态方法的实现，最终还是调用了SpringApplication的实例的run方法来完成了项目的启动。

/**
     * Static helper that can be used to run a {@link SpringApplication} from the
     * specified source using default settings.
     * @param source the source to load
     * @param args the application arguments (usually passed from a Java main method)
     * @return the running {@link ApplicationContext}
     */
    public static ConfigurableApplicationContext run(Object source, String... args) {
        return run(new Object[] { source }, args);
    }

    /**
     * Static helper that can be used to run a {@link SpringApplication} from the
     * specified sources using default settings and user supplied arguments.
     * @param sources the sources to load
     * @param args the application arguments (usually passed from a Java main method)
     * @return the running {@link ApplicationContext}
     */
    public static ConfigurableApplicationContext run(Object[] sources, String[] args) {
        return new SpringApplication(sources).run(args);
    }

SpringApplication构造函数

在构造SpringApplication实例的过程中，Springboot处理了如下几件事情，先上代码:

public SpringApplication(ResourceLoader resourceLoader, Object... sources) {
        this.resourceLoader = resourceLoader;
        initialize(sources);
    }

    @SuppressWarnings({ "unchecked", "rawtypes" })
    private void initialize(Object[] sources) {
        if (sources != null && sources.length > 0) {
            this.sources.addAll(Arrays.asList(sources));
        }
        // 是否是web环境
        this.webEnvironment = deduceWebEnvironment();
       // 设置初始化器
        setInitializers((Collection) getSpringFactoriesInstances(
                ApplicationContextInitializer.class));
       // 设置监听器
        setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
       // 设置有启动入口的Class类型
        this.mainApplicationClass = deduceMainApplicationClass();
    }

设置初始化器的步骤是遍历classpath下的所有的 META-INF/spring.factories 文件，查找key为org.springframework.context.ApplicationContextInitializer定义的所有类。有关 ApplicationContextInitializer的作用，可以参考下https://www.jianshu.com/p/3828e93be20d这边文章。

内存中调试得到的结果为:

初始化器内存调试结果

设置监听器的方式和设置初始化器的方式一致，也是遍历classpath下的所有的 META-INF/spring.factories 文件，不过它是查找key为org.springframework.context.ApplicationListener定义的所有类。

内存中得到的调试结果为:

应用监听器内存调试结果

当上述步骤搞定后，SpringApplication的实例就创建出来了。下一步是调用其实例的run方法。

SpringAppliation#run(String... args)

先看看该方法的代码:

public ConfigurableApplicationContext run(String... args) {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        ConfigurableApplicationContext context = null;
        FailureAnalyzers analyzers = null;
        configureHeadlessProperty();
        SpringApplicationRunListeners listeners = getRunListeners(args);
        listeners.starting();
        try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(
                    args);
            ConfigurableEnvironment environment = prepareEnvironment(listeners,
                    applicationArguments);
            Banner printedBanner = printBanner(environment);
            context = createApplicationContext();
            analyzers = new FailureAnalyzers(context);
            prepareContext(context, environment, listeners, applicationArguments,
                    printedBanner);
            refreshContext(context);
            afterRefresh(context, applicationArguments);
            listeners.finished(context, null);
            stopWatch.stop();
            if (this.logStartupInfo) {
                new StartupInfoLogger(this.mainApplicationClass)
                        .logStarted(getApplicationLog(), stopWatch);
            }
            return context;
        }
        catch (Throwable ex) {
            handleRunFailure(context, listeners, analyzers, ex);
            throw new IllegalStateException(ex);
        }
    }

configureHeadlessProperty

可以看看 java.awt.headless 详解

getRunListeners

和上面获取初始化器以及应用监听器的逻辑一样，不过这里是用来获取配置中key为org.springframework.boot.SpringApplicationRunListener的所有类。

prepareEnvironment

private ConfigurableEnvironment prepareEnvironment(
            SpringApplicationRunListeners listeners,
            ApplicationArguments applicationArguments) {
        // Create and configure the environment
       // 返回Environment对象，如果是web环境下的话，则返回StandardServletEnvironment，否则返回StandardEnvironment。
        ConfigurableEnvironment environment = getOrCreateEnvironment();
       // 对environment做一些配置文件和profile相关的配置
        configureEnvironment(environment, applicationArguments.getSourceArgs());
        listeners.environmentPrepared(environment);
        if (!this.webEnvironment) {
            environment = new EnvironmentConverter(getClassLoader())
                    .convertToStandardEnvironmentIfNecessary(environment);
        }
        return environment;
    }

printBanner

打印欢迎语

createApplicationContext

根据环境(web or 其他)来创建不同的context

protected ConfigurableApplicationContext createApplicationContext() {
        Class contextClass = this.applicationContextClass;
        if (contextClass == null) {
            try {
                contextClass = Class.forName(this.webEnvironment
                        ? DEFAULT_WEB_CONTEXT_CLASS : DEFAULT_CONTEXT_CLASS);
            }
            catch (ClassNotFoundException ex) {
                throw new IllegalStateException(
                        "Unable create a default ApplicationContext, "
                                + "please specify an ApplicationContextClass",
                        ex);
            }
        }
        return (ConfigurableApplicationContext) BeanUtils.instantiate(contextClass);
}

后续的过程就是spring容器开始初始化的过程，在后续的文章中我会详细的来讲解。