SpringBoot的启动流程分析

Posted 2021-11-08 wanghaoyang

通过分析我们可以找到 org.springframework.boot.SpringApplication 中如下，

    public static ConfigurableApplicationContext run(Object[] sources, String[] args) {
        return new SpringApplication(sources).run(args);
    }

可以看出，SpringBoot的启动分两步 1:实例化一个SpringApplication对象 2:run

初始化的方法如下：

private void initialize(Object[] sources) {
        if (sources != null && sources.length > 0) {
            this.sources.addAll(Arrays.asList(sources));
        }
        this.webEnvironment = deduceWebEnvironment();    //设置当前为web环境
        setInitializers((Collection) getSpringFactoriesInstances(
                ApplicationContextInitializer.class));
        setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
        this.mainApplicationClass = deduceMainApplicationClass();
    }

deduceWebEnvironment()方法会判断包路径下是否存在 "javax.servlet.Servlet" 和 "org.springframework.web.context.ConfigurableWebApplicationContext"，如果存在，则设置当前环境为Web环境，源码如下

private static final String[] WEB_ENVIRONMENT_CLASSES = { "javax.servlet.Servlet",
            "org.springframework.web.context.ConfigurableWebApplicationContext" };

    private boolean deduceWebEnvironment() {
        for (String className : WEB_ENVIRONMENT_CLASSES) {
            if (!ClassUtils.isPresent(className, null)) {
                return false;
            }
        }
        return true;
    }

setInitializers()方法用来设置SpringApplication启动时需要的类
setListeners()用来设置SpringApplication启动时需要的监听器，
这两个方法都用到了getSpringFactoriesInstances()方法，那我们来分析一下这个方法的相关源码

public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

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

    private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type,
            Class<?>[] parameterTypes, Object... args) {
        ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
        // Use names and ensure unique to protect against duplicates
        Set<String> names = new LinkedHashSet<String>(
                SpringFactoriesLoader.loadFactoryNames(type, classLoader));
        List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
                classLoader, args, names);
        AnnotationAwareOrderComparator.sort(instances);
        return instances;
    }

    public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
        String factoryClassName = factoryClass.getName();
        try {
            Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                    ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
            List<String> result = new ArrayList<String>();
            while (urls.hasMoreElements()) {
                URL url = urls.nextElement();
                Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
                String factoryClassNames = properties.getProperty(factoryClassName);
                result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
            }
            return result;
        }
        catch (IOException ex) {
            throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +
                    "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
        }
    }

通过上面的代码我们可以看出，getSpringFactoriesInstances() 方法会从 "META-INF/spring.factories" 这个文件中读取需要的类型实例化后注入到SpringApplication中，我们截取springboot中"META-INF/spring.factories" 中 ApplicationContextInitializer.class和 

ApplicationListener.class类型的类如下

# 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.context.embedded.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.liquibase.LiquibaseServiceLocatorApplicationListener,org.springframework.boot.logging.ClasspathLoggingApplicationListener,org.springframework.boot.logging.LoggingApplicationListener

deduceMainApplicationClass()将启动类设置为我们自己定义的启动类，它是用过读取程序的栈，并分析栈的方法是否为main来判断的，源码如下

    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;
    }

那么以上就是SpringApplication初始化完成的事情，限于篇幅，我们在下一章介绍run的过程