java 中，如何获取文件的MD5值呢？如何比较两个文件是否完全相同呢？

/**
	 * Get MD5 of one file:hex string,test OK!
	 * 
	 * @param file
	 * @return
	 */
	public static String getFileMD5(File file) {
		if (!file.exists() || !file.isFile()) {
			return null;
		}
		MessageDigest digest = null;
		FileInputStream in = null;
		byte buffer[] = new byte[1024];
		int len;
		try {
			digest = MessageDigest.getInstance("MD5");
			in = new FileInputStream(file);
			while ((len = in.read(buffer, 0, 1024)) != NEGATIVE_ONE) {
				digest.update(buffer, 0, len);
			}
			in.close();
		} catch (Exception e) {
			e.printStackTrace();
			return null;
		}
		BigInteger bigInt = new BigInteger(1, digest.digest());
		return bigInt.toString(16);
	}

	/***
	 * Get MD5 of one file！test ok!
	 * 
	 * @param filepath
	 * @return
	 */
	public static String getFileMD5(String filepath) {
		File file = new File(filepath);
		return getFileMD5(file);
	}

	/**
	 * MD5 encrypt,test ok
	 * 
	 * @param data
	 * @return byte[]
	 * @throws Exception
	 */
	public static byte[] encryptMD5(byte[] data) throws Exception {

		MessageDigest md5 = MessageDigest.getInstance(SystemUtil.KEY_MD5);
		md5.update(data);
		return md5.digest();
	}

	public static byte[] encryptMD5(String data) throws Exception {
		return encryptMD5(data.getBytes(SystemUtil.CHARSET_ISO88591));
	}
	/***
	 * compare two file by Md5
	 * 
	 * @param file1
	 * @param file2
	 * @return
	 */
	public static boolean isSameMd5(File file1,File file2){
		String md5_1=SystemUtil.getFileMD5(file1);
		String md5_2=SystemUtil.getFileMD5(file2);
		return md5_1.equals(md5_2);
	}
	/***
	 * compare two file by Md5
	 * 
	 * @param filepath1
	 * @param filepath2
	 * @return
	 */
	public static boolean isSameMd5(String filepath1,String filepath2){
		File file1=new File(filepath1);
		File file2=new File(filepath2);
		return isSameMd5(file1, file2);
	}

　　测试：

@Test
    public void test_getFileMD5() throws Exception{
        String filepath="D:\download\3_尚学堂_UML概览.avi";
//        File file=new File(filepath);
        String md5_1=SystemUtil.getFileMD5(filepath);
        System.out.println(md5_1);
        
        byte[]bytes=FileUtils.readBytes4file(filepath);
        byte[]md5=SystemUtil.encryptMD5(bytes);
        String md5_2=SystemUtil.toHexString(md5);
        System.out.println(md5_2);
        Assert.assertEquals(md5_1, md5_2);
    }