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This noncompliant code example creates a servlet that echos a parameter passed to it, as well as the previous parameter passed to itprompts the user for an email address, and then repeats the address back to the user. The previous parameter address is stored in the lastAddrAddr lastAddr variable, which is an instance field.
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Because the HttpServlet class is a singleton, there is only one lastAddr field that is shared by every client who accesses the servlet. Therefore the contents of the lastAddr field can be the previous setting of the field by a different client. Also, since there is no thread-safety, it is possible for the lastAddr field to take on a stale value should two clients request the parameter simultaneously. Consequently, this code example also violates VNA01-J. Ensure visibility of shared references to immutable objects.
Noncompliant Code Example
In this noncompliant code example, the lastAddr field is static. This more accurately reflects the fact that there is never more than a single instance of the field. This However, this code has the same behavior as the previous noncompliant code example, and also violates VNA01-J. Ensure visibility of shared references to immutable objects.
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public class SampleServlet extends HttpServlet {
private static String lastAddr = "nobody@nowhere.com";
// ... other methods unchanged
}
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Noncompliant Code Example
In this compliant solutionnoncompliant code example, the lastAddr field is static , and is protected from conemailAddr concurrent access by a separate lock object, as is recommended by LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code. This guarantees thread-safety in the servlet. The However, the servlet can still return the lastAddr parameter entered email address provicded by a different session, however.
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public class SampleServlet extends HttpServlet {
private static String lastAddr = "nobody@nowhere.com";
private static final Object lastAddrLock = new Object();
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<html>");
String emailAddr = request.getParameter("emailAddr");
if (emailAddr != null) {
out.println("Email Address::");
out.println(sanitize(emailAddr));
synchronized (lock) {
out.println("<br>Previous Email Address::");
out.println(sanitize(lastAddr));
}
};
out.println("<p>");
out.print("<form action=\"");
out.print("SampleServlet\" ");
out.println("method=POST>");
out.println("Parameter:");
out.println("<input type=text size=20 name=emailAddr>");
out.println("<br>");
out.println("<input type=submit>");
out.println("</form>");
synchronized (lock) {
lastAddr = emailAddr;
}
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
doGet(request, response);
}
// Filter the specified message string for characters
// that are sensitive in HTML.
public static String sanitize(String message) {
// ...
}
} |
Compliant Solution
This compliant solution stores the lastAddr parameter in the HttpSession object, which is provided as part of the HttpServletRequest. The servlet mechanism keeps track of the session, providing the client with the session's ID, which is stored as a cookie by the client's browser. The other information in the session, including the lastAddr attribute, are stored by the server. Consequently, the servlet provides the lastAddr value last email address that was presented to the servlet in the same session (avoiding race conditions with requests from other sessions). The local variables, which temporarily hold data in this example, are not vulnerable to race conditions in the singleton.
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public class SampleServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<html>");
String emailAddr = request.getParameter("emailAddr");
HttpSession session = request.getSession();
Object attr = session.getAttribute("lastAddr");
String lastAddr = (attr == null) ? "null" : attr.toString();
if (emailAddr != null) {
out.println("Email Address::");
out.println(sanitize(emailAddr));
out.println("<br>Previous Email Address::");
out.println(sanitize(lastAddr));
};
out.println("<p>");
out.print("<form action=\"");
out.print("SampleServlet\" ");
out.println("method=POST>");
out.println("Parameter:");
out.println("<input type=text size=20 name=emailAddr>");
out.println("<br>");
out.println("<input type=submit>");
out.println("</form>");
session.setAttribute("lastAddr", emailAddr);
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
doGet(request, response);
}
// Filter the specified message string for characters
// that are sensitive in HTML.
public static String sanitize(String message) {
// ...
}
}
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