Javaでbit.ly等の短縮URLを展開する方法

短縮URLを展開する必要があったため、高速な展開方法を実装しました。

短縮URLサービスは90個以上あるそうで、個別のAPIを使ってたら埒があかない。以下のコードは、bit.ly, t.co, goo.gl などでテスト済みだけど、あらゆる短縮URLサービスで利用可能だと思います。

public abstract class UrlUtility {
	public static URL expandUrl(URL aUrl) throws IOException, ProtocolException {
		final URLConnection tURLConnection = aUrl.openConnection(Proxy.NO_PROXY);
		if (!(tURLConnection instanceof HttpURLConnection)) {
			return aUrl;
		}
		final HttpURLConnection tHttpURLConnection = (HttpURLConnection) tURLConnection;
		tHttpURLConnection.setRequestMethod("HEAD");
		tHttpURLConnection.setInstanceFollowRedirects(false);
		tHttpURLConnection.connect();

		final String tExpandedUrl;
		final String tLocation = tHttpURLConnection.getHeaderField("Location");
		if (tLocation != null && tLocation.startsWith("http")) {
			final int tResponseCode = tHttpURLConnection.getResponseCode();
			if (tResponseCode == HttpURLConnection.HTTP_MOVED_PERM || tResponseCode == HttpURLConnection.HTTP_MOVED_TEMP) {
				return expandUrl(new URL(encode(tLocation))).toExternalForm();
			}
			tExpandedUrl = tLocation;
		} else {
			tExpandedUrl = tHttpURLConnection.getURL().toExternalForm();
		}

		return new URL(encode(tExpandedUrl));
	}

	// @formatter:off
	private static final String[] HEX = {
		"80","81","82","83","84","85","86","87","88","89","8A","8B","8C","8D","8E","8F",
		"90","91","92","93","94","95","96","97","98","99","9A","9B","9C","9D","9E","9F",
		"A0","A1","A2","A3","A4","A5","A6","A7","A8","A9","AA","AB","AC","AD","AE","AF",
		"B0","B1","B2","B3","B4","B5","B6","B7","B8","B9","BA","BB","BC","BD","BE","BF",
		"C0","C1","C2","C3","C4","C5","C6","C7","C8","C9","CA","CB","CC","CD","CE","CF",
		"D0","D1","D2","D3","D4","D5","D6","D7","D8","D9","DA","DB","DC","DD","DE","DF",
		"E0","E1","E2","E3","E4","E5","E6","E7","E8","E9","EA","EB","EC","ED","EE","EF",
		"F0","F1","F2","F3","F4","F5","F6","F7","F8","F9","FA","FB","FC","FD","FE","FF",
		"00","01","02","03","04","05","06","07","08","09","0A","0B","0C","0D","0E","0F",
		"10","11","12","13","14","15","16","17","18","19","1A","1B","1C","1D","1E","1F",
		"20","21","22","23","24","25","26","27","28","29","2A","2B","2C","2D","2E","2F",
		"30","31","32","33","34","35","36","37","38","39","3A","3B","3C","3D","3E","3F",
		"40","41","42","43","44","45","46","47","48","49","4A","4B","4C","4D","4E","4F",
		"50","51","52","53","54","55","56","57","58","59","5A","5B","5C","5D","5E","5F",
		"60","61","62","63","64","65","66","67","68","69","6A","6B","6C","6D","6E","6F",
		"70","71","72","73","74","75","76","77","78","79","7A","7B","7C","7D","7E","7F",
	};
	// @formatter:on

	private static String encode(String aUrl) throws UnsupportedEncodingException {
		final byte[] tBytes = aUrl.getBytes("ISO-8859-1");
		final int tLength = tBytes.length;
		final StringBuilder tBuilder = new StringBuilder(tLength * 3);
		for (int tIndex = 0; tIndex < tLength; tIndex++) {
			final int tIntAt = (int) tBytes[tIndex];
			if (tIntAt < 0) {
				tBuilder.append('%');
				tBuilder.append(HEX[tIntAt + 128]);
			} else {
				tBuilder.append((char) tIntAt);
			}
		}
		return tBuilder.toString();
	}
}

考慮したこと

以下のことを考慮する必要がありました。

• 展開結果のURLがマルチバイト文字を含んでいることがある
  →URLエンコードすることで対処
• 展開結果のURLのホストサーバがIISだった場合は、HttpURLConnection#getURL()の結果でパスが省略される
  →tHttpURLConnection.setInstanceFollowRedirects(false)として、IISへのアクセスを行わずLocationヘッダを取り出すことで対処
• 遅い
  →URL#openConnection(Proxy.NO_PROXY)を指定することで対処
  →HEADメソッドを使ってBODYを無視することで対処
• 展開不要なURLの判定が難しい
  →とりあえず展開を試みて、LocationヘッダかHttpURLConnection#getURL()のどちらかを利用することで対処
• 多段階のリダイレクトが行われることがある
  →HTTPステータスコードが301, 302あいだは再帰的にexpandUrlを呼び出すことで対処

参考URL

• 短縮URLサービス一覧
  多すぎる・・・orz
  %E7%B8%AE.jp, 1bps.biz, 3.ly, 30m.in, 5wd60.tk, a.r10.to, a5dyo.tk, a8.net, abf.to, ads.modiphi.com, am6.jp, amzn.to, artbeat.ly, bctiny.com, bit.ly, bkite.com, bt.io, dlvr.it, dw.am, fc2.in, feedburner.com, feedburner.jp, feedproxy.google.com, feeds.digitaldj.jp, flic.kr, flickr.com/p/, fnn-news.com, fon.gsyep.it, gigaz.in, go.2ch2.net, goo.gl, ht.ly, htn.to, icio.us, idek.net, instapaper.com, is.gd, j.mp, jpan.jp, labo.tv, liten.be, mfi.rep.tl, mjk.ac, moby.to, moi.st, mpr.hn, nhk.jp, oneclip.jp, ow.ly, pheedo.jp, pheedo.wiredvision.jp, pic.gd, post.ly, pr.cm, qurl.com, r.sm3.jp, rd.yahoo.co.jp, rss.rssad.jp, snipurl.com, snurl.com, sugowaza.jp/r, t.co, terra.es, tinyurl.com, tl.gdff.im, to.ly, tr.im, tumblr.com, tuna.be, twt.mx, twurl.nl, u.nu, url4.eu, urlbrief.com, urltea.com, urltea.me, ustre.am, vdh.bz, vriend.jp, xfs.jp, xrl.us, yfrog.com, youtu.be, z.la, 縮.jp
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