1  // Purpose.  Java calling C calling Java
 2  //           (Java passes object in, C++ object created, C++ object calls
 3  //           Java, Java returns String)
   
   
 4  ////////////////////////////// JNIdemos.java \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
   
 5  import java.awt.event.*;
 6  import javax.swing.*;
   
 7  class JNIdemos {
 8     private JFrame f;
 9     static {
10        System.loadLibrary( "JNIdemos" );
11     }
   
12     public JNIdemos() {
13        f = new JFrame( "JNI demo" );
14        f.addWindowListener( new WindowAdapter() {
15           public void windowClosing( WindowEvent e ) { System.exit(0); } } );
16        f.getContentPane().add( new JLabel( "The parent window" ) );
17        f.pack();
18        f.setVisible( true );
19        startCpp();
20        // startCpp( this );
21     }
22     public String getInput() {
23        System.out.println( "java: fetching input" );
24        String str = JOptionPane.showInputDialog( f, "Enter something" );
25        return str;
26     }
27     public native void startCpp();
28     // public native void startCpp( JNIdemos in );
   
29     public static void main( String[] args ) {
30        new JNIdemos();
31  }  }
   
   

























32  ////////////////////////////// JNIdemos.cpp \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
   
33  #include <iostream.h>
34  #include "JNIdemos.h"
   
35  class DemoGUI {
36  public:
37     DemoGUI( jobject gui, JNIEnv* env ) {
38        jclass      guiClass = env->GetObjectClass( gui );
39        jmethodID   getId    = env->GetMethodID( guiClass, "getInput",
40                                                 "()Ljava/lang/String;" );
41        const char* strCpp;
42        jstring     strJava;
43        for (int i=0; i < 3; i++) {
44           strJava = (jstring) env->CallObjectMethod( gui, getId );
45           strCpp  = env->GetStringUTFChars( strJava, 0 );
46           cout << "cpp:  input received --" << strCpp << "--" << endl;
47           env->ReleaseStringUTFChars( strJava, strCpp );
48        }
49     }
50  };
   
51  extern "C" {
52  JNIEXPORT void JNICALL Java_JNIdemos_startCpp( JNIEnv* env, jobject in ) {
53     new DemoGUI( in, env );
54  }
55  }
   
56  // extern "C" {
57  // JNIEXPORT void JNICALL Java_JNIdemos_startCpp( JNIEnv* env, jobject,
58  //                                                             jobject in ) {
59  //    new DemoGUI( in, env );
60  // }
61  // }
   
   
62  // C:> java JNIdemos
63  // java: fetching input
64  // cpp:  input received --first entry--
65  // java: fetching input
66  // cpp:  input received --second interaction--
67  // java: fetching input
68  // cpp:  input received --final input--