springboot源码解析-管中窥豹系列之总体结构

一、简介

• Springboot源码解析是一件大工程，逐行逐句的去研究代码，会很枯燥，也不容易坚持下去。
• 我们不追求大而全，而是试着每次去研究一个小知识点，最终聚沙成塔，这就是我们的springboot源码管中窥豹系列。

二、框架

我们先把springboot源码的框架了解清楚。

1、新建一个springboot项目

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class MyprojectApplication {

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

}

2、分析源码

相比于spring项目或者springmvc项目，springboot的入口很好找，就在main里面的run方法，我们进入run方法

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);
}

通过SpringApplication的静态方法，新建了一个SpringApplication类，调用它的run方法，我们先看SpringApplication的构造方法，再看run方法

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


@SuppressWarnings({ "unchecked", "rawtypes" })
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();
  setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
  setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
  this.mainApplicationClass = deduceMainApplicationClass();
}

构造方法主要做了这几件事：

• 确定web类型：webApplicationType
• 加载ApplicationContextInitializer
• 加载ApplicationListener
• 确定applicationcontext的实现类

实现细节我们先不探讨，接着看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);
  listeners.starting();
  try {
   ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
    ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
    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);
  }
  catch (Throwable ex) {
    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;
}

run方法做了几件事：

• stopwatch记录springboot启动耗时
• 不同阶段触发spring的listen事件
• 新建applicationcontext实现类
• 启动spring：refreshContext(context)
• 启动完成加载runner

最重要的就是refreshContext方法，延用的springframe的fresh方法

private void refreshContext(ConfigurableApplicationContext context) {
  refresh((ApplicationContext) context);
  if (this.registerShutdownHook) {
    try {
        context.registerShutdownHook();
    }
    catch (AccessControlException ex) {
        // Not allowed in some environments.
    }
  }
}

AbstractApplicationContext类的方法

public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
    // Prepare this context for refreshing.
    prepareRefresh();

    // Tell the subclass to refresh the internal bean factory.
    ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

    // Prepare the bean factory for use in this context.
    prepareBeanFactory(beanFactory);

    try {
        // Allows post-processing of the bean factory in context subclasses.
        postProcessBeanFactory(beanFactory);

        // Invoke factory processors registered as beans in the context.
        invokeBeanFactoryPostProcessors(beanFactory);

        // Register bean processors that intercept bean creation.
        registerBeanPostProcessors(beanFactory);

        // Initialize message source for this context.
        initMessageSource();

        // Initialize event multicaster for this context.
        initApplicationEventMulticaster();

        // Initialize other special beans in specific context subclasses.
        onRefresh();

        // Check for listener beans and register them.
        registerListeners();

        // Instantiate all remaining (non-lazy-init) singletons.
        finishBeanFactoryInitialization(beanFactory);

        // Last step: publish corresponding event.
        finishRefresh();
    }

    catch (BeansException ex) {
        if (logger.isWarnEnabled()) {
            logger.warn("Exception encountered during context initialization - " +
                    "cancelling refresh attempt: " + ex);
        }

        // Destroy already created singletons to avoid dangling resources.
        destroyBeans();

        // Reset 'active' flag.
        cancelRefresh(ex);

        // Propagate exception to caller.
        throw ex;
    }

    finally {
        // Reset common introspection caches in Spring's core, since we
        // might not ever need metadata for singleton beans anymore...
        resetCommonCaches();
    }
    }
}