Spring使用@Resource、@Autowired注入时出现空指针问题的原因

Myth丶恋晨 2022-06-07 12:39 414阅读 0赞

# Spring使用@Resource、@Autowired注入时出现空指针问题的原因 #

[http://blog.csdn.net/yzj99848873/article/details/45012193][http_blog.csdn.net_yzj99848873_article_details_45012193]

## 举例说明: ##

这是一个类,使用了@Component注解,里面有两个依赖注入的属性,使用@Autowired注解.

package cn.yearcon.shop.utils;
    
    import org.apache.http.NameValuePair;
    import org.apache.http.client.config.RequestConfig;
    import org.apache.http.client.entity.UrlEncodedFormEntity;
    import org.apache.http.client.methods.CloseableHttpResponse;
    import org.apache.http.client.methods.HttpGet;
    import org.apache.http.client.methods.HttpPost;
    import org.apache.http.client.utils.URIBuilder;
    import org.apache.http.impl.client.CloseableHttpClient;
    import org.apache.http.message.BasicNameValuePair;
    import org.apache.http.util.EntityUtils;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Component;
    
    import java.util.ArrayList;
    import java.util.List;
    import java.util.Map;
    
    
    @Component
    public class HttpClientUtil { 
    
        @Autowired
        private CloseableHttpClient httpClient;
    
        @Autowired
        private RequestConfig config;
    
    
        /** * 不带参数的get请求，如果状态码为200，则返回body，如果不为200，则返回null * * @param url * @return * @throws Exception */
        public String doGet(String url) throws Exception {
            // 声明 http get 请求
            HttpGet httpGet = new HttpGet(url);
    
            // 装载配置信息
            httpGet.setConfig(config);
    
            // 发起请求
            CloseableHttpResponse response = this.httpClient.execute(httpGet);
    
            // 判断状态码是否为200
            if (response.getStatusLine().getStatusCode() == 200) {
                // 返回响应体的内容
                return EntityUtils.toString(response.getEntity(), "UTF-8");
            }
            return null;
        }
    
        /** * 带参数的get请求，如果状态码为200，则返回body，如果不为200，则返回null * * @param url * @return * @throws Exception */
        public String doGet(String url, Map<String, Object> map) throws Exception {
            URIBuilder uriBuilder = new URIBuilder(url);
    
            if (map != null) {
                // 遍历map,拼接请求参数
                for (Map.Entry<String, Object> entry : map.entrySet()) {
                    uriBuilder.setParameter(entry.getKey(), entry.getValue().toString());
                }
            }
    
            // 调用不带参数的get请求
            return this.doGet(uriBuilder.build().toString());
    
        }
    
        /** * 带参数的post请求 * * @param url * @param map * @return * @throws Exception */
        public HttpResult doPost(String url, Map<String, Object> map) throws Exception {
            // 声明httpPost请求
            HttpPost httpPost = new HttpPost(url);
            // 加入配置信息
            httpPost.setConfig(config);
    
            // 判断map是否为空，不为空则进行遍历，封装from表单对象
            if (map != null) {
                List<NameValuePair> list = new ArrayList<NameValuePair>();
                for (Map.Entry<String, Object> entry : map.entrySet()) {
                    list.add(new BasicNameValuePair(entry.getKey(), entry.getValue().toString()));
                }
                // 构造from表单对象
                UrlEncodedFormEntity urlEncodedFormEntity = new UrlEncodedFormEntity(list, "UTF-8");
    
                // 把表单放到post里
                httpPost.setEntity(urlEncodedFormEntity);
            }
    
            // 发起请求
            CloseableHttpResponse response = this.httpClient.execute(httpPost);
            return new HttpResult(response.getStatusLine().getStatusCode(), EntityUtils.toString(
                    response.getEntity(), "UTF-8"));
        }
    
        /** * 不带参数post请求 * * @param url * @return * @throws Exception */
        public HttpResult doPost(String url) throws Exception {
            return this.doPost(url, null);
        }
    }

## 现在,我们要使用上面的类, ##

package cn.yearcon.shop.controller;
    
    import cn.yearcon.shop.utils.HttpClientUtil;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.beans.factory.annotation.Value;
    import org.springframework.web.bind.annotation.RequestMapping;
    import org.springframework.web.bind.annotation.RestController;
    
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    import java.util.HashMap;
    
    @RestController
    public class Weixin { 
    
    
    
        @Value("${weixin.appid}")
        String appid ;
        @Value("${weixin.secret}")
        String secret;
        @Value("${weixin.grant_type}")
        String grant_type;
    
    
        @RequestMapping(value = {
       "shop", ""})
        public String test(HttpServletRequest request, HttpServletResponse response) throws Exception {
            // 用户同意授权后，能获取到code
            String code = request.getParameter("code");
            System.out.println("===code==" + code);
    
            String url = "https://api.weixin.qq.com/sns/oauth2/access_token";
            HashMap<String, Object> map = new HashMap<String, Object>(16);
    
            map.put("appid",appid);
            map.put("secret",secret);
            map.put("grant_type",grant_type);
            map.put("code",code);
            HttpClientUtil httpClientUtil = new HttpClientUtil();
            String body = httpClientUtil.doGet(url, map);
    
            System.out.println(body);
    
    
            return "hello";
        }
    
    
    }

注意,HttpClientUtil 是我们手动new出来的,不是使用Spring自动装配进来的.

这时候运行就会出现 空指针异常,

追查原因是因为: HTTPclientUtil 类的

@Autowired
        private CloseableHttpClient httpClient;
    
        @Autowired
        private RequestConfig config;

这两个属性都是null,也就是没注入进来,既然是空,那调用它们的方法自然会产生空指针异常.

## Spring java配置及注解注入方法出现空指针异常的原因 ##

> 当通过new的方式创建一个对象的时候，虽然期望使用了注解@Autowired对这个对象进行装配，但是Spring是不会这么做的，因为Spring不会对任意一个new 出来的对象进行自动装配，只有这个对象也是一个在Spring中注册过的Bean，才会获得自动装配的功能。

## 使用Spring注入HttpClientUtil ##

package cn.yearcon.shop.controller;
    
    import cn.yearcon.shop.utils.HttpClientUtil;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.beans.factory.annotation.Value;
    import org.springframework.web.bind.annotation.RequestMapping;
    import org.springframework.web.bind.annotation.RestController;
    
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    import java.util.HashMap;
    
    @RestController
    public class Weixin { 
    
        @Autowired
        private HttpClientUtil httpClientUtil;
    
        @Value("${weixin.appid}")
        String appid ;
        @Value("${weixin.secret}")
        String secret;
        @Value("${weixin.grant_type}")
        String grant_type;
    
    
        @RequestMapping(value = {
       "shop", ""})
        public String test(HttpServletRequest request, HttpServletResponse response) throws Exception {
            // 用户同意授权后，能获取到code
            String code = request.getParameter("code");
            System.out.println("===code==" + code);
    
            String url = "https://api.weixin.qq.com/sns/oauth2/access_token";
            HashMap<String, Object> map = new HashMap<String, Object>(16);
    
            map.put("appid",appid);
            map.put("secret",secret);
            map.put("grant_type",grant_type);
            map.put("code",code);
    
            String body = httpClientUtil.doGet(url, map);
    
            System.out.println(body);
    
    
            return "hello";
        }
    
    
    }

这样我们让Spring来给我们注入HttpClientUtil示例,那么它里面的其他使用@Autowired 注解的属性,Spring才会帮助我们都注入进来

[http_blog.csdn.net_yzj99848873_article_details_45012193]: http://blog.csdn.net/yzj99848873/article/details/45012193