In Store.java, add two new methods to start and stop a watcher thread. These methods respectively return and take a long as parameter. This value helps us hold a native pointer on the Java side:

public class Store implements StoreListener {
    ...
    public native long startWatcher();
    public native void stopWatcher(long pPointer);
}

Create a new file, StoreThreadSafe.java. The StoreThreadSafe class inherits from the Store class and aims at making the Store instances thread-safe using synchronized Java blocks. Declare a static member field LOCK of type Object and define a default constructor:

package com.packtpub.store;

import com.packtpub.exception.InvalidTypeException;
import com.packtpub.exception.NotExistingKeyException;

public class StoreThreadSafe extends Store {
    protected static Object LOCK;
    
    public StoreThreadSafe(StoreListener...