一、场景

Springboot使用@ServerEndpoint来建立websocket链接。引入依赖。

<dependency>  
    <groupId>org.springframework.boot</groupId>  
    <artifactId>spring-boot-starter-websocket</artifactId>  
</dependency>

配置Websocket

@Configuration  
@EnableWebSocket  
public class WebSocketConfig {  
  
    @Bean  
    public ServerEndpointExporter serverEndpointExporter() {  
        return new ServerEndpointExporter();  
    }  
}

二、报错信息

springboot项目添加websocket依赖后运行测试类报如下错误：

Error starting ApplicationContext. To display the conditions report re-run your application with 'debug' enabled.
2024-01-15 10:27:30.908 ERROR 20552 --- [           main] o.s.boot.SpringApplication               : Application run failed

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'serverEndpointExporter' defined in class path resource [org/springblade/lab/external/webScoket/WebSocketConfig.class]: Invocation of init method failed; nested exception is java.lang.IllegalStateException: javax.websocket.server.ServerContainer not available
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1804)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:620)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:542)
	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335)
	at org.springframework.beans.factory.support.AbstractBeanFactory$$Lambda$211/1936375962.getObject(Unknown Source)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:955)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:929)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:591)
	at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:147)
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:732)
	at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:409)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:308)
	at org.springframework.boot.builder.SpringApplicationBuilder.run(SpringApplicationBuilder.java:164)
	at org.springblade.core.launch.BladeApplication.run(BladeApplication.java:49)
	at org.springblade.Application.main(Application.java:33)
Caused by: java.lang.IllegalStateException: javax.websocket.server.ServerContainer not available
	at org.springframework.util.Assert.state(Assert.java:76)
	at org.springframework.web.socket.server.standard.ServerEndpointExporter.afterPropertiesSet(ServerEndpointExporter.java:107)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1863)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1800)
	... 17 common frames omitted

三、排查思路

报的错误是创建ServerEndpointExporterBean失败，原因是ServerContainer不可用，那么我们就去看到ServerContainer在ServerEndpointExporter中是怎么注入的。

点进去ServerEndpointExporter()类，
在第49行代码打上debug跑一下

选中第50行代码，右键，执行计算，或者使用快捷键Crtl+U

计算结果发现是null

四、分析原因

为什么servletContext会返回null，定位到 ServerContainer 类，发现他是一个接口，那必定注入的时候是有相应的实现类，点击查看实现，居然有五个实现类，那就可以推断是依赖冲突导致不知道要注入哪个实现，最后获取Bean的时候返回了null。

点进去getAttribute方法，查看实现类

这里有五个实现类，三个是tomcat的，一个是undertow的，还有一个是springframework的，所以解决办法就是排除掉其他的实现类，只保留springframework这个就行。

五、解决办法

安装Maven Helper插件，打开项目的pom.xml文件，点击pom文件左下角的Dependency Analyzer排除掉多余的依赖。

最终结果：

      <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
            <!--添加以下排除方法-->
            <exclusions>
                <exclusion>
                    <groupId>org.springframework.boot</groupId>
                    <artifactId>spring-boot-starter-tomcat</artifactId>
                </exclusion>
            </exclusions>
        </dependency>

    <!--websocket服务-->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-websocket</artifactId>
             <!--添加以下排除方法-->
            <exclusions>
                <exclusion>
                    <artifactId>spring-boot-starter-web</artifactId>
                    <groupId>org.springframework.boot</groupId>
                </exclusion>
            </exclusions>
        </dependency>

再次启动，一切正常！