你用乘法得到y值是对的。我发现了一个非常方便的解决方案，它可以与本机java一起工作。
安全警告：示例程序没有异常处理，仅用于教育目的。
有一个愉快的周末！
下面是我的示例程序的（简短）结果：

Derive DSA PublicKey from PrivateKey
publicKey equals publicKeyDerived: true

完整代码：

import java.math.BigInteger;
import java.security.*;
import java.security.interfaces.DSAParams;
import java.security.interfaces.DSAPrivateKey;
import java.security.spec.DSAPublicKeySpec;
import java.util.Arrays;

public class DSA_RetrievePublicKeyFromPrivateKey {
    public static void main(String[] args) throws NoSuchProviderException, NoSuchAlgorithmException {
        System.out.println("Derive DSA PublicKey from PrivateKey");

        KeyPair keyPair = generateDsaKeyPair(2048);
        PublicKey publicKeyOriginal =  keyPair.getPublic(); // original for comparison
        PublicKey publicKeyDerived = deriveDsaPublicKeyFromPrivatekey(keyPair.getPrivate());
        System.out.println("publicKey equals publicKeyDerived: " + Arrays.equals(publicKeyOriginal.getEncoded(), publicKeyDerived.getEncoded()));
    }

    public static KeyPair generateDsaKeyPair(int keylengthInt)
            throws NoSuchAlgorithmException, NoSuchProviderException {
        KeyPairGenerator keypairGenerator = KeyPairGenerator.getInstance("DSA", "SUN");
        SecureRandom random = SecureRandom.getInstance("SHA1PRNG", "SUN");
        keypairGenerator.initialize(keylengthInt, random);
        return keypairGenerator.generateKeyPair();
    }

    public static PublicKey deriveDsaPublicKeyFromPrivatekey (PrivateKey privateKey) throws NoSuchAlgorithmException {
        DSAPrivateKey dsaPrivateKey = (DSAPrivateKey) privateKey;
        DSAParams params = dsaPrivateKey.getParams();
        BigInteger y = params.getG().modPow(dsaPrivateKey.getX(), params.getP());
        DSAPublicKeySpec keySpec = new DSAPublicKeySpec(y, params.getP(), params.getQ(), params.getG());
        PublicKey publicKey;
        KeyFactory keyFactory = KeyFactory.getInstance("DSA");
        try {
            publicKey = keyFactory.generatePublic(keySpec);
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
        return publicKey;
    }
}