Connecting Java to CGI

Bruce Eckel’s Thinking in Java Contents | Prev | Next

Encoding
data for CGI

In
this version, the name
and
the email address will be collected and stored in the file in the form:

First
Last <[email protected]>;

&lt;Form method="GET" ACTION="/cgi-bin/Listmgr2.exe"&gt;
&lt;P&gt;Name: &lt;INPUT TYPE = "text" NAME = "name"
VALUE = "" size = "40"&gt;&lt;/p&gt;
&lt;P&gt;Email Address: &lt;INPUT TYPE = "text"
NAME = "email" VALUE = "" size = "40"&gt;&lt;/p&gt;
&lt;p&gt;&lt;input type = "submit" name = "submit" &gt; &lt;/p&gt;
&lt;/Form&gt;

This
creates two data entry fields called
name
and
email,
along with a
submit
button that collects the data and sends it to a CGI program.
Listmgr2.exe
is the name of the executable program that resides in the directory
that’s typically called “cgi-bin” on your Web server.
[65]
(If the named program is not in the cgi-bin directory, you won’t see any
results.) If you fill out this form and press the “submit” button,
you will see in the URL address window of the browser something like:

http://www.myhome.com/cgi-bin/Listmgr2.exe?

name=First+Last&[email protected]&submit=Submit

(Without
the line break, of course). Here you see a little bit of the way that data is
encoded to send to CGI. For one thing, spaces are not allowed (since spaces
typically separate command-line arguments). Spaces are replaced by ‘
+
signs. In addition, each field contains the field name (which is determined by
the HTML page) followed by an ‘
=
and the field data, and terminated by a ‘
&’.

At
this point, you might wonder about the ‘
+’,

=,
and ‘
&’.
What if those are used in the field, as in “John & Marsha
Smith”? This is encoded to:

John+%26+Marsha+Smith

That
is, the special character is turned into a
‘%
followed by its ASCII value in hex.

Fortunately,
Java has a tool to perform this encoding for you. It’s a static method of
the class URLEncoder
called
encode( ).
You can experiment with this method using the following program:

//: EncodeDemo.java
// Demonstration of URLEncoder.encode()
import java.net.*;
 
public class EncodeDemo {
  public static void main(String[] args) {
    String s = "";
    for(int i = 0; i &lt; args.length; i++)
      s += args[i] + " ";
    s = URLEncoder.encode(s.trim());
    System.out.println(s);
  }
} ///:~ 

This
takes the command-line arguments and combines them into a string of words
separated by spaces (the final space is removed using
String.trim( )).
These are then encoded and printed.

The
applet

//: NameSender2.java
// An applet that sends an email address
// via a CGI GET, using Java 1.02.
import java.awt.*;
import java.applet.*;
import java.net.*;
import java.io.*;
 
public class NameSender2 extends Applet {
  final String CGIProgram = "Listmgr2.exe";
  Button send = new Button(
    "Add email address to mailing list");
  TextField name = new TextField(
    "type your name here", 40),
    email = new TextField(
    "type your email address here", 40);
  String str = new String();
  Label l = new Label(), l2 = new Label();
  int vcount = 0;
  public void init() {
    setLayout(new BorderLayout());
    Panel p = new Panel();
    p.setLayout(new GridLayout(3, 1));
    p.add(name);
    p.add(email);
    p.add(send);
    add("North", p);
    Panel labels = new Panel();
    labels.setLayout(new GridLayout(2, 1));
    labels.add(l);
    labels.add(l2);
    add("Center", labels);
    l.setText("Ready to send email address");
  }
  public boolean action (Event evt, Object arg) {
    if(evt.target.equals(send)) {
      l2.setText("");
      // Check for errors in data:
      if(name.getText().trim()
         .indexOf(' ') == -1) {
        l.setText(
          "Please give first and last name");
        l2.setText("");
        return true;
      }
      str = email.getText().trim();
      if(str.indexOf(' ') != -1) {
        l.setText(
          "Spaces not allowed in email name");
        l2.setText("");
        return true;
      }
      if(str.indexOf(',') != -1) {
        l.setText(
          "Commas not allowed in email name");
        return true;
      }
      if(str.indexOf('@') == -1) {
        l.setText("Email name must include '@'");
        l2.setText("");
        return true;
      }
      if(str.indexOf('@') == 0) {
        l.setText(
          "Name must preceed '@' in email name");
        l2.setText("");
        return true;
      }
      String end =
        str.substring(str.indexOf('@'));
      if(end.indexOf('.') == -1) {
        l.setText("Portion after '@' must " +
          "have an extension, such as '.com'");
        l2.setText("");
        return true;
      }
      // Build and encode the email data:
      String emailData =
        "name=" + URLEncoder.encode(
          name.getText().trim()) +
        "&amp;email=" + URLEncoder.encode(
          email.getText().trim().toLowerCase()) +
        "&amp;submit=Submit";
      // Send the name using CGI's GET process:
      try {
        l.setText("Sending...");
        URL u = new URL(
          getDocumentBase(), "cgi-bin/" +
          CGIProgram + "?" + emailData);
        l.setText("Sent: " + email.getText());
        send.setLabel("Re-send");
        l2.setText(
          "Waiting for reply " + ++vcount);
        DataInputStream server =
          new DataInputStream(u.openStream());
        String line;
        while((line = server.readLine()) != null)
          l2.setText(line);
      } catch(MalformedURLException e) {
        l.setText("Bad URl");
      } catch(IOException e) {
        l.setText("IO Exception");
      }
    }
    else return super.action(evt, arg);
    return true;
  }
} ///:~ 

The
name of the CGI program (which you’ll see later) is
Listmgr2.exe.
Many Web servers are Unix machines (mine runs Linux) that don’t
traditionally use the
.exe
extension for their executable programs, but you can call the program anything
you want under Unix. By using the
.exe
extension
the program can be tested without change under both Unix and Win32.

As
before, the applet sets up its user interface (with two fields this time
instead of one). The only significant difference occurs inside the
action( )
method,
which handles the button press. After the name has been checked, you see the
lines:

      String emailData =
        "name=" + URLEncoder.encode(
          name.getText().trim()) +
        "&amp;email=" + URLEncoder.encode(
          email.getText().trim().toLowerCase()) +
        "&amp;submit=Submit";
      // Send the name using CGI's GET process:
      try {
        l.setText("Sending...");
        URL u = new URL(
          getDocumentBase(), "cgi-bin/" +
          CGIProgram + "?" + emailData);
        l.setText("Sent: " + email.getText());
        send.setLabel("Re-send");
        l2.setText(
          "Waiting for reply " + ++vcount);
        DataInputStream server =
          new DataInputStream(u.openStream());
        String line;
        while((line = server.readLine()) != null)
          l2.setText(line); <p><tt>        // ... </tt></p>

The
CGI program you’re about to see returns only one line, a string
indicating success or failure (and the details of the failure). This line is
captured and placed into the second
Label
field so the user can see the results.


Displaying
a Web page from within an applet

It’s
also possible for the applet to display the result of the CGI program as a Web
page, just as if it were running in normal HTML mode. You can do this with the
following line:

getAppletContext().showDocument(u);

//: ShowHTML.java
import java.awt.*;
import java.applet.*;
import java.net.*;
import java.io.*;
 
public class ShowHTML extends Applet {
  static final String CGIProgram = "MyCGIProgram";
  Button send = new Button("Go");
  Label l = new Label();
  public void init() {
    add(send);
    add(l);
  }
  public boolean action (Event evt, Object arg) {
    if(evt.target.equals(send)) {
      try {
        // This could be an HTML page instead of
        // a CGI program. Notice that this CGI 
        // program doesn't use arguments, but 
        // you can add them in the usual way.
        URL u = new URL(
          getDocumentBase(),
          "cgi-bin/" + CGIProgram);
        // Display the output of the URL using
        // the Web browser, as an ordinary page:
        getAppletContext().showDocument(u);
      } catch(Exception e) {
        l.setText(e.toString());
      }
    }
    else return super.action(evt, arg);
    return true;
  }
} ///:~ 

The
CGI program in C++

At
this point you could follow the previous example and write the CGI program for
the server using ANSI C. One argument for doing this is that ANSI C can be
found virtually everywhere. However, C++ has become quite ubiquitous,
especially in the form of the
GNU
C++ Compiler
[66]
(
g++)
that
can be downloaded free from the Internet for virtually any platform (and often
comes pre-installed with operating systems such as Linux). As you will see,
this means that you can get the benefit of object-oriented programming in a CGI
program.

To
avoid throwing too many new concepts at you all at once, this program will not
be a “pure” C++ program; some code will be written in plain C even
though C++ alternatives exist. This isn’t a significant issue because the
biggest benefit in using C++ for this program is the ability to create classes.
Since what we’re concerned with when parsing the CGI information is the
field name-value pairs, one class (
Pair)
will
be used to represent a single name-value pair and a second class (
CGI_vector)
will automatically parse the CGI string into
Pair
objects that it will hold (as a
vector)
so you can fetch each
Pair
out at your leisure.

One
of the reasons for using C++ here is the convenience of the
C++
Standard
Template Library
.
Among other things, the STL contains a
vector
class. This is a C++
template,
which means that it will be configured at compile time so it will hold objects
of only a particular type (in this case,
Pair
objects). Unlike the Java
Vector,
which will accept anything, the C++
vector
template will cause a compile-time error message if you try to put anything but
a
Pair
object into the
vector,
and when you get something out of the
vector
it will automatically be a
Pair
object, without casting. Thus, the checking happens at compile time and
produces a more robust program. In addition, the program can run faster since
you don’t have to perform run-time casts. The
vector
also
overloads the
operator[]
so
you have a convenient syntax for extracting
Pair
objects.
The
vector
template will be used in the creation of
CGI_vector,
which you’ll see is a fairly short definition considering how powerful it
is.

On
the down side, look at the complexity of the definition of
Pair
in the following code.
Pair
has
more method definitions than you’re used to seeing in Java code, because
the C++ programmer must know how to control copying with the copy-constructor
and assignment with the overloaded
operator=.
As described in Chapter 12, occasionally you need to concern yourself with
similar things in Java, but in C++ you must be aware of them almost constantly.

The
project will start with a reusable portion, which consists of
Pair
and
CGI_vector
in a C++ header file. Technically, you shouldn’t cram this much into a
header file, but for these examples it doesn’t hurt anything and it will
also look more Java-like, so it will be easier for you to read:

//: CGITools.h
// Automatically extracts and decodes data
// from CGI GETs and POSTs. Tested with GNU C++ 
// (available for most server machines).
#include &lt;string.h&gt;
#include &lt;vector&gt; // STL vector
using namespace std;
 
// A class to hold a single name-value pair from
// a CGI query. CGI_vector holds Pair objects and
// returns them from its operator[].
class Pair {
  char* nm;
  char* val;
public:
  Pair() { nm = val = 0; }
  Pair(char* name, char* value) {
    // Creates new memory:
    nm = decodeURLString(name);
    val = decodeURLString(value);
  }
  const char* name() const { return nm; }
  const char* value() const { return val; }
  // Test for "emptiness"
  bool empty() const {
    return (nm == 0) || (val == 0);
  }
  // Automatic type conversion for boolean test:
  operator bool() const {
    return (nm != 0) &amp;&amp; (val != 0);
  }
  // The following constructors &amp; destructor are
  // necessary for bookkeeping in C++.
  // Copy-constructor:
  Pair(const Pair&amp; p) {
    if(p.nm == 0 || p.val == 0) {
      nm = val = 0;
    } else {
      // Create storage &amp; copy rhs values:
      nm = new char[strlen(p.nm) + 1];
      strcpy(nm, p.nm);
      val = new char[strlen(p.val) + 1];
      strcpy(val, p.val);
    }
  }
  // Assignment operator:
  Pair&amp; operator=(const Pair&amp; p) {
    // Clean up old lvalues:
    delete nm;
    delete val;
    if(p.nm == 0 || p.val == 0) {
      nm = val = 0;
    } else {
      // Create storage &amp; copy rhs values:
      nm = new char[strlen(p.nm) + 1];
      strcpy(nm, p.nm);
      val = new char[strlen(p.val) + 1];
      strcpy(val, p.val);
    }
    return *this;
  }
  ~Pair() { // Destructor
    delete nm; // 0 value OK
    delete val;
  }
  // If you use this method outide this class, 
  // you're responsible for calling 'delete' on
  // the pointer that's returned:
  static char*
  decodeURLString(const char* URLstr) {
    int len = strlen(URLstr);
    char* result = new char[len + 1];
    memset(result, len + 1, 0);
    for(int i = 0, j = 0; i &lt;= len; i++, j++) {
      if(URLstr[i] == '+')
        result[j] = ' ';
      else if(URLstr[i] == '%') {
        result[j] =
          translateHex(URLstr[i + 1]) * 16 +
          translateHex(URLstr[i + 2]);
        i += 2; // Move past hex code
      } else // An ordinary character
        result[j] = URLstr[i];
    }
    return result;
  }
  // Translate a single hex character; used by
  // decodeURLString():
  static char translateHex(char hex) {
    if(hex &gt;= 'A')
      return (hex &amp; 0xdf) - 'A' + 10;
    else
      return hex - '0';
  }
};
 
// Parses any CGI query and turns it
// into an STL vector of Pair objects:
class CGI_vector : public vector&lt;Pair&gt; {
  char* qry;
  const char* start; // Save starting position
  // Prevent assignment and copy-construction:
  void operator=(CGI_vector&amp;);
  CGI_vector(CGI_vector&amp;);
public:
  // const fields must be initialized in the C++
  // "Constructor initializer list":
  CGI_vector(char* query) :
      start(new char[strlen(query) + 1]) {
    qry = (char*)start; // Cast to non-const
    strcpy(qry, query);
    Pair p;
    while((p = nextPair()) != 0)
      push_back(p);
  }
  // Destructor:
  ~CGI_vector() { delete start; }
private:
  // Produces name-value pairs from the query 
  // string. Returns an empty Pair when there's 
  // no more query string left:
  Pair nextPair() {
    char* name = qry;
    if(name == 0 || *name == '')
      return Pair(); // End, return null Pair
    char* value = strchr(name, '=');
    if(value == 0)
      return Pair(); // Error, return null Pair
    // Null-terminate name, move value to start
    // of its set of characters:
    *value = '';
    value++;
    // Look for end of value, marked by '&amp;':
    qry = strchr(value, '&amp;');
    if(qry == 0) qry = ""; // Last pair found
    else {
      *qry = ''; // Terminate value string
      qry++; // Move to next pair
    }
    return Pair(name, value);
  }
}; ///:~ 

After
the
#include
statements, you see a line that says:

using
namespace std;

Namespaces
in C++ solve one of the problems taken care of by the
package
scheme in Java: hiding library names. The
std
namespace refers to the Standard C++ library, and
vector
is in this library so the line is required.

The
Pair
class starts out looking pretty simple: it just holds two (
private)
character
pointers, one for the name and one for the value. The default constructor
simply sets these pointers to zero, since in C++ an object’s memory
isn’t automatically zeroed. The second constructor calls the method
decodeURLString( )
that produces a decoded string in newly-allocated heap memory. This memory must
be managed and destroyed by the object, as you will see in the destructor. The
name( )
and
value( )
methods produce read-only pointers to the respective fields. The
empty( )
method is a way for you to ask the
Pair
object whether either of its fields are empty; it returns a
bool,
which is C++’s built-in primitive Boolean data type. The
operator
bool( )

uses a special case of
C++
operator
overloading
,
which allows you to control automatic type conversion. If you have a
Pair
object called
p
and you use it in an expression in which a Boolean result is expected, such as
if(p)
{ //…
,
then the compiler will recognize that it has a
Pair
and it needs a Boolean, so it will automatically call
operator
bool( )

to perform the necessary conversion.

The
copy-constructor
Pair(const
Pair&)

is automatically called whenever you pass an object into or out of a function
by
value
.
That is, you aren’t passing the address of the object you’re making
a copy of the whole object inside the function frame. This isn’t an
option in Java since you pass only handles, thus there’s no
copy-constructor in Java. (If you want to make a local duplicate, you
clone( )
the object – see Chapter 12.) Likewise, if you assign a handle in Java,
it’s simply copied. But assignment in C++ means that the entire object is
copied. In the copy-constructor, you create new storage and copy the source
data, but with the assignment operator you must release the old storage before
allocating new storage. What you’re seeing is probably the worst-case
complexity scenario for a C++ class, but it’s one of the reasons Java
proponents can argue that Java is a lot simpler than C++. In Java you pass
handles and there’s a garbage collector, so you don’t have to do
this kind of thing.

This
isn’t quite the whole story. The
Pair
class is using
char*
for
nm
and
val,
and the worst-case complexity occurs primarily around pointers. If you use the
more modern Standard C++
string
class instead of
char*,
things get much simpler (however, not all compilers have caught up enough to
come with
string).
Then, the first part of
Pair
looks like this:

class Pair {
  string nm;
  string val;
public:
  Pair() { }
  Pair(char* name, char* value) {
    // Creates new memory:
    nm = decodeURLString(name);
    val = decodeURLString(value);
  }
  const char* name() const { return nm.c_str(); }
  const char* value() const { return val.c_str(); }
  // Test for "emptiness"
  bool empty() const {
    return (nm.length() == 0) || (val.length() == 0);
  }
  // Automatic type conversion for boolean test:
  operator bool() const {
    return (nm.length() != 0) &amp;&amp; (val.length() != 0);
  }

(Also,
for this case
decodeURLString( )
returns a
string
instead of a
char*.)
You don’t need to define a copy-constructor,
operator=,
or destructor because the compiler does that for you, and does it correctly.
But even if it sometimes works automatically, C++ programmers must still know
the details of copy-construction and assignment.

The
remainder of the
Pair
class consists of the two methods
decodeURLString( )
and a helper method
translateHex( ),
which is used by
decodeURLString( ).
(Note that
translateHex( )
does not guard against bad user input such as “%1H.”) After
allocating adequate storage (which must be released by the destructor),
decodeURLString( )
moves through and replaces each ‘
+
with a space and each hex code (beginning with a ‘
%’)
with the appropriate character.

CGI_vector
parses and holds an entire CGI GET command. It is inherited from the STL
vector,
which is instantiated to hold
Pairs.
Inheritance in C++ is denoted by using a colon at the point you’d say
extends
in Java. In addition, inheritance defaults to
private
so you’ll almost always need to use the
public
keyword as was done here. You can also see that
CGI_vector
has a copy-constructor and an
operator=,
but they’re both declared as
private.
This is to prevent the compiler from synthesizing the two functions (which it
will do if you don’t declare them yourself), but it also prevents the
client programmer from passing a
CGI_vector
by value or from using assignment.

CGI_vector’s
job is to take the QUERY_STRING and parse it into name-value pairs, which it
will do with the aid of
Pair.
First it copies the string into locally-allocated memory and keeps track of the
starting address with the constant pointer
start.
(This
is later used in the destructor to release the memory.)

Then
it uses its method
nextPair( )
to parse the string into raw name-value pairs, delimited by ‘
=
and

&
signs. These are handed by
nextPair( )
to the
Pair
constructor so
nextPair( )
can return the
Pair
object, which is then added to the
vector
with
push_back( ).
When
nextPair( )
runs out of QUERY_STRING, it returns zero.

Now
that the basic tools are defined, they can easily be used in a CGI program,
like this:

//: Listmgr2.cpp
// CGI version of Listmgr.c in C++, which 
// extracts its input via the GET submission 
// from the associated applet. Also works as
// an ordinary CGI program with HTML forms.
#include &lt;stdio.h&gt;
#include "CGITools.h"
const char* dataFile = "list2.txt";
const char* notify = "[email protected]";
#undef DEBUG
 
// Similar code as before, except that it looks
// for the email name inside of '&lt;&gt;':
int inList(FILE* list, const char* emailName) {
  const int BSIZE = 255;
  char lbuf[BSIZE];
  char emname[BSIZE];
  // Put the email name in '&lt;&gt;' so there's no
  // possibility of a match within another name:
  sprintf(emname, "&lt;%s&gt;", emailName);
  // Go to the beginning of the list:
  fseek(list, 0, SEEK_SET);
  // Read each line in the list:
  while(fgets(lbuf, BSIZE, list)) {
    // Strip off the newline: 
    char * newline = strchr(lbuf, 'n');
    if(newline != 0)
      *newline = '';
    if(strstr(lbuf, emname) != 0)
      return 1;
  }
  return 0;
}
 
void main() {
  // You MUST print this out, otherwise the 
  // server will not send the response:
  printf("Content-type: text/plainnn");
  FILE* list = fopen(dataFile, "a+t");
  if(list == 0) {
    printf("error: could not open database. ");
    printf("Notify %s", notify);
    return;
  }
  // For a CGI "GET," the server puts the data
  // in the environment variable QUERY_STRING:
  CGI_vector query(getenv("QUERY_STRING"));
  #if defined(DEBUG)
  // Test: dump all names and values
  for(int i = 0; i &lt; query.size(); i++) {
    printf("query[%d].name() = [%s], ",
      i, query[i].name());
    printf("query[%d].value() = [%s]n",
      i, query[i].value());
  }
  #endif(DEBUG)
  Pair name = query[0];
  Pair email = query[1];
  if(name.empty() || email.empty()) {
    printf("error: null name or email");
    return;
  }
  if(inList(list, email.value())) {
    printf("Already in list: %s", email.value());
    return;
  }
  // It's not in the list, add it:
  fseek(list, 0, SEEK_END);
  fprintf(list, "%s &lt;%s&gt;;n",
    name.value(), email.value());
  fflush(list);
  fclose(list);
  printf("%s &lt;%s&gt; added to listn",
    name.value(), email.value());
} ///:~ 

The
alreadyInList( )
function is almost identical to the previous version, except that it assumes
all email names are inside ‘
<>’.

When
you use the
GET
approach (which is normally done in the HTML METHOD tag of the FORM directive,
but which is controlled here by the way the data is sent), the Web server grabs
everything after the ‘?’ and puts in into the environment variable
QUERY_STRING.
So to read that information you have to get the value of QUERY_STRING, which
you do using the standard C library function
getenv( ).
In
main( ),
notice
how simple the act of parsing the QUERY_STRING is: you just hand it to the
constructor for the
CGI_vector
object called
query
and
all the work is done for you. From then on you can pull out the names and
values from
query
as if it were an array. (This is because the
operator[]
is overloaded in
vector.)
You can see how this works in the debug code, which is surrounded by the
preprocessor directives
#if
defined(DEBUG)

and
#endif(DEBUG).

In
this case, we want the server to feed all the information directly back to the
client program (which is our applet, waiting for its reply). The information
should be unchanged, so the
content-type
is
text/plain.
Once the server sees this, it will echo all strings right back to the client.
So each of the strings you see, three for error conditions and one for a
successful add, will end up back at the applet.

Adding
the email name uses the same code. In the case of the CGI script, however,
there isn’t an infinite loop – the program just responds and then
terminates. Each time a CGI request comes in, the program is started in
response to that request, and then it shuts down. Thus there is no possibility
of CPU hogging, and the only performance issue concerns starting the program up
and opening the file, which are dwarfed by the overhead of the Web server as it
handles the CGI request.

What
about POST?

Using
a GET is fine for many applications. However, GET passes its data to the CGI
program through an environment variable, and some Web servers can run out of
environment space with long GET strings (you should start worrying at about 200
characters). CGI provides a solution for this: POST. With POST, the data is
encoded and concatenated the same way as a GET, but POST uses standard input to
pass the encoded query string to the CGI program. All you have to do is
determine the length of the query string, and this length is stored in the
environment variable CONTENT_LENGTH. Once you know the length, you can allocate
storage and read the precise number of bytes from standard input.

The
Pair
and
CGI_vector
from
CGITools.h
can be used as is for a CGI program that handles POSTs. The following listing
shows how simple it is to write such a CGI program. In this example,
“pure” C++ will be used so the
stdio.h
library will be dropped in favor of
iostreams.
With
iostreams,
two predefined objects are available:
cin,
which connects to standard input, and
cout,
which connects to standard output. There are several ways to read from
cin
and write to
cout,
but the following program take the common approach of using ‘
<<
to send information to
cout,
and the use of a member function (in this case,
read( ))
to read from
cin.

//: POSTtest.cpp
// CGI_vector works as easily with POST as it
// does with GET. Written in "pure" C++.
#include &lt;iostream.h&gt;
#include "CGITools.h"
 
void main() {
  cout &lt;&lt; "Content-type: text/plainn" &lt;&lt; endl;
  // For a CGI "POST," the server puts the length
  // of the content string in the environment 
  // variable CONTENT_LENGTH:
  char* clen = getenv("CONTENT_LENGTH");
  if(clen == 0) {
    cout &lt;&lt; "Zero CONTENT_LENGTH" &lt;&lt; endl;
    return;
  }
  int len = atoi(clen);
  char* query_str = new char[len + 1];
  cin.read(query_str, len);
  query_str[len] = '';
  CGI_vector query(query_str);
  // Test: dump all names and values
  for(int i = 0; i &lt; query.size(); i++)
    cout &lt;&lt; "query[" &lt;&lt; i &lt;&lt; "].name() = [" &lt;&lt;
      query[i].name() &lt;&lt; "], " &lt;&lt;
      "query[" &lt;&lt; i &lt;&lt; "].value() = [" &lt;&lt;
      query[i].value() &lt;&lt; "]" &lt;&lt; endl;
  delete query_str; // Release storage
} ///:~ 

The
getenv( )
function returns a pointer to a character string representing the content
length. If this pointer is zero, the CONTENT_LENGTH environment variable has
not been set, so something is wrong. Otherwise, the character string must be
converted to an integer using the ANSI C library function
atoi( ).
The length is used with
new
to
allocate enough storage to hold the query string (plus its null terminator),
and then
read( )
is called for
cin.
The
read( )
function takes a pointer to the destination buffer and the number of bytes to
read. The
query_str
is then null-terminated to indicate the end of the character string.

At
this point, the query string is no different from a GET query string, so it is
handed to the constructor for
CGI_vector.
The different fields in the vector are then available just as in the previous
example.

To
test this program, you must compile it in the cgi-bin directory of your host
Web server. Then you can perform a simple test by writing an HTML page like this:

&lt;HTML&gt;
&lt;HEAD&gt;
&lt;META CONTENT="text/html"&gt;
&lt;TITLE&gt;A test of standard HTML POST&lt;/TITLE&gt;
&lt;/HEAD&gt;
Test, uses standard html POST
&lt;Form method="POST" ACTION="/cgi-bin/POSTtest"&gt;
&lt;P&gt;Field1: &lt;INPUT TYPE = "text" NAME = "Field1"
VALUE = "" size = "40"&gt;&lt;/p&gt;
&lt;P&gt;Field2: &lt;INPUT TYPE = "text" NAME = "Field2"
VALUE = "" size = "40"&gt;&lt;/p&gt;
&lt;P&gt;Field3: &lt;INPUT TYPE = "text" NAME = "Field3"
VALUE = "" size = "40"&gt;&lt;/p&gt;
&lt;P&gt;Field4: &lt;INPUT TYPE = "text" NAME = "Field4"
VALUE = "" size = "40"&gt;&lt;/p&gt;
&lt;P&gt;Field5: &lt;INPUT TYPE = "text" NAME = "Field5"
VALUE = "" size = "40"&gt;&lt;/p&gt;
&lt;P&gt;Field6: &lt;INPUT TYPE = "text" NAME = "Field6"
VALUE = "" size = "40"&gt;&lt;/p&gt;
&lt;p&gt;&lt;input type = "submit" name = "submit" &gt; &lt;/p&gt;
&lt;/Form&gt;
&lt;/HTML&gt;

When
you fill this out and submit it, you’ll get back a simple text page
containing the parsed results, so you can see that the CGI program works
correctly.

//: POSTtest.java
// An applet that sends its data via a CGI POST
import java.awt.*;
import java.applet.*;
import java.net.*;
import java.io.*;
 
public class POSTtest extends Applet {
  final static int SIZE = 10;
  Button submit = new Button("Submit");
  TextField[] t = new TextField[SIZE];
  String query = "";
  Label l = new Label();
  TextArea ta = new TextArea(15, 60);
  public void init() {
    Panel p = new Panel();
    p.setLayout(new GridLayout(t.length + 2, 2));
    for(int i = 0; i &lt; t.length; i++) {
      p.add(new Label(
        "Field " + i + "  ", Label.RIGHT));
      p.add(t[i] = new TextField(30));
    }
    p.add(l);
    p.add(submit);
    add("North", p);
    add("South", ta);
  }
  public boolean action (Event evt, Object arg) {
    if(evt.target.equals(submit)) {
      query = "";
      ta.setText("");
      // Encode the query from the field data:
      for(int i = 0; i &lt; t.length; i++)
         query += "Field" + i + "=" +
           URLEncoder.encode(
             t[i].getText().trim()) +
           "&amp;";
      query += "submit=Submit";
      // Send the name using CGI's POST process:
      try {
        URL u = new URL(
          getDocumentBase(), "cgi-bin/POSTtest");
        URLConnection urlc = u.openConnection();
        urlc.setDoOutput(true);
        urlc.setDoInput(true);
        urlc.setAllowUserInteraction(false);
        DataOutputStream server =
          new DataOutputStream(
            urlc.getOutputStream());
        // Send the data
        server.writeBytes(query);
        server.close();
        // Read and display the response. You
        // cannot use 
        // getAppletContext().showDocument(u);
        // to display the results as a Web page!
        DataInputStream in =
          new DataInputStream(
            urlc.getInputStream());
        String s;
        while((s = in.readLine()) != null) {
          ta.appendText(s + "n");
        }
        in.close();
      }
      catch (Exception e) {
        l.setText(e.toString());
      }
    }
    else return super.action(evt, arg);
    return true;
  }
} ///:~ 

Once
the information is sent to the server, you can call getInputStream( )
and wrap the return value in a
DataInputStream
so that you can read the results. One thing you’ll notice is that the
results are displayed as lines of text in a
TextArea.
Why not simply use
getAppletContext().showDocument(u)?
Well, this is one of those mysteries. The code above works fine, but if you try
to use
showDocument( )
instead,
everything stops working – almost. That is,
showDocument( )
does
work, but what you get back from
POSTtest
is “Zero CONTENT_LENGTH.” So somehow,
showDocument( )
prevents the POST query from being passed on to the CGI program. It’s
difficult to know whether this is a bug that will be fixed, or some lack of
understanding on my part (the books I looked at were equally abstruse). In any
event, if you can stand to limit yourself to looking at the text that comes
back from the CGI program, the above applet works fine.


[65]
You can test this under Windows32 using the Microsoft Personal Web Server that
comes with Microsoft Office 97 and some of their other products. This is a nice
way to experiment since you can perform local tests (and it’s also fast). If
you’re on a different platform or if you don’t have Office 97, you might be
able to find a freeware Web server for testing by searching the Internet.

[66]
GNU stands for “Gnu’s Not Unix.” The project, created by the
Free Software Foundation, was originally intended to replace the Unix operating
system with a free version of that OS. Linux appears to have replaced this
initiative, but the GNU tools have played an integral part in the development
of Linux, which comes packaged with many GNU components.

[67]
My book
Thinking
in C++

(Prentice-Hall, 1995) devotes an entire chapter to this subject. Refer to this
if you need further information on the subject.

[68]
I can’t say I really understand what’s going on here, but I managed
to get it working by studying
Java
Network Programming

by Elliotte Rusty Harold (O’Reilly 1997). He alludes to a number of
confusing bugs in the Java networking libraries, so this is an area in which
you can’t just write code and have it work right away. Be warned.

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