Java获取文件MD5的方式

MD5是一种常用的散列算法，它可以将任意长度的输入数据映射为一个128位的输出，这个输出就是一个固定长度的数字，通常用来校验数据的完整性，它的特点是不可逆，即无法从散列值反推出原来的数据。

相关思路

用JDK中自带的Security包中的MessageDigest类可以实现MD5算法。所以基本的实现流程是 选择文件 -> 读取二进制流 -> MD5信息摘要 -> 转换为String返回输出。

代码

Main.java

import java.io.File;
import java.util.Scanner;

/*
A tool to get the MD5 of a File.
Author: Titan
 */
public class Main {

    public static void main(String[] args) {
        File file = null;
        Scanner scanner = new Scanner(System.in);
        System.out.println("Please input the file name.");
        String fileName = scanner.nextLine();
        file = new File(fileName);
        // Jude if the file is exist
        if (!file.isFile()) {
            System.out.println("File Not Found!");
            System.exit(-1);
        }
        // Handle the MD5 Operation
        Handler handler = new Handler(file);
        // Get MD5
        String md5Text = handler.getMD5();
        if (md5Text != null) {
            System.out.println("Get the MD5 Successfully");
            System.out.println("File Name: " + fileName);
            System.out.println("MD5 Text: " + md5Text);
        } else {
            System.out.println("Get MD5 Failed. ");
        }
    }
}

Handler.java

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.math.BigInteger;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class Handler {
    private final byte[] buffer = new byte[1024];
    private MessageDigest md5 = null;
    private File file = null;

    public Handler(File file) {
        if (!file.isFile()) {
            System.out.println("File not found!");
        }
        try {
            this.md5 = MessageDigest.getInstance("MD5");
        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace();
            System.out.println("MD5 Method Error!");
        }
        this.file = file;
    }

    public String getMD5() {
        int length = 0;
        try (FileInputStream inputStream = new FileInputStream(file)) {
            while ((length = inputStream.read(buffer)) != -1) {
                md5.update(buffer, 0, length);
            }
            return new BigInteger(1, md5.digest()).toString(16);
        } catch (IOException e) {
            e.printStackTrace();
            return null;
        }
    }
}