1  // Purpose.  Java calling C throwing exception
 2  //           (Java passes int in, C returns nothing or throws exception)
   
   
 3  ////////////////////////////// JNIdemos.java \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
   
 4  class JNIdemos {
 5     private double balance = 100;
 6     static {
 7        System.loadLibrary( "JNIdemos" );
 8     }
 9     static class InsufficientFunds extends Exception { }
   
10     public double getBalance() {
11        return balance;
12     }
13     public native void withdraw( int amt ) throws InsufficientFunds;
   
14     public static void main( String[] args ) {
15        JNIdemos account = new JNIdemos();
16        System.out.println( "balance was " + account.getBalance() );
17        try {
18           account.withdraw( 50 );
19           account.withdraw( 70 );
20        } catch (InsufficientFunds ex) {
21           System.out.println( "withdraw failed");
22        }
23        System.out.println( "balance is  " + account.getBalance() );
24     }
25  }
   
   
26  ////////////////////////////// JNIdemos.cpp \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
   
27  #include <iostream.h>
28  #include "JNIdemos.h"
   
29  extern "C" {
30  JNIEXPORT void JNICALL Java_JNIdemos_withdraw( JNIEnv* env, jobject this_ptr,
31                                                 jint amt ) {
32     jclass   accountClass = env->GetObjectClass( this_ptr );
33     jfieldID balanceId    = env->GetFieldID( accountClass, "balance",  "D" );
34     jdouble  balance      = env->GetDoubleField( this_ptr, balanceId );
35     if (amt > balance) {
36        jclass     excepClass = env->FindClass(  "JNIdemos$InsufficientFunds" );
37        jmethodID  ctorId     = env->GetMethodID( excepClass, "<init>", "()V" );
38        jthrowable excepObj   = (jthrowable)env->NewObject( excepClass,ctorId );
39        env->Throw( excepObj );
40        return;
41     }
42     balance -= amt;
43     env->SetDoubleField( this_ptr, balanceId, balance );
44  }
45  }
   
   
46  // C:> java JNIdemos
47  // balance was 100.0
48  // withdraw failed
49  // balance is  50.0