Get object from an object with JNI in C

Question

public class Student
{
   private People people;
   private Result result;
   private int amount;
}

Here is the sample of the class in Java; in C, I tried to get the "people" in "Student", but I failed. However, I am able to get int type "amount" from "Student".

jobject getObjectFromObject(JNIEnv *env, jobject obj, const char * fieldName)
{
    jfieldID fid; /* store the field ID */
    jobject i;

    /* Get a reference to obj's class */
    jclass cls = (*env)->GetObjectClass(env, obj);

    /* Look for the instance field s in cls */
    fid = (*env)->GetFieldID(env, cls, fieldName, "L");
    if (fid == NULL)
    {
        return 0; /* failed to find the field */
    }

    /* Read the instance field s */
    i = (*env)->GetObjectField(env, obj, fid);

    return i;
}

I am trying to pass "people" as a fieldName into the method, but it still gives the following error: "java.lang.NoSuchFieldError: people"

i have fix the type signature by passing the package name.But still unable to fix my issue — user1151874, Apr 03 '13 at 10:44

mbrenon · Accepted Answer · 2013-04-03T10:40:06.623

8

As documented here, in the GetFieldID method you can't use "L" alone as a type signature, you have to specify the class name after that.

For example if you want to specify that the argument is a String, you'll have to use Ljava/lang/String; (The final semicolon is part of the signature!).

For your custom class named People, supposing it's in the package your.package.name, you'll have to use Lyour/package/name/People; as a type signature.

edited Apr 03 '13 at 10:40

answered Apr 03 '13 at 09:30

mbrenon

4,851
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25

fid = (*env)->GetFieldID(env, cls, fieldName, "LCalculateResult/src/student/jni/Student;"); still unable to find, should i put the inner class"People" or "Student", but i tried both also failed. I just realise both project is different path, how it able to locate it? – user1151874 Apr 03 '13 at 10:05
You have to use the *package name*, not the path. – mbrenon Apr 03 '13 at 10:15
Look into your `People.java` file, it must start with something like `package your.package.name;`. Convert the dots to "/" and you'll have a valid type signature. If `People` is an inner class of `Student`, I think you'll have to separate the "external" class from the inner class using a `$ `: use `Lyour/package/name/Student$People;` – mbrenon Apr 03 '13 at 10:20
package test.jni.bean; `code` these is my package name, so i converted into fid = (*env)->GetFieldID(env, cls, fieldName, "Ltest/jni/bean/Student$People;"); `code` but still java.lang.NoSuchFieldError: people – user1151874 Apr 03 '13 at 10:38
1

anyways i fixed my issue,`code`fid = (*env)->GetFieldID(env, cls, fieldName, "Ltest/jni/bean/People;"); `code` just give the package of the class, forget the outer class of "Student" – user1151874 Apr 04 '13 at 01:39

Get object from an object with JNI in C

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