Springboot註冊Servlet幾種方式你都知道？內部實現原理解析
阿新 • • 發佈：2021-07-09
環境：springboot2.3.9.RELEASE
1 Servlet註冊
方式1：
在配置類（啟動類）上新增@ServletComponentScan註解
@SpringBootApplication @ServletComponentScan public class SpringBootComprehensiveApplication }
Servlet類上新增@WebServlet註解介面
@WebServlet("/hello")
public class MyServlet extends HttpServlet {
}
對應的Filter， Linstener有：@WebFilter, @WebListener
方式2：
通過向IOC容器新增ServletRegistrationBean方式；該種方式可以在Servlet中注入其它Bean或者讀取application.properties配置資訊等。對應的filter, Listener有對應的bean；FilterRegistrationBean,
ServletListenerRegistrationBean
@Bean
public ServletRegistrationBean<MyServlet> servlet() {
ServletRegistrationBean<MyServlet> servlet = new ServletRegistrationBean<>(new MyServlet()) ;
servlet.addUrlMappings("/hello") ;
return servlet ;
}方式3：
動態註冊Servlet
@Component
public class DynamicRegServlet implements ServletContextInitializer {
@Override
public void onStartup(ServletContext servletContext) throws ServletException {
ServletRegistration initServlet = servletContext.addServlet("myServlet", MyServlet.class) ;
initServlet.addMapping("/hello") ;
}
}該種方式是利用的Servlet3.0開始才有的功能，通過SPI技術在容器啟動的時候做一些初始化工作，比如註冊Servlet等。在Servle規範中通過
ServletContainerInitializer實現該功能。
該Servlet規範的開發流程如下；
1、配置
ServletContainerInitializer
在src/META-INF/services下新建
javax.servlet.ServletContainerInitializer檔案，檔案內容為ServletContainerInitializer介面的實現類（完整的包名+類名）如下：
com.pack.container.config.CustomServletInitializerCustomServletInitializer類
@HandlesTypes({ScoreWebInit.class})
public class CustomServletInitializer implements ServletContainerInitializer {
@Override
public void onStartup(Set<Class<?>> c, ServletContext ctx) throws ServletException {
c.forEach(web -> {
try {
((ScoreWebInit)web.newInstance()).pay() ;
} catch (InstantiationException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
});
ServletRegistration.Dynamic reg = ctx.addServlet("MyServlet", com.pack.servlet.MyServlet.class) ;
reg.setLoadOnStartup(1) ;
reg.addMapping("/hello") ;
}
}
注意：@HandlesTypes該註解會把屬性value配置的值（ScoreWebInt.class）對應的所有類都收集上然後在onStartup方法中的Set<Class>集合中應用。
在spring-web-xxxx.jar下就有通過該技術實現的相應功能。
2 掃描Servlet實現原理
在方式1中的實現原理就是掃描類路徑下所有@WebServlet，@WebFilter，@WebListener。找到所有的類後再通過方式2的方式進行註冊。下面將核心原始碼貼出
2.1 匯入核心類
// 該註解如果沒有配置basePackages或者basePackageClasses屬性，那麼會讀取當前新增@ServletComponentScan註解的類的包路徑進行掃描。
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(ServletComponentScanRegistrar.class)
public @interface ServletComponentScan {
}
// 註冊BeanDefinition
class ServletComponentScanRegistrar implements ImportBeanDefinitionRegistrar{
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
Set<String> packagesToScan = getPackagesToScan(importingClassMetadata);
if (registry.containsBeanDefinition(BEAN_NAME)) {
updatePostProcessor(registry, packagesToScan);
} else {
// 當前容器中沒有對應的Bean時執行該方法
addPostProcessor(registry, packagesToScan);
}
}
}
2.2 註冊BeanFactory 處理器
ServletComponentScanRegistrar最核心的功能就是註冊BeanFactoryPostProcessor
class ServletComponentScanRegistrar implements ImportBeanDefinitionRegistrar{
private void addPostProcessor(BeanDefinitionRegistry registry, Set<String> packagesToScan) {
GenericBeanDefinition beanDefinition = new GenericBeanDefinition();
beanDefinition.setBeanClass(ServletComponentRegisteringPostProcessor.class);
beanDefinition.getConstructorArgumentValues().addGenericArgumentValue(packagesToScan);
beanDefinition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
registry.registerBeanDefinition(BEAN_NAME, beanDefinition);
}
}
2.3 例項化掃描元件
進入
ServletComponentRegisteringPostProcessor類中首先這個類有個static程式碼塊
class ServletComponentRegisteringPostProcessor implements BeanFactoryPostProcessor, ApplicationContextAware {
private static final List<ServletComponentHandler> HANDLERS;
static {
List<ServletComponentHandler> servletComponentHandlers = new ArrayList<>();
servletComponentHandlers.add(new WebServletHandler());
servletComponentHandlers.add(new WebFilterHandler());
servletComponentHandlers.add(new WebListenerHandler());
HANDLERS = Collections.unmodifiableList(servletComponentHandlers);
}
}
這段程式碼分別是新增相應Servlet, Filter, Listener的處理控制代碼，分別處理@WebServlet，@WebFilter，@WebListener 註解。
檢視WebServletHandler
class WebServletHandler extends ServletComponentHandler {
WebServletHandler() {
super(WebServlet.class);
}
// 看到該方法也就十分清楚了最終找到所有的Class以後，通過ServletRegistrationBean進行註冊為Bean
@Override
public void doHandle(Map<String, Object> attributes, AnnotatedBeanDefinition beanDefinition,BeanDefinitionRegistry registry) {
BeanDefinitionBuilder builder = BeanDefinitionBuilder.rootBeanDefinition(ServletRegistrationBean.class);
builder.addPropertyValue("asyncSupported", attributes.get("asyncSupported"));
builder.addPropertyValue("initParameters", extractInitParameters(attributes));
builder.addPropertyValue("loadOnStartup", attributes.get("loadOnStartup"));
String name = determineName(attributes, beanDefinition);
builder.addPropertyValue("name", name);
builder.addPropertyValue("servlet", beanDefinition);
builder.addPropertyValue("urlMappings", extractUrlPatterns(attributes));
builder.addPropertyValue("multipartConfig", determineMultipartConfig(beanDefinition));
registry.registerBeanDefinition(name, builder.getBeanDefinition());
}
// other code
}
// 父類ServletComponentHandler;在父類總新增相應的過濾器（分別查詢相應註解的類,@WebServlet等。）
abstract class ServletComponentHandler {
private final Class<? extends Annotation> annotationType;
private final TypeFilter typeFilter;
protected ServletComponentHandler(Class<? extends Annotation> annotationType) {
this.typeFilter = new AnnotationTypeFilter(annotationType);
this.annotationType = annotationType;
}
}
接下來執行BeanFactoryPostProcessor對應的回撥方法了
class ServletComponentRegisteringPostProcessor implements BeanFactoryPostProcessor, ApplicationContextAware {
@Override
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
if (isRunningInEmbeddedWebServer()) {
ClassPathScanningCandidateComponentProvider componentProvider = createComponentProvider();
for (String packageToScan : this.packagesToScan) {
scanPackage(componentProvider, packageToScan);
}
}
}
}
createComponentProvider方法進行建立掃描相應符合條件的Bean掃描類
private ClassPathScanningCandidateComponentProvider createComponentProvider() {
ClassPathScanningCandidateComponentProvider componentProvider = new ClassPathScanningCandidateComponentProvider(false);
componentProvider.setEnvironment(this.applicationContext.getEnvironment());
componentProvider.setResourceLoader(this.applicationContext);
for (ServletComponentHandler handler : HANDLERS) {
componentProvider.addIncludeFilter(handler.getTypeFilter());
}
return componentProvider;
}
在該方法中為當前的
ClassPathScanningCandidateComponentProvider類掃描設定過濾器；過濾器在上面的static靜態程式碼塊中已經設定了WebServletHandler，WebFilterHandler，WebListenerHandler在父類中分別建立不同註解的new AnnotationTypeFilter(annotationType)過濾類。
建立完類掃描類以後開始掃描通過該類掃描相應包（路徑）下的所有類檔案 檢查是否有對應的註解。
2.4 查詢及註冊Bean
class ServletComponentRegisteringPostProcessor implements BeanFactoryPostProcessor, ApplicationContextAware {
private void scanPackage(ClassPathScanningCandidateComponentProvider componentProvider, String packageToScan) {
for (BeanDefinition candidate : componentProvider.findCandidateComponents(packageToScan)) {
if (candidate instanceof AnnotatedBeanDefinition) {
for (ServletComponentHandler handler : HANDLERS) {
handler.handle(((AnnotatedBeanDefinition) candidate), (BeanDefinitionRegistry) this.applicationContext);
}
}
}
}
}
findCandidateComponents方法查詢候選的（符合條件）的類並例項化為BeanDefinition物件。
方法執行鏈findCandidateComponents ---》scanCandidateComponents
在scanCandidateComponents方法中查詢符合條件的類，然後例項化為BeanDefinition
public class ClassPathScanningCandidateComponentProvider implements EnvironmentCapable, ResourceLoaderAware {
private Set<BeanDefinition> scanCandidateComponents(String basePackage) {
Set<BeanDefinition> candidates = new LinkedHashSet<>();
MetadataReader metadataReader = getMetadataReaderFactory().getMetadataReader(resource);
if (isCandidateComponent(metadataReader)) {
ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);
sbd.setSource(resource);
if (isCandidateComponent(sbd)) {
candidates.add(sbd);
}
}
return candidates;
}
protected boolean isCandidateComponent(MetadataReader metadataReader) throws IOException {
for (TypeFilter tf : this.excludeFilters) {
if (tf.match(metadataReader, getMetadataReaderFactory())) {
return false;
}
}
for (TypeFilter tf : this.includeFilters) {
if (tf.match(metadataReader, getMetadataReaderFactory())) {
return isConditionMatch(metadataReader);
}
}
return false;
}
}
在第二個for開始匹配所有的類是否有相關的註解。如果匹配上就相應的建立BeanDefinition物件放入集合Set中。
查詢到所有的類以後分別呼叫相應的Web*Handler（ServletComponentHandler）進行註冊Bean。
返回到上面的scanPackage方法中執行handler.handle方法。
abstract class ServletComponentHandler {
void handle(AnnotatedBeanDefinition beanDefinition, BeanDefinitionRegistry registry) {
Map<String, Object> attributes = beanDefinition.getMetadata().getAnnotationAttributes(this.annotationType.getName());
if (attributes != null) {
doHandle(attributes, beanDefinition, registry);
}
}
}
doHandle方法分別在子類（WebServletHandler，WebFilterHandler，WebListenerHandler）中實現，如上面已經提到的WebServletHandler類的doHandler方法。
到這裡我們知道了註解的方式最終也是被註冊為ServletRegistrationBean 例項。那這個ServletRegistrationBean又是如何被容器（Tomcat）所感知的呢？
2.5 Tomcat註冊Servlet
在Web容器下 BeanFactory使用的是
AnnotationConfigServletWebServerApplicationContext
Spring IOC容器核心方法是refresh方法
public abstract class AbstractApplicationContext extends DefaultResourceLoader implements ConfigurableApplicationContext {
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
try {
// Initialize other special beans in specific context subclasses.
onRefresh();
} catch (BeansException ex) {
throw ex;
} finally {
resetCommonCaches();
}
}
}
}
這裡只留了onRefresh方法，進入該方法：
onRefresh會進入到子類的方法
public class ServletWebServerApplicationContext extends GenericWebApplicationContext implements ConfigurableWebServerApplicationContext {
protected void onRefresh() {
super.onRefresh();
try {
createWebServer();
} catch (Throwable ex) {
throw new ApplicationContextException("Unable to start web server", ex);
}
}
}
進入createWebServer方法
public class ServletWebServerApplicationContext extends GenericWebApplicationContext implements ConfigurableWebServerApplicationContext {
private void createWebServer() {
WebServer webServer = this.webServer;
ServletContext servletContext = getServletContext();
if (webServer == null && servletContext == null) {
ServletWebServerFactory factory = getWebServerFactory();
this.webServer = factory.getWebServer(getSelfInitializer());
getBeanFactory().registerSingleton("webServerGracefulShutdown",new WebServerGracefulShutdownLifecycle(this.webServer));
getBeanFactory().registerSingleton("webServerStartStop", new WebServerStartStopLifecycle(this, this.webServer));
} else if (servletContext != null) {
try {
getSelfInitializer().onStartup(servletContext);
} catch (ServletException ex) {
throw new ApplicationContextException("Cannot initialize servlet context", ex);
}
}
initPropertySources();
}
}
這裡會進入factory.getWebServer(getSelfInitializer())方法執行
進入getSelfInitializer()方法
public class ServletWebServerApplicationContext extends GenericWebApplicationContext implements ConfigurableWebServerApplicationContext {
private org.springframework.boot.web.servlet.ServletContextInitializer getSelfInitializer() {
return this::selfInitialize;
}
private void selfInitialize(ServletContext servletContext) throws ServletException {
prepareWebApplicationContext(servletContext);
registerApplicationScope(servletContext);
WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);
for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
beans.onStartup(servletContext);
}
}
}
這裡的for迴圈是遍歷當前容器中所有ServletContextInitializer型別的Bean。ServletRegistrationBean就是繼承ServletContextInitializer
到這裡分別呼叫ServletContextInitializer的onStartup方法，進入onStartup方法：
public abstract class RegistrationBean implements ServletContextInitializer, Ordered {
@Override
public final void onStartup(ServletContext servletContext) throws ServletException {
String description = getDescription();
if (!isEnabled()) {
logger.info(StringUtils.capitalize(description) + " was not registered (disabled)");
return;
}
register(description, servletContext);
}
// 在子類中實現
protected abstract void register(String description, ServletContext servletContext);
}
進入子類DynamicRegistrationBean
1 public abstract class DynamicRegistrationBean<D extends Registration.Dynamic> extends RegistrationBean {
2 @Override
3 protected final void register(String description, ServletContext servletContext) {
4 D registration = addRegistration(description, servletContext);
5 if (registration == null) {
6 logger.info(StringUtils.capitalize(description) + " was not registered (possibly already registered?)");
7 return;
8 }
9 // 配置Servlet Mapping相關的資訊（在子類ServletRegistrationBean中實現的）
10 configure(registration);
11 }
12 // 子類中實現
13 protected abstract D addRegistration(String description, ServletContext servletContext);
14 }
進入子類ServletRegistrationBean
public class ServletRegistrationBean<T extends Servlet> extends DynamicRegistrationBean<ServletRegistration.Dynamic> {
@Override
protected ServletRegistration.Dynamic addRegistration(String description, ServletContext servletContext) {
String name = getServletName();
return servletContext.addServlet(name, this.servlet);
}
}
到這裡就是通過ServletContext來動態註冊Servlet（Servilet3.0）。
這裡返回了
ServletRegistration.Dynamic物件會繼續執行configure方法配置urlMapping等資訊。
public class ServletRegistrationBean<T extends Servlet> extends DynamicRegistrationBean<ServletRegistration.Dynamic> {
@Override
protected void configure(ServletRegistration.Dynamic registration) {
super.configure(registration);
String[] urlMapping = StringUtils.toStringArray(this.urlMappings);
if (urlMapping.length == 0 && this.alwaysMapUrl) {
urlMapping = DEFAULT_MAPPINGS;
}
if (!ObjectUtils.isEmpty(urlMapping)) {
registration.addMapping(urlMapping);
}
registration.setLoadOnStartup(this.loadOnStartup);
if (this.multipartConfig != null) {
registration.setMultipartConfig(this.multipartConfig);
}
}
}
到此在Springboot環境下Servlet如何被註冊到Servlet容器中就已經清晰了。這動態註冊Servlet的相關API都是在Servlet3.0規範中才有的。
完畢！！！
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