剖析Spring MVC,介绍Spring MVC的结构与创建过程。
首先看Spring MVC的结构。我们知道Spring MVC从创建DispatcherServlet开始:
graph BT
DispatcherServlet --> FrameworkServlet
DispatcherServlet -.- SuppressWarnings
FrameworkServlet --> HttpServletBean
FrameworkServlet -.-> ApplicationContextAware
HttpServletBean --> HttpServlet
HttpServletBean -.-> EnvironmentAware
HttpServletBean -.-> EnvironmentCapable
HttpServlet --> GenericServlet
EnvironmentAware --> Aware
ApplicationContextAware --> Aware
GenericServlet -.->ServletConfig
GenericServlet -.->Servlet
GenericServlet -.->Serializable可以看到DispatcherServlet继承FrameworkServlet,FrameworkServlet继承HttpServletBean,HttpServletBean继承Servlet中的HttpServlet。
1. 接口
这三个类实现了EnvironmentCapable,EnvironmentAware.ApplicationContextAware接口。可以实现这些接口获取Environment,ApplicationContext。
Environment：应用运行的环境，包括应用环境的：Profiles和Properties
public interface Environment extends PropertyResolver {
//获取激活的Profiles
String[] getActiveProfiles();
//没有找到激活的则找默认的Profiles
String[] getDefaultProfiles();
@Deprecated
boolean acceptsProfiles(String... profiles);
boolean acceptsProfiles(Profiles profiles);
}public interface PropertyResolver {
boolean containsProperty(String key);
@Nullable
String getProperty(String key);
String getProperty(String key, String defaultValue);
@Nullable
<T> T getProperty(String key, Class<T> targetType);
<T> T getProperty(String key, Class<T> targetType, T defaultValue);
String getRequiredProperty(String key) throws IllegalStateException;
<T> T getRequiredProperty(String key, Class<T> targetType) throws IllegalStateException;
String resolvePlaceholders(String text);
String resolveRequiredPlaceholders(String text) throws IllegalArgumentException;
}
1. Profiles
profile配置是一个被命名的、bean定义的逻辑组,当profile激活时对应的bean会被注册到容器中，这样就能实现不同环境不同配置的切换，如开发中dev,sit,pro环境。可以使用xml,@Profile,环境变量，JVM参数，Maven等设置。
//注解
@Component
@Profile("dev")
public class DevDatasourceConfig
//xml
<beans profile="dev">
<bean id="devDatasourceConfig"
class="DevDatasourceConfig" />
</beans>
//WebApplicationInitializer
servletContext.setInitParameter("spring.profiles.active", "dev");
//JVM参数
java -jar xxx.jar -Dspring.profiles.active=dev
//环境变量
export spring_profiles_active=dev
//maven
<profiles>
<profile>
<id>dev</id>
<activation>
<activeByDefault>true</activeByDefault>
</activation>
<properties>
<spring.profiles.active>dev</spring.profiles.active>
</properties>
</profile>
<profile>
<id>pro</id>
<properties>
<spring.profiles.active>pro</spring.profiles.active>
</properties>
</profile>
</profiles>
mvn install -Pdev2. Properties
properties配置属性，常见的.properties文件，JVM properties,system环境
2. HttpServletBean
@Override
public final void init() throws ServletException {
//设置参数
PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
if (!pvs.isEmpty()) {
try {
BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
//抽象方法。
initBeanWrapper(bw);
bw.setPropertyValues(pvs, true);
}
catch (BeansException ex) {
if (logger.isErrorEnabled()) {
logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
}
throw ex;
}
}
// 抽象方法。初始化工作，调用子类（FrameworkServlet）initServletBean()方法
initServletBean();
}3. FrameworkServlet
从HttpServletBean可以看到调用initServletBean()方法。
/**
* Overridden method of {@link HttpServletBean}, invoked after any bean properties
* have been set. Creates this servlet's WebApplicationContext.
*/
@Override
protected final void initServletBean() throws ServletException {
getServletContext().log("Initializing Spring " + getClass().getSimpleName() + " '" + getServletName() + "'");
if (logger.isInfoEnabled()) {
logger.info("Initializing Servlet '" + getServletName() + "'");
}
long startTime = System.currentTimeMillis();
try {
//初始化webApplicationContext
this.webApplicationContext = initWebApplicationContext();
//抽象方法，子类并没有实现
initFrameworkServlet();
}
catch (ServletException | RuntimeException ex) {
logger.error("Context initialization failed", ex);
throw ex;
}
if (logger.isDebugEnabled()) {
String value = this.enableLoggingRequestDetails ?
"shown which may lead to unsafe logging of potentially sensitive data" :
"masked to prevent unsafe logging of potentially sensitive data";
logger.debug("enableLoggingRequestDetails='" + this.enableLoggingRequestDetails +
"': request parameters and headers will be " + value);
}
if (logger.isInfoEnabled()) {
logger.info("Completed initialization in " + (System.currentTimeMillis() - startTime) + " ms");
}
}来看initWebApplicationContext()
protected WebApplicationContext initWebApplicationContext() {
//获取rootContext
//String ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT";
//Object attr = sc.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
//从ServletContext中获取名为WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE的Object
WebApplicationContext rootContext =
WebApplicationContextUtils.getWebApplicationContext(getServletContext());
WebApplicationContext wac = null;
//如果已经设置了webApplicationContext
if (this.webApplicationContext != null) {
// A context instance was injected at construction time -> use it
wac = this.webApplicationContext;
if (wac instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
if (!cwac.isActive()) {
// The context has not yet been refreshed -> provide services such as
// setting the parent context, setting the application context id, etc
if (cwac.getParent() == null) {
// The context instance was injected without an explicit parent -> set
// the root application context (if any; may be null) as the parent
cwac.setParent(rootContext);
}
configureAndRefreshWebApplicationContext(cwac);
}
}
}
if (wac == null) {
//如果WebApplicationContext未设置，从ServletContext中获取
//WebApplicationContext wac = WebApplicationContextUtils.getWebApplicationContext(getServletContext(), attrName);
wac = findWebApplicationContext();
}
if (wac == null) {
//如果还未获取到，创建一个
//设置一些东西，特别是设置了一个ContextRefreshListener监听
//public void onApplicationEvent(ContextRefreshedEvent event) {
// this.refreshEventReceived = true; //表示已经onRefresh
// synchronized (this.onRefreshMonitor) {
// onRefresh(event.getApplicationContext());//onRefresh
// }
//}
//onApplicationEvent()->
wac = createWebApplicationContext(rootContext);
}
//如果没有onRefresh过则onRefreshÒÒÒÒÒÒ
if (!this.refreshEventReceived) {
synchronized (this.onRefreshMonitor) {
//抽象方法。子类重写
onRefresh(wac);
}
}
//把WebApplicationContext设置到ServletContext中
if (this.publishContext) {
// Publish the context as a servlet context attribute.
String attrName = getServletContextAttributeName();
getServletContext().setAttribute(attrName, wac);
return wac;
}4. DispatcherServlet
从FrameworkServlet看出，调用DispatcherServlet中onRefresh方法。onRefresh()->initStrategies()
/**
* This implementation calls {@link #initStrategies}.
*/
@Override
protected void onRefresh(ApplicationContext context) {
initStrategies(context);
}
/**
* 初始化组件
* Initialize the strategy objects that this servlet uses.
* <p>May be overridden in subclasses in order to initialize further strategy objects.
*/
protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context);
initViewResolvers(context);
initFlashMapManager(context);
}初始化组件会从context中获取，如果获取不到会调用getDefaultStrategies()。通过默认文件DispatcherServlet.properties初始化组件
private static final Properties defaultStrategies;
private static final String DEFAULT_STRATEGIES_PATH = "DispatcherServlet.properties";
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, DispatcherServlet.class);
defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
}
catch (IOException ex) {
throw new IllegalStateException("Could not load '" + DEFAULT_STRATEGIES_PATH + "': " + ex.getMessage());
}
}
/**
* Create a List of default strategy objects for the given strategy interface.
* <p>The default implementation uses the "DispatcherServlet.properties" file (in the same
* package as the DispatcherServlet class) to determine the class names. It instantiates
* the strategy objects through the context's BeanFactory.
* @param context the current WebApplicationContext
* @param strategyInterface the strategy interface
* @return the List of corresponding strategy objects
*/
@SuppressWarnings("unchecked")
protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) {
String key = strategyInterface.getName();
String value = defaultStrategies.getProperty(key);
if (value != null) {
String[] classNames = StringUtils.commaDelimitedListToStringArray(value);
List<T> strategies = new ArrayList<>(classNames.length);
for (String className : classNames) {
try {
Class<?> clazz = ClassUtils.forName(className, DispatcherServlet.class.getClassLoader());
Object strategy = createDefaultStrategy(context, clazz);
strategies.add((T) strategy);
}
catch (ClassNotFoundException ex) {
throw new BeanInitializationException(
"Could not find DispatcherServlet's default strategy class [" + className +
"] for interface [" + key + "]", ex);
}
catch (LinkageError err) {
throw new BeanInitializationException(
"Unresolvable class definition for DispatcherServlet's default strategy class [" +
className + "] for interface [" + key + "]", err);
}
}
return strategies;
}
else {
return new LinkedList<>();
}
}