public class WxAccessToken {
	private static final long MAX_TIME = 7200 * 1000;// 微信允許最長Access_token有效時間（ms）

	/**
	 * 獲取Access_token 儲存並且只儲存2小時Access_token。如果超過兩個小時重新獲取；如果沒有超過兩個小時，直接獲取
	 * 思路:將獲取到的Access_token和當前時間儲存到file裡，
	 * 取出時判斷當前時間和儲存裡面的記錄的時間的時間差，如果大於MAX_TIME,重新獲取，並且將獲取到的儲存到file替換原來的內容
	 * ，如果小於MAX_TIME，直接獲取。
	 * 
	 * @throws IOException
	 */
	public static String getSavedAccess_token() throws IOException {
		Gson gson = new Gson();
		String mAccess_token = null;// 需要獲取的Access_token；
		File file = new File("temp_access_token.temp");// Access_token儲存的位置
		// 如果檔案不存在，建立
		if (!file.exists())
			file.createNewFile();
		// 如果檔案大小等於0，說明第一次使用，存入Access_token
		if (file.length() == 0) {
			mAccess_token = getAccessToken();
			FileOutputStream fos = new FileOutputStream(file, false);// 不允許追加
			Access_token at = new Access_token();
			at.setAccess_token(mAccess_token);
			at.setExpires_in(System.currentTimeMillis() + "");
			String json = gson.toJson(at);
			fos.write((json).getBytes());
			fos.close();
		} else {
			// 讀取檔案內容
			FileInputStream fis = new FileInputStream(file);
			byte[] b = new byte[2048];
			int len = fis.read(b);
			String mJsonAccess_token = new String(b, 0, len);// 讀取到的檔案內容
			Access_token access_token = gson.fromJson(mJsonAccess_token, new Access_token().getClass());
			if (access_token.getExpires_in() != null) {
				long saveTime = Long.parseLong(access_token.getExpires_in());
				long nowTime = System.currentTimeMillis();
				long remianTime = nowTime - saveTime;
				//System.out.println(TAG + "時間差：" + remianTime);
				if (remianTime < MAX_TIME) {
					Access_token at = gson.fromJson(mJsonAccess_token, new Access_token().getClass());
					mAccess_token = at.getAccess_token();
				} else {
					mAccess_token = getAccessToken();
					FileOutputStream fos = new FileOutputStream(file, false);// 不允許追加
					Access_token at = new Access_token();
					at.setAccess_token(mAccess_token);
					at.setExpires_in(System.currentTimeMillis() + "");
					String json = gson.toJson(at);
					fos.write((json).getBytes());
					fos.close();
				}

			}
		}
		//System.out.println("獲取到的Access_token是：" + mAccess_token);
		return mAccess_token;
		
	}

	public static String getAccessToken() {
		String urlString = "https://api.weixin.qq.com/cgi-bin/token?grant_type=client_credential&appid="
				+ Constant.APPID + "&secret=" + Constant.SERCRET;
		String reslut = null;
		try {
			URL reqURL = new URL(urlString);
			HttpsURLConnection httpsConn = (HttpsURLConnection) reqURL.openConnection();
			InputStreamReader isr = new InputStreamReader(httpsConn.getInputStream());
			char[] chars = new char[1024];
			reslut = "";
			int len;
			while ((len = isr.read(chars)) != -1) {
				reslut += new String(chars, 0, len);
			}
			isr.close();
		} catch (IOException e) {

			e.printStackTrace();
		}
		Gson gson = new Gson();
		Access_token access_token = gson.fromJson(reslut, new Access_token().getClass());
		if (access_token.getAccess_token() != null) {
			return access_token.getAccess_token();
		} else {
			return null;
		}
	}
}