IWXAPI api = WXAPIFactory.createWXAPI(this, Values.WXPAYAPPID);
api.registerApp(Values.WXPAYAPPID); // 注册APPID，这句很重要，不然不会发起调用

PayReq req = new PayReq();
req.appId = wxPay.appid;// 也就是Values.WXPAYAPPID，如果二者不一致的话是没法发起支付的。
req.partnerId = json.getString("partnerid");
req.prepayId = json.getString("prepayid");
req.nonceStr = json.getString("noncestr");
req.timeStamp = json.getString("timestamp");
req.packageValue = json.getString("package");
req.sign = json.getString("sign");
//req.extData = "";// "app data"; // optional

boolean r = api.sendReq(req);
if(!r) {
 Toast.makeText(this,"打开微信支付失败!",Toast.LENGTH_SHORT).show();
}

<activity
 android:name=".wxapi.WXPayEntryActivity"
 android:screenOrientation="portrait" android:exported="true"
 android:launchMode="singleTop">
 <intent-filter>
  <action android:name="android.intent.action.VIEW"/><category android:name="android.intent.category.DEFAULT"/><data android:scheme="你的APPID"/>
 </intent-filter>/>
</activity>

@Override
public void onResp(BaseResp resp) {
 Log.d("onPayFinish", "onPayFinish, errCode = " + resp.errCode);
 if(resp.errCode==0){
  Toast.makeText(this,"支付成功!",Toast.LENGTH_SHORT).show();
 }else if(resp.errCode==-1){
  Toast.makeText(this,"支付失败!",Toast.LENGTH_SHORT).show();
 }else if(resp.errCode==-2){
  Toast.makeText(this,"取消支付!",Toast.LENGTH_SHORT).show();
 }
}