SPI机制
Tomcat启动的时候会去启动ServletContainerInitializer的实现类的onStartup方法,然后这个onStartup方法里面又会去调用HandlesTypes注解里面的接口的所有实现类的onStartup方法
/**
* 类的描述:Spring应用-启动(tomcat启动的时候,就回去扫描当前应用下导入jar包的META-INF/services)
* 的javax.servlet.ServiceContainerInitializer名称中的内容,改内容就是ServletContainerInitializer的实现类
* 全类名路径,然后会调用该实现的onstartup方法,并且我们可以在ServletContainerInitializer的实现类
* 上标注@HandlesTypes,配置WebApplicationInitiaizer接口,那么所有的WebApplicationInitiaizer接口
* 的实现类都会被传递到onStartup方法的入参中,然后判断传递进来的WebApplicationInitiaizer的实现是不是接口
* 或者不是抽象类,那么就会进行通过反射调用生成对象
*
*/
@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {
/**
* 这个onStartup方法是 Tomcat启动的时候就会调用的
*/
@Override
public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
throws ServletException {
List<WebApplicationInitializer> initializers = new LinkedList<>();
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 {
initializers.add((WebApplicationInitializer)
ReflectionUtils.accessibleConstructor(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");
// 对那些实现类进行排序
AnnotationAwareOrderComparator.sort(initializers);
for (WebApplicationInitializer initializer : initializers) {
// 调用@HandlesTypes注解里面的接口的所有实现类的onStartup方法
initializer.onStartup(servletContext);
}
}
}