以下代码将CA证书文件yourcert.cer插入到您的密钥存储库中，而不使用keytool：

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.security.Key;
import java.security.KeyStore;
import java.security.cert.Certificate;
import java.io.IOException;
import java.io.InputStream;
import java.io.DataInputStream;
import java.io.ByteArrayInputStream;
import java.security.spec.*;
import java.security.cert.Certificate;
import java.security.cert.CertificateFactory;
import java.util.Collection;

public class ImportCA {

    public static void main(String[] argv) throws Exception {
        String certfile = "yourcert.cer"; /*your cert path*/
        FileInputStream is = new FileInputStream("yourKeyStore.keystore");

        KeyStore keystore = KeyStore.getInstance(KeyStore.getDefaultType());
        keystore.load(is, "yourKeyStorePass".toCharArray());

        String alias = "youralias";
        char[] password = "yourKeyStorePass".toCharArray();

        //////

        CertificateFactory cf = CertificateFactory.getInstance("X.509");
        InputStream certstream = fullStream (certfile);
        Certificate certs =  cf.generateCertificate(certstream);

        ///
        File keystoreFile = new File("yourKeyStorePass.keystore");
        // Load the keystore contents
        FileInputStream in = new FileInputStream(keystoreFile);
        keystore.load(in, password);
        in.close();

        // Add the certificate
        keystore.setCertificateEntry(alias, certs);

        // Save the new keystore contents
        FileOutputStream out = new FileOutputStream(keystoreFile);
        keystore.store(out, password);
        out.close();
    }

    private static InputStream fullStream ( String fname ) throws IOException {
        FileInputStream fis = new FileInputStream(fname);
        DataInputStream dis = new DataInputStream(fis);
        byte[] bytes = new byte[dis.available()];
        dis.readFully(bytes);
        ByteArrayInputStream bais = new ByteArrayInputStream(bytes);
        return bais;
    }
}

对不起，这个答案没有带来什么新的，但在接受的答案代码是如此可怕，我只是要张贴。这只是一个完美的版本，仅此而已。因此，考虑从这里复制/粘贴，但对接受的答案投赞成票，而不是这个答案。

public static void addX509CertificateToTrustStore(String certPath, String certAlias, String storePath, String storePassword, String storeType)
            throws FileNotFoundException, KeyStoreException, CertificateException, IOException, NoSuchAlgorithmException {

        char[] storePasswordCharArr = Objects.requireNonNull(storePassword, "").toCharArray();

        KeyStore keystore;
        try (FileInputStream storeInputStream = new FileInputStream(storePath);
                FileInputStream certInputStream = new FileInputStream(certPath)) {
            keystore = KeyStore.getInstance(storeType);
            keystore.load(storeInputStream, storePasswordCharArr);

            CertificateFactory certificateFactory = CertificateFactory.getInstance("X.509");
            Certificate certificate = certificateFactory.generateCertificate(certInputStream);

            keystore.setCertificateEntry(certAlias, certificate);
        } finally {
        }

        try (FileOutputStream storeOutputStream = new FileOutputStream(storePath)) {
            keystore.store(storeOutputStream, storePasswordCharArr);
        } finally {
        }
    }

从链接下载证书并存储到特定路径。然后在运行时使用下面的代码将该文件加载到trustStore中。我希望这个例子对你有帮助。

String filename = "D:\\certs_path\\cacerts"; // cerrtification file path
System.setProperty("javax.net.ssl.trustStore", fileName);