教你详解JAVA中获取文件MD5值的四种方法。

1. 使用MessageDigest类，2. 使用Apache Commons Codec库，3. 使用Java NIO和MessageDigest类，4. 使用第三方库，如Apache Commons Codec或Bouncy Castle

在Java中，获取文件的MD5值是一种常见的需求，例如在文件校验、数据一致性检查等方面，MD5是一种广泛使用的加密哈希函数，它可以生成一个128位（16字节）的哈希值，通常用32个十六进制数字表示，下面将介绍四种在Java中获取文件MD5值的方法。

方法一：使用java.security.MessageDigest类

java.security.MessageDigest类是Java提供的一个用于计算消息摘要的类，它可以实现多种摘要算法，包括MD5，以下是使用MessageDigest类计算文件MD5值的示例代码：

import java.io.FileInputStream;
import java.io.IOException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
public class MD5Util {
    public static String getFileMD5(String filePath) throws IOException, NoSuchAlgorithmException {
        MessageDigest md = MessageDigest.getInstance("MD5");
        FileInputStream fis = new FileInputStream(filePath);
        byte[] buffer = new byte[1024];
        int len;
        while ((len = fis.read(buffer)) != -1) {
            md.update(buffer, 0, len);
        }
        fis.close();
        byte[] digest = md.digest();
        StringBuilder sb = new StringBuilder();
        for (byte b : digest) {
            sb.append(String.format("%02x", b));
        }
        return sb.toString();
    }
}

方法二：使用Apache Commons Codec库

Apache Commons Codec库提供了一种简单的方式来计算文件的MD5值，需要添加Apache Commons Codec库的依赖到项目中，可以使用DigestUtils类的md5Hex方法来计算文件的MD5值，以下是使用Apache Commons Codec库计算文件MD5值的示例代码：

import org.apache.commons.codec.digest.DigestUtils;
import java.io.File;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
public class MD5Util {
    public static String getFileMD5(String filePath) throws IOException {
        Path path = Paths.get(filePath);
        byte[] bytes = Files.readAllBytes(path);
        return DigestUtils.md5Hex(bytes);
    }
}

方法三：使用第三方库Bouncy Castle

Bouncy Castle是一个开源的Java安全和密码学库，它提供了许多加密算法的实现，包括MD5，需要添加Bouncy Castle库的依赖到项目中，可以使用org.bouncycastle.crypto.digests.MD5Digest类来计算文件的MD5值，以下是使用Bouncy Castle库计算文件MD5值的示例代码：

import org.bouncycastle.crypto.digests.MD5Digest;
import org.bouncycastle.util.encoders.Hex;
import java.io.FileInputStream;
import java.io.IOException;
import java.security.NoSuchAlgorithmException;
import java.security.Security;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.KeySpec;
import javax.crypto.SecretKeyFactory;
import javax.crypto.spec.PBEKeySpec;
import javax.crypto.spec.SecretKeySpec;
import java.math.BigInteger;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.security.*;
import java.util.*;
import javafx.util.*; // For Base64 encoding and decoding in JavaFX applications only! Use Java's built-in classes instead in non-JavaFX applications!
private static final int ITERATION_COUNT = 65536; // Number of times to hash the data before returning the result (more iterations can increase security but also slow down the process)
private static final int SALT_LENGTH = 8; // The length of the salt used for generating the key (must be at least 8 characters long)
private static final int KEY_LENGTH = 128; // The length of the key generated by hashing the data with the salt (must be at least 128 bits long)
private static final int PBE_ITERATION_COUNT = 65536; // Number of times to hash the data before returning the result (more iterations can increase security but also slow down the process)
private static final int PBE_SALT_LENGTH = 16; // The length of the salt used for generating the key (must be at least 16 characters long)
private static final int PBE_KEY_LENGTH = 128; // The length of the key generated by hashing the data with the salt (must be at least 128 bits long)
private static final String ALGORITHM = "PBKDF2WithHmacSHA1"; // The algorithm used to generate the key from the salt and password (must be a valid algorithm name)
private static final String CIPHER_ALGORITHM = "AES/CBC/PKCS7Padding"; // The algorithm used to encrypt and decrypt the data (must be a valid algorithm name)
private static final String PADDING_MODE = "PKCS7Padding"; // The mode used to pad the data before encryption or decryption (must be a valid mode name)
private static final String PROVIDER = "BC"; // The provider used to initialize the cryptographic objects (must be a valid provider name)
private static final char[] PASSWORD = "password".toCharArray(); // The password used to generate the key from the salt (must be at least 8 characters long) private static final byte[] SALT = "somesalt".getBytes(); // The salt used to generate the key from the password (must be at least 8 characters long) private static final byte[] IV = new byte[16]; // The initialization vector used for encryption and decryption (must be at least 16 bytes long) private static final SecretKeyFactory SECRET_KEY_FACTORY = SecretKeyFactory