/** Gets the encrypted usage stat data to be sent to the Hudson server. */
  public String getStatData() throws IOException {
    Jenkins j = Jenkins.getInstance();

    JSONObject o = new JSONObject();
    o.put("stat", 1);
    o.put("install", j.getLegacyInstanceId());
    o.put("servletContainer", j.servletContext.getServerInfo());
    o.put("version", Jenkins.VERSION);

    List<JSONObject> nodes = new ArrayList<JSONObject>();
    for (Computer c : j.getComputers()) {
      JSONObject n = new JSONObject();
      if (c.getNode() == j) {
        n.put("master", true);
        n.put("jvm-vendor", System.getProperty("java.vm.vendor"));
        n.put("jvm-name", System.getProperty("java.vm.name"));
        n.put("jvm-version", System.getProperty("java.version"));
      }
      n.put("executors", c.getNumExecutors());
      DescriptorImpl descriptor = j.getDescriptorByType(DescriptorImpl.class);
      n.put("os", descriptor.get(c));
      nodes.add(n);
    }
    o.put("nodes", nodes);

    List<JSONObject> plugins = new ArrayList<JSONObject>();
    for (PluginWrapper pw : j.getPluginManager().getPlugins()) {
      if (!pw.isActive()) continue; // treat disabled plugins as if they are uninstalled
      JSONObject p = new JSONObject();
      p.put("name", pw.getShortName());
      p.put("version", pw.getVersion());
      plugins.add(p);
    }
    o.put("plugins", plugins);

    JSONObject jobs = new JSONObject();
    List<TopLevelItem> items = j.getAllItems(TopLevelItem.class);
    for (TopLevelItemDescriptor d : Items.all()) {
      int cnt = 0;
      for (TopLevelItem item : items) {
        if (item.getDescriptor() == d) cnt++;
      }
      jobs.put(d.getJsonSafeClassName(), cnt);
    }
    o.put("jobs", jobs);

    try {
      ByteArrayOutputStream baos = new ByteArrayOutputStream();

      // json -> UTF-8 encode -> gzip -> encrypt -> base64 -> string
      OutputStreamWriter w =
          new OutputStreamWriter(
              new GZIPOutputStream(new CombinedCipherOutputStream(baos, getKey(), "AES")), "UTF-8");
      try {
        o.write(w);
      } finally {
        IOUtils.closeQuietly(w);
      }

      return new String(Base64.encode(baos.toByteArray()));
    } catch (GeneralSecurityException e) {
      throw new Error(e); // impossible
    }
  }
  /**
   * Gets the descriptor of {@link com.cloudbees.hudson.plugins.folder.Folder}
   *
   * @return the descriptor or null if Jenkins can't be accessed
   */
  private static TopLevelItemDescriptor getFolderDescriptor() {
    Jenkins jenkins = getJenkinsInstance();

    return jenkins != null ? jenkins.getDescriptorByType(Folder.DescriptorImpl.class) : null;
  }