如何在Web应用程序中使用Spring自动装配注释

Question

New to Spring, I have rushed into it due to the needs of my project. I was successfully able to use most of the features of Spring when using it as a Java application but am not getting the desired results in a web-application.

I am trying to initialize my datasource in applicationContext.xml:

<context:annotation-config />
<context:component-scan base-package="learn.spring.webapp.dao" />

<bean id="dataSource"
    class="org.springframework.jdbc.datasource.DriverManagerDataSource">
    <property name="driverClassName" value="org.postgresql.Driver" />
    <property name="url" value="jdbc:postgresql://localhost:5432/postgres" />
    <property name="username" value="postgres" />
    <property name="password" value="postgres" />
</bean>

<bean id="template" class="org.springframework.jdbc.core.JdbcTemplate"
    autowire="constructor" />

web.xml:

 <context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>WEB-INF/spring/applicationContext.xml</param-value>
  </context-param>

  <listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
  </listener>

I was successfully able to initialize Spring in my application but fail to autowire jdbctemplate in my dao class.

package learn.spring.webapp.dao;

import learn.spring.webapp.model.Manga;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.stereotype.Component;

@Component
public class MangaDao {
    @Autowired
    private JdbcTemplate jdbcTemplate;

    public void insert(Manga manga){
        String sql = "insert into manga_list values (?,?)";
        jdbcTemplate.update(sql,
                new Object[]{manga.getName(), manga.getAuthor()}); //Null Pointer exception
    }
}

I want to autowire the template bean with JdbcTemplate in my dao package. I get a NullPointerException when I try to use the JdbcTemplate . MangaDao is being called from a servlet. I think I might not be using the autowired annotations correctly. Please advice.

Edit:

The servlet code that calls the DAO is:

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        MangaDao md=new MangaDao();
        Manga manga=new Manga();
        manga.setName("Bleach");
        manga.setAuthor("Kubo Tite");
        md.insert(manga);
    }

And the stackTrace is as follows:

SEVERE: Servlet.service() for servlet TesterServlet threw exception
java.lang.NullPointerException
        at learn.spring.webapp.dao.MangaDao.insert(MangaDao.java:16)
        at learn.spring.webapp.controller.TesterServlet.doGet(TesterServlet.java:33)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
        at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:857)
        at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
        at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
        at java.lang.Thread.run(Thread.java:662)

Answer 1

I suppose code in your servlet is like this:

Manga manga = new Manga();
manga.setName(request.getParameter("name"));
manga.setAuthor(request.getParameter("author"));
MangaDao dao = new MangaDao();
dao.insert(manga); // here you get exception;

But autowiring works only if you get bean fron context.

MangaDao dao = appContext.getBean(MangaDao.class);
dao.insert(manga); // should be ok;

So where you can get appContext. You should load it from file and store for furure use. ContextLoaderListener does part of this job for you. Then you mere store it in static variable, for example:

@Component
public class AppContext implements ApplicationContextAware {
    private static MangaDao mangaDao = null;
    public static MangaDao getMangaDao() {
        if(mangaDao == null){
            throw new ApplicationContextException("context not initialized");
        }
        return mangaDao;
    }

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        mangaDao = applicationContext.getBean(MangaDao.class);
    }
}

Make sure class AppContext goes in package, defined with context:component-scan , it will learn.spring.webapp.dao in your case

And i strongly suggest you to use transactions.

Answer 2

To answer your updated post:

You have to use the bean you created in your context rather than creating a new instance. Try autowiring your new component and then using that rather than creating a new one in your controller.

@Autowired MangaDao mangaDao;
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    Manga manga=new Manga();
    manga.setName("Bleach");
    manga.setAuthor("Kubo Tite");
    mangaDao.insert(manga);
}

In answer to your original post:

Looks like you are just missing the right package name in your component scan. Your @Component annotated class is in the learn.spring.webapp.model package and your scan isn't reaching it. Try using a more generic package in the annotation-scan:

<context:component-scan base-package="learn.spring.webapp" />

Answer 3

I would recommend using spring-mvc rather than raw Servlets.

With spring-mvc you define Controllers instead of Servlets. Based on your code a Spring Controller may look something like this:

@Controller 
public class MangaController {

    @Autowired
    private MangaDao dao;

    @RequestMapping(value = "/manga/insert", method = RequestMethod.GET)
    public void doGet() {
        Manga manga = new Manga();
        manga.setName("Bleach");
        manga.setAuthor("Kubo Tite");
        dao.insert(manga);
    }
}

Because the Controller is managed by Spring all required dependencies will be injected and ready for use by your code - eliminating the NullPointerException .

Within your applicationContext.xml , you'd have to include the controller package in the component-scan and also enable the mvc features:

<mvc:annotation-driven />

I strongly recommend reading this article on getting started with spring-mvc . It covers all the required steps along with the Spring dependencies you need.

I'd also recommend taking a look through the Spring Reference documentation , as you'll see, it's very comprehensive!

Answer 4

I had got same "null" exception until I user "SpringBeanAutowiringSupport" abstract class. You should extand the class with this abstract class like that:

public class SomeClass extends SpringBeanAutowiringSupport {}