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오류 Cannot invoke "jakarta.servlet.http.HttpSession.getServletContext()" because "session" is null

오류
java.lang.NullPointerException: Cannot invoke "jakarta.servlet.http.HttpSession.getServletContext()" because "session" is null
at com.ezen.demo.ws.HttpSessionConfig.modifyHandshake(HttpSessionConfig.java:17) ~[classes/:na]
at org.apache.tomcat.websocket.server.UpgradeUtil.doUpgrade(UpgradeUtil.java:227) ~[tomcat-embed-websocket-10.1.4.jar:10.1.4]
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:78) ~[tomcat-embed-websocket-10.1.4.jar:10.1.4]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:185) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:158) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) ~[spring-web-6.0.3.jar:6.0.3]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.0.3.jar:6.0.3]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:185) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:158) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) ~[spring-web-6.0.3.jar:6.0.3]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.0.3.jar:6.0.3]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:185) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:158) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) ~[spring-web-6.0.3.jar:6.0.3]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.0.3.jar:6.0.3]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:185) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:158) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:177) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:542) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:119) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:357) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:400) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:859) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1734) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:52) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) ~[tomcat-embed-core-10.1.4.jar:10.1.4]
at java.base/java.lang.Thread.run(Thread.java:833) ~[na:na]

내용
/** ServerEndPoint 클래스(웹소켓 서버)에서 접근할 수 있는 설정자 클래스 */
public class HttpSessionConfig extends Configurator {
@Override
public void modifyHandshake(
/* 아래의 config 에 저장한 내용은 ServerEndPoint 클래스에 전달된다 */
ServerEndpointConfig config, HandshakeRequest request, HandshakeResponse response) {
HttpSession session = (HttpSession) request.getHttpSession();
ServletContext context = session.getServletContext();

/* 위에서 구한 HttpSession 객체의 참조를 config에 저장한다 */
config.getUserProperties().put("session", session);
// 아래처럼 다수개의 값도 저장 가능
config.getUserProperties().put("context", context);
}
}

해결
@WebListener
public class RequestListener implements ServletRequestListener {

    @Override
    public void requestDestroyed(ServletRequestEvent servletRequestEvent) {
    }

    @Override
    public void requestInitialized(ServletRequestEvent servletRequestEvent) {
        ((HttpServletRequest)servletRequestEvent.getServletRequest()).getSession();
    }
}
@SpringBootApplication
@ServletComponentScan
public class Application {

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}
출처:https://stackoverflow.com/questions/20240591/websocket-httpsession-returns-null

추가
웹소켓
- Thymeleaf, Websocket
- HttpSession 전달(Controller -> WebSocket)
- Interceptor 방식으로 ServletContext 전달
- page, request, session, application
- application : 웹서버에 한개만 존재
- application = ServletContext
- Controller에서 ServletContext에 HttpSession의 참조를 저장
- 인터셉터를 통해 ServletContext를 WebSocket으로 전달
- WebSocket에서는 WebSocketSession을 통해서 전달된 속성 값을 추출할 수 있다
- WebSocket에서는 먼저 ServletContext를 추출하고 ServletContext를 통해 HttpSession객체를 추출할 수 있다.

클래스 4개
- Controller : 로그인, ServletContext객체에 HttpSession객체를 저장
- Configurer : @Configuration, WebSocketHandler 등록, Interceptor 등록
설정파일
- pom.xml : dependency추가

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