JNI call to Java function returning an Object results in NoSuchMethodError

Question

I have a Java function which returns a singleton instance of a Class

public static synchronized MyClass getInstance() throws MyClassException{
    if (instance == NULL){
        // create
    } 
    return instance;
}

I want to call this through C++ code, but when I do, it returns a NoSuchMethodError.

cls = jenv->FindClass("MyClass");
if (cls == NULL)
{
//error handling
}
mid = jenv->GetStaticMethodID(cls, "getInstance", "()LMyClass");
if (mid == NULL)
{
//error handling
}

When I run:

javap -s -p on MyClass, I get the signature:
public static synchronized MyClass getInstance()   throws MyClassException;
Signature: ()LMyClass;

If I change the function signature to void in the Java class, the GetStaticMethodID call works as expected.

Do I need to setup a jobject to expect the return value from the call?

Is this possible without calling GetStaticMethodID first?

When I run the C++ app, I get: Exception in thread "main" java.lang.NoSuchMethodError: getInstance — donalmg
– donalmg, Commented Feb 1, 2012 at 20:16
I wonder if you need to specify the fully qualified path to "MyClass" in the call to GetStaticMethodId. For example: jenv->GetStaticMethodID(cls, "getInstance", "()Lcom/work/MyClass;"); Depending on where the CPP code lives relative to the Java code? Just a thought. — eternaln00b
– eternaln00b, Commented Feb 1, 2012 at 20:19

eternaln00b · Accepted Answer · 2012-02-01 20:37:55Z

1

I believe the problem is that it's unable to resolve the output argument specified. If your java class were in the package: "com/work/", you would say:

jenv->GetStaticMethodID(cls, "getInstance", "()Lcom/work/MyClass;");

That should do it.

EDIT:

It looks like the answer is in the output of javap isn't it? You should be doing:

jenv->GetStaticMethodID(cls, "getInstance", "()LMyClass;");

edited Feb 1, 2012 at 20:37

answered Feb 1, 2012 at 20:26

eternaln00b

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4 Comments

donalmg Over a year ago

Typo in my original question. I had this as ()LMyClass

eternaln00b Over a year ago

And you explicitly said "()LMyClass;" with the semi-colon at the end?

eternaln00b Over a year ago

For example, I can do: getStaticMethodID(cls, "currentThread", "()Ljava/lang/Thread;"); And that works fine. Does that work for you?

donalmg Over a year ago

Yep - missed the semi-colon. Thanks!

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