SpringBoot 使用外部Tomcat方法及啓動原理
基於 SpringBoot 2.x
方法
- 一、必須是一個war項目,利用IDEA可以直接創建,或者是修改pom.xml文件
<packaging>war</packaging>
- 二、將內置Tomcat的作用範圍修改成provided
dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-tomcat</artifactId>
<scope>provided</scope>
</dependency>
- 三、自定義一個類繼承 SpringBootServletInitializer 重寫其configure()方法
public class ServletInitializer extends SpringBootServletInitializer {
@Override
protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
// 傳入SpringBoot的主程序類
return builder.sources(SbdemoApplication.class);
}
}
啓動原理
前提
- 根據Servlet3.0規範,服務器啓動(web應用啓動)會創建當前web應用裏面每一個jar包裏面ServletContainerInitializer實例。
- ServletContainerInitializer的實現放在jar包的META-INF/services文件夾下,有一個名爲javax.servlet.ServletContainerInitializer的文件,內容就是ServletContainerInitializer的實現類的全類名。
- 使用@HandlesTypes,在應用啓動的時候加載我們感興趣的類。
SpringBoot中啓動流程
- 一、SpringBoot 中的 ServletContainerInitializer 實現類位置在spring-web模塊下
文件內容:
org.springframework.web.SpringServletContainerInitializer
- 二、SpringServletContainerInitializer類
@HandlesTypes({WebApplicationInitializer.class})
public class SpringServletContainerInitializer implements ServletContainerInitializer {
public SpringServletContainerInitializer() {
}
public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext) throws ServletException {
List<WebApplicationInitializer> initializers = new LinkedList();
Iterator var4;
if (webAppInitializerClasses != null) {
var4 = webAppInitializerClasses.iterator();
while(var4.hasNext()) {
Class<?> waiClass = (Class)var4.next();
if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) && WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
try {
// 將@HandlesTypes(WebApplicationInitializer.class)標註的所有這個類型的類都傳入到onStartup方法的Set<Class<?>>;爲這些WebApplicationInitializer類型的類創建實例。
initializers.add((WebApplicationInitializer)ReflectionUtils.accessibleConstructor(waiClass, new Class[0]).newInstance());
} catch (Throwable var7) {
throw new ServletException("Failed to instantiate WebApplicationInitializer class", var7);
}
}
}
}
if (initializers.isEmpty()) {
servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
} else {
servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
AnnotationAwareOrderComparator.sort(initializers);
var4 = initializers.iterator();
while(var4.hasNext()) {
WebApplicationInitializer initializer = (WebApplicationInitializer)var4.next();
// 每一個WebApplicationInitializer都調用自己的onStartup()
initializer.onStartup(servletContext);
}
}
}
}
- 三、Tomcat 啓動引導類
// ServletInitializer 繼承 SpringBootServletInitializer
public class ServletInitializer extends SpringBootServletInitializer {
@Override
protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
return builder.sources(SbdemoApplication.class);
}
}
SpringBootServletInitializer 繼承 WebApplicationInitializer 類,就是SpringServletContainerInitializer類的@HandlesTypes({WebApplicationInitializer.class}), 下面展示的是 onStartup() 、createRootApplicationContext() 和 configure() 方法
public abstract class SpringBootServletInitializer implements WebApplicationInitializer {
public void onStartup(ServletContext servletContext) throws ServletException {
this.logger = LogFactory.getLog(this.getClass());
// 創建web應用容器
WebApplicationContext rootAppContext = this.createRootApplicationContext(servletContext);
if (rootAppContext != null) {
servletContext.addListener(new ContextLoaderListener(rootAppContext) {
public void contextInitialized(ServletContextEvent event) {
}
});
} else {
this.logger.debug("No ContextLoaderListener registered, as createRootApplicationContext() did not return an application context");
}
}
protected WebApplicationContext createRootApplicationContext(ServletContext servletContext) {
// 1、創建SpringApplicationBuilder
SpringApplicationBuilder builder = this.createSpringApplicationBuilder();
StandardServletEnvironment environment = new StandardServletEnvironment();
// 2、準備環境
environment.initPropertySources(servletContext, (ServletConfig)null);
builder.environment(environment);
builder.main(this.getClass());
ApplicationContext parent = this.getExistingRootWebApplicationContext(servletContext);
if (parent != null) {
this.logger.info("Root context already created (using as parent).");
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, (Object)null);
builder.initializers(new ApplicationContextInitializer[]{new ParentContextApplicationContextInitializer(parent)});
}
// 3、初始化
builder.initializers(new ApplicationContextInitializer[]{new ServletContextApplicationContextInitializer(servletContext)});
builder.contextClass(AnnotationConfigServletWebServerApplicationContext.class);
// 4、調用configure方法,子類重寫了這個方法,將SpringBoot的主程序類傳入了進來
builder = this.configure(builder);
SpringApplication application = builder.build();
if (application.getAllSources().isEmpty() && AnnotationUtils.findAnnotation(this.getClass(), Configuration.class) != null) {
application.addPrimarySources(Collections.singleton(this.getClass()));
}
Assert.state(!application.getAllSources().isEmpty(), "No SpringApplication sources have been defined. Either override the configure method or add an @Configuration annotation");
if (this.registerErrorPageFilter) {
application.addPrimarySources(Collections.singleton(ErrorPageFilterConfiguration.class));
}
// 5、啓動Spring應用
return this.run(application);
}
}
// 此方法被啓動引導類 ServletInitializer有方法重寫, 傳入的是應用構建器SpringApplicationBuilder, 也就是SpringBoot的主程序類
protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
return builder;
}
個人隨筆,如有錯誤,歡迎指正!