Spring基础系列-容器启动流程(1)

原创作品，可以转载，但是请标注出处地址：https://www.cnblogs.com/V1haoge/p/9870339.html

概述

我说的容器启动流程涉及两种情况，SSM开发模式和Springboot开发模式。

　　SSM开发模式中，需要配置web.xml文件用作启动配置文件，而Springboot开发模式中由main方法直接启动。

　　下面是web项目中容器启动的流程，起点是web.xml中配置的ContextLoaderListener监听器。

调用流程图（右键可查看大图）

流程解析

　　Tomcat服务器启动时会读取项目中

web.xml

ServletContext

ContextLoaderListener

ServletContextListener

ServletContext

ServletContext

contextInitialized()

　　源码1-来自：ContextLoaderListener

/**
* Initialize the root web application context.
*/
@Override
public void contextInitialized(ServletContextEvent event) {
initWebApplicationContext(event.getServletContext());
}

ContextLoaderListener

root WebApplicationContext

ServletContextListener

contextInitialized

contextDestoryed

　　上面方法中

contextInitialized

initWebApplicationContext

　　源码2-来自：ContextLoader

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
//SpringIOC容器的重复性创建校验
if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {
throw new IllegalStateException(
"Cannot initialize context because there is already a root application context present - " +
"check whether you have multiple ContextLoader* definitions in your web.xml!");
}

Log logger = LogFactory.getLog(ContextLoader.class);
servletContext.log("Initializing Spring root WebApplicationContext");
if (logger.isInfoEnabled()) {
logger.info("Root WebApplicationContext: initialization started");
}
//记录Spring容器创建开始时间
long startTime = System.currentTimeMillis();

try {
// Store context in local instance variable, to guarantee that
// it is available on ServletContext shutdown.
if (this.context == null) {
//创建Spring容器实例
this.context = createWebApplicationContext(servletContext);
}
if (this.context instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
if (!cwac.isActive()) {
//容器只有被刷新至少一次之后才是处于active（激活）状态
if (cwac.getParent() == null) {
//此处是一个空方法，返回null,也就是不设置父级容器
ApplicationContext parent = loadParentContext(servletContext);
cwac.setParent(parent);
}
//重点操作：配置并刷新容器
configureAndRefreshWebApplicationContext(cwac, servletContext);
}
}
//将创建完整的Spring容器作为一条属性添加到Servlet容器中
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);

ClassLoader ccl = Thread.currentThread().getContextClassLoader();
if (ccl == ContextLoader.class.getClassLoader()) {
//如果当前线程的类加载器是ContextLoader类的类加载器的话，也就是说如果是当前线程加载了ContextLoader类的话，则将Spring容器在ContextLoader实例中保留一份引用
currentContext = this.context;
}
else if (ccl != null) {
//添加一条ClassLoader到Springweb容器的映射
currentContextPerThread.put(ccl, this.context);
}

if (logger.isDebugEnabled()) {
logger.debug("Published root WebApplicationContext as ServletContext attribute with name [" +
WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]");
}
if (logger.isInfoEnabled()) {
long elapsedTime = System.currentTimeMillis() - startTime;
logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms");
}

return this.context;
}
catch (RuntimeException ex) {
logger.error("Context initialization failed", ex);
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
throw ex;
}
catch (Error err) {
logger.error("Context initialization failed", err);
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, err);
throw err;
}
}

　　这里十分清晰的显示出了

ServletContext

Spring root ApplicationContext

ServletContext

Spring

ServletContext

ServletContext

　　源码中

WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE

WebApplicationContext.class.getName() + ".ROOT"

ServletContext

　　在这段源码中主要是概述Spring容器的创建和初始化，分别由两个方法实现：createWebApplicationContext方法和configureAndRefreshWebApplicationContext方法。

　　首先，我们需要创建Spring容器，我们需要决定使用哪个容器实现。

　　源码3-来自：ContextLoader

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
//决定使用哪个容器实现
Class<?> contextClass = determineContextClass(sc);
if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
"] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
}
//反射方式创建容器实例
return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
}

//我们在web.xml中以 <context-param> 的形式设置contextclass参数来指定使用哪个容器实现类，
//若未指定则使用默认的XmlWebApplicationContext，其实这个默认的容器实现也是预先配置在一个
//叫ContextLoader.properties文件中的
protected Class<?> determineContextClass(ServletContext servletContext) {
//获取Servlet容器中配置的系统参数contextClass的值，如果未设置则为null
String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
if (contextClassName != null) {
try {
return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
}
catch (ClassNotFoundException ex) {
throw new ApplicationContextException(
"Failed to load custom context class [" + contextClassName + "]", ex);
}
}
else {
//获取预先配置的容器实现类
contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());
try {
return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());
}
catch (ClassNotFoundException ex) {
throw new ApplicationContextException(
"Failed to load default context class [" + contextClassName + "]", ex);
}
}
}

BeanUtils

instantiateClass

　　我们可以在

web.xml

<context-param>

contextclass

ContextLoader.properties

org.springframework.web.context.WebApplicationContext

　　源码4-来自：ContextLoader.properties

# Default WebApplicationContext implementation class for ContextLoader.
# Used as fallback when no explicit context implementation has been specified as context-param.
# Not meant to be customized by application developers.

org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext

　　可见，一般基于Spring的web应用默认使用的都是

XmlWebApplicationContext

　　到此位置容器实例就创建好了，下一步就是配置和刷新了。

　　源码5-来自：ContextLoader

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {
if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
// The application context id is still set to its original default value
// -> assign a more useful id based on available information
String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);
if (idParam != null) {
wac.setId(idParam);
}
else {
// Generate default id...
wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
ObjectUtils.getDisplayString(sc.getContextPath()));
}
}
//在当前Spring容器中保留对Servlet容器的引用
wac.setServletContext(sc);
//设置web.xml中配置的contextConfigLocation参数值到当前容器中
String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
if (configLocationParam != null) {
wac.setConfigLocation(configLocationParam);
}

// The wac environment's #initPropertySources will be called in any case when the context
// is refreshed; do it eagerly here to ensure servlet property sources are in place for
// use in any post-processing or initialization that occurs below prior to #refresh
//在容器刷新之前，提前进行属性资源的初始化，以备使用，将ServletContext设置为servletContextInitParams
ConfigurableEnvironment env = wac.getEnvironment();
if (env instanceof ConfigurableWebEnvironment) {
((ConfigurableWebEnvironment) env).initPropertySources(sc, null);
}
//取得web.xml中配置的contextInitializerClasses和globalInitializerClasses对应的初始化器，并执行初始化操作，需自定义初始化器
customizeContext(sc, wac);
//刷新容器
wac.refresh();
}

　　首先将

ServletContext

ServletContext

ServletContext

contextConfigLocation

web.xml

<context-param>

Spring

<context-param>

web.xml

ServletContext

ServletContext

ServletContext

ServletContext

contextConfigLocation

ServletContext

<context-param>

applicationContext.xml

<context-param>

DispatchServlet

spring-mvc.xml

　　Spring容器的

Environment

Profile

Properties

initPropertySources

ConfigurableWebEnvironment

StubPropertySource

StubPropertySource

​ 源码6-来自：StandardServletEnvironment

@Override
public void initPropertySources(@Nullable ServletContext servletContext, @Nullable ServletConfig servletConfig) {
WebApplicationContextUtils.initServletPropertySources(getPropertySources(), servletContext, servletConfig);
}

​ 源码7-来自：WebApplicationContextUtils

public static void initServletPropertySources(MutablePropertySources sources,
@Nullable ServletContext servletContext, @Nullable ServletConfig servletConfig) {

Assert.notNull(sources, "'propertySources' must not be null");
String name = StandardServletEnvironment.SERVLET_CONTEXT_PROPERTY_SOURCE_NAME;
if (servletContext != null && sources.contains(name) && sources.get(name) instanceof StubPropertySource) {
sources.replace(name, new ServletContextPropertySource(name, servletContext));
}
name = StandardServletEnvironment.SERVLET_CONFIG_PROPERTY_SOURCE_NAME;
if (servletConfig != null && sources.contains(name) && sources.get(name) instanceof StubPropertySource) {
sources.replace(name, new ServletConfigPropertySource(name, servletConfig));
}
}

​ 从上面的源码中可以看出，这里只进行了有关Servlet的占位资源的替换操作：

ServletContext

ServletConfig

　　上面源码中

customizeContext

ApplicationContextInitializer<C extends ConfigurableApplicationContext>

web.xml

　　源码8-来自：ContextLoader

protected void customizeContext(ServletContext sc, ConfigurableWebApplicationContext wac) {
//获取初始化器类集合
List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> initializerClasses =
determineContextInitializerClasses(sc);

for (Class<ApplicationContextInitializer<ConfigurableApplicationContext>> initializerClass : initializerClasses) {
Class<?> initializerContextClass =
GenericTypeResolver.resolveTypeArgument(initializerClass, ApplicationContextInitializer.class);
if (initializerContextClass != null && !initializerContextClass.isInstance(wac)) {
throw new ApplicationContextException(String.format(
"Could not apply context initializer [%s] since its generic parameter [%s] " +
"is not assignable from the type of application context used by this " +
"context loader: [%s]", initializerClass.getName(), initializerContextClass.getName(),
wac.getClass().getName()));
}
//实例化初始化器并添加到集合中
this.contextInitializers.add(BeanUtils.instantiateClass(initializerClass));
}
//排序并执行，编号越小越早执行
AnnotationAwareOrderComparator.sort(this.contextInitializers);
for (ApplicationContextInitializer<ConfigurableApplicationContext> initializer : this.contextInitializers) {
initializer.initialize(wac);
}
}

protected List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>>
determineContextInitializerClasses(ServletContext servletContext) {
List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> classes =
new ArrayList<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>>();
//通过<context-param>属性配置globalInitializerClasses获取全局初始化类名
String globalClassNames = servletContext.getInitParameter(GLOBAL_INITIALIZER_CLASSES_PARAM);
if (globalClassNames != null) {
for (String className : StringUtils.tokenizeToStringArray(globalClassNames, INIT_PARAM_DELIMITERS)) {
classes.add(loadInitializerClass(className));
}
}
//通过<context-param>属性配置contextInitializerClasses获取容器初始化类名
String localClassNames = servletContext.getInitParameter(CONTEXT_INITIALIZER_CLASSES_PARAM);
if (localClassNames != null) {
for (String className : StringUtils.tokenizeToStringArray(localClassNames, INIT_PARAM_DELIMITERS)) {
classes.add(loadInitializerClass(className));
}
}
return classes;
}

​

customizeContext

ApplicationContextInitializer

ApplicationContextInitializer

<context-param>

web.xml

ApplicationContextInitializer

　　到达refresh操作我们先暂停。refresh操作是容器初始化的操作。是通用操作，而到达该点的方式确实有多种，每种就是一种Spring的开发方式。

　　除了此处的web开发方式，还有Springboot开发方式，貌似就两种。。。下面说说Springboot启动的流程，最后统一说refresh流程。