springboot的依赖注入报null的问题

最近使用springboot开发项目，使用到了依赖注入，频繁的碰到注入的对象报空指针,错误如下

java.lang.NullPointerException: null
    at com.mayihc.audit.controller.MaterialNkDetailController.download(MaterialNkDetailController.java:)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:)
    at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:)
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:)
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:)
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:)
    at org.apache.shiro.web.servlet.ProxiedFilterChain.doFilter(ProxiedFilterChain.java:)
    at org.apache.shiro.web.servlet.AdviceFilter.executeChain(AdviceFilter.java:)
    at org.apache.shiro.web.servlet.AdviceFilter.doFilterInternal(AdviceFilter.java:)
    at org.apache.shiro.web.servlet.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:)
    at org.apache.shiro.web.servlet.ProxiedFilterChain.doFilter(ProxiedFilterChain.java:)
    at org.apache.shiro.web.servlet.AbstractShiroFilter.executeChain(AbstractShiroFilter.java:)
    at org.apache.shiro.web.servlet.AbstractShiroFilter$.call(AbstractShiroFilter.java:)
    at org.apache.shiro.subject.support.SubjectCallable.doCall(SubjectCallable.java:)
    at org.apache.shiro.subject.support.SubjectCallable.call(SubjectCallable.java:)
    at org.apache.shiro.subject.support.DelegatingSubject.execute(DelegatingSubject.java:)
    at org.apache.shiro.web.servlet.AbstractShiroFilter.doFilterInternal(AbstractShiroFilter.java:)
    at org.apache.shiro.web.servlet.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:)
    at com.alibaba.druid.support.http.WebStatFilter.doFilter(WebStatFilter.java:)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:)
    at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:)
    at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:)
    at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:)
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:)
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:)
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:)
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:)
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:)
    at java.lang.Thread.run(Unknown Source)

第一次碰见的情况是：在new出来的对象中使用@Autowired来获取注入的对象

 DateCheckUtils dateCheckUtils = new DateCheckUtils();

我要在这个类中使用依赖注入，刚开始写的是注入方式如下

public class DateCheckUtils {
     @Autowired
    private CompanyInfoService companyInfoService;
 @Autowired
private MaterialNkDetailService materialNkDetailService;

.........
}

这是会报空指针异常，后俩修改代码就解决了问题，修改后的代码如下

1、创建一个类实现ApplicationContextAware，用来获取applicationContext 和bean

package com.mayihc.audit.core.utils;

import org.springframework.beans.BeansException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.stereotype.Component;

@Component
public class SpringUtils implements ApplicationContextAware{

    private static ApplicationContext applicationContext = null; 

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        if(SpringUtils.applicationContext == null){
            SpringUtils.applicationContext  = applicationContext;
        } 

    }
    //获取applicationContext
    public static ApplicationContext getApplicationContext() {
        return applicationContext;
    }  

    //通过name获取 Bean.
    public static Object getBean(String name){
        return getApplicationContext().getBean(name);
    }  

    //通过class获取Bean.
    public static <T> T getBean(Class<T> clazz){
        return getApplicationContext().getBean(clazz);
    }  

    //通过name,以及Clazz返回指定的Bean
    public static <T> T getBean(String name,Class<T> clazz){
        return getApplicationContext().getBean(name, clazz);
    }  

}

2、在new出来的DateCheckUtils类中获取ApplicationContext的实例

public class DateCheckUtils {
　　//获取ApplicationContext的实例
    private ApplicationContext applicationContext = SpringUtils.getApplicationContext();
　　//通过applicationContext和反射机制来获取对象
    private CompanyInfoService companyInfoService = applicationContext.getBean(CompanyInfoService.class);
    private MaterialNkDetailService materialNkDetailService = applicationContext.getBean(MaterialNkDetailService.class);
    private MaterialNkLogbookService materialNkLogbookService = applicationContext
            .getBean(MaterialNkLogbookService.class);
    private MaterialNkLedgerService materialNkLedgerService = applicationContext.getBean(MaterialNkLedgerService.class);
    private MaterialGjChangeDetailService materialGjChangeDetailService = applicationContext
            .getBean(MaterialGjChangeDetailService.class);
    private MaterialGjLedgerService materialGjLedgerService = applicationContext.getBean(MaterialGjLedgerService.class);
    .....

}

这样就可以获取到要注入的对象

报空指针的原因：当对象声明为bean组件的时候，他是交给spring容器去管理的，容器会帮你进行初始化；但是如果使用new方法来调用对象是=时，会跳过spring容器生成的对象，这是就无法进行初始化，所以在调用的时候就会出现对象为null，并且对象里面以注入方式引用的对象也为null，被声明为bean对象的组件必修使用注入的方式进行调用。

上述的问题也可以通过将  DataCheckUtils dataCheckUtils=new DataCheckUtils();改为：

@Autowired

private DataCheckUtils dataCheckUtils;来实现

第二种碰见null的情况：在同一个类中只有某个方法在被前端调用的时候报null，其余方法都正常。

错误代码如下

  @RequestMapping("/download")
     private void download(HttpServletResponse response, MaterialNkDetail materialNkDetail) throws Exception {
         SysUser u = (SysUser) SecurityUtils.getSubject().getPrincipal();
         if(!"1".equals(u.getType())) {
             materialNkDetail.setCompanyId(u.getCompanyInfoId());
         }
         CompanyInfo companyInfo = companyInfoService.selectByPrimaryKey(u.getCompanyInfoId());
         if(companyInfo == null) {
             companyInfo = new CompanyInfo();
             companyInfo.setFullName("全部车辆");
         }
     .....
 }    

当运行这个方法时，注入的对象总是报null，经排查后，此方法是用private修饰的，改为public修饰后就OK了。

原因：容器扫描bean生成代理类的时候，public和protected方法可以被正常代理，而private方法不会被正常代理，属性的注入是在代理类中完成的，所以public和protected方法获取的注入属相是完成注入的属性，private方法获取的是为完成注入的属性，所以是null