如何提高BufferedInputStream的转换速度？

你的问题主要有这么几处：

1. 从HTTP读取到的数据，等待全部读取完放在了内存中，等待耗时，文件很大并发时可能会内存溢出
2. 阻塞读取，将数据全部存buffer过程中，response一直在等待，什么也没做，可以边读边写
3. HTTP请求连接未复用，建议使用OK-http等库复用连接
4. 资源未释放，response会等待超时，且内存会泄露

方案1：原始流复制，这里的buffer越大，效率越快，但是注意内存占用

HttpURLConnection connection = null;
try {
    URL url = new URL(imageUrl);
    connection = (HttpURLConnection) url.openConnection();
    connection.setRequestMethod("GET");
    connection.setConnectTimeout(5000);
    try(InputStream bis = new BufferedInputStream(connection.getInputStream());
        OutputStream out = response.getOutputStream()) {
        response.setContentType("image/jpg");
        
        // buffer 越大，效率越快
        byte[] buffer = new byte[1024];
        int len;
        while ((len = bis.read(buffer)) != -1) {
            out.write(buffer, 0, len);
            
            out.flush();
            
        }
    }
} catch (Exception e) {
    throw new RuntimeException(e);
} finally {
    if(null != connection){
        connection.disconnect();
    }
}

方案2:使用一些三方库的COPY工具类

HttpURLConnection connection = null;
try {
    URL url = new URL(imageUrl);
    connection = (HttpURLConnection) url.openConnection();
    connection.setRequestMethod("GET");
    connection.setConnectTimeout(5000);
    try(InputStream bis = new BufferedInputStream(connection.getInputStream());
        OutputStream out = response.getOutputStream()) {
        response.setContentType("image/jpg");

        IoUtil.copy(bis, out);
        
    }
} catch (Exception e) {
    throw new RuntimeException(e);
} finally {
    if(null != connection){
        connection.disconnect();
    }
}

方案3：使用NIO非阻塞传输，同样缓冲区越大，效率越快

HttpURLConnection connection = null;
try {
    URL url = new URL(imageUrl);
    connection = (HttpURLConnection) url.openConnection();
    connection.setRequestMethod("GET");
    connection.setConnectTimeout(5000);
    try (ReadableByteChannel in = Channels.newChannel(new BufferedInputStream(connection.getInputStream()));
         WritableByteChannel out = Channels.newChannel(response.getOutputStream())) {
        response.setContentType("image/jpg");

        ByteBuffer byteBuffer = ByteBuffer.allocate(8192);
        while (in.read(byteBuffer) != -1) {
            // 写转读
            byteBuffer.flip();
            out.write(byteBuffer);
            byteBuffer.clear();
        }
    }
} catch (Exception e) {
    throw new RuntimeException(e);
} finally {
    if (null != connection) {
        connection.disconnect();
    }
}