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When a web servlet receives a request from a client, it must produce some suitable response. Java's HttpServlet provides the HttpServletResponse object to capture a suitable response. This response can be built using an output stream provided by getOutputStream() or a writer provided by getWriter().

A response is said to be committed if its status code and HTML headers have been sent [J2EE API 2013]. After a response is committed, further data may be added to the response, but certain behaviors become impossible. For example, it is impossible to change the character encoding, because the encoding is included in the HTML header. Some of these illegal operations will yield a IllegalStateException, while others will have no effect. These illegal behaviors include the following:

  • Resetting the stream or recommitting to the stream
  • Flushing the stream's or writer's buffer
  • Invoking either getWriter() or getOutputStream()
  • Redirecting an HttpServletResponse to another server
  • Modifying the stream's character encoding, content type, or buffer size

Noncompliant Code Example

This noncompliant code example illustrates a servlet that indicates if an internal error occurs by using the HttpServletResponse.sendError() method to indicate an internal server error.

public void doGet(HttpServletRequest request, HttpServletResponse response)
  throws IOException, ServletException {

  PrintWriter out = response.getWriter();
  try {
    out.println("<html>");

    // ... Write some response text

    out.flush();  // Commits the stream

    // ... More work

  } catch (IOException x) {
    response.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);
  }
}

If an IOException occurs after flushing the stream, the stream will be committed when the catch clause executes. Consequently, the sendError() operation will throw an IllegalStateException.

Noncompliant Code Example

This noncompliant code example illustrates a servlet that indicates if an internal error occurs by printing an error message to the output stream and flushing it:

public void doGet(HttpServletRequest request, HttpServletResponse response)
  throws IOException, ServletException {

  PrintWriter out = response.getWriter();
  try {
    out.println("<html>");

    // ... Write some response text

    out.flush();  // Commits the stream

    // ... More work

  } catch (IOException x) {
    out.println(x.getMessage());
    out.flush();
  }
}

If an IOException occurs after flushing the stream, the stream will be reflushed in the catch clause.

Compliant Solution

The sendError() method should be used only before an output stream or writer has been created because it overwrites any output. Once the output stream or writer has been created, errors should be output alongside valid HTML. This compliant solution uses both strategies, ensuring that the stream is flushed only once, in the finally clause.

public void doGet(HttpServletRequest request, HttpServletResponse response)
  throws IOException, ServletException {

  try {
    // Do work that doesn't require the output writer
  } catch (IOException x) {
    response.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);
  }

  PrintWriter out = response.getWriter();
  try {
    out.println("<html>");

    // ... All work

  } catch (IOException ex) {
    out.println(ex.getMessage());
  } finally {
    out.flush();
  }
}

Risk Assessment

If a servlet's output stream is reset after it has been committed, an IllegalStateException usually results, which can cause the servlet's response to be truncated.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

FIO15-J

Low

Probable

Medium

P4

L3

Automated Detection

Fortify6.10.0120

Multiple_Stream_Commits

Implemented
Tool
Version
Checker
Description

Bibliography