How Set Java Plugin on Unity?

I recently started developing a Java plugin. I developed the Java code at Eclipse, and tested it, its works on Android just fine. After that, I checked the projet as 'Library', and dragged my jar file to the Assets/Plugins/Android folder at my Unity project. After that I did a C# code, to call the Android functions. It should work, but do far I have not figured out the problem.

When I play it at the editor, with the remote and the device, it apears that "JNI: Init'd AndroidJavaClass with null ptr!", and when I copy the apk directly at the device and install it, it just doesn't work.

Bellow, my C# code:(the void GetAlarm is the one triggered by a button)

 using UnityEngine;
 using System.Runtime.InteropServices;
 using System.Collections;
 
 public class Awaker : MonoBehaviour {
 
 #if UNITY_ANDROID
     public AndroidJavaClass pluginWaker;
     public int num;
 
         public void GetAlarm(){
             
             AndroidJNI.AttachCurrentThread();
             pluginWaker = new AndroidJavaClass("com.example.awaker.Main"            
             SetAlarm();
             SetInt();
             numLabel.text = num.ToString();
         }
         
             public void SetAlarm(){
             AndroidJNI.AttachCurrentThread();
             pluginWaker = new AndroidJavaClass("com.example.awaker.Main");
             pluginWaker.Call("setAlarm",14,22);
         }
     
         public void SetInt(){
             num = pluginWaker.Call<int>("getNumber");
         }    
 #endif    
 }