How to invoke APIs from Java using https client with ssl certificate

I am invoking rest API from Java file using HttpClient. By using that I am able to call http API but not https API.

I am getting below error, while calling httpsapi.

javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
        at sun.security.ssl.Alerts.getSSLException(Alerts.java:192)
        at sun.security.ssl.SSLSocketImpl.fatal(SSLSocketImpl.java:1946)
        at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:316)
        at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:310)
        at sun.security.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1639)
        at sun.security.ssl.ClientHandshaker.processMessage(ClientHandshaker.java:223)
        at sun.security.ssl.Handshaker.processLoop(Handshaker.java:1037)
        at sun.security.ssl.Handshaker.process_record(Handshaker.java:965)
        at sun.security.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:1064)
        at sun.security.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1367)
        at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1395)
        at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1379)

I want to call https API using CloseableHttpClient.

I am having certificate file that is having private key. Please let me know how I can use this private key to call API?

To call API using https you need to configure SSLContext and set it to your HttpClient. Refer below sample code. This is just the sample, you can load keystore and truststore in different way like from classpath, form file system etc.., make the changes accordingly.

    KeyStore trustStore = KeyStore.getInstance(KeyStore.getDefaultType());

    KeyStore identity = KeyStore.getInstance(KeyStore.getDefaultType());
    TrustManagerFactory trustManagerFactory = TrustManagerFactory.getInstance(TrustManagerFactory.getDefaultAlgorithm());
    trustManagerFactory.init(trustStore);
    KeyManagerFactory keyManagerFactory = KeyManagerFactory.getInstance(KeyManagerFactory.getDefaultAlgorithm());
    keyManagerFactory.init(identity, "password".toCharArray());
    SSLContext sslContext = SSLContext.getInstance("TLSv1.3");
    sslContext.init(
        keyManagerFactory.getKeyManagers(), 
        trustManagerFactory.getTrustManagers(), 
        null
    );
    HttpClient httpClient = HttpClients.custom()
            .setSSLContext(sslContext)
            .setSSLHostnameVerifier(new DefaultHostnameVerifier())
            .build();