java如何读取sm2的pem文件获取公钥，并获取base64编码的公钥

要读取SM2的PEM格式文件并获取公钥，可以使用Bouncy Castle库中的SM2PublicKey类。

以下是Java代码示例：

import java.io.BufferedReader;
import java.io.FileReader;
import java.security.Security;

import org.bouncycastle.asn1.pkcs.PrivateKeyInfo;
import org.bouncycastle.crypto.params.ECPrivateKeyParameters;
import org.bouncycastle.crypto.params.ECPublicKeyParameters;
import org.bouncycastle.crypto.util.PrivateKeyInfoFactory;
import org.bouncycastle.crypto.util.PrivateKeyInfoParser;
import org.bouncycastle.crypto.util.PublicKeyFactory;
import org.bouncycastle.crypto.util.SubjectPublicKeyInfoFactory;
import org.bouncycastle.crypto.util.SubjectPublicKeyInfoParser;
import org.bouncycastle.jce.provider.BouncyCastleProvider;
import org.bouncycastle.util.io.pem.PemObject;
import org.bouncycastle.util.io.pem.PemReader;

public class SM2PublicKeyReader {

    public static void main(String[] args) throws Exception {
        // 添加Bouncy Castle提供程序
        Security.addProvider(new BouncyCastleProvider());

        // 读取PEM文件
        String pemFile = "sm2_public_key.pem";
        BufferedReader reader = new BufferedReader(new FileReader(pemFile));
        PemReader pemReader = new PemReader(reader);
        PemObject pemObject = pemReader.readPemObject();
        byte[] pemContent = pemObject.getContent();

        // 解析SubjectPublicKeyInfo
        SubjectPublicKeyInfoParser subjectPublicKeyInfoParser = new SubjectPublicKeyInfoParser(pemContent);
        ECPublicKeyParameters ecPublicKeyParameters = (ECPublicKeyParameters) PublicKeyFactory.createKey(subjectPublicKeyInfoParser.getSubjectPublicKeyInfo());

        // 获取公钥
        byte[] publicKeyBytes = ecPublicKeyParameters.getQ().getEncoded(true);
        String publicKeyBase64 = java.util.Base64.getEncoder().encodeToString(publicKeyBytes);
        System.out.println("Public key in base64: " + publicKeyBase64);
    }

}

这个示例代码假设PEM文件中只包含一个SM2公钥。如果PEM文件中包含多个公钥或私钥，则需要根据具体情况进行调整。