Android Question MQTT Subscribe problem with reconnect sample. Null object reference

josejad · Sep 24, 2021

Hi there¡¡

EDITED: The error is easiliest reproduced if you call ConnectAndReconnect from B4XPage_Created instead of B4XPage_Appear. If not, the error just happens from time to time.
EDITED 2: A quick way of getting the error. Disconnect wifi, when it reconnects througth mobile connection, the error happens

I'm using Erel's sample Connect and Reconnect sample in a B4XPages project.
My problem is subscribing to a Topic.

Code: Copy

Sub ConnectAndReconnect
    Do While working
        If mqtt.IsInitialized Then mqtt.Close
        mqtt.Initialize("mqtt", "tcp://test.mosquitto.org", "B4X" & Rnd(0, 999999999))
        'Dim mo As MqttConnectOptions
        'mo.Initialize(username, password)
        Log("Trying to connect")
        'mqtt.Connect2(mo)
        mqtt.Connect
        Wait For mqtt_Connected (Success As Boolean)
        If Success Then
            Log(mqtt.IsInitialized)               '<-Added¡¡¡¡ The log is TRUE
            mqtt.Subscribe("B4XTest", 1)    '<-Added¡¡¡¡ Error MqttAsyncClient.subscribe(java.lang.String, int)' on a null object reference
            Log("Mqtt connected")
            Do While working And mqtt.Connected
                mqtt.Publish2("ping", Array As Byte(0), 1, False) 'change the ping topic as needed
                Sleep(5000)
            Loop
            Log("Disconnected")
        Else
            Log("Error connecting.")
        End If
        Sleep(5000)
    Loop
End Sub

From time to time you get this error:

*** Service (starter) Create ***
** Service (starter) Start **
** Activity (main) Create, isFirst = true **
Call B4XPages.GetManager.LogEvents = True to enable logging B4XPages events.
** Activity (main) Resume **
Trying to connect
Mqtt connected
** Activity (main) Pause event (activity is not paused). **
** Activity (main) Resume **
Trying to connect
Disconnected
Trying to connect
True
Error occurred on line: 52 (B4XMainPage)
java.lang.NullPointerException: Attempt to invoke virtual method 'org.eclipse.paho.client.mqttv3.IMqttToken org.eclipse.paho.client.mqttv3.MqttAsyncClient.subscribe(java.lang.String, int)' on a null object reference
at anywheresoftware.b4j.objects.MqttAsyncClientWrapper.Subscribe(MqttAsyncClientWrapper.java:116)
at java.lang.reflect.Method.invoke(Native Method)
at anywheresoftware.b4a.shell.Shell.runVoidMethod(Shell.java:777)
at anywheresoftware.b4a.shell.Shell.raiseEventImpl(Shell.java:354)
at anywheresoftware.b4a.shell.Shell.raiseEvent(Shell.java:255)
at java.lang.reflect.Method.invoke(Native Method)
at anywheresoftware.b4a.ShellBA.raiseEvent2(ShellBA.java:144)
at anywheresoftware.b4a.BA.raiseEvent(BA.java:193)
at anywheresoftware.b4a.shell.DebugResumableSub$RemoteResumableSub.resume(DebugResumableSub.java:22)
at anywheresoftware.b4a.BA.checkAndRunWaitForEvent(BA.java:267)
at anywheresoftware.b4a.ShellBA.raiseEvent2(ShellBA.java:137)
at anywheresoftware.b4a.BA$2.run(BA.java:387)
at android.os.Handler.handleCallback(Handler.java:938)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:246)
at android.app.ActivityThread.main(ActivityThread.java:8587)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130)

Sometimes the error is "The client is not connected", despite it happens when mqtt_Connected is Success.

See attached a project with the problem.
EDITED: To "accelerate" the error, change the call to ConnectAndReconnect from B4XPage_Created to B4XPage_Appear
Just open the app, press home button and go back to the app one or two times.

Thanks in advance

Erel · Sep 26, 2021

It should definitely not go in B4XPage_Appear. You don't want that ConnectAndReconnect will be called multiple times.

1. Add Log("ConnectAndReconnect") to the top of that sub.
2. Call it from B4XPage_Created.
3. Post the logs if it crashes.

josejad · Sep 26, 2021

Erel said:
1. Add Log("ConnectAndReconnect") to the top of that sub.

Done

Erel said:
Call it from B4XPage_Created.

Done.

Now I run the app, and change the way it's connected (i.e. connected with wifi, then disconnect wifi and wait the mobile data try to reconnect). Connected with USB debug

Code: Copy

Copying updated assets files (1)
*** Service (starter) Create ***
** Service (starter) Start **
** Activity (main) Create, isFirst = true **
*** mainpage: B4XPage_Created
ConnectAndReconnect
Trying to connect
*** mainpage: B4XPage_Appear
** Activity (main) Resume **
true
Mqtt connected
Disconnected mqtt
Disconnected
Trying to connect
false
Error occurred on line: 59 (B4XMainPage)             '<---------------------------- mqtt.Subscribe("B4XTest", 1)
java.lang.NullPointerException: Attempt to invoke virtual method 'org.eclipse.paho.client.mqttv3.IMqttToken org.eclipse.paho.client.mqttv3.MqttAsyncClient.subscribe(java.lang.String, int)' on a null object reference
    at anywheresoftware.b4j.objects.MqttAsyncClientWrapper.Subscribe(MqttAsyncClientWrapper.java:116)
    at b4a.example.b4xmainpage$ResumableSub_ConnectAndReconnect.resume(b4xmainpage.java:197)
    at anywheresoftware.b4a.shell.DebugResumableSub$DelegatableResumableSub.resumeAsUserSub(DebugResumableSub.java:48)
    at java.lang.reflect.Method.invoke(Native Method)
    at anywheresoftware.b4a.shell.Shell.runMethod(Shell.java:732)
    at anywheresoftware.b4a.shell.Shell.raiseEventImpl(Shell.java:348)
    at anywheresoftware.b4a.shell.Shell.raiseEvent(Shell.java:255)
    at java.lang.reflect.Method.invoke(Native Method)
    at anywheresoftware.b4a.ShellBA.raiseEvent2(ShellBA.java:144)
    at anywheresoftware.b4a.BA.raiseEvent(BA.java:193)
    at anywheresoftware.b4a.shell.DebugResumableSub$DelegatableResumableSub.resume(DebugResumableSub.java:43)
    at anywheresoftware.b4a.BA.checkAndRunWaitForEvent(BA.java:267)
    at anywheresoftware.b4a.ShellBA.raiseEvent2(ShellBA.java:137)
    at anywheresoftware.b4a.BA$2.run(BA.java:387)
    at android.os.Handler.handleCallback(Handler.java:938)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:246)
    at android.app.ActivityThread.main(ActivityThread.java:8587)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130)
** Activity (main) Pause event (activity is not paused). **

Code: Copy

Registro conectado a: RZ8N11K70XX
--------- beginning of crash
--------- beginning of system
--------- beginning of main
*** Service (starter) Create ***
** Service (starter) Start **
** Activity (main) Create, isFirst = true **
*** mainpage: B4XPage_Created
ConnectAndReconnect
Trying to connect
*** mainpage: B4XPage_Appear
** Activity (main) Resume **
true
Mqtt connected
Disconnected mqtt
Disconnected
Trying to connect
false
b4xmainpage$ResumableSub_ConnectAndReconnectresume (java line: 181)
java.lang.NullPointerException: Attempt to invoke virtual method 'org.eclipse.paho.client.mqttv3.IMqttToken org.eclipse.paho.client.mqttv3.MqttAsyncClient.subscribe(java.lang.String, int)' on a null object reference
    at anywheresoftware.b4j.objects.MqttAsyncClientWrapper.Subscribe(MqttAsyncClientWrapper.java:116)
    at b4a.example.b4xmainpage$ResumableSub_ConnectAndReconnect.resume(b4xmainpage.java:181)
    at anywheresoftware.b4a.BA.checkAndRunWaitForEvent(BA.java:267)
    at anywheresoftware.b4a.BA.raiseEvent2(BA.java:207)
    at anywheresoftware.b4a.BA$2.run(BA.java:387)
    at android.os.Handler.handleCallback(Handler.java:938)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:246)
    at android.app.ActivityThread.main(ActivityThread.java:8587)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130)

Erel · Sep 27, 2021

Try it with this code:

Code: Copy

Sub ConnectAndReconnect
    Log("ConnectAndReconnect")
    Do While working
        If mqtt.IsInitialized Then mqtt.Close
        Do While mqtt.IsInitialized
            Sleep(100)
        Loop
        mqtt.Initialize("mqtt", "tcp://test.mosquitto.org", "B4X" & Rnd(0, 999999999))
        'Dim mo As MqttConnectOptions
        'mo.Initialize(username, password)
        Log("Trying to connect")
        'mqtt.Connect2(mo)
        mqtt.Connect
        Wait For mqtt_Connected (Success As Boolean)
        If Success Then
            Log(mqtt.IsInitialized)
            mqtt.Subscribe("B4XTest", 1)
            Button1.Color = xui.Color_Green
            Log("Mqtt connected")
            Do While working And mqtt.Connected
                mqtt.Publish2("ping", Array As Byte(0), 1, False) 'change the ping topic as needed
                Sleep(5000)
            Loop
            Button1.Color = xui.Color_Red
            Log("Disconnected")
        Else
            Log("Error connecting.")
        End If
        Sleep(5000)
    Loop
End Sub

josejad · Sep 27, 2021

Thanks¡¡ It seems to avoid the error. I will test for a few days¡¡

Code: Copy

        Do While mqtt.IsInitialized
            Sleep(100)
        Loop

Gabino A. de la Gala · Sep 27, 2021

I think it is something that has changed between version 1.00 and 1.01.

With 1.00 it never "breaks" me and yet the same code with version 1.01 always ...

This example is compiled with version 1.00 of jMQTT.

josejad · Sep 27, 2021

Hi @Erel:

Gabino A. de la Gala said:
This example is compiled with version 1.00 of jMQTT.

Yes, it runs without errors

Erel · Sep 27, 2021

The suggested fix will solve it.

Gabino A. de la Gala · Sep 27, 2021

Erel said:
The suggested fix will solve it.

Yes. It works!!!

Lakhtin_V · Oct 31, 2022

José J. Aguilar said:
Thanks¡¡ It seems to avoid the error. I will test for a few days¡¡

Code: Copy

Do While mqtt.IsInitialized Sleep(100) Loop

Erel said:

Try it with this code:

Code: Copy

Sub ConnectAndReconnect
    Log("ConnectAndReconnect")
    Do While working
        If mqtt.IsInitialized Then mqtt.Close
        Do While mqtt.IsInitialized
            Sleep(100)
        Loop
        mqtt.Initialize("mqtt", "tcp://test.mosquitto.org", "B4X" & Rnd(0, 999999999))
        'Dim mo As MqttConnectOptions
        'mo.Initialize(username, password)
        Log("Trying to connect")
        'mqtt.Connect2(mo)
        mqtt.Connect
        Wait For mqtt_Connected (Success As Boolean)
        If Success Then
            Log(mqtt.IsInitialized)
            mqtt.Subscribe("B4XTest", 1)
            Button1.Color = xui.Color_Green
            Log("Mqtt connected")
            Do While working And mqtt.Connected
                mqtt.Publish2("ping", Array As Byte(0), 1, False) 'change the ping topic as needed
                Sleep(5000)
            Loop
            Button1.Color = xui.Color_Red
            Log("Disconnected")
        Else
            Log("Error connecting.")
        End If
        Sleep(5000)
    Loop
End Sub

I want to know the time it took for a signal to reach a specific server. How can I use this library for this or maybe you know other ways to get the ping time.

josejad · Oct 31, 2022

You should create a new thread for your question

Android Question MQTT Subscribe problem with reconnect sample. Null object reference

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