解决使用@ResponseBody后返回500错误的问题

在springmvc+mybatis的项目中，利用mybatis分页插件mybatis-paginator进行分页查询，结果出现了500异常，后端又没有明显的报错。

原来的写法，返回Map对象，由springmvc里面的机制转为json对象,这样就会导致，在转json过程中的报错，都隐藏了，无法抛出，前端获取不到正确的数据，

最后就出现了500的异常。

 @RequestMapping(value = "/query")
 @ResponseBody
 public Map<String, Object> data(HttpServletRequest request, HttpServletResponse response, CreditloanInfoParams params) {
 Map<String, Object> data = new HashMap<String, Object>();
 if(params==null){
 params=new CreditloanInfoParams();
 }
 PageList<CreditloanInfo> list = (PageList<CreditloanInfo>) creditloanInfoService.getCreditloanInfoListData(params);
 data.put("total", list.getPaginator().getTotalCount());
 data.put("rows", list);
 return data;
 }

改成使用fastJson主动转化为json格式的字符串，这样的好处是，转json过程中如果出现错误，会有很明确的提示。

 @SuppressWarnings("unchecked")
 @RequestMapping(value = "/query")
 @ResponseBody
 public String data(HttpServletRequest request, HttpServletResponse response, BlackParams params) {
 //Map<String, Object> data = new HashMap<String, Object>();
 /*PageList<Blacklist> blackDatas = (PageList<Blacklist>) blackService.getPageData(params);
 data.put("total", blackDatas.getPaginator().getTotalCount());
 data.put("rows", blackDatas);*/
 //blackService.getPageData(params);
/* Map data = new HashMap();
 PageList<Blacklist> blackDatas = blackService.getBlackListData(params);
 data.put("total", blackDatas.getPaginator().getTotalCount());
 data.put("rows", blackDatas);*/
 return JSON.toJSONString(blackService.getPageData(params));
 }

最后发现是转json中出现了空指针异常。修复后，问题解决。

补充知识：springboot 使用过滤器获取response内容保存接口访问日志

一、创建过滤器

1. 在spring boot的启动入口出添加注解 @ServletComponentScan

@SpringBootApplication
@ServletComponentScan
public class Application {
 public static void main(String[] args) {
  SpringApplication.run(Applicatioin.class, args);
 }
}

2.新建过滤器AccessLogFilter.java

@WebFilter(filterName = "accessLog", urlPatterns = "/api/*")
public class AccessLogFilter implements Filter {

 @Autowired
 AccessLogMapper accessLogMapper;
 public void destroy() {
 }
 public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
 long startTime = System.currentTimeMillis();
 ResponseWrapper wrapper = new ResponseWrapper((HttpServletResponse) response);
 HttpServletRequest req = (HttpServletRequest) request;

 chain.doFilter(request, wrapper);
 long endTime = System.currentTimeMillis();
 Gson gson = new Gson();
 // 获取response返回的内容并重新写入response

 String result = wrapper.getResponseData(response.getCharacterEncoding());
 response.getOutputStream().write(result.getBytes());
 String uri = req.getRequestURI();
 AccessLog log = new AccessLog();
 log.setMethod(req.getMethod());
 log.setUrl(uri);
 log.setParameters(gson.toJson(req.getParameterMap()));
 log.setResponseCode(String.valueOf(wrapper.getStatus()));
 log.setResult(result);
 log.setCreateDatetime(new Date());
 log.setTimeConsuming((int)(endTime - startTime));
 accessLogMapper.insertSelective(log);
 }
 public void init(FilterConfig fConfig) throws ServletException {
 }
}

这个过滤器使用了注解@WebFilter(filterName = "accessLog", urlPatterns = "/api/*") 进行配置，指定了url进入规则，只有以/api/开头的url才能进入到此过滤器中。在doFilter方法中使用了自定义的ResponseWrapper对response进行封装。Controller接口走完之后获取到接口返回的数据并再次封装到response。

3. ResponseWrapper.java 类

import java.io.ByteArrayOutputStream;
import java.io.CharArrayWriter;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.io.UnsupportedEncodingException;

import javax.servlet.ServletOutputStream;
import javax.servlet.WriteListener;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpServletResponseWrapper;
public class ResponseWrapper extends HttpServletResponseWrapper {
 private ByteArrayOutputStream buffer = null;
 private ServletOutputStream out = null;
 private PrintWriter writer = null; 

 public ResponseWrapper(HttpServletResponse response) throws IOException{
 super(response);

  buffer = new ByteArrayOutputStream();
  out = new WapperedOutputStream(buffer);
  writer = new PrintWriter(new OutputStreamWriter(buffer, "UTF-8"));
 }

 //重载父类获取outputstream的方法
 @Override
 public ServletOutputStream getOutputStream() throws IOException {
  return out;
 }

 @Override
 public PrintWriter getWriter() throws IOException {
  return writer;
 }

 @Override
 public void flushBuffer() throws IOException {
  if (out != null) {
   out.flush();
  }
  if (writer != null) {
   writer.flush();
  }
 }

 @Override
 public void reset() {
  buffer.reset();
 }

 public String getResponseData(String charset) throws IOException {
  flushBuffer();//将out、writer中的数据强制输出到WapperedResponse的buffer里面，否则取不到数据
  byte[] bytes = buffer.toByteArray();
  try {
   return new String(bytes, "UTF-8");
  } catch (UnsupportedEncodingException e) {
   return "";
  }
 } 

 //内部类，对ServletOutputStream进行包装，指定输出流的输出端

 private class WapperedOutputStream extends ServletOutputStream {
  private ByteArrayOutputStream bos = null;
  public WapperedOutputStream(ByteArrayOutputStream stream) throws IOException {
   bos = stream;
  }

  //将指定字节写入输出流bos
  @Override
  public void write(int b) throws IOException {
   bos.write(b);
  }

 @Override
 public boolean isReady() {
 // TODO Auto-generated method stub
 return false;
 }

 @Override
 public void setWriteListener(WriteListener listener) {
 // TODO Auto-generated method stub

 }
 }
}

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