问题
Bluetooth app won't run due to Null pointer exceptions. No errors, but I can't see to connect to the emulator as the app just crashes every time. Even if I try sample GitHub codes, it won't run either. I'll post code and catlog and anything else you need. Thank you for your help
package com.example.bluetooth_demoproject;
import android.app.Activity;
import android.bluetooth.BluetoothA2dp;
import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothDevice;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;
import org.w3c.dom.Text;
import java.util.Set;
public class MainActivity extends Activity {
private static final int REQUEST_ENABLE_BT = 0;
private static final int REQUEST_DISCOVER_BT_ = 1;
TextView mBluetoothStatus, mPairedDevicelist;
ImageView mBluetoothIcon;
Button mOnButton, mOffButton, mDiscoverableButton, mPairedDevices,
mSendImage;
BluetoothAdapter mBlueAdapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
java.text.DateFormat dateFormat = android.text.format.DateFormat.getDateFormat(getApplicationContext());
mBluetoothStatus = findViewById(R.id.BluetoothStatus);
mPairedDevicelist = findViewById(R.id.pairedDeviceList);
mBluetoothIcon = findViewById(R.id.bluetoothIcon);
mOnButton = findViewById(R.id.onButton);
mOffButton = findViewById(R.id.offButton);
mDiscoverableButton = findViewById(R.id.discoverableButton);
mPairedDevices = findViewById(R.id.pairedDevices);
mSendImage = findViewById(R.id.sendImage);
//adapter
BluetoothAdapter mBluetoothAdapter =
BluetoothAdapter.getDefaultAdapter();
// check if bluetooth is available
if(mBluetoothAdapter == null){
mBluetoothStatus.setText("Bluetooth is not available");
}
else {
mBluetoothStatus.setText("Bluetooth is available");
}
//if Bluetooth isnt enabled, enable it
if (!mBluetoothAdapter.isEnabled()) {
Intent enableBtIntent = new
Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT);
}
//on button Click
mOnButton.setOnClickListener(new View.OnClickListener(){
@Override
public void onClick(View v) {
if (!mBlueAdapter.isEnabled()) {
showToast("Turning Bluetooth on...");
// intent to on bluetooth
Intent intent = new
Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
startActivityForResult(intent, REQUEST_ENABLE_BT);
}
else {
showToast("Bluetooth is already on");
}
}
});
//discover Bluetooth button
mDiscoverableButton.setOnClickListener(new View.OnClickListener(){
@Override
public void onClick(View v){
if (!mBlueAdapter.isDiscovering()) {
showToast("Discovering devices..");
Intent intent = new
Intent(BluetoothAdapter.ACTION_REQUEST_DISCOVERABLE);
startActivityForResult(intent, REQUEST_DISCOVER_BT_);
}
}
});
// off button click
mOffButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
if (mBlueAdapter.isEnabled()) {
showToast("Turning Bluetooth off...");
// intent to turn off bluetooth
mBluetoothIcon.setImageResource(R.drawable.action_off);
}
else{
showToast("Bluetooth is already off");
}
}
});
//get paired device button click
Set<BluetoothDevice> pairedDevices =
mBluetoothAdapter.getBondedDevices();
if (pairedDevices.size() > 0) {
// Retrieve the name and address of the paired devices
for (BluetoothDevice device: pairedDevices) {
String deviceName = device.getName();
String deviceAddr = device.getAddress();
}
}
}
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent
data) {
switch (requestCode) {
case REQUEST_ENABLE_BT:
if (resultCode == RESULT_OK){
//bluetooth is on
mBluetoothIcon.setImageResource(R.drawable.action_on);
showToast("Bluetooth is on");
}
else {
//user is denied turning on Bluetooth
showToast("Unable to turn on Bluetooth");
}
break;
}
super.onActivityResult(requestCode, resultCode, data);
}
//toast message function
private void showToast(String msg) {
Toast.makeText(this, msg, Toast.LENGTH_SHORT) .show();
}
}
9-08 22:03:46.371 1510-2830/system_process W/ActivityManager:
getRunningAppProcesses: caller 10060 does not hold REAL_GET_TASKS;
limiting output
09-08 22:03:46.372 5869-5869/? I/InstantRun: starting instant run server: is main process
09-08 22:03:46.416 5869-5869/? E/BluetoothAdapter: Bluetooth binder is null
09-08 22:03:46.416 5869-5869/? D/AndroidRuntime: Shutting down VM
09-08 22:03:46.418 5869-5869/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.bluetooth_demoproject, PID: 5869
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.bluetooth_demoproject/com.example.bluetooth_demoproject.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.bluetooth.BluetoothAdapter.isEnabled()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
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:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.bluetooth.BluetoothAdapter.isEnabled()' on a null object reference
at com.example.bluetooth_demoproject.MainActivity.onCreate(MainActivity.java:57)
at android.app.Activity.performCreate(Activity.java:5990)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
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:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
09-08 22:03:46.431 1510-2232/system_process W/ActivityManager: Force finishing activity 1 com.example.bluetooth_demoproject/.MainActivityenter code here
回答1:
It looks like the problem is from line 59: if (!mBluetoothAdapter.isEnabled()) {
Note: The error message says line 57, but in my text editor it says 59.
A few lines above, you check whether the object mBluetoothAdapter was successfully created by checking if it's == null. But after that, you call its method: .isEnabled(). I think the problem is that mBluetoothAdapter is becoming null and therefore throws a NullPointerException when you try to access one of its methods.
If I understand correctly, I think the solution would be something like this:
// check if bluetooth is available
if(mBluetoothAdapter == null){
mBluetoothStatus.setText("Bluetooth is not available");
}
else {
mBluetoothStatus.setText("Bluetooth is available");
//if Bluetooth isnt enabled, enable it
if (!mBluetoothAdapter.isEnabled()) {
Intent enableBtIntent = new
Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT);
}
}
This would keep the enabling code away from a mBluetoothAdapter with a value of null.
Alternativly, you could use return to exit the method if BlueTooth isn't available.
// check if bluetooth is available
if(mBluetoothAdapter == null){
mBluetoothStatus.setText("Bluetooth is not available");
return;
}
else {
mBluetoothStatus.setText("Bluetooth is available");
}
//if Bluetooth isnt enabled, enable it
if (!mBluetoothAdapter.isEnabled()) {
Intent enableBtIntent = new
Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT);
}
This would do the same in a little bit of a cleaner way.
I think the next step for you is to find out why BluetoothAdapter mBluetoothAdapter = BluetoothAdapter.getDefaultAdapter(); is returning null instead of the object you want. I don't think the answer to that is in your MainActivity method. The device isn't finding a default adapter and that could be the result of numerous problems.
来源：https://stackoverflow.com/questions/52239820/android-bluetooth-app-wont-run-due-to-null-pointer-error