1  // Purpose.  C calling Java
 2  //           (C creates JVM, C passes nothing, Java returns String)
 3  //           [see file:///C:/jdk1.2/docs/guide/jni/jni-12.html]
   
   
 4  ////////////////////////////// JNIdemos.java \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
   
 5  import java.awt.event.*;
 6  import javax.swing.*;
   
 7  class JNIdemos {
 8     private JFrame f;
 9     public JNIdemos() {
10        f = new JFrame( "JNI demo" );
11        f.addWindowListener( new WindowAdapter() {
12           public void windowClosing( WindowEvent e ) { System.exit(0); } } );
13        f.getContentPane().add( new JLabel( "The parent window" ) );
14        f.pack();
15        f.setVisible( true );
16     }
17     public String getInput() {
18        System.out.println( "java: fetching input" );
19        String str = JOptionPane.showInputDialog( f, "Enter something" );
20        return str;
21     }
22  }
   
   
23  ////////////////////////////// JNIdemos.cpp \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
   
24  #include <iostream.h>
25  #include <jni.h>
   
26  void main( void ) {
27     JavaVMOption   options[2];  // arbitrary VM start-up strings
28     JavaVMInitArgs vm_args;     // JVM initialization arguments
29     JavaVM*        jvm;         // pointer to receive JVM struct
30     JNIEnv*        env;         // pointer to receive JNI struct
   
31     // disable JIT
32     options[0].optionString = "-Djava.compiler=NONE";
33     // CLASSPATH
34     options[1].optionString = "-Djava.class.path=c:\\jdk1.2\\vlh";
   
35     // set native library path - had no effect (vlh)
36     // options[2].optionString =
37     //                 "-Djava.library.path=c:\\jdk1.2\\jre\\bin\\classic";
38     // print JNI-related messages
39     // options[3].optionString = "-verbose:jni";
   
40     vm_args.version            = JNI_VERSION_1_2;
41     vm_args.options            = options;
42     vm_args.nOptions           = 2;
43     vm_args.ignoreUnrecognized = true;
   









44     int res = JNI_CreateJavaVM( &jvm, (void**) &env, &vm_args );
45     if (res < 0) cout << "create failed" << endl;
   
46     jclass    clas = env->FindClass( "JNIdemos" );
47     jmethodID ctor = env->GetMethodID( clas, "<init>", "()V" );
48     jobject   gui  = env->NewObject( clas, ctor );
49     jmethodID get  = env->GetMethodID( clas, "getInput",
50                                              "()Ljava/lang/String;");
51     const char* strCpp;
52     jstring     strJava;
53     for (int i=0; i < 3; i++) {
54        strJava = (jstring) env->CallObjectMethod( gui, get );
55        strCpp  = env->GetStringUTFChars( strJava, 0 );
56        cout << "cpp:  input received --" << strCpp << "--" << endl;
57        env->ReleaseStringUTFChars( strJava, strCpp );
58     }
   
59     jvm->DestroyJavaVM();
60  }
   
   
61  // C:> jnidemos
62  // java: fetching input
63  // cpp:  input received --first entry--
64  // java: fetching input
65  // cpp:  input received --second interaction--
66  // java: fetching input
67  // cpp:  input received --final input--