如果要尝试编写ssl配置代码，可以使用提供给您的p12文件，而无需将其转换为jks。此外，您还需要在p12中使用私钥，而不仅仅是复制到jks中的证书。不确定这是否能直接满足您的需求，但这可能会让您走上正确的道路：

KeyStore clientStore = KeyStore.getInstance("PKCS12");
        clientStore.load(new FileInputStream("test.p12"), "testPass".toCharArray());

        KeyManagerFactory kmf = KeyManagerFactory.getInstance(KeyManagerFactory.getDefaultAlgorithm());
        kmf.init(clientStore, "testPass".toCharArray());
        KeyManager[] kms = kmf.getKeyManagers();

        KeyStore trustStore = KeyStore.getInstance("JKS");
        trustStore.load(new FileInputStream("cacerts"), "changeit".toCharArray());

        TrustManagerFactory tmf = TrustManagerFactory.getInstance(TrustManagerFactory.getDefaultAlgorithm());
        tmf.init(trustStore);
        TrustManager[] tms = tmf.getTrustManagers();

        SSLContext sslContext = null;
        sslContext = SSLContext.getInstance("TLS");
        sslContext.init(kms, tms, new SecureRandom());

        HttpsURLConnection.setDefaultSSLSocketFactory(sslContext.getSocketFactory());
        URL url = new URL("https://www.testurl.com");

        HttpsURLConnection urlConn = (HttpsURLConnection) url.openConnection();

以这种方式配置信任库是可选的。您可以使用p12链中的所有证书创建一个jks，或者确保它们位于jre的cacerts文件中。至于keytool，作为参考，您可以在p12上运行keytool命令（指定-storetype pkcs12），但不能将p12导入jks。也不能使用keytool命令仅从p12导出密钥。
我目前没有服务器设置来测试这段代码，所以试一试，看看你是否仍然收到403错误。

添加这个作为答案，因为我需要更多的空间来写。
首先，一个问题：证书是否由可信机构（如verisign）签名？如果不是，信任库应该有ca证书（通常是一个.pem文件），使p12证书“有效”。默认的java信任存储包含来自大公司（如verisign和thawte）的大部分（如果不是全部的话）ca证书。
此外，您可以测试应用程序以连接到安全服务器，而无需对ssl配置进行编码，但需要使用一些命令行参数，例如：

java -Djavax.net.ssl.keyStore=[path_to_p12_cert] \
 -Djavax.net.ssl.keyStorePassword=[p12_password] \
 -Djavax.net.ssl.keyStoreType=PKCS12 \
 -Djavax.net.ssl.trustStore=[path_to_trust_store_with_CA_certificates] \
 -Djavax.net.ssl.trustStorePassword=[trust_store_password] \
 [MainClass]

然后你的代码就变成了

HttpsURLConnection con = (HttpsURLConnection) (new URL(urlString)).openConnection();
con.connect();
con.getInputStream();
con.disconnect();

如果你觉得受虐狂，jsse参考指南是非常有趣的。

这就是我的工作原理：

KeyStore keyStore  = KeyStore.getInstance("PKCS12");
    FileInputStream instream = new FileInputStream(new File("client-p12-keystore.p12"));
    try {
        keyStore.load(instream, "password".toCharArray());
    } finally {
        instream.close();
    }

    // Trust own CA and all self-signed certs
    SSLContext sslcontext = SSLContexts.custom()
        .loadKeyMaterial(keyStore, "password".toCharArray())
        //.loadTrustMaterial(trustStore, new TrustSelfSignedStrategy())
        .build();
    // Allow TLSv1 protocol only
    SSLConnectionSocketFactory sslsf = new SSLConnectionSocketFactory(
        sslcontext,
        SSLConnectionSocketFactory.ALLOW_ALL_HOSTNAME_VERIFIER);
    CloseableHttpClient httpclient = HttpClients.custom()
        .setHostnameVerifier(SSLConnectionSocketFactory.ALLOW_ALL_HOSTNAME_VERIFIER)
        .setSSLSocketFactory(sslsf)
        .build();
    try {

        HttpGet httpget = new HttpGet("https://localhost:8443/secure/index");

        System.out.println("executing request" + httpget.getRequestLine());

        CloseableHttpResponse response = httpclient.execute(httpget);
        try {
            HttpEntity entity = response.getEntity();

            System.out.println("----------------------------------------");
            System.out.println(response.getStatusLine());
            if (entity != null) {
                System.out.println("Response content length: " + entity.getContentLength());
            }
            EntityUtils.consume(entity);
        } finally {
            response.close();
        }
    } finally {
        httpclient.close();
    }
}

如果您使用的是spring，则可以通过resttemplate访问它：

public RestTemplate restTemplate() throws Exception {
    KeyStore clientStore = KeyStore.getInstance("PKCS12");
    InputStream resource = this.getClass().getClassLoader().getResourceAsStream("path_to_certificate.p12");
    clientStore.load(resource, "p12_password".toCharArray());

    SSLContextBuilder sslContextBuilder = new SSLContextBuilder();
    sslContextBuilder.setProtocol("TLS");
    sslContextBuilder.loadKeyMaterial(clientStore, "p12_password".toCharArray());
    sslContextBuilder.loadTrustMaterial(new TrustSelfSignedStrategy());

    SSLConnectionSocketFactory sslConnectionSocketFactory = new SSLConnectionSocketFactory(sslContextBuilder.build());
    CloseableHttpClient httpClient = HttpClients.custom()
        .setSSLSocketFactory(sslConnectionSocketFactory)
        .build();
    HttpComponentsClientHttpRequestFactory requestFactory = new HttpComponentsClientHttpRequestFactory(httpClient);
    return new RestTemplate(requestFactory);
}

简单的keytool命令会将.p12密钥库导出到.jks密钥库： keytool -importkeystore -srckeystore keystore.p12 -srcstoretype PKCS12 -deststoretype JKS -destkeystore keystore.jks