像百度網盤有秒傳的功能，其實他的原理主要是比較需要上傳的檔案和網盤中的檔案是否一樣，如果不一樣則上傳，如果一樣就沒必要上傳檔案，只是將網盤中的檔案指向你的使用者名稱即可，從而實現了秒傳。

通過計算檔案的MD5或SHA-1是否一致，程式碼如下

public static String getFileMD5(File file) {
	    if (!file.isFile()) {
	        return null;
	    }
	    MessageDigest digest = null;
	    FileInputStream in = null;
	    byte buffer[] = new byte[8192];
	    int len;
	    try {
	        digest =MessageDigest.getInstance("MD5");
	        in = new FileInputStream(file);
	        while ((len = in.read(buffer)) != -1) {
	            digest.update(buffer, 0, len);
	        }
	        BigInteger bigInt = new BigInteger(1, digest.digest());
	        return bigInt.toString(16);
	    } catch (Exception e) {
	        e.printStackTrace();
	        return null;
	    } finally {
	        try {
	            in.close();
	        } catch (Exception e) {
	            e.printStackTrace();
	        }
	    }
}
 

public static String getFileSha1(File file) {
	    if (!file.isFile()) {
	        return null;
	    }
	    MessageDigest digest = null;
	    FileInputStream in = null;
	    byte buffer[] = new byte[8192];
	    int len;
	    try {
	        digest =MessageDigest.getInstance("SHA-1");
	        in = new FileInputStream(file);
	        while ((len = in.read(buffer)) != -1) {
	            digest.update(buffer, 0, len);
	        }
	        BigInteger bigInt = new BigInteger(1, digest.digest());
	        return bigInt.toString(16);
	    } catch (Exception e) {
	        e.printStackTrace();
	        return null;
	    } finally {
	        try {
	            in.close();
	        } catch (Exception e) {
	            e.printStackTrace();
	        }
	    }
}
 

獲取網路檔案的MD5

public static String getNetFileMD5(String path) throws Exception{
		URL url = new URL(path);
		HttpURLConnection con = (HttpURLConnection) url.openConnection();
		InputStream inStream = con.getInputStream();  
		
	    MessageDigest digest = null;
	    byte buffer[] = new byte[8192];
	    int len;
	    try {
	    	digest =MessageDigest.getInstance("MD5");
	        while ((len = inStream.read(buffer)) != -1) {
	            digest.update(buffer, 0, len);
	        }
	        BigInteger bigInt = new BigInteger(1, digest.digest());
	        return bigInt.toString(16);
	    } catch (Exception e) {
	        e.printStackTrace();
	        return null;
	    } finally {
	        try {
	        	inStream.close();
	        } catch (Exception e) {
	            e.printStackTrace();
	        }
	    }
	}