SpringBoot 使用外部 Tomcat 容器运行

继承 SpringBootServletInitializer  类，重写 configure 方法

@SpringBootApplication
public class SpisApplication extends SpringBootServletInitializer {

    public static void main(String[] args) {
        SpringApplication.run(SpisApplication.class, args);
    }

    @Override
    protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
        return builder.sources(SpisApplication.class);
    }
}

原理

servlet 部分

从 servlet 3.0 开始，Tomcat 启动时会自动加载实现了 ServletContainerInitializer  接口的类（需要在 META-INF/services 目录下新建配置文件）。也称为 SPI 机制

package javax.servlet;

import java.util.Set;

public interface ServletContainerInitializer {
    public void onStartup(Set<Class<?>> c, ServletContext ctx) throws ServletException; 
}

spring 部分

SpringServletContainerInitializer 的 onStartup 会被执行，webAppInitializerClasses 参数是所有实现了 WebApplicationInitializer 接口的类（servlet 注解 @HandlesTypes(WebApplicationInitializer.class) 的作用）

package org.springframework.web;

import java.lang.reflect.Modifier;
import java.util.LinkedList;
import java.util.List;
import java.util.ServiceLoader;
import java.util.Set;

import javax.servlet.ServletContainerInitializer;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.HandlesTypes;

import org.springframework.core.annotation.AnnotationAwareOrderComparator;
import org.springframework.lang.Nullable;
import org.springframework.util.ReflectionUtils;

@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {


    @Override
    public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext) throws ServletException {

        List<WebApplicationInitializer> initializers = new LinkedList<>();

        if (webAppInitializerClasses != null) {
            for (Class<?> waiClass : webAppInitializerClasses) {
                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");
        AnnotationAwareOrderComparator.sort(initializers);
        for (WebApplicationInitializer initializer : initializers) {
            initializer.onStartup(servletContext);
        }
    }
}

再看 SpringBoot 启动类所需要继承的类

public abstract class SpringBootServletInitializer implements WebApplicationInitializer

就是这个接口，所以 Tomcat 启动时 SpringBoot 也就跟着启动了

public interface WebApplicationInitializer {
    void onStartup(ServletContext servletContext) throws ServletException;
}

附上利用 Servlet 该特性的 SSM 整合：https://www.cnblogs.com/jhxxb/p/10512553.html

SpringBoot 内嵌 Tomcat 的启动不是 SPI 机制，是直接启动的，可以参看这里的内嵌 Tomcat 启动类：https://www.cnblogs.com/jhxxb/p/13564329.html

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https://spring.io/blog/2014/03/07/deploying-spring-boot-applications

https://blog.csdn.net/f641385712/article/details/87474907