我正在尝试从导航抽屉注销并返回登录屏幕，我不断收到此logcat错误：
java.lang.NullPointerException: Attempt to invoke virtual method 'boolean com.google.android.gms.common.api.GoogleApiClient.isConnected()' on a null object reference
at com.gijbelssiemen.werkstukandroid.MainActivity.onNavigationItemSelected(MainActivity.java:97)
at android.support.design.widget.NavigationView$1.onMenuItemSelected(NavigationView.java:150)
at android.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:811)
at android.support.v7.view.menu.SubMenuBuilder.dispatchMenuItemSelected(SubMenuBuilder.java:84)
at android.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:152)
at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:958)
at android.support.design.internal.NavigationMenuPresenter$1.onClick(NavigationMenuPresenter.java:308)
at android.view.View.performClick(View.java:5204)
at android.view.View$PerformClick.run(View.java:21153)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
这是导航抽屉的主要活动：
package com.gijbelssiemen.werkstukandroid;
import android.content.Intent;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.view.View;
import android.support.design.widget.NavigationView;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.MenuItem;
import com.google.android.gms.common.api.GoogleApiClient;
import com.google.android.gms.maps.CameraUpdateFactory;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.OnMapReadyCallback;
import com.google.android.gms.maps.SupportMapFragment;
import com.google.android.gms.maps.model.LatLng;
import com.google.android.gms.maps.model.MarkerOptions;
public class MainActivity extends AppCompatActivity
implements NavigationView.OnNavigationItemSelectedListener, OnMapReadyCallback {
private GoogleMap mMap;
private GoogleApiClient mGoogleApiClient;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
.setAction("Action", null).show();
}
});
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
drawer.setDrawerListener(toggle);
toggle.syncState();
NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
navigationView.setNavigationItemSelectedListener(this);
SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager()
.findFragmentById(R.id.map);
mapFragment.getMapAsync(this);
}
public void onMapReady(GoogleMap googleMap) {
mMap = googleMap;
// Add a marker in Ghent and move the camera
LatLng ghent = new LatLng(51.055855, 3.72076);
mMap.addMarker(new MarkerOptions().position(ghent).title("Marker in Ghent"));
mMap.moveCamera(CameraUpdateFactory.newLatLng(ghent));
mMap.moveCamera(CameraUpdateFactory.zoomTo(13f));
}
@Override
public void onBackPressed() {
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
if (drawer.isDrawerOpen(GravityCompat.START)) {
drawer.closeDrawer(GravityCompat.START);
} else {
super.onBackPressed();
}
}
@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {
// Handle navigation view item clicks here.
int id = item.getItemId();
if (id == R.id.nav_map) {
Intent intent = new Intent(this, MainActivity.class);
startActivity(intent);
} else if (id == R.id.nav_gallery) {
} else if (id == R.id.nav_slideshow) {
} else if (id == R.id.nav_manage) {
} else if (id == R.id.nav_share) {
} else if (id == R.id.nav_logout) {
if (mGoogleApiClient.isConnected()) {
mGoogleApiClient.disconnect();
}
Intent intent = new Intent(this, SignInActivity.class);
startActivity(intent);
}
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
drawer.closeDrawer(GravityCompat.START);
return true;
}
}
这是我的SignInActivity：
package com.gijbelssiemen.werkstukandroid;
import android.app.ProgressDialog;
import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.TextView;
import com.google.android.gms.auth.api.Auth;
import com.google.android.gms.auth.api.signin.GoogleSignInAccount;
import com.google.android.gms.auth.api.signin.GoogleSignInOptions;
import com.google.android.gms.auth.api.signin.GoogleSignInResult;
import com.google.android.gms.common.ConnectionResult;
import com.google.android.gms.common.SignInButton;
import com.google.android.gms.common.api.GoogleApiClient;
import com.google.android.gms.common.api.OptionalPendingResult;
import com.google.android.gms.common.api.ResultCallback;
import com.google.android.gms.common.api.Status;
/**
* Activity to demonstrate basic retrieval of the Google user's ID, email address, and basic
* profile.
*/
public class SignInActivity extends AppCompatActivity implements
GoogleApiClient.OnConnectionFailedListener,
View.OnClickListener {
private static final String TAG = "SignInActivity";
private static final int RC_SIGN_IN = 9001;
private GoogleApiClient mGoogleApiClient;
private TextView mStatusTextView;
private ProgressDialog mProgressDialog;
private static final String DIALOG_FRAGMENT_TAG = "myFragment";
private static final String SECRET_MESSAGE = "Very secret message";
/**
* Alias for our key in the Android Key Store
*/
private static final String KEY_NAME = "my_key";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.sign_in);
// Views
mStatusTextView = (TextView) findViewById(R.id.status);
// Button listeners
findViewById(R.id.sign_in_button).setOnClickListener(this);
findViewById(R.id.sign_out_button).setOnClickListener(this);
findViewById(R.id.disconnect_button).setOnClickListener(this);
// [START configure_signin]
// Configure sign-in to request the user's ID, email address, and basic
// profile. ID and basic profile are included in DEFAULT_SIGN_IN.
GoogleSignInOptions gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
.requestEmail()
.build();
// [END configure_signin]
// [START build_client]
// Build a GoogleApiClient with access to the Google Sign-In API and the
// options specified by gso.
mGoogleApiClient = new GoogleApiClient.Builder(this)
.enableAutoManage(this /* FragmentActivity */, this /* OnConnectionFailedListener */)
.addApi(Auth.GOOGLE_SIGN_IN_API, gso)
.build();
// [END build_client]
// [START customize_button]
// Customize sign-in button. The sign-in button can be displayed in
// multiple sizes and color schemes. It can also be contextually
// rendered based on the requested scopes. For example. a red button may
// be displayed when Google+ scopes are requested, but a white button
// may be displayed when only basic profile is requested. Try adding the
// Scopes.PLUS_LOGIN scope to the GoogleSignInOptions to see the
// difference.
SignInButton signInButton = (SignInButton) findViewById(R.id.sign_in_button);
signInButton.setSize(SignInButton.SIZE_STANDARD);
signInButton.setScopes(gso.getScopeArray());
// [END customize_button]
}
@Override
public void onStart() {
super.onStart();
OptionalPendingResult<GoogleSignInResult> opr = Auth.GoogleSignInApi.silentSignIn(mGoogleApiClient);
if (opr.isDone()) {
// If the user's cached credentials are valid, the OptionalPendingResult will be "done"
// and the GoogleSignInResult will be available instantly.
Log.d(TAG, "Got cached sign-in");
GoogleSignInResult result = opr.get();
handleSignInResult(result);
} else {
// If the user has not previously signed in on this device or the sign-in has expired,
// this asynchronous branch will attempt to sign in the user silently. Cross-device
// single sign-on will occur in this branch.
showProgressDialog();
opr.setResultCallback(new ResultCallback<GoogleSignInResult>() {
@Override
public void onResult(GoogleSignInResult googleSignInResult) {
hideProgressDialog();
handleSignInResult(googleSignInResult);
}
});
}
}
// [START onActivityResult]
@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
super.onActivityResult(requestCode, resultCode, data);
// Result returned from launching the Intent from GoogleSignInApi.getSignInIntent(...);
if (requestCode == RC_SIGN_IN) {
GoogleSignInResult result = Auth.GoogleSignInApi.getSignInResultFromIntent(data);
handleSignInResult(result);
}
}
// [END onActivityResult]
// [START handleSignInResult]
private void handleSignInResult(GoogleSignInResult result) {
Log.d(TAG, "handleSignInResult:" + result.isSuccess());
if (result.isSuccess()) {
// Signed in successfully, show authenticated UI.
GoogleSignInAccount acct = result.getSignInAccount();
mStatusTextView.setText(getString(R.string.signed_in_fmt, acct.getDisplayName()));
updateUI(true);
} else {
// Signed out, show unauthenticated UI.
updateUI(false);
}
}
// [END handleSignInResult]
// [START signIn]
public void signIn() {
Intent signInIntent = Auth.GoogleSignInApi.getSignInIntent(mGoogleApiClient);
startActivityForResult(signInIntent, RC_SIGN_IN);
}
// [END signIn]
// [START signOut]
public void signOut() {
Auth.GoogleSignInApi.signOut(mGoogleApiClient).setResultCallback(
new ResultCallback<Status>() {
@Override
public void onResult(Status status) {
// [START_EXCLUDE]
updateUI(false);
// [END_EXCLUDE]
}
});
}
// [END signOut]
// [START revokeAccess]
private void revokeAccess() {
Auth.GoogleSignInApi.revokeAccess(mGoogleApiClient).setResultCallback(
new ResultCallback<Status>() {
@Override
public void onResult(Status status) {
// [START_EXCLUDE]
updateUI(false);
// [END_EXCLUDE]
}
});
}
// [END revokeAccess]
@Override
public void onConnectionFailed(ConnectionResult connectionResult) {
// An unresolvable error has occurred and Google APIs (including Sign-In) will not
// be available.
Log.d(TAG, "onConnectionFailed:" + connectionResult);
}
private void showProgressDialog() {
if (mProgressDialog == null) {
mProgressDialog = new ProgressDialog(this);
mProgressDialog.setMessage(getString(R.string.loading));
mProgressDialog.setIndeterminate(true);
}
mProgressDialog.show();
}
private void hideProgressDialog() {
if (mProgressDialog != null && mProgressDialog.isShowing()) {
mProgressDialog.hide();
}
}
private void updateUI(boolean signedIn) {
if (signedIn) {
findViewById(R.id.sign_in_button).setVisibility(View.GONE);
Intent intent = new Intent(this, MainActivity.class);
startActivity(intent);
findViewById(R.id.sign_out_and_disconnect).setVisibility(View.VISIBLE);
} else {
mStatusTextView.setText(R.string.signed_out);
setContentView(R.layout.sign_in);
findViewById(R.id.sign_in_button).setVisibility(View.VISIBLE);
findViewById(R.id.sign_out_and_disconnect).setVisibility(View.GONE);
}
}
@Override
public void onClick(View v) {
switch (v.getId()) {
case R.id.sign_in_button:
signIn();
break;
case R.id.sign_out_button:
signOut();
break;
case R.id.disconnect_button:
revokeAccess();
break;
}
}
}