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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 re-committing recommitting to the stream
- Flushing the stream's or writer's buffer
- Invoking either
getWriter()orgetOutputStream() - Redirecting an
HttpServletResponseto another server - Modifying the stream's character encoding, content type, or buffer size
Noncompliant Code Example
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| Code Block | ||||
|---|---|---|---|---|
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public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
PrintWriterServletOutputStream out = response.getWritergetOutputStream();
try {
out.println("<html>");
// ... writeWrite some response text
out.flush(); // commitsCommits the stream
// ... moreMore work
} catch (IOException x) {
response.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);
}
}
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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.
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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.:
| Code Block | ||||
|---|---|---|---|---|
| ||||
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
PrintWriterServletOutputStream out = response.getWritergetOutputStream();
try {
out.println("<html>");
// ... writeWrite some response text
out.flush(); // commitsCommits the stream
// ... moreMore work
} catch (IOException x) {
out.println(x.getMessage());
out.flush();
}
}
|
If an IOException occurs after flushing the stream, the stream will be re-flushed reflushed in the catch clause.
Compliant Solution
The sendError() method should only be used only before an output stream or writer has been created , as 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.
| Code Block | ||||
|---|---|---|---|---|
| ||||
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
try {
// doDo work that doesn't require the output writerstream
} catch (IOException x) {
response.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);
}
PrintWriterServletOutputStream out = response.getWritergetOutputStream();
try {
out.println("<html>");
// ... allAll work
} catch (IOException ex) {
out.println(ex.getMessage());
} finally {
out.flush();
}
}
|
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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
| Fortify | 6.10.0120 | Multiple_Stream_Commits | Implemented |
Tool | Version | Checker | Description |
|---|
Bibliography
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