apache 的HttpClient很強大，據說可以承受一萬左右的高併發，但是在做專案的時候用HttpClient進行附件上傳，併發1000不到的時候都導致了記憶體溢位，核心程式碼為：

HttpPost post = new HttpPost(url.toString()); post.setEntity(multipartEntityBuilder.build());

HttpResponse response = httpClient.execute(post);

經過除錯和檢視httpClient的原始碼發現，由於httpClient.execute(post)返回值response並不能close，因此在上傳大檔案而且高併發的情況下會導致執行緒一直被佔用，導致資源不足

用CloseableHttpClient來替代HttpClient，httpClient.execute(post)得到CloseableHttpResponse可以被關閉，從而避免了上述問題

  HttpPost post = new HttpPost(url.toString());
        post.setEntity(multipartEntityBuilder.build());
        // try with resource語法response.close()會被自動呼叫
        try (CloseableHttpResponse response = httpClient.execute(post)) { 

        } catch
 
 (Exception ex) {

        } finally {

        }