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你一定不知道的有关HttpServletResponse和HttpServletRequest取值的两个坑

前言

有时候,我们需要用拦截器对Request或者Response流里面的数据进行拦截,读取里面的一些信息,也许是作为日志检索,也许是做一些校验,但是当我们读取里请求或者回调的流数据后,会发现这些流数据在下游就无法再次被消费了,这里面是其实存在着两个潜在的坑。

坑一

Request的 getInputStream()、getReader()、getParameter()方法互斥,也就是使用了其中一个,再使用另外的两,是获取不到数据的。除了互斥外,getInputStream()和getReader()都只能使用一次,getParameter单线程上可重复使用。

三个方法互斥原因

org.apache.catalina.connector.Request方法实现了javax.servlet.http.HttpServletRequest接口,我们来看看这三个方法的实现:

getInputStream

@Override
public ServletInputStream getInputStream() throws IOException {

    if (usingReader) {
        throw new IllegalStateException
            (sm.getString("coyoteRequest.getInputStream.ise"));
    }

    usingInputStream = true;
    if (inputStream == null) {
        inputStream = new CoyoteInputStream(inputBuffer);
    }
    return inputStream;

}
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getReader

@Override
public BufferedReader getReader() throws IOException {

    if (usingInputStream) {
        throw new IllegalStateException
            (sm.getString("coyoteRequest.getReader.ise"));
    }

    usingReader = true;
    inputBuffer.checkConverter();
    if (reader == null) {
        reader = new CoyoteReader(inputBuffer);
    }
    return reader;

}
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首先来看getInputStream()和getReader()这两个方法,可以看到,在读流时分别用usingReader和usingInputStream标志做了限制,这两个方法的互斥很好理解。下面看一看getParameter()方法是怎么跟他们互斥的。

getParameter

@Override
public String getParameter(String name) {
		// 只会解析一遍Parameter
    if (!parametersParsed) {
        parseParameters();
    }
  	// 从coyoteRequest中获取参数
    return coyoteRequest.getParameters().getParameter(name);

}
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粗略一看好像没有互斥,别着急,继续往下看,我们进到parseParameters()方法中来看一看(可以直接看源码中间部分):

protected void parseParameters() {
		//标识位,标志已经被解析过。
    parametersParsed = true;
		
    Parameters parameters = coyoteRequest.getParameters();
    boolean success = false;
    try {
        // Set this every time in case limit has been changed via JMX
        parameters.setLimit(getConnector().getMaxParameterCount());

        // getCharacterEncoding() may have been overridden to search for
        // hidden form field containing request encoding
        String enc = getCharacterEncoding();

        boolean useBodyEncodingForURI = connector.getUseBodyEncodingForURI();
        if (enc != null) {
            parameters.setEncoding(enc);
            if (useBodyEncodingForURI) {
                parameters.setQueryStringEncoding(enc);
            }
        } else {
            parameters.setEncoding
                (org.apache.coyote.Constants.DEFAULT_CHARACTER_ENCODING);
            if (useBodyEncodingForURI) {
                parameters.setQueryStringEncoding
                    (org.apache.coyote.Constants.DEFAULT_CHARACTER_ENCODING);
            }
        }

        parameters.handleQueryParameters();
				// 重点看这里:这里会判断是否有读取过流。如果有,则直接return。
        if (usingInputStream || usingReader) {
            success = true;
            return;
        }

        if( !getConnector().isParseBodyMethod(getMethod()) ) {
            success = true;
            return;
        }

        String contentType = getContentType();
        if (contentType == null) {
            contentType = "";
        }
        int semicolon = contentType.indexOf(';');
        if (semicolon >= 0) {
            contentType = contentType.substring(0, semicolon).trim();
        } else {
            contentType = contentType.trim();
        }

        if ("multipart/form-data".equals(contentType)) {
            parseParts(false);
            success = true;
            return;
        }

        if (!("application/x-www-form-urlencoded".equals(contentType))) {
            success = true;
            return;
        }

        int len = getContentLength();

        if (len > 0) {
            int maxPostSize = connector.getMaxPostSize();
            if ((maxPostSize > 0) && (len > maxPostSize)) {
                Context context = getContext();
                if (context != null && context.getLogger().isDebugEnabled()) {
                    context.getLogger().debug(
                            sm.getString("coyoteRequest.postTooLarge"));
                }
                checkSwallowInput();
                return;
            }
            byte[] formData = null;
            if (len < CACHED_POST_LEN) {
                if (postData == null) {
                    postData = new byte[CACHED_POST_LEN];
                }
                formData = postData;
            } else {
                formData = new byte[len];
            }
            try {
                if (readPostBody(formData, len) != len) {
                    return;
                }
            } catch (IOException e) {
                // Client disconnect
                Context context = getContext();
                if (context != null && context.getLogger().isDebugEnabled()) {
                    context.getLogger().debug(
                            sm.getString("coyoteRequest.parseParameters"),
                            e);
                }
                return;
            }
            parameters.processParameters(formData, 0, len);
        } else if ("chunked".equalsIgnoreCase(
                coyoteRequest.getHeader("transfer-encoding"))) {
            byte[] formData = null;
            try {
                formData = readChunkedPostBody();
            } catch (IOException e) {
                // Client disconnect or chunkedPostTooLarge error
                Context context = getContext();
                if (context != null && context.getLogger().isDebugEnabled()) {
                    context.getLogger().debug(
                            sm.getString("coyoteRequest.parseParameters"),
                            e);
                }
                return;
            }
            if (formData != null) {
                parameters.processParameters(formData, 0, formData.length);
            }
        }
        success = true;
    } finally {
        if (!success) {
            parameters.setParseFailed(true);
        }
    }

}
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这样一来,就说明了getParameter()方法也不能随意读取的。那么为什么它们都只能读取一次呢?

只能读取一次的原因

getInputStream()和getReader()方法都只能读取一次,而getParameter()是在单线程上可重复使用,主要是因为getParameter()中会解析流中的数据后存放在了一个LinkedHashMap中,相关的内容可以看Parameters类中的封装,在上面parseParameters()方法的源码中也可以看到一开始就生成了一个Parameters对象。后续读取的数据都存在了这个对象中。但是getInputStream()和getReader()方法就没有这样做,getInputStream()方法返回CoyoteInputStream,getReader()返回CoyoteReader,CoyoteInputStream继承了InputStream,CoyoteReader继承了BufferedReader,从源码看InputStream和BufferedReader在读取数据后,记录数据读取的坐标不会被重置,因为CoyoteInputStream和CoyoteReader都没有实现reset方法,这导致数据只能被读取一次。

坑二

Response与Request一样,getOutputStream()和getWriter()方法也是互斥的,并且Response中的body数据也只能消费一次。

互斥原因

getOutputStream

@Override
public ServletOutputStream getOutputStream()
    throws IOException {

    if (usingWriter) {
        throw new IllegalStateException
            (sm.getString("coyoteResponse.getOutputStream.ise"));
    }

    usingOutputStream = true;
    if (outputStream == null) {
        outputStream = new CoyoteOutputStream(outputBuffer);
    }
    return outputStream;

}
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getWriter

@Override
public PrintWriter getWriter()
    throws IOException {

    if (usingOutputStream) {
        throw new IllegalStateException
            (sm.getString("coyoteResponse.getWriter.ise"));
    }

    if (ENFORCE_ENCODING_IN_GET_WRITER) {
        setCharacterEncoding(getCharacterEncoding());
    }

    usingWriter = true;
    outputBuffer.checkConverter();
    if (writer == null) {
        writer = new CoyoteWriter(outputBuffer);
    }
    return writer;
}
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只能读取一次的原因

在Response中,读取是指从OutputStream中重新把body数据读出来,而OutputStream也和InputStream存在同样的问题,流只能读取一次,这里就不展开讲了。

解决方案

在Spring库中,提供了ContentCachingResponseWrapper和ContentCachingRequestWrapper两个类,分别解决了Response和Request不能重复读以及方法互斥问题。我们可以直接用ContentCachingRequestWrapper来包装Request,ContentCachingResponseWrapper来包装Response,包装后,在读取流数据的时候会将这个数据缓存一份,等读完以后,再将流数据重新写入Request或者Response就可以了。下面是一个简单的使用示例:

ContentCachingResponseWrapper responseToCache = new ContentCachingResponseWrapper(response);
String responseBody = new String(responseToCache.getContentAsByteArray());
responseToCache.copyBodyToResponse();
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缓存一份流数据,这就是基本的解决思路,下面我们从源码层面来看一看,主要关注getContentAsByteArray()、copyBodyToResponse()方法就行:

public class ContentCachingResponseWrapper extends HttpServletResponseWrapper {

   private final FastByteArrayOutputStream content = new FastByteArrayOutputStream(1024);

   private final ServletOutputStream outputStream = new ResponseServletOutputStream();

   private PrintWriter writer;

   private int statusCode = HttpServletResponse.SC_OK;

   private Integer contentLength;


   /**
    * Create a new ContentCachingResponseWrapper for the given servlet response.
    * @param response the original servlet response
    */
   public ContentCachingResponseWrapper(HttpServletResponse response) {
      super(response);
   }


   @Override
   public void setStatus(int sc) {
      super.setStatus(sc);
      this.statusCode = sc;
   }

   @SuppressWarnings("deprecation")
   @Override
   public void setStatus(int sc, String sm) {
      super.setStatus(sc, sm);
      this.statusCode = sc;
   }

   @Override
   public void sendError(int sc) throws IOException {
      copyBodyToResponse(false);
      try {
         super.sendError(sc);
      }
      catch (IllegalStateException ex) {
         // Possibly on Tomcat when called too late: fall back to silent setStatus
         super.setStatus(sc);
      }
      this.statusCode = sc;
   }

   @Override
   @SuppressWarnings("deprecation")
   public void sendError(int sc, String msg) throws IOException {
      copyBodyToResponse(false);
      try {
         super.sendError(sc, msg);
      }
      catch (IllegalStateException ex) {
         // Possibly on Tomcat when called too late: fall back to silent setStatus
         super.setStatus(sc, msg);
      }
      this.statusCode = sc;
   }

   @Override
   public void sendRedirect(String location) throws IOException {
      copyBodyToResponse(false);
      super.sendRedirect(location);
   }

   @Override
   public ServletOutputStream getOutputStream() throws IOException {
      return this.outputStream;
   }

   @Override
   public PrintWriter getWriter() throws IOException {
      if (this.writer == null) {
         String characterEncoding = getCharacterEncoding();
         this.writer = (characterEncoding != null ? new ResponsePrintWriter(characterEncoding) :
               new ResponsePrintWriter(WebUtils.DEFAULT_CHARACTER_ENCODING));
      }
      return this.writer;
   }

   @Override
   public void flushBuffer() throws IOException {
      // do not flush the underlying response as the content as not been copied to it yet
   }

   @Override
   public void setContentLength(int len) {
      if (len > this.content.size()) {
         this.content.resize(len);
      }
      this.contentLength = len;
   }

   // Overrides Servlet 3.1 setContentLengthLong(long) at runtime
   public void setContentLengthLong(long len) {
      if (len > Integer.MAX_VALUE) {
         throw new IllegalArgumentException("Content-Length exceeds ContentCachingResponseWrapper's maximum (" +
               Integer.MAX_VALUE + "): " + len);
      }
      int lenInt = (int) len;
      if (lenInt > this.content.size()) {
         this.content.resize(lenInt);
      }
      this.contentLength = lenInt;
   }

   @Override
   public void setBufferSize(int size) {
      if (size > this.content.size()) {
         this.content.resize(size);
      }
   }

   @Override
   public void resetBuffer() {
      this.content.reset();
   }

   @Override
   public void reset() {
      super.reset();
      this.content.reset();
   }

   /**
    * Return the status code as specified on the response.
    */
   public int getStatusCode() {
      return this.statusCode;
   }

   /**
    * Return the cached response content as a byte array.
    */
   public byte[] getContentAsByteArray() {
      return this.content.toByteArray();
   }

   /**
    * Return an {@link InputStream} to the cached content.
    * @since 4.2
    */
   public InputStream getContentInputStream() {
      return this.content.getInputStream();
   }

   /**
    * Return the current size of the cached content.
    * @since 4.2
    */
   public int getContentSize() {
      return this.content.size();
   }

   /**
    * Copy the complete cached body content to the response.
    * @since 4.2
    */
   public void copyBodyToResponse() throws IOException {
      copyBodyToResponse(true);
   }

   /**
    * Copy the cached body content to the response.
    * @param complete whether to set a corresponding content length
    * for the complete cached body content
    * @since 4.2
    */
   protected void copyBodyToResponse(boolean complete) throws IOException {
      if (this.content.size() > 0) {
         HttpServletResponse rawResponse = (HttpServletResponse) getResponse();
         if ((complete || this.contentLength != null) && !rawResponse.isCommitted()) {
            rawResponse.setContentLength(complete ? this.content.size() : this.contentLength);
            this.contentLength = null;
         }
         this.content.writeTo(rawResponse.getOutputStream());
         this.content.reset();
         if (complete) {
            super.flushBuffer();
         }
      }
   }


   private class ResponseServletOutputStream extends ServletOutputStream {

      @Override
      public void write(int b) throws IOException {
         content.write(b);
      }

      @Override
      public void write(byte[] b, int off, int len) throws IOException {
         content.write(b, off, len);
      }
   }


   private class ResponsePrintWriter extends PrintWriter {

      public ResponsePrintWriter(String characterEncoding) throws UnsupportedEncodingException {
         super(new OutputStreamWriter(content, characterEncoding));
      }

      @Override
      public void write(char buf[], int off, int len) {
         super.write(buf, off, len);
         super.flush();
      }

      @Override
      public void write(String s, int off, int len) {
         super.write(s, off, len);
         super.flush();
      }

      @Override
      public void write(int c) {
         super.write(c);
         super.flush();
      }
   }

}
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而ContentCachingRequestWrapper的解决思路也是差不多,我这里就不展开了,有兴趣的可以直接查看源码。

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