// clear the cache before time numDays
private int clearCacheFolder(File dir, long numDays) {
  int deletedFiles = 0;
  if (dir!= null && dir.isDirectory()) {
    try {
      for (File child:dir.listFiles()) {
        if (child.isDirectory()) {
          deletedFiles += clearCacheFolder(child, numDays);
        }
        if (child.lastModified() < numDays) {
          if (child.delete()) {
            deletedFiles++;
          }
        }
      }
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
  return deletedFiles;
}

//优先使用缓存：
WebView.getSettings().setCacheMode(WebSettings.LOAD_CACHE_ELSE_NETWORK);
//不使用缓存：
WebView.getSettings().setCacheMode(WebSettings.LOAD_NO_CACHE);

File file = CacheManager.getCacheFileBaseDir();
  if (file != null && file.exists() && file.isDirectory()) {
  for (File item : file.listFiles()) {
   item.delete();
  }
  file.delete();
  }
 context.deleteDatabase("webview.db");
 context.deleteDatabase("webviewCache.db");