How to Enable Multiple Reads of POST Request Body in Java Servlet Filters?

Multiple Reads of Post Request Body in Java Servlet Filters

Problem:

In Java Servlet filters, accessing HTTP request parameters from a POST request body can result in the parameters being unavailable later in the filter chain. This occurs because the request body is consumed during the initial read.

Solution:

To overcome this issue and enable multiple reads of the request body parameters, a solution involves extending HttpServletRequestWrapper and caching the input stream bytes. Here's a custom request wrapper class:

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);
  }

  /* An input stream which reads the cached request body */
  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();
    }

    @Override
    public boolean isFinished() {
      return buffer.available() == 0;
    }

    @Override
    public boolean isReady() {
      return true;
    }

    @Override
    public void setReadListener(ReadListener listener) {
  throw new RuntimeException("Not implemented");
    }
  }
}

Usage:

Wrap the original request with the custom wrapper before passing it through the filter chain:

public class MyFilter implements Filter {
  @Override
  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);
  }
}

This allows for multiple reads of the request body, either through the getInputStream() or getReader() methods.

Note:

For newer versions of the ServletInputStream interface, additional methods like isReady() and setReadListener() need to be implemented in the custom input stream class. Refer to the link provided in the answer for details.

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