尝试从 Apache Tomcat 上的 Eclipse 运行 Servlet 项目时出现 java.lang.NullPointerException

Question

尝试使用 Apache Tomcat 7(http://localhost:8080/MyProject/project/) 运行我的项目时不断出现此错误，并且不知道为什么它仍然是 NullPointer:

java.lang.NullPointerException com.myproject.doGet(MyProjectListServlet.java:36) javax.servlet.http.HttpServlet.service(HttpServlet.java:621) javax.servlet.http.HttpServlet.service(HttpServlet.java:722)

MyProjectListServlet.java:

package com.myproject;

import java.io.IOException;

import javax.inject.Inject;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet("/project/")
public class MyProjectListServlet extends HttpServlet {

    @Inject
    private MyProjectRepository itemsRepo;

    public MyProjectListServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        request.setAttribute("items", itemsRepo.listItems());

        getServletContext().getRequestDispatcher("/WEB-INF/pages/item-list.jsp").forward(request, response);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
    }

}

MyProjectRepositoryImpl.java

package com.myproject;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import javax.enterprise.context.ApplicationScoped;

@ApplicationScoped
public class MyProjectRepositoryImpl implements MyProjectRepository {

    private int count;
    private Map<String, MyItem> idToMyItemMap = new HashMap<String, MyItem>();

    public MyProjectRepositoryImpl(){
        synchronized(this){
            items(item("aaa", "bbb", "111", "222", "zzz"),
                    item("ccc", "ddd", "333", "444", "xxx"));
        }
    }

    private MyItem item(String str1, String str2, String str3, String str4, String str5){
        return new MyItem("" + (count++), str1, str2, str3, str4, str5);
    }

    private void items(MyItem... items){
                for(MyItem item : items){
                    doAddMyItem(item);
                }
    }

    private void doAddMyItem(MyItem item){
        synchronized(this){
            this.idToMyItemMap.put(item.getId(), item);
        }
    }

    @Override
    public MyItem lookupMyItemById(String id) {
        // TODO Auto-generated method stub
        synchronized(this){
            return this.idToMyItemMap.get(id).cloneMe();
        }
    }

    @Override
    public void addMyItem(String str1, String str2, String str3, String str4, String str5) {
        // TODO Auto-generated method stub
        doAddMyItem(item(str1, str2, str3, str4, str5));

    }

    @Override
    public void updateMyITtem(String id, String str1, String str2, String str3, String str4, String str5) {
        // TODO Auto-generated method stub
        MyItem item = item(str1, str2, str3, str4, str5);
        synchronized(this){
            item.setId(id);
            this.idToMyItemMap.put(id, item);
        }
    }

    private List<MyItem> doListMyItems(){
        List<MyItem> items;
        synchronized(this){
            items = new ArrayList<MyItem>(this.idToMyItemMap.size());
            for(MyItem item : this.idToMyItemMap.values()){
                items.add(item.cloneMe());
            }
        }
        return items;
    }

    @Override
    public void removeMyItem(String id) {
        // TODO Auto-generated method stub
        synchronized(this){
            this.idToMyItemMap.remove(id);
        }
    }

    @Override
    public List<MyItem> listItems() {
        // TODO Auto-generated method stub
        List<MyItem> items = doListMyItems();
         Collections.sort(items, new Comparator<MyItem>(){
             public int compare(MyItem itemA, MyItem itemB){
                 return itemA.getId().compareTo(itemB.getId());
             }
         });
         return items;
    }

}

Answer 1

您的 private MyProjectRepository itemsRepo; 尚未实例化。