Client is not connected, so not sending message in MQTT - android-studio

Question

This code is written by Android Studio, I want to connect to server and publish my message.
When I connect the MQTT server , server show my phone's IP Address connected.
However, when I click publish, logcat show me message like "Client is not connected, so not sending message".
logcat:
15643-15643/com.example.project001 I/MqttConnection: Client is not connected, so not sending message
How to fix this problem ?
Connect to server:
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
String clientId = MqttClient.generateClientId();
//client setup
client = new MqttAndroidClient(this, MQTTHOST, clientId);
Button connectButton = findViewById(R.id.button2);
connectButton.setOnClickListener(new Button.OnClickListener() {
#Override
public void onClick(View v) {
Log.d(TAG, "onClick2");
MqttConnectOptions options = new MqttConnectOptions();
options.setUserName(USERNAME);
options.setPassword(PASSWORD.toCharArray());
try {
IMqttToken token = client.connect();
if (client.isConnected() == false) {
Log.d(TAG, "client.isConnected() == false");
}
token.setActionCallback(new IMqttActionListener() {
#Override
public void onSuccess(IMqttToken asyncActionToken) {
// We are connected
Log.d(TAG, "onSuccess");
}
#Override
public void onFailure(IMqttToken asyncActionToken, Throwable exception) {
// Something went wrong e.g. connection timeout or firewall problems
Log.d(TAG, "onFailure");
}
});
} catch (MqttException e) {
e.printStackTrace();
}
}
});
//publish
Button publishButton = findViewById(R.id.button1);
publishButton.setOnClickListener(new Button.OnClickListener() {
#Override
public void onClick(View v) {
publish("test");
Log.d(TAG, "onClick");
}
});
}
publish function:
public void publish(String payload) {
String topic = topicName;
try {
if (client.isConnected() == false) {
client.connect();
}
IMqttToken token = client.connect();
MqttMessage message = new MqttMessage();
message.setPayload(payload.getBytes());
message.setQos(0);
client.publish(topic, message, null, new IMqttActionListener() {
#Override
public void onSuccess(IMqttToken asyncActionToken) {
Log.i(TAG, "publish succeed!");
}
#Override
public void onFailure(IMqttToken asyncActionToken, Throwable exception) {
Log.i(TAG, "publish failed!");
}
});
} catch (MqttException e) {
Log.e(TAG, e.toString());
e.printStackTrace();
}
}