獲取文件的哈希碼是一項常見的編程任務。MD5 是一種非常流行和常用的哈希算法。使用 Java 可以輕松獲取文件的 MD5 哈希碼，如下面的代碼所示：

import java.io.BufferedInputStream; import java.io.File; import java.io.FileInputStream; import java.security.MessageDigest;public class MD5file {public static String md5OfFile(File file) throws Exception {MessageDigest md = MessageDigest.getInstance("MD5");FileInputStream fs = new FileInputStream(file);BufferedInputStream bs = new BufferedInputStream(fs);byte[] buffer = new byte[1024];int bytesRead;while ((bytesRead = bs.read(buffer, 0, buffer.length)) != -1) {md.update(buffer, 0, bytesRead);}byte[] digest = md.digest();StringBuilder sb = new StringBuilder();for (byte bite : digest) {sb.append(String.format("%02x", bite & 0xff));}return sb.toString();}public static void main(String[] args) {File md5file = new File("f:\\Fedora-KDE-Live-x86_64-34-1.2.iso");try {String out = md5OfFile(md5file);System.out.println(out);} catch (Exception e) {// TODO Auto-generated catch blocke.printStackTrace();}}}

上面的代碼做了幾件事：

• 創建一個MessageDigest最終會創建 MD5 哈希碼的對象
• 使用FileInputStream和BufferedInputStream緩沖來自文件的輸入
• 每讀取 1024 個字節更新MessageDigest對象update()
• 使用digest()方法返回文件的MD5哈希碼
• 將此 MD5 哈希碼轉換為十六進制格式

import java.io.BufferedInputStream; import java.io.File; import java.io.FileInputStream; import java.io.FileNotFoundException; import java.io.IOException; import java.security.MessageDigest; import java.security.NoSuchAlgorithmException;public class MD5file2 {public static void main(String[] args) throws NoSuchAlgorithmException, IOException {File file = new File("f:\\Fedora-KDE-Live-x86_64-34-1.2.iso");MessageDigest md = MessageDigest.getInstance("MD5");FileInputStream fs = new FileInputStream(file);BufferedInputStream bs = new BufferedInputStream(fs);byte[] buffer = new byte[1024];int bytesRead;while ((bytesRead = bs.read(buffer, 0, buffer.length)) != -1) {md.update(buffer, 0, bytesRead);}byte[] digest = md.digest();StringBuilder sb = new StringBuilder();for (byte bite : digest) {sb.append(String.format("%02x", bite & 0xff));}System.out.println(sb.toString());}} import java.io.BufferedInputStream; import java.io.File; import java.io.FileInputStream; import java.io.FileNotFoundException; import java.io.IOException; import java.security.MessageDigest; import java.security.NoSuchAlgorithmException;public class MD5file3 {public static void main(String[] args) {File file = new File("f:\\Fedora-KDE-Live-x86_64-34-1.2.iso");MessageDigest md = null;try {md = MessageDigest.getInstance("MD5");} catch (NoSuchAlgorithmException e) {// TODO Auto-generated catch blocke.printStackTrace();}FileInputStream fs = null;try {fs = new FileInputStream(file);} catch (FileNotFoundException e) {// TODO Auto-generated catch blocke.printStackTrace();}BufferedInputStream bs = new BufferedInputStream(fs);byte[] buffer = new byte[1024];int bytesRead;try {while ((bytesRead = bs.read(buffer, 0, buffer.length)) != -1) {md.update(buffer, 0, bytesRead);}} catch (IOException e) {// TODO Auto-generated catch blocke.printStackTrace();}byte[] digest = md.digest();StringBuilder sb = new StringBuilder();for (byte bite : digest) {sb.append(String.format("%02x", bite & 0xff));}System.out.println(sb.toString());}}

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