Google Apps Script中getThumbnail函数为何运行不稳定？

问题根源 #

你遇到的Exception: Argument cannot be null: blob错误，是因为新创建的PDF文件在Google Drive中还未完成缩略图生成流程，此时调用getThumbnail()会返回null。Google Drive的文件处理（包括缩略图生成）是异步的，刚创建文件时可能还没完成这一步，导致偶尔失败。

解决方案 #

方案1：添加等待与重试机制 #

在创建PDF后，等待一段时间让Drive生成缩略图，或循环重试直到获取到有效的Blob：

function createJPG() {
  let nome = "Filipe Pereira";
  let grad = "3º Sargento";
  let quadro = "QPMP-C";
  let funcao = "Fotógrafo";
  let lotacao = "Assessoria de Comunicação";
  let telefone = "(27) 3636-8717";
  let email = "secretaria@pm.es.gov.br";
  const docFile = DriveApp.getFileById("1EhDglV8aumu__zAOfasdfasrukNKYFDas");
  const folder = DriveApp.getFolderById("1fwAhuCdoYyUdHfasdfaPyjRq5iN97");
  const jpgFolder = DriveApp.getFolderById("14iZ4sS4jR5sadfasdKainJTnurZ9cgvpH");
  const tempFile = docFile.makeCopy(jpgFolder);
  const tempDocFile = SlidesApp.openById(tempFile.getId());
  const slide = tempDocFile.getSlides()[0];
  slide.replaceAllText("{nome}",nome);
  slide.replaceAllText("{grad}",grad);  
  slide.replaceAllText("{quadro}",quadro);
  slide.replaceAllText("{funcao}",funcao);
  slide.replaceAllText("{lotacao}",lotacao);
  slide.replaceAllText("{telefone}",telefone);
  slide.replaceAllText("{email}",email);
  tempDocFile.saveAndClose();
    
  // 创建PDF
  const pdfContentBlob = tempFile.getAs(MimeType.PDF);
  const newPDF = jpgFolder.createFile(pdfContentBlob).setName(nome);
  
  // 等待并重试获取缩略图
  let blob = null;
  const maxRetries = 5;
  let retryCount = 0;
  while (blob === null && retryCount < maxRetries) {
    Utilities.sleep(2000); // 每次等待2秒
    blob = newPDF.getThumbnail();
    retryCount++;
  }
  
  if (blob !== null) {
    const newJPG = jpgFolder.createFile(blob).setName(nome);
  } else {
    console.error("无法获取PDF缩略图，重试次数已达上限");
  }
  
  folder.removeFile(tempFile);
  jpgFolder.removeFile(newPDF);
}

方案2：直接从幻灯片导出图片（更可靠） #

跳过PDF转换步骤，直接将编辑后的幻灯片导出为图片，避免依赖Drive的缩略图生成机制：

function createJPG() {
  let nome = "Filipe Pereira";
  let grad = "3º Sargento";
  let quadro = "QPMP-C";
  let funcao = "Fotógrafo";
  let lotacao = "Assessoria de Comunicação";
  let telefone = "(27) 3636-8717";
  let email = "secretaria@pm.es.gov.br";
  const docFile = DriveApp.getFileById("1EhDglV8aumu__zAOfasdfasrukNKYFDas");
  const folder = DriveApp.getFolderById("1fwAhuCdoYyUdHfasdfaPyjRq5iN97");
  const jpgFolder = DriveApp.getFolderById("14iZ4sS4jR5sadfasdKainJTnurZ9cgvpH");
  const tempFile = docFile.makeCopy(jpgFolder);
  const tempDocFile = SlidesApp.openById(tempFile.getId());
  const slide = tempDocFile.getSlides()[0];
  slide.replaceAllText("{nome}",nome);
  slide.replaceAllText("{grad}",grad);  
  slide.replaceAllText("{quadro}",quadro);
  slide.replaceAllText("{funcao}",funcao);
  slide.replaceAllText("{lotacao}",lotacao);
  slide.replaceAllText("{telefone}",telefone);
  slide.replaceAllText("{email}",email);
  tempDocFile.saveAndClose();
  
  // 直接将幻灯片导出为JPG Blob
  const slideId = slide.getObjectId();
  const url = `https://docs.google.com/presentation/d/${tempFile.getId()}/export/png?id=${tempFile.getId()}&pageid=${slideId}`;
  const options = {
    headers: {
      'Authorization': 'Bearer ' + ScriptApp.getOAuthToken()
    }
  };
  const response = UrlFetchApp.fetch(url, options);
  const blob = response.getBlob().setName(`${nome}.jpg`);
  
  // 保存图片到指定文件夹
  jpgFolder.createFile(blob);
  
  // 清理临时文件
  jpgFolder.removeFile(tempFile);
}

说明 #

• 方案2无需依赖PDF转换，直接通过Google Slides的导出接口生成图片，成功率更高且流程更简洁。
• 若必须保留PDF转换步骤，方案1的重试机制能有效降低异步处理导致的失败概率。

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