以下是使用Java进行AES 256 CBC模式加密字符串并解密的代码示例：

import javax.crypto.Cipher;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;
import java.nio.charset.StandardCharsets;
import java.util.Base64;

public class AESCBCExample {

    private static final String AES_ALGORITHM = "AES/CBC/PKCS5Padding";
    private static final String AES_KEY = "0123456789abcdef0123456789abcdef";
    private static final String AES_IV = "abcdef9876543210";

    public static void main(String[] args) {
        String plaintext = "Hello World";

        String encryptedText = encrypt(plaintext);
        System.out.println("Encrypted Text: " + encryptedText);

        String decryptedText = decrypt(encryptedText);
        System.out.println("Decrypted Text: " + decryptedText);
    }

    public static String encrypt(String plaintext) {
        try {
            SecretKeySpec secretKey = new SecretKeySpec(AES_KEY.getBytes(), "AES");
            IvParameterSpec iv = new IvParameterSpec(AES_IV.getBytes());

            Cipher cipher = Cipher.getInstance(AES_ALGORITHM);
            cipher.init(Cipher.ENCRYPT_MODE, secretKey, iv);

            byte[] encryptedBytes = cipher.doFinal(plaintext.getBytes(StandardCharsets.UTF_8));
            return Base64.getEncoder().encodeToString(encryptedBytes);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

    public static String decrypt(String encryptedText) {
        try {
            SecretKeySpec secretKey = new SecretKeySpec(AES_KEY.getBytes(), "AES");
            IvParameterSpec iv = new IvParameterSpec(AES_IV.getBytes());

            Cipher cipher = Cipher.getInstance(AES_ALGORITHM);
            cipher.init(Cipher.DECRYPT_MODE, secretKey, iv);

            byte[] encryptedBytes = Base64.getDecoder().decode(encryptedText);
            byte[] decryptedBytes = cipher.doFinal(encryptedBytes);
            return new String(decryptedBytes, StandardCharsets.UTF_8);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }
}

在上面的示例中，使用AES/CBC/PKCS5Padding算法来加密和解密字符串。AES密钥为256位，由32个十六进制字符组成。初始向量（IV）为128位，由16个十六进制字符组成。

请注意，加密和解密使用相同的密钥和初始向量。确保在实际使用时使用不同的密钥和初始向量来确保安全性。