ServletContext-Attribut ist null, wenn auf es über erholsame Web-Service in Jersey-Implementierung zugreifen

Question

Ich habe eine Java MVC-Anwendung mit einem einzigen Controller. Die Datenbank wird in der Servletkontext-Listener-Klasse initialisiert. Ich übergebe das Datenbankobjekt als Servlet-Context-Attribut an das Controller-Servlet und die Webservice-Klasse. Im Controller-Servlet funktioniert das "db" -Attribut gut, aber in der Web-Service-Klasse erhalte ich eine ungültige und ungültige Status-Ausnahme.

Fehler Stacktrace -

javax.servlet.ServletException: A MultiException has 2 exceptions. They 
are: 
1. java.lang.NullPointerException 
2. java.lang.IllegalStateException: Unable to perform operation: create on 
com.home.mystorywriter.UserWS 

org.glassfish.jersey.servlet.WebComponent.serviceImpl(WebComponent.java:489) 
org.glassfish.jersey.servlet.WebComponent.service(WebComponent.java:427) 
org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java: 388) 
org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java: 341) 
org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java: 228) 
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) 
root cause 
java.lang.NullPointerException 
at com.home.mystorywriter.UserWS.<init>(UserWS.java:36) 
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) 

WebService-Klasse

package com.home.mystorywriter; 
import java.util.ArrayList; 
import java.util.List; 
import javax.servlet.ServletContext; 
import javax.servlet.http.HttpServletRequest; 
import javax.servlet.http.HttpServletResponse; 
import javax.ws.rs.GET; 
import javax.ws.rs.Path; 
import javax.ws.rs.Produces; 
import javax.ws.rs.core.Context; 
import javax.ws.rs.core.MediaType; 
import com.google.gson.Gson; 

@Path("/users") 

public class UserWS { 

@Context 
private HttpServletRequest req; 
private HttpServletResponse res; 
private ServletContext context ; 
DAOdb db = (DAOdb)req.getServletContext().getAttribute("db"); 

@GET 
@Produces(MediaType.APPLICATION_JSON) 
@Path("allUsers") 
public String getAllUsers() { 
List<Profile> userlist = db.getProfileListOfAllUsers(); 
Gson gson = new Gson(); 
String jsonUsers = gson.toJson(userlist); 
return jsonUsers; 
} 

} 

Web.xml

<?xml version="1.0" encoding="ISO-8859-1"?> 
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xmlns="http://xmlns.jcp.org/xml/ns/javaee"  
si:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee 
http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" version="3.1"> 
<listener> 
    <description>MyServletContextListener</description> 
    <listener- 
     class>com.home.mystorywriter.MyServletContextListener</listener-class> 
</listener> 
<servlet> 
<servlet-name>StoryServlet</servlet-name> 
<servlet-class>com.home.mystorywriter.StoryServlet</servlet-class> 
</servlet> 
<servlet> 
<servlet-name>Jersey REST Service</servlet-name> 
<servlet-class>org.glassfish.jersey.servlet.ServletContainer</servlet-class> 
<init-param> 
    <param-name>jersey.config.server.provider.packages</param-name> 
    <param-value>com.home.mystorywriter</param-value> 
</init-param> 
<init-param> 
    <param-name>com.sun.jersey.api.json.POJOMappingFeature</param-name> 
    <param-value>true</param-value> 
</init-param> 
</servlet> 
<servlet-mapping> 
<servlet-name>StoryServlet</servlet-name> 
<url-pattern>/StoryServlet</url-pattern> 
</servlet-mapping> 
<servlet-mapping> 
<servlet-name>Jersey REST Service</servlet-name> 
<url-pattern>/rest/*</url-pattern> 
</servlet-mapping> 
<welcome-file-list> 
<welcome-file>login.html</welcome-file> 
</welcome-file-list> 
</web-app> 

Answer 1

@Context Injektion innerhalb der Methode Web-Service-Klasse oder der Klasse geschieht. Der folgende Code funktioniert einwandfrei.

@Path("/users") 

public class UserWS { 
private ServletContext context ; 
DAOdb db; 
public UserWS() { 
System.out.println("Inside Constructor " + context); 
} 
@Context 
public void setServletContext(ServletContext context) { 
System.out.println("servlet context set here"); 
this.context = context; 
db = (DAOdb)context.getAttribute("db"); 
} 
@GET 
@Produces(MediaType.APPLICATION_JSON) 
@Path("/allUsers") 
public String getAllUsers() { 
System.out.println("Inside get method:"+ context); 
System.out.println("db value"+ db); 
Profile userlist = db.getProfileWithoutPic(); 
Gson gson = new Gson(); 
String jsonUsers = gson.toJson(userlist); 
return jsonUsers; 
} 
}