支付宝支付表单提示"参数无效：买家不能为空"

{"alipay_trade_create_response":{"msg":"Business Failed","code":"40004","sub_msg":"参数无效：买家不能为空","sub_code":"ACQ.INVALID_PARAMETER"},"sign":"Ma0IcDxUcSGEgQ0u6gxw+C7Rx/G5JoEllGAF3tzQT6mFEAnby8Ge293xcNoWQjZDbOP5ol8Lxfz3O7tYLKxAz3aLWYnRSq2i8uIUg9zgCD0+mJtasXggIhT7jSRbyfk6F3m2EjqiPwYV4c+vJng9Wdwxrcuj8yL8FDQtQutTz7kjMoKWAjPA0K64xpSHlNPr98+XEBw7tDNAGH3oJZj41/mjJvCx7rNPs7DFvMSneZI8aR8itNdOTMHA0hfZVA4E3GSA7ucuhFPojVOhOoxlfsbD7+Zabqim7Z8tIZGBZzTJHEBtFh+zFyzEWZkX8tYMJuHSloJ5P1yvp0kd+3svGQ=="}

原因：创建交易请求时用的接口（alipay.trade.create）要求必须指定买家信息。
AlipayTradeCreateRequest request = new AlipayTradeCreateRequest();
改成：AlipayTradePagePayRequest request = new AlipayTradePagePayRequest();