为什么 Spring 依赖注入 @Autowired 在 servlet 中抛出 NullPointerException？

Question

项目结构：

TestController.java

:

package com.mypack;

import java.io.IOException;
import java.io.PrintWriter;

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 org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;

/**
 * Servlet implementation class Test
 */
@WebServlet("/test")
@Controller
public class TestController extends HttpServlet {
    private static final long serialVersionUID = 1L;
    
    @Autowired
    private TestManager testManager;

    
    public TestManager getTestManager() {
        return testManager;
    }

    public void setTestManager(TestManager testManager) {
        this.testManager = testManager;
    }

    public TestController() {
        super();
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.getWriter().append("Served at: ").append(request.getContextPath());

        String name=testManager.getName();
        PrintWriter pw=response.getWriter();
        pw.write(name);
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }

}

TestManager.java

:

package com.mypack;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Configurable;
import org.springframework.stereotype.Service;

@Service("testManager")
public class TestManager {
    
    @Autowired
    private TestDao testDao;
    
    public String getName(){
        return testDao.getName();
    }

    public TestDao getTestDao() {
        return testDao;
    }

    public void setTestDao(TestDao testDao) {
        this.testDao = testDao;
    }
}

TestDao.java

:

package com.mypack;

import org.springframework.stereotype.Repository;

@Repository("testDao")
public class TestDao {
    public String getName(){
        return "Randhir";
    }
}

spring.xml

:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context-2.5.xsd">

    <!-- Enable Spring Application Context -->
    <context:spring-configured />
    
    <!-- Scan class file in class path for annotated component -> @Component, @Repository, @Service, and @Controller  -->
    <context:component-scan base-package="com.mypack" />
    
</beans>

web.xml

:

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">
  <display-name>SpringIOCWeb</display-name>
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
  
  <listener>
     <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

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

服务器控制台：

INFO: 1 Spring WebApplicationInitializers detected on classpath
Jan 21, 2017 7:50:39 PM org.apache.catalina.core.ApplicationContext log
INFO: Initializing Spring root WebApplicationContext
Jan 21, 2017 7:50:39 PM org.springframework.web.context.ContextLoader initWebApplicationContext
INFO: Root WebApplicationContext: initialization started
Jan 21, 2017 7:50:39 PM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing Root WebApplicationContext: startup date [Sat Jan 21 19:50:39 IST 2017]; root of context hierarchy
Jan 21, 2017 7:50:39 PM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from ServletContext resource [/WEB-INF/spring.xml]
Jan 21, 2017 7:50:40 PM org.springframework.web.context.ContextLoader initWebApplicationContext
INFO: Root WebApplicationContext: initialization completed in 731 ms
Jan 21, 2017 7:50:40 PM org.apache.coyote.AbstractProtocol start
INFO: Starting ProtocolHandler ["http-nio-8090"]
Jan 21, 2017 7:50:40 PM org.apache.coyote.AbstractProtocol start
INFO: Starting ProtocolHandler ["ajp-nio-8009"]
Jan 21, 2017 7:50:40 PM org.apache.catalina.startup.Catalina start
INFO: Server startup in 2836 ms
Jan 21, 2017 7:50:45 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [com.mypack.TestController] in context with path [] threw exception
java.lang.NullPointerException
    at com.mypack.TestController.doGet(TestController.java:43)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:618)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:725)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:291)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:506)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:142)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:610)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:537)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1081)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:658)
    at org.apache.coyote.http11.Http11NioProtocol$Http11ConnectionHandler.process(Http11NioProtocol.java:222)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1566)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1523)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:724)

IOC容器将实例化bean并使用setter方法分配对象引用。我的想法对吗？

但是这里

@Autowired

并没有分配对象引用。

我使用了

main

方法和

ApplicationContext

和

getBean()

来学习Spring。

但是 bean 配置如何与 servlet 一起工作？

Answer 1

您的

testManager

为空。 您正在混合 Servlet 和 Spring 控制器。

您应该使用 Spring 调度程序 servlet :

<servlet>
    <servlet-name>mvc-dispatcher</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <load-on-startup>1</load-on-startup>
</servlet>

<servlet-mapping>
    <servlet-name>mvc-dispatcher</servlet-name>
    <url-pattern>/spring/*</url-pattern>
</servlet-mapping>

DispatcherServlet 是一个独特的 Servlet，类似于前端控制器（在 MVC2 模式中），它将请求分派到 Spring 控制器（一个 Spring 组件）。

然后在你的控制器TestController中，删除HttpServlet的继承并重写方法（doGet、doPost..），并使用

@RequestMapping

。

这样你就可以使用Spring框架的依赖注入了。

@Controller
@RequestMapping("/test")
public class TestController {

    @Autowired
    private TestManager testManager;

    @RequestMapping
    @ResponseBody
    public String handleRequest(final HttpServletRequest request, final HttpServletResponse response) {
        String name=testManager.getName();
        return name;
    }

}

Answer 2

由于您使用的是 servlet 而不是控制器，您可以使用代码自动装配它

@WebServlet("/test")
public class TestController extends HttpServlet {
    private static final long serialVersionUID = 1L;

    private AutowireCapableBeanFactory ctx;

    @Autowired
    private TestManager testManager;

    @Override
    public void init() throws ServletException {
        super.init();

        WebApplicationContext context = WebApplicationContextUtils
                .getWebApplicationContext(getServletContext());
        ctx = context.getAutowireCapableBeanFactory();
        ctx.autowireBean(this);
    }
    ...
}

您已经使用 Spring 自己的上下文监听器集成了 Spring

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