如何无消耗地多次访问HTTP Servlet请求参数？

无消耗访问HTTP Servlet请求参数

问题：

访问多个HTTP请求时Servlet 过滤器中的参数，初始参数访问会消耗这些参数，使其无法用于后续使用。这个问题在参数驻留在请求正文中的 POST 请求中尤其容易出现。

解决方案：

要访问参数而不消耗它们，您可以扩展 HttpServletRequestWrapper 并实现自定义方法缓存输入流：

public class MultiReadHttpServletRequest extends HttpServletRequestWrapper {
  private ByteArrayOutputStream cachedBytes;

  public MultiReadHttpServletRequest(HttpServletRequest request) {
    super(request);
  }

  @Override
  public ServletInputStream getInputStream() throws IOException {
    if (cachedBytes == null)
      cacheInputStream();

    return new CachedServletInputStream(cachedBytes.toByteArray());
  }

  @Override
  public BufferedReader getReader() throws IOException{
    return new BufferedReader(new InputStreamReader(getInputStream()));
  }

  private void cacheInputStream() throws IOException {
    cachedBytes = new ByteArrayOutputStream();
    IOUtils.copy(super.getInputStream(), cachedBytes);
  }

  private static class CachedServletInputStream extends ServletInputStream {
    private final ByteArrayInputStream buffer;

    public CachedServletInputStream(byte[] contents) {
      this.buffer = new ByteArrayInputStream(contents);
    }

    @Override
    public int read() {
      return buffer.read();
    }
    // ... Implementation for newer versions of ServletInputStream interface
  }
}

通过使用此类包装原始请求，您可以访问在过滤器链内外多次缓存输入流，允许 getParameterXXX 和 doMyThing 等自定义方法读取参数：

public class MyFilter implements Filter {
  public void doFilter(ServletRequest request, ServletResponse response,
        FilterChain chain) throws IOException, ServletException {
    MultiReadHttpServletRequest multiReadRequest = new MultiReadHttpServletRequest((HttpServletRequest) request);
    doMyThing(multiReadRequest.getInputStream());
    chain.doFilter(multiReadRequest, response);
  }
}

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