Nashorn环境下不落地磁盘将Base64文件以Multipart形式POST至URL

运行在Nashorn环境中，需将Base64编码的文件内容通过HTTP以multipart表单形式发送，现有代码在字节处理上存在问题：发送至httpbin.org时返回异常文件内容，发送至真实服务器时返回500错误，且环境中无ByteArrayBody类，只能使用基础Java API。

原代码 #

//获取Bearer Token的前置代码无问题
var boundary = java.util.UUID.randomUUID().toString();
var headers = {
   "Authorization": "Bearer "+ token,
   "Transfer-Encoding": "chunked",
   "Content-Type": "multipart/form-data; boundary=" + boundary
};
var con = new java.net.URL("theUrl").openConnection();
con.requestMethod = "POST";
var hk = Object.keys(headers);
for (var key of hk){
    con.setRequestProperty(key, headers[key]);
}
// 发送POST请求
con.doOutput=true;
var rn="\r\n";
var dos = new java.io.DataOutputStream(con.outputStream);
var b64 = Java.type("java.util.Base64");
dos.writeBytes("--" + boundary + rn);
dos.writeBytes("Content-Disposition: form-data; name=\"mycontent\"; filename=\"mycontent.zip\"" + rn);
dos.writeBytes("Content-Type: application/octet-stream" + rn);
dos.writeBytes("Content-Transfer-Encoding: binary" + rn);
dos.writeBytes(rn);
mydata = b64.getDecoder().decode(content_pack).toString().getBytes(); //此处可能存在问题
dos.write(mydata, 0, mydata.length); //或者此处有问题？
dos.writeBytes(rn); 
dos.flush(); 
// 结束multipart/form-data
dos.writeBytes("--" + boundary + "--" + rn);
dos.flush();
dos.close();    

asResponse(con); //读取响应并返回JSON对象

httpbin.org响应结果 #

{
"data": "{  \"args\": {},   \"data\": \"\",   \"files\": {    \"content-pack\": \"[B@5e3b67a5\"  },   \"form\": {},   \"headers\": {    \"Accept\": \"text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2\",     \"Authorization\": \"Bearer Dab35ZB60lIdgI\",     \"Content-Length\": \"254\",     \"Content-Type\": \"multipart/form-data;boundary=b9aff414-4066-4e4b-9165-fc315ca4e02e\",     \"Host\": \"httpbin.org\",     \"User-Agent\": \"Java/11.0.20\",     \"X-Amzn-Trace-Id\": \"Root=1-64e5cbc3-3feefbcc4bcb86c374402dc8\"  },   \"json\": null,   \"origin\": \"111.1.26.12\",   \"url\": \"https://httpbin.org/post\"}",
"headers": {
    "null": "HTTP/1.1 200 OK",
    "Server": "gunicorn/19.9.0",
    "Access-Control-Allow-Origin": "*",
    "Access-Control-Allow-Credentials": "true",
    "Connection": "keep-alive",
    "Content-Length": "587",
    "Date": "Wed, 23 Aug 2023 09:05:08 GMT",
    "Content-Type": "application/json"
},
"statusCode": 200}

问题分析与修正 #

核心错误在于字节数组的错误转换：
b64.getDecoder().decode(content_pack)已经得到原始文件的字节数组，调用toString()会将其转为[B@xxxx格式的字符串（这是Java字节数组的默认toString输出），再调用getBytes()得到的是这个字符串的字节，而非原始文件的字节，导致发送的文件内容完全错误。

修正后的代码 #

//获取Bearer Token的前置代码无问题
var boundary = java.util.UUID.randomUUID().toString();
var headers = {
   "Authorization": "Bearer "+ token,
   "Content-Type": "multipart/form-data; boundary=" + boundary
};
var con = new java.net.URL("theUrl").openConnection();
con.requestMethod = "POST";
var hk = Object.keys(headers);
for (var key of hk){
    con.setRequestProperty(key, headers[key]);
}
// 发送POST请求
con.doOutput=true;
var rn="\r\n";
var dos = new java.io.DataOutputStream(con.outputStream);
var b64 = Java.type("java.util.Base64");
dos.writeBytes("--" + boundary + rn);
dos.writeBytes("Content-Disposition: form-data; name=\"mycontent\"; filename=\"mycontent.zip\"" + rn);
dos.writeBytes("Content-Type: application/octet-stream" + rn);
dos.writeBytes(rn); // 移除Content-Transfer-Encoding，multipart默认支持二进制传输
// 直接使用解码后的原始字节数组
var mydata = b64.getDecoder().decode(content_pack);
dos.write(mydata, 0, mydata.length);
dos.writeBytes(rn); 
// 结束multipart/form-data
dos.writeBytes("--" + boundary + "--" + rn);
dos.flush();
dos.close();    

asResponse(con); //读取响应并返回JSON对象

关键修正点 #

1. 移除错误的字节转换：直接使用Base64解码后的字节数组，跳过toString().getBytes()的无效转换
2. 移除不必要的请求头：删除Transfer-Encoding: chunked，Java的URLConnection会根据内容自动处理传输编码，手动设置可能引发冲突
3. 简化Content-Transfer-Encoding：multipart/form-data默认支持二进制传输，可省略该头或保持为binary
4. 确保格式完整性：保证multipart的边界、换行符格式完全符合HTTP规范

内容的提问来源于stack exchange，提问作者user2360915