先将字符串base64转为字节数组输入流，再转为输入流

java

 public InputStream base64ToInputStream(String base64) {
        ByteArrayInputStream stream = null;
        try {
            byte[] bytes = Base64.getDecoder().decode(base64);
            stream = new ByteArrayInputStream(bytes);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return stream;
    }