程式碼：

import org.springframework.security.crypto.password.PasswordEncoder;
import java.math.BigInteger;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class MD5Utils implements PasswordEncoder {
	/**
	 * 使用md5的演算法進行加密
	 */
	public static String md5(String plainText) {
		byte[] secretBytes = null;
		try {
			secretBytes = MessageDigest.getInstance("md5").digest(
					plainText.getBytes());
		} catch (NoSuchAlgorithmException e) {
			throw new RuntimeException("沒有md5這個演算法！");
		}
		String md5code = new BigInteger(1, secretBytes).toString(16);// 16進位制數字
		// 如果生成數字未滿32位，需要前面補0
		for (int i = 0; i < 32 - md5code.length(); i++) {
			md5code = "0" + md5code;
		}
		return md5code;
	}
	public static void main(String[] args) {
		System.out.println(md5("1234"));
	}
	@Override
	public String encode(CharSequence charSequence) {
		return null;
	}
	@Override
	public boolean matches(CharSequence charSequence, String s) {
		return false;
	}
}