Question

This is the Java class i’m accessing through JNIEnv in my monodroid application

package mypackage;

import android.util.Log;

public class JavaScriptInterface {


    public String submitAns = "";

    // The JNI in the original question uses a default constructor.
    // Either provide one explicitly or use the implicit one...
    public JavaScriptInterface() 
    {

    }   

    public String getSelctd()
    {
        return submitAns;
    }
}

I’m able to instantiate the class by the following statements:

Java.Lang.Object jclassWrp_;

IntPtr JavaScriptInterface_Class = JNIEnv.FindClass("mypackage.JavaScriptInterface");

IntPtr JavaScriptInterface_ctor = JNIEnv.GetMethodID(JavaScriptInterface_Class, "<init>", "()V"); //(Landroid/context/Context;)V

IntPtr jsInterfaceinstance_ = JNIEnv.NewObject(JavaScriptInterface_Class, JavaScriptInterface_ctor);

jclassWrp_ = new Java.Lang.Object(jsInterfaceinstance_, JniHandleOwnership.TransferGlobalRef);

But when i try to create the object to access the getSelctd() method:

IntPtr ipApid = JNIEnv.GetMethodID(jclassWrp_, "getSelctd", "()Ljava/lang/String;");

It throws NoSuchMethodExist Exception…
Please tell me whether i’m doing it the right way and what i’m missing here…

Answer 1

I’m able to instantiate the class by the following statements:

Java.Lang.Object jclassWrp_;
IntPtr JavaScriptInterface_Class = JNIEnv.FindClass("mypackage.JavaScriptInterface");

JNI use should use JNI conventions, thus mypackage/JavaScriptInterface (note / instead of .).

IntPtr JavaScriptInterface_ctor = JNIEnv.GetMethodID(JavaScriptInterface_Class, "<init>", "()V");
IntPtr jsInterfaceinstance_ = JNIEnv.NewObject(JavaScriptInterface_Class, JavaScriptInterface_ctor);
jclassWrp_ = new Java.Lang.Object(jsInterfaceinstance_, JniHandleOwnership.TransferGlobalRef);

JNIEnv.NewObject() returns a local ref, not a global ref, so you want JniHandleOwnership.TransferLocalRef.

But when i try to create the object to access the getSelctd() method:

IntPtr ipApid = JNIEnv.GetMethodID(jclassWrp_, "getSelctd", "()Ljava/lang/String;");

JNIEnv.GetMethodID() takes a class handle, not an instance. Firstly, the above shouldn’t compile (Java.Lang.Object != IntPtr). Secondly, jclassWrp contains a mypackage.JavaScriptInterface instance, not the mypackage.JavaScriptInterface Class instance.

Instead, do:

IntPtr ipApid = JNIEnv.GetMethodID(JavaScriptInterface_Class, "getSelctd", "()Ljava/lang/String;");

Finally, don’t forget to JNIEnv.DeleteGlobalRef(JavaScriptInterface_Class) when you don’t need it anymore, otherwise you’ll leak the gref.

Complete code:

// FindClass() returns a gref; must be freed (see below)
IntPtr JavaScriptInterface_Class     = JNIEnv.FindClass("mypackage/JavaScriptInterface");

// MethodIDs do not need to be freed
IntPtr JavaScriptInterface_ctor      = JNIEnv.GetMethodID(JavaScriptInterface_Class,
        "<init>", "()V");
IntPtr JavaScriptInterface_getSelctd = JNIEnv.GetMethodID(JavaScriptInterface_Class,
        "getSelctd", "()Ljava/lang/String;");

// JNIEnv.NewObject() & JNIEnv.CallObjectMethod() return lrefs; freed below
IntPtr lrefInstance                  = JNIEnv.NewObject(JavaScriptInterface_Class,
        JavaScriptInterface_ctor);
IntPtr lrefSelectd                   = JNIEnv.CallObjectMethod(jsInterfaceinstance_, ipApid);

// JniHandleOwnership.TransferLocalRef causes lrefSelectd to be released for us
string selected                      = JNIEnv.GetString(lrefSelectd, JniHandleOwnership.TransferLocalRef);

// Resource cleanup
JNIEnv.DeleteLocalRef(lrefInstance);
JNIEnv.DeleteGlobalRef(JavaScriptInterface_Class);