支付宝支付下单服务端java代码

当然可以！以下是一个简单的支付宝支付下单服务端的Java代码示例：

import com.alipay.api.AlipayApiException;
import com.alipay.api.AlipayClient;
import com.alipay.api.DefaultAlipayClient;
import com.alipay.api.request.AlipayTradePagePayRequest;
import com.alipay.api.response.AlipayTradePagePayResponse;

public class AlipayService {

    // 支付宝网关地址
    private static final String ALIPAY_GATEWAY = "https://openapi.alipay.com/gateway.do";
    
    // 应用ID
    private static final String APP_ID = "your_app_id";
    
    // 商户私钥
    private static final String PRIVATE_KEY = "your_private_key";
    
    // 支付宝公钥
    private static final String ALIPAY_PUBLIC_KEY = "alipay_public_key";
    
    // 编码格式
    private static final String CHARSET = "UTF-8";
    
    // 返回格式
    private static final String FORMAT = "json";
    
    // 签名类型
    private static final String SIGN_TYPE = "RSA2";

    public String createOrder(String outTradeNo, String totalAmount, String subject, String body) throws AlipayApiException {
        AlipayClient alipayClient = new DefaultAlipayClient(ALIPAY_GATEWAY, APP_ID, PRIVATE_KEY, FORMAT, CHARSET, ALIPAY_PUBLIC_KEY, SIGN_TYPE);
        AlipayTradePagePayRequest request = new AlipayTradePagePayRequest();
        request.setReturnUrl("your_return_url");
        request.setNotifyUrl("your_notify_url");
        request.setBizContent("{" +
                "\"out_trade_no\":\"" + outTradeNo + "\"," +
                "\"total_amount\":\"" + totalAmount + "\"," +
                "\"subject\":\"" + subject + "\"," +
                "\"body\":\"" + body + "\"," +
                "\"product_code\":\"FAST_INSTANT_TRADE_PAY\"" +
                "}");
        AlipayTradePagePayResponse response = alipayClient.pageExecute(request);
        return response.getBody();
    }
}

这段代码使用了支付宝提供的Java SDK，首先需要在支付宝开放平台注册应用并获取到应用ID、商户私钥和支付宝公钥。然后，通过`AlipayClient`创建一个支付宝客户端实例，设置相关参数，最后调用`pageExecute`方法发起支付请求，并返回支付页面的HTML代码。

请注意，上述代码中的`your_app_id`、`your_private_key`、`alipay_public_key`、`your_return_url`和`your_notify_url`需要替换为你自己的实际值。