java如何生成sm2公钥pem文件

要生成SM2公钥PEM文件，需要使用Java的Bouncy Castle库。下面是一个示例代码，可以生成SM2公钥PEM文件：

import java.io.FileWriter;
 java.io.IOException;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.Security;

import org.bouncycastle.asn1.x500.X500Name;
import org.bouncycastle.cert.X509v3CertificateBuilder;
import org.bouncycastle.cert.jcajce.JcaX509CertificateConverter;
import org.bouncycastle.cert.jcajce.JcaX509v3CertificateBuilder;
import org.bouncycastle.crypto.generators.ECKeyPairGenerator;
import org.bouncycastle.crypto.params.ECKeyGenerationParameters;
import org.bouncycastle.crypto.params.ECPrivateKeyParameters;
import org.bouncycastle.crypto.params.ECPublicKeyParameters;
import org.bouncycastle.crypto.util.PrivateKeyFactory;
import org.bouncycastle.crypto.util.SubjectPublicKeyInfoFactory;
import org.bouncycastle.jce.provider.BouncyCastleProvider;
import org.bouncycastle.util.encoders.Hex;

public class SM2KeyPairGenerator {

    static {
        Security.addProvider(new BouncyCastleProvider());
    }

    public static void main(String[] args) throws Exception {

        // Generate SM2 key pair
        KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("EC", "BC");
        ECKeyGenerationParameters ecParams = new ECKeyGenerationParameters(SM2Curve.SM2P256V1_DOMAIN_PARAMS, null);
        ECKeyPairGenerator keyPairGen = new ECKeyPairGenerator();
        keyPairGen.init(ecParams);
        keyPairGenerator.initialize(ecParams);
        KeyPair keyPair = keyPairGenerator.generateKeyPair();
        ECPublicKeyParameters publicKey = (ECPublicKeyParameters) keyPair.getPublic();
        ECPrivateKeyParameters privateKey = (ECPrivateKeyParameters) keyPair.getPrivate();

        // Output public key in PEM format
        String pemFileName = "public_key.pem";
        FileWriter pemFileWriter = new FileWriter(pemFileName);
        pemFileWriter.write("-----BEGIN PUBLIC KEY-----\n");
        byte[] publicKeyBytes = SubjectPublicKeyInfoFactory.createSubjectPublicKeyInfo(publicKey).getEncoded();
        pemFileWriter.write(new String(Hex.encode(publicKeyBytes)).toUpperCase());
        pemFileWriter.write("\n-----END PUBLIC KEY-----\n");
        pemFileWriter.close();

        System.out.println("Public key generated: " + pemFileName);
    }
}

在上面的代码中，我们首先生成了一个SM2密钥对，然后将公钥转换为PEM格式并将其写入文件。这里我们使用了Bouncy Castle库提供的SubjectPublicKeyInfoFactory类来将ECPublicKeyParameters对象转换为SubjectPublicKeyInfo对象，然后将其编码为DER格式，最后将DER格式的公钥用Hex编码并写入PEM文件中。