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);
}

@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);
}