Question

我创建一个单例类并在其中调用一个方法。我使用spring mvc SimpleJdbcTemplate从数据库中检索数据，但我得到了NullPointerException。我写的代码是：

package in.dewsolutions.jc.utils;

import in.dewsolutions.jc.dao.ReportsDAO;
import in.dewsolutions.jc.dto.ReportsView;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.List;

import javax.annotation.PostConstruct;
import javax.sql.DataSource;

import org.apache.log4j.Logger;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.config.PropertiesFactoryBean;
import org.springframework.jdbc.core.RowMapper;
import org.springframework.jdbc.core.simple.ParameterizedRowMapper;
import org.springframework.jdbc.core.simple.SimpleJdbcTemplate;
import org.springframework.stereotype.Component;
import org.springframework.stereotype.Repository;
import org.springframework.stereotype.Service;

public class SingletonFilterData {
    private static final Logger logger = Logger.getLogger(SingletonFilterData.class);

    @Autowired
    private SimpleJdbcTemplate simpleJdbcTemplate;

    @Autowired
    public void setDataSource(DataSource dataSource) {
        this.simpleJdbcTemplate = new SimpleJdbcTemplate(dataSource);
    }

    private SingletonFilterData() // private defaul constructor
    {
    }

    private static SingletonFilterData reference = null;

    public static SingletonFilterData getInstance() {

        if (reference == null) {
            synchronized (SingletonFilterData.class) {
                if (reference == null) {
                    reference = new SingletonFilterData();

                }
            }

        }
        return reference;
    }

    public List<ReportsView> findManuFacturer() {
        // TODO Auto-generated method stub
        List<ReportsView> manufacturer = new ArrayList<ReportsView>();

        ReportsView rv = new ReportsView();
        rv.setProductName("None");
        rv.setProductId(0);
        manufacturer.add(rv);

        String query = "SELECT * FROM manufacturer ORDER BY name";
        ParameterizedRowMapper<ReportsView> mapper = new ParameterizedRowMapper<ReportsView>() {
            public ReportsView mapRow(ResultSet rs, int rowNum)
                    throws SQLException {
                ReportsView reportsView = new ReportsView();
                reportsView.setProductName(rs.getString("name"));
                reportsView.setProductId(rs.getLong("id_manufacturer"));

                return reportsView;
            }
        };
        manufacturer.addAll(simpleJdbcTemplate.query(query, mapper));
        return manufacturer;
    }

我得到的例外是：

type Exception report

message

description The server encountered an internal error () that prevented it from fulfilling this request.

exception

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is java.lang.NullPointerException
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:656)
    org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:549)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    org.displaytag.filter.ResponseOverrideFilter.doFilter(ResponseOverrideFilter.java:125)

root cause

java.lang.NullPointerException
    in.dewsolutions.jc.utils.SingletonFilterData.findManuFacturer(SingletonFilterData.java:77)
    in.dewsolutions.jc.controller.ReportsController.filterQuantity(ReportsController.java:169)
    sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    java.lang.reflect.Method.invoke(Method.java:597)
    org.springframework.web.bind.annotation.support.HandlerMethodInvoker.invokeHandlerMethod(HandlerMethodInvoker.java:176)
    org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.invokeHandlerMethod(AnnotationMethodHandlerAdapter.java:426)
    org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.handle(AnnotationMethodHandlerAdapter.java:414)
    org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:790)
    org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:719)
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:644)
    org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:549)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    org.displaytag.filter.ResponseOverrideFilter.doFilter(ResponseOverrideFilter.java:125)

note The full stack trace of the root cause is available in the Apache Tomcat/6.0.29 logs.

如何删除例外？

Answer 1

使用new SingletonFilterData()不会调用自动装配，因此simpleJdbcTemplate仍为null。

您可以配置Spring，而不是实现getInstance()，以便SingletonFilterData是单身并注入ReportsController。

在spring mvc中使用Singleton Class来获取数据

1 个答案: