下面是一个使用Java实现AES 256 GCM加密的示例代码：

import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import javax.crypto.spec.GCMParameterSpec;
import javax.crypto.spec.SecretKeySpec;
import java.security.SecureRandom;

public class AESGCMExample {

    private static final int GCM_TAG_LENGTH = 128;
    private static final int GCM_IV_LENGTH = 12;
    private static final int AES_KEY_SIZE = 256;

    public static void main(String[] args) throws Exception {
        // 生成随机密钥
        SecretKey secretKey = generateRandomKey();

        // 加密
        byte[] ciphertext = encrypt(secretKey, "Hello, World!");

        // 解密
        String plaintext = decrypt(secretKey, ciphertext);

        System.out.println("Plaintext: " + plaintext);
    }

    public static SecretKey generateRandomKey() throws Exception {
        KeyGenerator keyGenerator = KeyGenerator.getInstance("AES");
        keyGenerator.init(AES_KEY_SIZE);
        return keyGenerator.generateKey();
    }

    public static byte[] encrypt(SecretKey secretKey, String plaintext) throws Exception {
        // 生成随机IV
        byte[] iv = new byte[GCM_IV_LENGTH];
        SecureRandom secureRandom = new SecureRandom();
        secureRandom.nextBytes(iv);

        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
        GCMParameterSpec gcmParameterSpec = new GCMParameterSpec(GCM_TAG_LENGTH, iv);
        cipher.init(Cipher.ENCRYPT_MODE, secretKey, gcmParameterSpec);

        byte[] plaintextBytes = plaintext.getBytes();
        byte[] ciphertext = cipher.doFinal(plaintextBytes);

        // 将IV与密文组合在一起
        byte[] ivAndCiphertext = new byte[iv.length + ciphertext.length];
        System.arraycopy(iv, 0, ivAndCiphertext, 0, iv.length);
        System.arraycopy(ciphertext, 0, ivAndCiphertext, iv.length, ciphertext.length);

        return ivAndCiphertext;
    }

    public static String decrypt(SecretKey secretKey, byte[] ivAndCiphertext) throws Exception {
        // 提取IV和密文
        byte[] iv = new byte[GCM_IV_LENGTH];
        byte[] ciphertext = new byte[ivAndCiphertext.length - GCM_IV_LENGTH];
        System.arraycopy(ivAndCiphertext, 0, iv, 0, iv.length);
        System.arraycopy(ivAndCiphertext, iv.length, ciphertext, 0, ciphertext.length);

        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
        GCMParameterSpec gcmParameterSpec = new GCMParameterSpec(GCM_TAG_LENGTH, iv);
        cipher.init(Cipher.DECRYPT_MODE, secretKey, gcmParameterSpec);

        byte[] plaintextBytes = cipher.doFinal(ciphertext);
        return new String(plaintextBytes);
    }
}

此示例使用javax.crypto包中的Cipher类来执行AES加密和解密操作。在加密过程中，随机生成一个12字节的IV并与密文一起存储，以便在解密时使用。密钥长度为256位。加密算法使用AES-GCM模式，不使用填充。在解密过程中，从密文中提取出IV并使用它来初始化Cipher对象，然后执行解密操作。最后，将解密后的明文作为字符串返回。

请注意，此示例代码仅供参考，并且可能需要根据您的具体需求进行适当的修改和优化。