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<TITLE>Writing Advanced Applications, Chapter 5: JNI Technology
Issues</TITLE>
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<FONT SIZE="-1"><A HREF="/developer/onlineTraining/index.html">Training Index</A></FONT>
<H2>Writing Advanced Applications<BR>
<FONT SIZE="3">Chapter 5 Continued: Other Programming Issues</FONT></H2>
<FONT SIZE="-1">[<A HREF="jnistring.html"><<BACK</A>] [<A HREF="index.html#contents">CONTENTS</A>] [<A HREF="swing.html">NEXT>></A>]</FONT></DIV>
<P>
This section presents information on accessing classes, methods, and fields,
and covers threading, memory, and Java<A HREF="#TJVM"><SUP>1</SUP></A> virtual machine
issues.
<UL>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#add">Language Issues</A></FONT>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#call">Calling Methods</A></FONT>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#field">Accessing Fields</A></FONT>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#thrd">Threads and Synchronization</A></FONT>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#memory">Memory Issues</A></FONT>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#invoke">Invocation</A></FONT>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#attach">Attaching Threads</A></FONT>
</UL>
<HR>
<A NAME="add"></A>
<H3>Language issues</H3>
So far, the native method examples have covered calling standalone C and
C++ functions that either return a result or modify parameters
passed into the function. However, C++ like the Java language uses
instances of classes. If you create a class in one native method, the
reference to this class does not have an equivalent class in the Java language,
which makes it difficult to call functions on the C++ class that was
first created.
<P>
One way to handle this situation is to keep a record of the C++ class
reference and pass that back to a proxy or to the calling
program. To ensure the C++ class persists across native method calls, use
the C++ <CODE>new</CODE> operator to create a reference to the C++
object on the stack.
<P>
The following code provides a mapping between the Xbase database and Java
language code. The Xbase database has a C++ API and uses an initialization
class to perform subsequent database operations. When the class object is
created, a pointer to this object is returned as a Java language
<CODE>int</CODE> value. You can use a <CODE>long</CODE> or larger value for
machines with greater than 32 bits.
<PRE>
public class CallDB {
public native int initdb();
public native short opendb(String name, int ptr);
public native short GetFieldNo(
String fieldname, int ptr);
static {
System.loadLibrary("dbmaplib");
}
public static void main(String args[]) {
String prefix=null;
CallDB db=new CallDB();
int res=db.initdb();
if(args.length>=1) {
prefix=args[0];
}
System.out.println(db.opendb("MYFILE.DBF", res));
System.out.println(db.GetFieldNo("LASTNAME", res));
System.out.println(db.GetFieldNo("FIRSTNAME", res));
}
}
</PRE>
The return result from the call to the <CODE>initdb</CODE> native
method, the <CODE>int</CODE> value,
is passed to subsequent native method calls. The native code
included in the <CODE>dbmaplib.cc</CODE> library de--references the
Java language object passed in as a parameter and retrieves the object
pointer. The line <CODE>xbDbf* Myfile=(xbDbf*)ptr;</CODE> casts the
<CODE>int</CODE> pointer value to be a pointer of Xbase type
<CODE>xbDbf</CODE>.
<PRE>
#include <jni.h>
#include <xbase/xbase.h>
#include "CallDB.h"
JNIEXPORT jint JNICALL Java_CallDB_initdb(
JNIEnv *env, jobject jobj) {
xbXBase* x;
x= new xbXBase();
xbDbf* Myfile;
Myfile =new xbDbf(x);
return ((jint)Myfile);
}
JNIEXPORT jshort JNICALL Java_CallDB_opendb(
JNIEnv *env, jobject jobj,
jstring dbname, jint ptr) {
xbDbf* Myfile=(xbDbf*)ptr;
return((*Myfile).OpenDatabase( "MYFILE.DBF"));
}
JNIEXPORT jshort JNICALL Java_CallDB_GetFieldNo
(JNIEnv *env, jobject jobj,
jstring fieldname,
jint ptr) {
xbDbf* Myfile=(xbDbf*)ptr;
return((*Myfile).GetFieldNo(
env->GetStringUTFChars(fieldname,0)));
}
</PRE>
<A NAME="call"></A>
<H3>Calling Methods</H3>
The section on arrays highlighted some reasons for calling Java
language methods from within native code; for example, when you need to
free the result you intend to return. Other uses for calling Java native
methods from within your native code would be if you need to return more
than one result or you just simply
want to modify Java language values from within native code.
<P>
Calling a Java language method from
within native code involves the following three steps:
<OL>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">Retrieve a class reference </FONT>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">Retrieve a method identifier</FONT>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">Call the Methods</FONT>
</OL>
<H4>Retrieve a Class Reference</H4>
The first step is to retrieve a reference to
the class that contains the methods you want to access.
To retrieve a reference, you can either use the <CODE>FindClass</CODE> method
or access the <CODE>jobject</CODE> or <CODE>jclass</CODE> argument to
the native method.
<PRE>
<STRONG>Use the FindClass method:</STRONG>
JNIEXPORT void JNICALL Java_ArrayHandler_returnArray
(JNIEnv *env, jobject jobj){
jclass cls = (*env)->FindClass(env, "ClassName");
}
</PRE>
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