Null pointer exception during instruction 'monitor-enter v1'

Question

I get this error now everytime
inside public void onResponse(Call call, Response response){:

mContext.runOnUiThread(new Runnable() {
            @Override
            public void run() {

    LocalDBUtility local=LocalDBUtility.getInstance();
//code
}

inside my singleton class LocalDBUtility

static LocalDBUtility localDBUtility;
    private LocalDBUtility(){
    }

    public static LocalDBUtility getInstance(){

        if(localDBUtility==null){
            synchronized (localDBUtility) {
                localDBUtility = new LocalDBUtility();
            }
            return localDBUtility;

        }else {
            return localDBUtility;
        }
    }

I want to know what caused NPE?

Stack Trace:

 java.lang.NullPointerException: Null pointer exception during instruction 'monitor-enter v1'
                                                                   at com.utilities.LocalDBUtility.getInstance(LocalDBUtility.java:34)
                                                                   at com.MainActivity$4.run(MainActivity.java:386)
                                                                   at android.app.Activity.runOnUiThread(Activity.java:5506)
at 

    retrofit2.ExecutorCallAdapterFactory$ExecutorCallbackCall$1$1.run(ExecutorCallAdapterFactory.java:68)
                                                                   at android.os.Handler.handleCallback(Handler.java:815)
                                                                   at android.os.Handler.dispatchMessage(Handler.java:104)
                                                                   at android.os.Looper.loop(Looper.java:194)
                                                                   at android.app.ActivityThread.main(ActivityThread.java:5763)
                                                                   at java.lang.reflect.Method.invoke(Native Method)
                                                                   at java.lang.reflect.Method.invoke(Method.java:372)
                                                                   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:960)
                                                                   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755

Answer 1

    if(localDBUtility==null){
        synchronized (localDBUtility) {

If localDBUtility is null, you can't call synchronize on it. You can only synchronize on an instantiated object.

Answer 2

In Java every instance of a reference type has its own monitor associated with it. The synchronized block uses this monitor, locking it when a thread enters and unlocking it when this same thread leaves it. If you pass a null reference to the synchronized block, the operation that tries to get the associated monitor will be called on a null reference that results in a NullPointerException. Hence, you can not synchronize on null references.

You may already see that the code you are to running is trying to use a null reference ("localDBUtility==null") for the synchronized block which obviously results in an NPE.

Note: A monitor is a synchronization construct that is used to implement mutual exclusion.