public long nativeSession;

public void native jniLogin(String server, String userName, String password);

	public void login(String server, String userName, String password) {
		jniLogin(server, userName, String password);
	}

Listing 1 provides a nativeSession variable, which will hold the session object that actually gets created in C through the native method jniLogin. Once we create the session object in C, we'll fill this nativeSession variable that's declared in Listing 1 with the memory location of the C object. I'll discuss this in-depth later.

I won't explain the step about creating the header file out of a Java file using the javah command (see Resources for this information), so I'll jump directly to the C implementation of this native method.

Let's see the C implementation of the native method jniLogin:

	Session msession = NULL;
            connect(mserver, muserName, mpassword, &msession);

jlong sessionLocation = (jlong)msession;

jclass cls = (*env)->GetObjectClass(env, obj);

	jfieldID fid =(*env)->GetFieldID(env, cls,"nativeSession","J");
            (*env)->SetLongField(env,obj,fid,sessionLocation);

In line 8 of Listing 2, I'm declaring the Session(C) object. In line 14, to connect to a particular mail server, we're passing the values of the server, the user name, and the password to the connect method. If the given user name and password is valid, the msession object gets filled. Note that the function "Connect" is already defined in the C API of the mail server. Now we have the session object that logged into the server in C. In line 15 we're getting the memory location of the msession object. When you do (jlong)msession, it gives the memory location of the msession object. In line 20 we are getting the Java object that called this C function. In line 21, we're getting the long field(variable) "nativeSession" that we declared in Session(Java). In line 20 we're setting the long variable with the variable sessionLocation. Now the variable nativeSession of Session(Java) object holds the memory location of the msession(C) object.

Getting the C Object Back Out of the Java Object
Figure 2 explains the different steps involved in this process.

The following example provides a clearer picture:

public void native jniSendMail(Object o, String to, String subject, String body);

public void sendMail(String to, String subject, String body) {
	jniSendMail(this, to, subject, body);
}

Let's say you wanted to send an e-mail with the session that logged in through the login method. For that we are passing the session object (with "this" keyword) to the jniSendMail method in Listing 3. Notice that session object has the nativeSession variable that holds the memory location of the native session object. With that we can send the e-mail. Let's see how we can get the native session back from this object in the next steps:

jclass cls = (*env)->GetObjectClass(env, sessionobj);

	jfieldID fid =(*env)->GetFieldID(env, cls,"nativeSession","J");
            jlong session = (*env)->GetLongField(env,obj,fid);

	Session *  msession = (Session)session;
            sendMail(msession, mto, msubject, mbody);

In line 14 of Listing 4, we're getting the Session(Java) class. In line 15, we're getting the variable nativeSession. Line 16 provides the value of the nativeSession variable that has the memory location of the native session. Line 18 has the C object from the memory location, since the long variable session holds the memory location of the native session object. When you typecast with (Session)session, it reads the object from the memory location. Now we have the C object. With this in line 19 we are calling the SendMail function with the values msession, mto, msubject, and mbody, which sends the e-mail to the address specified in the field "mto". Note that "SendMail" is the function that's already defined in the C API of the mail server.

Resources