在 doGet 或 doPost 之外调用 EJB 会话

Question

我在弄清楚我是否可以在不采用 HttpServletRequest 和 HttpServletResponse 参数的方法中调用会话 Bean 的方法时遇到了一些问题？

意识到代码在doGet和doPost方法中工作后，我修改了一个定制的方法来获取请求和响应的参数，突然它开始工作了。

我的目标是让方法工作（调用会话 bean 方法），而无需在方法参数中发出请求和响应。

然而，这是有问题的版本的代码，它在itemRegistrationSession.openConnection（）上授予NullPointerException;

package web;
import java.io.IOException;
import java.util.ArrayList;
import javax.ejb.EJB;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import auction.itemRegistrationRemote;
import auction.userRegistrationRemote;
/**
 * Servlet implementation class processItem
 */
@WebServlet("/processItem")
public class processItem extends HttpServlet {
    private static final long serialVersionUID = 1L;
    @EJB itemRegistrationRemote itemRegistrationSession;
    @EJB userRegistrationRemote processRegistrationSession;
    /**
     * @see HttpServlet#HttpServlet()
     */
    public processItem() {
        super();
        getCategories();
    }
    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //ArrayList<String> categories = getCategories(request, response);
    }
    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    }
    protected ArrayList<String> getCategories() {
        System.out.println("Hello from getCategories()");
        itemRegistrationSession.openConnection();
        System.out.println("Hello2 from getCategories()");
        ArrayList<String> categories = itemRegistrationSession.getCategories();
        itemRegistrationSession.closeConnection();
        return categories;
    }
}

---

package auction;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import javax.ejb.LocalBean;
import javax.ejb.Singleton;
/**
 * Session Bean implementation class itemRegistration
 */
@Singleton
public class itemRegistration implements itemRegistrationRemote {
    Connection con;
    PreparedStatement prepStat;
    ResultSet resultSet;
    final String ITEM_NAME = "ITEM_NAME";
    final String CATEGORY_NAME = "CATEGORY_NAME";
    final String USER_NAME = "USER_NAME";
    final String ITEM_MODEL = "ITEM_MODEL";
    final String ITEM_DESCRIPTION = "ITEM_DESCRIPTION";
    /**
     * Default constructor. 
     */
    public itemRegistration() {
        // TODO Auto-generated constructor stub
    }
    @Override
    public void openConnection() {
        System.out.println("Openning connection for itemRegistration");
        try {
            String driver = "org.apache.derby.jdbc.EmbeddedDriver";
            Class.forName(driver).newInstance();
        } catch (InstantiationException e) {
            System.out.println("Openning database connection - Instantiation Exception " + e.getMessage());
        } catch (IllegalAccessException e) {
            System.out.println("Openning database connection - IllegalAccessException " + e.getMessage());
        } catch (ClassNotFoundException e) {
            System.out.println("Openning database connection - ClassNotFoundException " + e.getMessage());
        }
        try {
            con = DriverManager.getConnection("jdbc:derby:C:/Users/Adrian/MyDB;upgrade=true", "adrian", "testero");
        } catch (SQLException e) {
            System.out.println("Openning database connection - SQLException " + e.getMessage());
        }
    } // End of openConnection method
    @Override
    public void closeConnection() {
        try {
            resultSet.close();
            prepStat.close();
            con.close();
        } catch (SQLException e) {
            System.out.println("Closing database connection - SQLException " + e.getMessage());
        }
    } // End of closeConnection method
    @Override
    public ArrayList<String> getCategories() {
        ArrayList<String> categories = new ArrayList<String>();
        try {
            prepStat = con.prepareStatement("SELECT * FROM CATEGORY");
            resultSet = prepStat.executeQuery();
            int i = 1;
            while (resultSet.next()) {
                categories.add(resultSet.getString(i));
                i++;
            }
        } catch (SQLException e) {
            System.out.println("getCategories SQLException - " + e.getMessage());
        }
        return categories;
    }
}

Answer 1

听起来像

是

@EJB itemRegistrationRemote itemRegistrationSession 

不起作用。 尝试添加显示 itemRegistrationSession 对象值的日志语句，以查看该成员是否已正确初始化。

Answer 2

你得到的是异常，因为当你调用getCategories()时，EJB 还没有被注入。

在构造函数调用时，注入尚未发生，请记住，对象刚刚创建，因此容器没有时间注入任何内容，执行此操作的正确方法是使用 @PostConstruct 注释。来自 javadoc：

PostConstruct注释用于需要在完成依赖注入以执行任何初始化后执行的方法...

因此，您需要将getCategories()调用移动到@PostConstruct注释的方法，如下所示：

@WebServlet("/processItem")
public class processItem extends HttpServlet {
private static final long serialVersionUID = 1L;
@EJB itemRegistrationRemote itemRegistrationSession;
@EJB userRegistrationRemote processRegistrationSession;
/**
 * @see HttpServlet#HttpServlet()
 */
public processItem() {
    super();
}
@Postconstruct
void init() {
  getCategories();
}...