Can't create handler inside thread that has not called Looper.prepare()

                    Looper.prepare();  // Can't create handler inside thread that has not called Looper.prepare()。 
                    Toast.makeText(context,"登录超时",Toast.LENGTH_SHORT).show();
                    Looper.loop();

这样子写就不会报错了；

但是他为什么报错？？？你是不是在子线程显示Toast了；来看一下Toast的源码：

    /**
     * Constructs an empty Toast object.  If looper is null, Looper.myLooper() is used.
     * @hide
     */
    public Toast(@NonNull Context context, @Nullable Looper looper) {
        mContext = context;
        mTN = new TN(context.getPackageName(), looper);
        mTN.mY = context.getResources().getDimensionPixelSize(
                com.android.internal.R.dimen.toast_y_offset);
        mTN.mGravity = context.getResources().getInteger(
                com.android.internal.R.integer.config_toastDefaultGravity);
    }

TN : 就看最主要这一点吧！

  if (looper == null) {
                // Use Looper.myLooper() if looper is not specified.
                looper = Looper.myLooper();
                if (looper == null) {
                    throw new RuntimeException(
                            "Can't toast on a thread that has not called Looper.prepare()");
                }
            }
            mHandler = new Handler(looper, null) {
                @Override
                public void handleMessage(Message msg) {
                    switch (msg.what) {
                        case SHOW: {
                            IBinder token = (IBinder) msg.obj;
                            handleShow(token);
                            break;
                        }
                        case HIDE: {
                            handleHide();
                            // Don't do this in handleHide() because it is also invoked by
                            // handleShow()
                            mNextView = null;
                            break;
                        }
                        case CANCEL: {
                            handleHide();
                            // Don't do this in handleHide() because it is also invoked by
                            // handleShow()
                            mNextView = null;
                            try {
                                getService().cancelToast(mPackageName, TN.this);
                            } catch (RemoteException e) {
                            }
                            break;
                        }
                    }
                }
            };

报错的原因是因为你的looper == null ，myLooper也是null;

Toast的内部会创建Handler，而Handler又依赖于Looper；Looper不存在，自然会报错；

Android主线程会默认创建Looper，而子线程则不会默认创建Looper；所以我们需要手动去创建Looper并开启循环；