嵌入式Servlet容器:
优点:简单,便携;
缺点:默认不支持jsp,优化定制比较复杂;
使用外置Servlet容器的步骤:
1)、必须创建一个war项目,需要建立好web项目的目录结构,特别是webapp/WEB-INF/web.xml;
2)、嵌入式的Tomcat依赖的scope指定为provided;
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-tomcat</artifactId>
<scope>provided</scope>
</dependency>
3)、必须编写一个SpringBootServletInitializer类的子类,并重写configure方法;
public class ServletInitializer extends SpringBootServletInitializer {
@Override
protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {
return application.sources(SpringBoot04WebJspApplication.class);
}
}
4)、启动服务器。
jar包和war包启动区别:
jar包:执行SpringBootApplication的run方法,启动IOC容器,然后创建嵌入式的Servlet容器。
war包:启动Servlet服务器,服务器启动SpringBoot应用【SpringBootServletInitializer】,然后才启动IOC容器。
Servlet3.0+规则:
1)、服务器启动(web应用启动)会创建当前web应用里面每一个jar包里面ServletContainerInitializer实例;
2)、ServletContainerInitializer的实现放在jar包的META-INF/services文件夹下,有个名为javax.servlet.ServletContainerInitializer的文件,内容就是ServletContainerInitializer实现类的全类名;
3)、还可以使用@HandlesTypes注解,在应用启动的时候加载指定的类。
流程&原理:
1)、启动Tomcat;
2)、根据上述描述的Servlet3.0+规则,可以在Spring的web模块里面找到有个文件名为javax.servlet.ServletContainerInitializer的文件,而文件的内容为org.springframework.web.SpringServletContainerInitializer,用于加载SpringServletContainerInitializer类;
3)、看看SpringServletContainerInitializer类定义:
@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {
/**
* Delegate the {@code ServletContext} to any {@link WebApplicationInitializer}
* implementations present on the application classpath.
* <p>Because this class declares @{@code HandlesTypes(WebApplicationInitializer.class)},
* Servlet 3.0+ containers will automatically scan the classpath for implementations
* of Spring's {@code WebApplicationInitializer} interface and provide the set of all
* such types to the {@code webAppInitializerClasses} parameter of this method.
* <p>If no {@code WebApplicationInitializer} implementations are found on the classpath,
* this method is effectively a no-op. An INFO-level log message will be issued notifying
* the user that the {@code ServletContainerInitializer} has indeed been invoked but that
* no {@code WebApplicationInitializer} implementations were found.
* <p>Assuming that one or more {@code WebApplicationInitializer} types are detected,
* they will be instantiated (and <em>sorted</em> if the @{@link
* org.springframework.core.annotation.Order @Order} annotation is present or
* the {@link org.springframework.core.Ordered Ordered} interface has been
* implemented). Then the {@link WebApplicationInitializer#onStartup(ServletContext)}
* method will be invoked on each instance, delegating the {@code ServletContext} such
* that each instance may register and configure servlets such as Spring's
* {@code DispatcherServlet}, listeners such as Spring's {@code ContextLoaderListener},
* or any other Servlet API componentry such as filters.
* @param webAppInitializerClasses all implementations of
* {@link WebApplicationInitializer} found on the application classpath
* @param servletContext the servlet context to be initialized
* @see WebApplicationInitializer#onStartup(ServletContext)
* @see AnnotationAwareOrderComparator
*/
@Override
public void onStartup(Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
throws ServletException {
List<WebApplicationInitializer> initializers = new LinkedList<WebApplicationInitializer>();
if (webAppInitializerClasses != null) {
for (Class<?> waiClass : webAppInitializerClasses) {
// Be defensive: Some servlet containers provide us with invalid classes,
// no matter what @HandlesTypes says...
if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&
WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
try {
//为所有的WebApplicationInitializer类型的类创建实例,并加入到集合中
initializers.add((WebApplicationInitializer) waiClass.newInstance());
}
catch (Throwable ex) {
throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);
}
}
}
}
if (initializers.isEmpty()) {
servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
return;
}
servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
AnnotationAwareOrderComparator.sort(initializers);
//调用每一个WebApplicationInitializer实例的onStartup方法
for (WebApplicationInitializer initializer : initializers) {
initializer.onStartup(servletContext);
}
}
}
在上面一段长长的注释中可以看到,SpringServletContainerInitializer将@HandlesTypes(WebApplicationInitializer.class)标注的所有WebApplicationInitializer这个类型的类都传入到onStartup方法的Set参数中,并通过反射为这些WebApplicationInitializer类型的类创建实例;
4)、函数最后,每一个WebApplicationInitializer实例都调用自己的onStartup方法;
public interface WebApplicationInitializer {
void onStartup(ServletContext servletContext) throws ServletException;
}
5)、而WebApplicationInitializer有个抽象实现类SpringBootServletInitializer(记住我们继承了该抽象类),则会调用每一个WebApplicationInitializer实例(包括SpringBootServletInitializer)的onStartup方法:
public abstract class SpringBootServletInitializer implements WebApplicationInitializer {
//other code...
@Override
public void onStartup(ServletContext servletContext) throws ServletException {
// Logger initialization is deferred in case a ordered
// LogServletContextInitializer is being used
this.logger = LogFactory.getLog(getClass());
//创建IOC容器
WebApplicationContext rootAppContext = createRootApplicationContext(
servletContext);
if (rootAppContext != null) {
servletContext.addListener(new ContextLoaderListener(rootAppContext) {
@Override
public void contextInitialized(ServletContextEvent event) {
// no-op because the application context is already initialized
}
});
}
else {
this.logger.debug("No ContextLoaderListener registered, as "
+ "createRootApplicationContext() did not "
+ "return an application context");
}
}
protected WebApplicationContext createRootApplicationContext(
ServletContext servletContext) {
//创建Spring应用构建器,并进行相关属性设置
SpringApplicationBuilder builder = createSpringApplicationBuilder();
StandardServletEnvironment environment = new StandardServletEnvironment();
environment.initPropertySources(servletContext, null);
builder.environment(environment);
builder.main(getClass());
ApplicationContext parent = getExistingRootWebApplicationContext(servletContext);
if (parent != null) {
this.logger.info("Root context already created (using as parent).");
servletContext.setAttribute(
WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, null);
builder.initializers(new ParentContextApplicationContextInitializer(parent));
}
builder.initializers(
new ServletContextApplicationContextInitializer(servletContext));
builder.contextClass(AnnotationConfigEmbeddedWebApplicationContext.class);
//调用configure方法,创建war类型的web项目后,由于编写SpringBootServletInitializer的子类重写configure方法,所以此处调用的是我们定义的子类重写的configure方法
builder = configure(builder);
//通过构建器构建了一个Spring应用
SpringApplication application = builder.build();
if (application.getSources().isEmpty() && AnnotationUtils
.findAnnotation(getClass(), Configuration.class) != null) {
application.getSources().add(getClass());
}
Assert.state(!application.getSources().isEmpty(),
"No SpringApplication sources have been defined. Either override the "
+ "configure method or add an @Configuration annotation");
// Ensure error pages are registered
if (this.registerErrorPageFilter) {
application.getSources().add(ErrorPageFilterConfiguration.class);
}
//启动Spring应用
return run(application);
}
//Spring应用启动,创建并返回IOC容器
protected WebApplicationContext run(SpringApplication application) {
return (WebApplicationContext) application.run();
}
}
SpringBootServletInitializer实例执行onStartup方法的时候会通过createRootApplicationContext方法来执行run方法,接下来的过程就同以jar包形式启动的应用的run过程一样了,在内部会创建IOC容器并返回,只是以war包形式的应用在创建IOC容器过程中,不再创建Servlet容器了。