为什么整合jsp后必须通过spring-boot:run方式启动？

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背景

在Spring Boot - 整合Jsp/FreeMarker这篇文章中，我们用了两种启动方式

  1. mvn clean spring-boot:run

  2. main方法启动
    测试发现，通过maven启动能够正常渲染jsp页面，而通过main方法启动无法渲染，本文分析下原因。

分析

我们代码没有调整，只是启动方式不同，那么怀疑是classpath不一致！

  • mvn启动classpath

/Users/wanye/Code/springboot/target/classes//Users/wanye/.m2/repository/ch/qos/logback/logback-classic/1.1.9/logback-classic-1.1.9.jar/Users/wanye/.m2/repository/ch/qos/logback/logback-core/1.1.9/logback-core-1.1.9.jar/Users/wanye/.m2/repository/com/fasterxml/classmate/1.3.3/classmate-1.3.3.jar/Users/wanye/.m2/repository/com/fasterxml/jackson/core/jackson-annotations/2.8.0/jackson-annotations-2.8.0.jar/Users/wanye/.m2/repository/com/fasterxml/jackson/core/jackson-core/2.8.6/jackson-core-2.8.6.jar/Users/wanye/.m2/repository/com/fasterxml/jackson/core/jackson-databind/2.8.6/jackson-databind-2.8.6.jar/Users/wanye/.m2/repository/javax/servlet/jstl/1.2/jstl-1.2.jar/Users/wanye/.m2/repository/javax/validation/validation-api/1.1.0.Final/validation-api-1.1.0.Final.jar/Users/wanye/.m2/repository/org/apache/tomcat/embed/tomcat-embed-core/8.5.11/tomcat-embed-core-8.5.11.jar/Users/wanye/.m2/repository/org/apache/tomcat/embed/tomcat-embed-el/8.5.11/tomcat-embed-el-8.5.11.jar/Users/wanye/.m2/repository/org/apache/tomcat/embed/tomcat-embed-jasper/8.5.11/tomcat-embed-jasper-8.5.11.jar/Users/wanye/.m2/repository/org/apache/tomcat/embed/tomcat-embed-websocket/8.5.11/tomcat-embed-websocket-8.5.11.jar/Users/wanye/.m2/repository/org/hibernate/hibernate-validator/5.3.4.Final/hibernate-validator-5.3.4.Final.jar/Users/wanye/.m2/repository/org/jboss/logging/jboss-logging/3.3.0.Final/jboss-logging-3.3.0.Final.jar/Users/wanye/.m2/repository/org/slf4j/jcl-over-slf4j/1.7.22/jcl-over-slf4j-1.7.22.jar/Users/wanye/.m2/repository/org/slf4j/jul-to-slf4j/1.7.22/jul-to-slf4j-1.7.22.jar/Users/wanye/.m2/repository/org/slf4j/log4j-over-slf4j/1.7.22/log4j-over-slf4j-1.7.22.jar/Users/wanye/.m2/repository/org/slf4j/slf4j-api/1.7.22/slf4j-api-1.7.22.jar/Users/wanye/.m2/repository/org/springframework/boot/spring-boot-autoconfigure/1.5.1.RELEASE/spring-boot-autoconfigure-1.5.1.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/boot/spring-boot-starter-logging/1.5.1.RELEASE/spring-boot-starter-logging-1.5.1.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/boot/spring-boot-starter-tomcat/1.5.1.RELEASE/spring-boot-starter-tomcat-1.5.1.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/boot/spring-boot-starter-web/1.5.1.RELEASE/spring-boot-starter-web-1.5.1.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/boot/spring-boot-starter/1.5.1.RELEASE/spring-boot-starter-1.5.1.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/boot/spring-boot/1.5.1.RELEASE/spring-boot-1.5.1.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/spring-aop/4.3.6.RELEASE/spring-aop-4.3.6.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/spring-beans/4.3.6.RELEASE/spring-beans-4.3.6.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/spring-context/4.3.6.RELEASE/spring-context-4.3.6.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/spring-core/4.3.6.RELEASE/spring-core-4.3.6.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/spring-expression/4.3.6.RELEASE/spring-expression-4.3.6.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/spring-web/4.3.6.RELEASE/spring-web-4.3.6.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/spring-webmvc/4.3.6.RELEASE/spring-webmvc-4.3.6.RELEASE.jar/Users/wanye/.m2/repository/org/yaml/snakeyaml/1.17/snakeyaml-1.17.jar

  • Main启动classpath

/Users/wanye/Code/springboot/target/classes//Users/wanye/.m2/repository/ch/qos/logback/logback-classic/1.1.9/logback-classic-1.1.9.jar/Users/wanye/.m2/repository/ch/qos/logback/logback-core/1.1.9/logback-core-1.1.9.jar/Users/wanye/.m2/repository/com/fasterxml/classmate/1.3.3/classmate-1.3.3.jar/Users/wanye/.m2/repository/com/fasterxml/jackson/core/jackson-annotations/2.8.0/jackson-annotations-2.8.0.jar/Users/wanye/.m2/repository/com/fasterxml/jackson/core/jackson-core/2.8.6/jackson-core-2.8.6.jar/Users/wanye/.m2/repository/com/fasterxml/jackson/core/jackson-databind/2.8.6/jackson-databind-2.8.6.jar/Users/wanye/.m2/repository/javax/servlet/jstl/1.2/jstl-1.2.jar/Users/wanye/.m2/repository/javax/validation/validation-api/1.1.0.Final/validation-api-1.1.0.Final.jar/Users/wanye/.m2/repository/org/apache/tomcat/embed/tomcat-embed-core/8.5.11/tomcat-embed-core-8.5.11.jar/Users/wanye/.m2/repository/org/apache/tomcat/embed/tomcat-embed-el/8.5.11/tomcat-embed-el-8.5.11.jar/Users/wanye/.m2/repository/org/apache/tomcat/embed/tomcat-embed-websocket/8.5.11/tomcat-embed-websocket-8.5.11.jar/Users/wanye/.m2/repository/org/hibernate/hibernate-validator/5.3.4.Final/hibernate-validator-5.3.4.Final.jar/Users/wanye/.m2/repository/org/jboss/logging/jboss-logging/3.3.0.Final/jboss-logging-3.3.0.Final.jar/Users/wanye/.m2/repository/org/slf4j/jcl-over-slf4j/1.7.22/jcl-over-slf4j-1.7.22.jar/Users/wanye/.m2/repository/org/slf4j/jul-to-slf4j/1.7.22/jul-to-slf4j-1.7.22.jar/Users/wanye/.m2/repository/org/slf4j/log4j-over-slf4j/1.7.22/log4j-over-slf4j-1.7.22.jar/Users/wanye/.m2/repository/org/slf4j/slf4j-api/1.7.22/slf4j-api-1.7.22.jar/Users/wanye/.m2/repository/org/springframework/boot/spring-boot-autoconfigure/1.5.1.RELEASE/spring-boot-autoconfigure-1.5.1.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/boot/spring-boot-starter-logging/1.5.1.RELEASE/spring-boot-starter-logging-1.5.1.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/boot/spring-boot-starter-tomcat/1.5.1.RELEASE/spring-boot-starter-tomcat-1.5.1.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/boot/spring-boot-starter-web/1.5.1.RELEASE/spring-boot-starter-web-1.5.1.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/boot/spring-boot-starter/1.5.1.RELEASE/spring-boot-starter-1.5.1.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/boot/spring-boot/1.5.1.RELEASE/spring-boot-1.5.1.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/spring-aop/4.3.6.RELEASE/spring-aop-4.3.6.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/spring-beans/4.3.6.RELEASE/spring-beans-4.3.6.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/spring-context/4.3.6.RELEASE/spring-context-4.3.6.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/spring-core/4.3.6.RELEASE/spring-core-4.3.6.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/spring-expression/4.3.6.RELEASE/spring-expression-4.3.6.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/spring-web/4.3.6.RELEASE/spring-web-4.3.6.RELEASE.jar/Users/wanye/.m2/repository/org/springframework/spring-webmvc/4.3.6.RELEASE/spring-webmvc-4.3.6.RELEASE.jar/Users/wanye/.m2/repository/org/yaml/snakeyaml/1.17/snakeyaml-1.17.jar

  • 对比

192:~ wanye$ diff Desktop/mainsort Desktop/mvnsort12a13
> /Users/wanye/.m2/repository/org/apache/tomcat/embed/tomcat-embed-jasper/8.5.11/tomcat-embed-jasper-8.5.11.jar

对比后发现，通过main启动后classpath缺少tomcat-embed-jasper.jar；定位到这里，我们就可以解决这个问题了。

  • 解决方法：去掉将pom.xml中tomcat-embed-jasper依赖<scope>provided</scope>，依赖调整如下

<dependency>
    <groupId>org.apache.tomcat.embed</groupId>
    <artifactId>tomcat-embed-jasper</artifactId>
    <!--
    <scope>provided</scope>
    --></dependency>

还原现场，分析Spring Boot启动流程

在IDE里，直接运行的main函数：

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

Embead Tomcat的Servlet加载流程

  1. 判断是否在web环境，略，大家自己查阅相关资料，不是本文重点。

  2. spring boot通过TomcatEmbeddedServletContainerFactory来启动对应的web服务器。

//TomcatEmbeddedServletContainerFactory
    @Override
    public EmbeddedServletContainer getEmbeddedServletContainer(            ServletContextInitializer... initializers) {        Tomcat tomcat = new Tomcat();        File baseDir = (this.baseDirectory != null ? this.baseDirectory
                : createTempDir("tomcat"));        tomcat.setBaseDir(baseDir.getAbsolutePath());        Connector connector = new Connector(this.protocol);        tomcat.getService().addConnector(connector);        customizeConnector(connector);        tomcat.setConnector(connector);        tomcat.getHost().setAutoDeploy(false);        configureEngine(tomcat.getEngine());        for (Connector additionalConnector : this.additionalTomcatConnectors) {            tomcat.getService().addConnector(additionalConnector);
        }        // 初始化上下文，加载Servlet
        prepareContext(tomcat.getHost(), initializers);        return getTomcatEmbeddedServletContainer(tomcat);
    }

初始化Servlet

protected void prepareContext(Host host, ServletContextInitializer[] initializers) {
    ……    if (isRegisterDefaultServlet()) {
        addDefaultServlet(context);
    }    // 初始化JspServlet
    if (shouldRegisterJspServlet()) {
        addJspServlet(context);
        addJasperInitializer(context);        context.addLifecycleListener(new StoreMergedWebXmlListener());
    }
    ……
}// 判断是否加载jspServletprotected boolean shouldRegisterJspServlet() {    return this.jspServlet != null && this.jspServlet.getRegistered() && ClassUtils            .isPresent(this.jspServlet.getClassName(), getClass().getClassLoader());
}

shouldRegisterJspServlet是重点了，这里ClassUtils.isPresent会去classLoader里面加载jspServlet（org.apache.jasper.servlet.JspServlet)，但是classpath里面没有这个类。addJspServlet没有被执行

被吃掉的异常

技术分享

继续跟进（如果addJspServlet被执行）

技术分享

jspServlet对象被添加到Tomcat上下文中，并且以jsp扩展名为key放到HashMap中，接下来猜想，当有web请求会通过jsp扩展名去找到jspServlet对象，然后执行jsp。

验证

  • 访问http://localhost:8080/jsp/home，请求首先被tomcat容器拦截到，然后查找适合的servlet来处理（Tomcat本身是Servlet容器，通过servlet来响应请求，Spring也是通过注册servlet到tomcat才能处理请求的，当然Jsp也就是servlet），我们来看下核心类org.apache.catalina.mapper.Mapper

public void map(MessageBytes host, MessageBytes uri, String version,
                MappingData mappingData) throws IOException {    if (host.isNull()) {
        host.getCharChunk().append(defaultHostName);
    }
    host.toChars();
    uri.toChars();        // 通过uri，找servlet映射
    internalMap(host.getCharChunk(), uri.getCharChunk(), version,
            mappingData);
}// 匹配servlet的核心方法private final void internalMapWrapper(ContextVersion contextVersion,
                                      CharChunk path,
                                      MappingData mappingData) throws IOException {      // 很多匹配规则，这我们关注 Extension Match （扩展名）
    // Rule 3 -- Extension Match
    MappedWrapper[] extensionWrappers = contextVersion.extensionWrappers;    if (mappingData.wrapper == null && !checkJspWelcomeFiles) {
        internalMapExtensionWrapper(extensionWrappers, path, mappingData,                true);
    }    // Rule 7 -- Default servlet
    if (mappingData.wrapper == null && !checkJspWelcomeFiles) {        if (contextVersion.defaultWrapper != null) {              // 默认使用org.springframework.web.servlet.DispatcherServlet
            mappingData.wrapper = contextVersion.defaultWrapper.object;
     }}
}

我们的请求path是：/jsp/home，所以没有匹配Rule 3，而是使用默认servlet，那么这个请求被spring的dispatcherServlet接管，然后执行controller

@RequestMapping("/jsp/home")public String home() {    return "home";
}

方法执行完毕，spring会去加载home这个view，由于我们整合了jsp，所以通过配置spring.mvc.view.prefix=/WEB-INF/jsp/ 这个路径下，寻找home.jsp文件，然后将请求转发（这里大家需要了解一下，请求转发和重定向的区别）到/WEB-INF/jsp/home.jsp。

  • 处理jsp请求
    请求（WEB-INF/jsp/home.jsp）被Tomcat再次拦截，匹配Rule 3，通过扩展名jsp，在map中获取到jspServlet对象，给大家截图

技术分享

总结

简单总结一下，本文阐述的问题并不是日常开发中的主要问题（可能连主要问题都算不上，谁会用main去调试？？），但是遇到了就花时间来研究一下，还是有所收获的。

  1. 分析问题思路

  2. Spring Boot 初始化的部分流程

  3. 请求转发和重定向的区别

另外大家注意如果pom文件中<scope>去掉，再正常部署到tomcat容器中，会有jar冲突，建议大家试验过后，修改回去。

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