参照这个写就行了。

public static void main(String[] args) {
    try {
        //商户号
        String mchid = "";
        //证书序列号
        String serial_no = "";
        //商户私钥（拷贝apiclient_key.pem文件里-----BEGIN PRIVATE KEY-----和-----END PRIVATE KEY-----之间的内容）
        String rsaPrivateKey = "";
        //微信支付平台公钥
        String rsaPublicKeyFile = "";
        //时间戳
        String timestamp = Long.toString(System.currentTimeMillis()/1000);
        //随机数
        String nonce_str = "liujinliang8848";

        //图片文件
        String filePath = "";//文件路径

        String boundary = "";

        File file = new File(filePath);
        String filename = file.getName();//文件名
        String fileSha256 = DigestUtils.sha256Hex(new FileInputStream(file));//文件sha256

        //拼签名串
        StringBuffer sb = new StringBuffer();
        sb.append("POST").append("\n");
        sb.append("/v3/merchant/media/upload").append("\n");
        sb.append(timestamp).append("\n");
        sb.append(nonce_str).append("\n");
        sb.append("{\"filename\":\""+filename+"\",\"sha256\":\""+fileSha256+"\"}").append("\n");
        System.out.println("签名原串:"+sb.toString());

        //计算签名
        String sign = new String(Base64.encodeBase64(signRSA(sb.toString(),rsaPrivateKey)));
        System.out.println("签名sign值:"+sign);

        //拼装http头的Authorization内容
        String authorization ="WECHATPAY2-SHA256-RSA2048 mchid=\""+mchid+"\",nonce_str=\""+nonce_str+"\",signature=\""+sign+"\",timestamp=\""+timestamp+"\",serial_no=\""+serial_no+"\"";
        System.out.println("authorization值:"+authorization);

        //接口URL
        String url = "https://api.mch.weixin.qq.com/v3/merchant/media/upload";
        CloseableHttpClient httpclient = HttpClients.createDefault();
        HttpPost httpPost = new HttpPost(url);

        //设置头部
        httpPost.addHeader("Charset", "UTF-8");
        httpPost.addHeader("Accept", "application/json");
        httpPost.addHeader("Content-Type", "multipart/form-data; boundary =" + boundary);
        httpPost.addHeader("Authorization", authorization);

        //创建MultipartEntityBuilder
        MultipartEntityBuilder multipartEntityBuilder = MultipartEntityBuilder.create().setMode(HttpMultipartMode.RFC6532);
        //设置boundary
        multipartEntityBuilder.setBoundary("boundary");
        multipartEntityBuilder.setCharset(Charset.forName("UTF-8"));
        //设置meta内容
        multipartEntityBuilder.addTextBody("meta", "{\"filename\":\""+filename+"\",\"sha256\":\""+fileSha256+"\"}", ContentType.APPLICATION_JSON);
        //设置图片内容
        multipartEntityBuilder.addBinaryBody("file", file, ContentType.create("image/jpg"), filename);
        //放入内容
        httpPost.setEntity(multipartEntityBuilder.build());

        //获取返回内容
        CloseableHttpResponse response = httpclient.execute(httpPost);
        HttpEntity httpEntity = response.getEntity();
        String rescontent = new String(InputStreamTOByte(httpEntity.getContent()));
        System.out.println("返回内容:" + rescontent);
        //获取返回的http header
        Header headers[] = response.getAllHeaders();
        int i = 0;
        while (i < headers.length) {
            System.out.println(headers[i].getName() + ":  " + headers[i].getValue());
            i++;
        }

        //验证微信支付返回签名
        String Wtimestamp = response.getHeaders("Wechatpay-Timestamp")[0].getValue();
        String Wnonce = response.getHeaders("Wechatpay-Nonce")[0].getValue();
        String Wsign = response.getHeaders("Wechatpay-Signature")[0].getValue();
        //拼装待签名串
        StringBuffer ss = new StringBuffer();
        ss.append(Wtimestamp).append("\n");
        ss.append(Wnonce).append("\n");
        ss.append(rescontent).append("\n");
        //验证签名
        if(verifyRSA(ss.toString(), Base64.decodeBase64(Wsign.getBytes()), rsaPublicKeyFile)) {
            System.out.println("签名验证成功");
        } else {
            System.out.println("签名验证失败");
        }

        EntityUtils.consume(httpEntity);
        response.close();

    } catch (Exception e) {
        System.out.println("发送POST请求异常！" + e);
        e.printStackTrace();
    }
}

public static byte[] InputStreamTOByte(InputStream in) throws IOException {

    int BUFFER_SIZE = 4096;
    ByteArrayOutputStream outStream = new ByteArrayOutputStream();
    byte[] data = new byte[BUFFER_SIZE];
    int count = -1;

    while((count = in.read(data,0,BUFFER_SIZE)) != -1)
        outStream.write(data, 0, count);

    data = null;
    byte[] outByte = outStream.toByteArray();
    outStream.close();

    return outByte;
}


public static byte[] signRSA(String data, String priKey) throws Exception {
    //签名的类型
    Signature sign = Signature.getInstance("SHA256withRSA");
    //读取商户私钥,该方法传入商户私钥证书的内容即可
    byte[] keyBytes = Base64.decodeBase64(priKey);
    PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(keyBytes);
    KeyFactory keyFactory = KeyFactory.getInstance("RSA");
    PrivateKey privateKey = keyFactory.generatePrivate(keySpec);
    sign.initSign(privateKey);
    sign.update(data.getBytes("UTF-8"));
    return sign.sign();
}

public static boolean verifyRSA(String data, byte[] sign, String pubKey) throws Exception{
    if(data == null || sign == null || pubKey == null){
        return false;
    }
    CertificateFactory cf = CertificateFactory.getInstance("X.509");
    FileInputStream in = new FileInputStream(pubKey);
    Certificate c = cf.generateCertificate(in);
    in.close();
    PublicKey publicKey = c.getPublicKey();
    Signature signature = Signature.getInstance("SHA256WithRSA");
    signature.initVerify(publicKey);
    signature.update(data.getBytes("UTF-8"));
    return signature.verify(sign);
}