在spring Web中，需要初始化IOC容器，用于存放我们注入的各种对象。当tomcat启动时首先会初始化一个web对应的IOC容器，用于初始化和注入各种我们在web运行过程中需要的对象。当tomcat启动的时候是如何初始化IOC容器的，我们先看一下在web.xml中经常看到的配置：
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>
classpath:applicationContext.xml
</param-value>
</context-param>
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
ContextLoaderListener是一个监听器，其实现了ServletContextListener接口，其用来监听Servlet，当tomcat启动时会初始化一个Servlet容器，这样ContextLoaderListener会监听到Servlet的初始化，这样在Servlet初始化之后我们就可以在ContextLoaderListener中也进行一些初始化操作。看下面的ServletContextListener的源码也是比较简单的，ContextLoaderListener实现了ServletContextListener接口，所以会有两个方法contextInitialized和contextDestroyed。web容器初始化时会调用方法contextInitialized，web容器销毁时会调用方法contextDestroyed。
public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
public ContextLoaderListener() {
}
public ContextLoaderListener(WebApplicationContext context) {
super(context);
}
/**
* Initialize the root web application context.
*/
@Override
public void contextInitialized(ServletContextEvent event) {
//在父类ContextLoader中实现
initWebApplicationContext(event.getServletContext());
}
/**
* Close the root web application context.
*/
@Override
public void contextDestroyed(ServletContextEvent event) {
closeWebApplicationContext(event.getServletContext());
ContextCleanupListener.cleanupAttributes(event.getServletContext());
}
}
ContextLoaderListener的方法contextInitialized（）的默认实现是在他的父类ContextLoader的initWebApplicationContext方法中实现的，意思就是初始化web应用上下文。他的主要流程就是创建一个IOC容器，并将创建的IOC容器存到servletContext中，ContextLoader的核心实现如下：
initWebApplicationContext函数：
//初始化WebApplicationContext，IOC容器
public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
//判断web容器中是否已经有WebApplicationContext
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");
}
long startTime = System.currentTimeMillis();
try {
// Store context in local instance variable, to guarantee that
// it is available on ServletContext shutdown.
//创建WebApplicationContext
if (this.context == null) {
//最终得到的是XmlWebApplicationContext
this.context = createWebApplicationContext(servletContext);
}
if (this.context instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
//判断应用上下文是否激活
if (!cwac.isActive()) {
// The context has not yet been refreshed -> provide services such as
// setting the parent context, setting the application context id, etc
//判断是否已经有父容器
if (cwac.getParent() == null) {
// The context instance was injected without an explicit parent ->
// determine parent for root web application context, if any.
//获得父容器
ApplicationContext parent = loadParentContext(servletContext);
cwac.setParent(parent);
}
//设置并刷新WebApplicationContext容器
configureAndRefreshWebApplicationContext(cwac, servletContext);
}
}
//将初始化的WebApplicationContext设置到servletContext中
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
ClassLoader ccl = Thread.currentThread().getContextClassLoader();
if (ccl == ContextLoader.class.getClassLoader()) {
currentContext = this.context;
}
else if (ccl != null) {
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");
}
//初始化 WebApplicationContext完成并返回
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;
}
}
springMVC父容器初始化流程图（盗图）如下：
完整的ContextLoader源码如下：
/**
* 用于初始化ApplicationContext
*
*/
public class ContextLoader {
//WebApplicationContext id
public static final String CONTEXT_ID_PARAM = "contextId";
//配置文件
public static final String CONFIG_LOCATION_PARAM = "contextConfigLocation";
public static final String CONTEXT_CLASS_PARAM = "contextClass";
public static final String CONTEXT_INITIALIZER_CLASSES_PARAM = "contextInitializerClasses";
public static final String GLOBAL_INITIALIZER_CLASSES_PARAM = "globalInitializerClasses";
public static final String LOCATOR_FACTORY_SELECTOR_PARAM = "locatorFactorySelector";
public static final String LOCATOR_FACTORY_KEY_PARAM = "parentContextKey";
private static final String INIT_PARAM_DELIMITERS = ",; \t\n";
private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";
private static final Properties defaultStrategies;
static {
// Load default strategy implementations from properties file.
// This is currently strictly internal and not meant to be customized
// by application developers.
try {
ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, ContextLoader.class);
defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
}
catch (IOException ex) {
throw new IllegalStateException("Could not load ‘ContextLoader.properties‘: " + ex.getMessage());
}
}
private static final Map<ClassLoader, WebApplicationContext> currentContextPerThread =
new ConcurrentHashMap<ClassLoader, WebApplicationContext>(1);
private static volatile WebApplicationContext currentContext;
private WebApplicationContext context;
private BeanFactoryReference parentContextRef;
private final List<ApplicationContextInitializer<ConfigurableApplicationContext>> contextInitializers =
new ArrayList<ApplicationContextInitializer<ConfigurableApplicationContext>>();
public ContextLoader() {
}
public ContextLoader(WebApplicationContext context) {
this.context = context;
}
@SuppressWarnings("unchecked")
public void setContextInitializers(ApplicationContextInitializer<?>... initializers) {
if (initializers != null) {
for (ApplicationContextInitializer<?> initializer : initializers) {
this.contextInitializers.add((ApplicationContextInitializer<ConfigurableApplicationContext>) initializer);
}
}
}
//初始化WebApplicationContext，IOC容器
public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
//判断web容器中是否已经有WebApplicationContext
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");
}
long startTime = System.currentTimeMillis();
try {
// Store context in local instance variable, to guarantee that
// it is available on ServletContext shutdown.
//创建WebApplicationContext
if (this.context == null) {
//最终得到的是XmlWebApplicationContext
this.context = createWebApplicationContext(servletContext);
}
if (this.context instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
//判断应用上下文是否激活
if (!cwac.isActive()) {
// The context has not yet been refreshed -> provide services such as
// setting the parent context, setting the application context id, etc
//判断是否已经有父容器
if (cwac.getParent() == null) {
// The context instance was injected without an explicit parent ->
// determine parent for root web application context, if any.
//获得父容器
ApplicationContext parent = loadParentContext(servletContext);
cwac.setParent(parent);
}
//设置并刷新WebApplicationContext容器
configureAndRefreshWebApplicationContext(cwac, servletContext);
}
}
//将初始化的WebApplicationContext设置到servletContext中
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
ClassLoader ccl = Thread.currentThread().getContextClassLoader();
if (ccl == ContextLoader.class.getClassLoader()) {
currentContext = this.context;
}
else if (ccl != null) {
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");
}
//初始化 WebApplicationContext完成并返回
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;
}
}
//在web容器加载的时候初始化根WebApplicationContext
protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
//通过反射获得上下文类
Class<?> contextClass = determineContextClass(sc);
if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
"] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
}
//初始化WebApplicationContext，是其实现类XmlWebApplicationContext
return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
}
protected Class<?> determineContextClass(ServletContext servletContext) {
//查看servlet是否已经存在应用上下文类名
String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
if (contextClassName != null) {
try {
//类名不为空则反射获得类
return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
}
catch (ClassNotFoundException ex) {
throw new ApplicationContextException(
"Failed to load custom context class [" + contextClassName + "]", ex);
}
}
else {
//如果应用上下文类名为空，则获得默认的context上下文名
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);
}
}
}
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);
//设置应用上下文id,在web.xml中可以进行配置
if (idParam != null) {
wac.setId(idParam);
}
else {
// Generate default id...
//如果没有配置则设置默认名称
wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
ObjectUtils.getDisplayString(sc.getContextPath()));
}
}
//设置ServletContext到容器中
wac.setServletContext(sc);
//获得web.xml中配置的contextConfigLocation的值，一般是classpath:applicationContext.xml
String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);
if (configLocationParam != null) {
//设置加载文件地址
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
ConfigurableEnvironment env = wac.getEnvironment();
if (env instanceof ConfigurableWebEnvironment) {
((ConfigurableWebEnvironment) env).initPropertySources(sc, null);
}
customizeContext(sc, wac);
//这一步是特别关键的，这样application.xml中的所有配置都会被初始化到XMLWebApplication中
wac.refresh();
}
protected void customizeContext(ServletContext sc, ConfigurableWebApplicationContext wac) {
List<Class<ApplicationContextInitializer<ConfigurableApplicationContext>>> initializerClasses =
determineContextInitializerClasses(sc);
for (Class<ApplicationContextInitializer<ConfigurableApplicationContext>> initializerClass : initializerClasses) {
Class<?> initializerContextClass =
GenericTypeResolver.resolveTypeArgument(initializerClass, ApplicationContextInitializer.class);
if (initializerContextClass != null) {
Assert.isAssignable(initializerContextClass, wac.getClass(), String.format(
"Could not add 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()));
}
this.contextInitializers.add(BeanUtils.instantiateClass(initializerClass));
}
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>>>();
String globalClassNames = servletContext.getInitParameter(GLOBAL_INITIALIZER_CLASSES_PARAM);
if (globalClassNames != null) {
for (String className : StringUtils.tokenizeToStringArray(globalClassNames, INIT_PARAM_DELIMITERS)) {
classes.add(loadInitializerClass(className));
}
}
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;
}
@SuppressWarnings("unchecked")
private Class<ApplicationContextInitializer<ConfigurableApplicationContext>> loadInitializerClass(String className) {
try {
Class<?> clazz = ClassUtils.forName(className, ClassUtils.getDefaultClassLoader());
Assert.isAssignable(ApplicationContextInitializer.class, clazz);
return (Class<ApplicationContextInitializer<ConfigurableApplicationContext>>) clazz;
}
catch (ClassNotFoundException ex) {
throw new ApplicationContextException("Failed to load context initializer class [" + className + "]", ex);
}
}
//从ServletContext中获取父容器
protected ApplicationContext loadParentContext(ServletContext servletContext) {
ApplicationContext parentContext = null;
String locatorFactorySelector = servletContext.getInitParameter(LOCATOR_FACTORY_SELECTOR_PARAM);
String parentContextKey = servletContext.getInitParameter(LOCATOR_FACTORY_KEY_PARAM);
if (parentContextKey != null) {
// locatorFactorySelector may be null, indicating the default "classpath*:beanRefContext.xml"
BeanFactoryLocator locator = ContextSingletonBeanFactoryLocator.getInstance(locatorFactorySelector);
Log logger = LogFactory.getLog(ContextLoader.class);
if (logger.isDebugEnabled()) {
logger.debug("Getting parent context definition: using parent context key of ‘" +
parentContextKey + "‘ with BeanFactoryLocator");
}
this.parentContextRef = locator.useBeanFactory(parentContextKey);
parentContext = (ApplicationContext) this.parentContextRef.getFactory();
}
return parentContext;
}
//通过给定的ServletContext来关闭IOC容器
public void closeWebApplicationContext(ServletContext servletContext) {
servletContext.log("Closing Spring root WebApplicationContext");
try {
if (this.context instanceof ConfigurableWebApplicationContext) {
((ConfigurableWebApplicationContext) this.context).close();
}
}
finally {
ClassLoader ccl = Thread.currentThread().getContextClassLoader();
if (ccl == ContextLoader.class.getClassLoader()) {
currentContext = null;
}
else if (ccl != null) {
currentContextPerThread.remove(ccl);
}
servletContext.removeAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
if (this.parentContextRef != null) {
this.parentContextRef.release();
}
}
}
//在当前线程中获取WebApplicationContext
public static WebApplicationContext getCurrentWebApplicationContext() {
ClassLoader ccl = Thread.currentThread().getContextClassLoader();
if (ccl != null) {
WebApplicationContext ccpt = currentContextPerThread.get(ccl);
if (ccpt != null) {
return ccpt;
}
}
return currentContext;
}
}
转自：http://blog.csdn.net/qq924862077/article/details/52769754
原文地址：https://www.cnblogs.com/gllegolas/p/11661255.html
时间： 2024-10-09 16:19:50