Passing data types from Java to C (or vice versa) using JNI

Question

Using JNI can we pass custom data types from Java to C (or vice versa)? I see a mapping of primitive datatypes to types in C however not too sure if we can send across our own data types (e.g. Send across or return an Employee object or something!).

Answer 1

If you're going to be doing this with a lot of objects, something like Swig would be best. You could use jobject type to pass around custom objects. The syntax isn't nice, perhaps there is a better way to write this.

Example Employee object:

public class Employee {     private int age;      public Employee(int age) {         this.age = age;     }      public int getAge() {         return age;     } } 

Call this code from some client:

public class Client {     public Client() {         Employee emp = new Employee(32);          System.out.println("Pass employee to C and get age back: "+getAgeC(emp));          Employee emp2 = createWithAge(23);          System.out.println("Get employee object from C: "+emp2.getAge());     }      public native int getAgeC(Employee emp);     public native Employee createWithAge(int age); } 

You could have JNI functions like this for passing an employee object from Java to C, as a jobject method argument:

JNIEXPORT jint JNICALL Java_Client_getAgeC(JNIEnv *env, jobject callingObject, jobject employeeObject) {     jclass employeeClass = (*env)->GetObjectClass(env, employeeObject);     jmethodID midGetAge = (*env)->GetMethodID(env, employeeClass, "getAge", "()I");     int age =  (*env)->CallIntMethod(env, employeeObject, midGetAge);     return age; } 

Passing an employee object back from C to Java as a jobject, you could use:

JNIEXPORT jobject JNICALL Java_Client_createWithAge(JNIEnv *env, jobject callingObject, jint age) {     jclass employeeClass = (*env)->FindClass(env,"LEmployee;");     jmethodID midConstructor = (*env)->GetMethodID(env, employeeClass, "<init>", "(I)V");     jobject employeeObject = (*env)->NewObject(env, employeeClass, midConstructor, age);     return employeeObject; }