compile 'com.tencent.mm.opensdk:wechat-sdk-android-with-mta:+'

public class WXPayEntryActivity extends Activity implements IWXAPIEventHandler {

 private IWXAPI api;

 @Override
 public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.wx_pay_result);
  api = WXAPIFactory.createWXAPI(this, "你的APP_ID");
  api.handleIntent(getIntent(), this);
 }

 @Override
 protected void onNewIntent(Intent intent) {
  super.onNewIntent(intent);
  setIntent(intent);
  api.handleIntent(intent, this);
 }

 @Override
 public void onResp(BaseResp resp) {
  int errCode = resp.errCode;
 }
}

public class MD5 {

 private MD5() {
 }

 public final static String getMessageDigest(byte[] buffer) {
  char hexDigits[] = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};
  try {
   MessageDigest mdTemp = MessageDigest.getInstance("MD5");
   mdTemp.update(buffer);
   byte[] md = mdTemp.digest();
   int j = md.length;
   char str[] = new char[j * 2];
   int k = 0;
   for (int i = 0; i < j; i++) {
    byte byte0 = md[i];
    str[k++] = hexDigits[byte0 >>> 4 & 0xf];
    str[k++] = hexDigits[byte0 & 0xf];
   }
   return new String(str);
  } catch (Exception e) {
   return null;
  }
 }
}

/**
 * 注册微信支付
 */
private void registerWx() {
 IWXAPI wxapi = WXAPIFactory.createWXAPI(context, null);
 wxapi.registerApp("你的APP_ID");
}

/**
 * 发起微信支付
 */
private void wxPay() {
 PayReq payReq = new PayReq();
 payReq.appId = "后台返回的appId";
 payReq.partnerId = "后台返回的partnerId";
 payReq.prepayId = "后台返回的prepayId";
 payReq.packageValue = "后台返回的package";
 payReq.nonceStr = "后台返回的nonceStr";
 payReq.timeStamp = "后台返回的timeStamp";
 payReq.signType = "后台返回的signType";
 payReq.sign = signNum();
 wxapi.sendReq(payReq);
}

/**
 * 拼接sign
 */
public static String signNum() {
 String stringA = "appid=" + "后台返回的appid" +
   "&noncestr=" + "后台返回的noncestr" +
   "&package=" + "后台返回的package" +
   "&partnerid=" + "后台返回的partnerid" +
   "&prepayid=" + "后台返回的prepayid" +
   "×tamp=" + "后台返回的timestamp";
 String stringSignTemp = stringA + "&key=" + "后台商户服务端的key";
 String sign = MD5.getMessageDigest(stringSignTemp.getBytes()).toUpperCase();
 return sign;
}