20170704unable to find valid certification path to requested target

有时候用Java代码访问某些HTTPS网站时，会抛出以下异常信息：
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

原因是缺少访问这类网站的安全证书

解决方法是：将要访问的网站的安全证书导入到Java客户端

生成证书的class如下：

 * Copyright 2006 Sun Microsystems, Inc.  All Rights Reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * *   - Redistributions of source code must retain the above copyright *     notice, this list of conditions and the following disclaimer. * *   - Redistributions in binary form must reproduce the above copyright *     notice, this list of conditions and the following disclaimer in the *     documentation and/or other materials provided with the distribution. * *   - Neither the name of Sun Microsystems nor the names of its *     contributors may be used to endorse or promote products derived *     from this software without specific prior written permission. * * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */import java.io.BufferedReader;import java.io.File;import java.io.FileInputStream;import java.io.FileOutputStream;import java.io.InputStream;import java.io.InputStreamReader;import java.io.OutputStream;import java.security.KeyStore;import java.security.MessageDigest;import java.security.cert.CertificateException;import java.security.cert.X509Certificate;import javax.net.ssl.SSLContext;import javax.net.ssl.SSLException;import javax.net.ssl.SSLSocket;import javax.net.ssl.SSLSocketFactory;import javax.net.ssl.TrustManager;import javax.net.ssl.TrustManagerFactory;import javax.net.ssl.X509TrustManager;public class InstallCert {    public static void main(String[] args) throws Exception {        String host;        int port;        char[] passphrase;        if ((args.length == 1) || (args.length == 2)) {            String[] c = args[0].split(":");            host = c[0];            port = (c.length == 1) ? 443 : Integer.parseInt(c[1]);            String p = (args.length == 1) ? "changeit" : args[1];            passphrase = p.toCharArray();        } else {            System.out                    .println("Usage: java InstallCert <host>[:port] [passphrase]");            return;        }        File file = new File("jssecacerts");        if (file.isFile() == false) {            char SEP = File.separatorChar;            File dir = new File(System.getProperty("java.home") + SEP + "lib"                    + SEP + "security");            file = new File(dir, "jssecacerts");            if (file.isFile() == false) {                file = new File(dir, "cacerts");            }        }        System.out.println("Loading KeyStore " + file + "...");        InputStream in = new FileInputStream(file);        KeyStore ks = KeyStore.getInstance(KeyStore.getDefaultType());        ks.load(in, passphrase);        in.close();        SSLContext context = SSLContext.getInstance("TLS");        TrustManagerFactory tmf = TrustManagerFactory                .getInstance(TrustManagerFactory.getDefaultAlgorithm());        tmf.init(ks);        X509TrustManager defaultTrustManager = (X509TrustManager) tmf                .getTrustManagers()[0];        SavingTrustManager tm = new SavingTrustManager(defaultTrustManager);        context.init(null, new TrustManager[] { tm }, null);        SSLSocketFactory factory = context.getSocketFactory();        System.out                .println("Opening connection to " + host + ":" + port + "...");        SSLSocket socket = (SSLSocket) factory.createSocket(host, port);        socket.setSoTimeout(10000);        try {            System.out.println("Starting SSL handshake...");            socket.startHandshake();            socket.close();            System.out.println();            System.out.println("No errors, certificate is already trusted");        } catch (SSLException e) {            System.out.println();            e.printStackTrace(System.out);        }        X509Certificate[] chain = tm.chain;        if (chain == null) {            System.out.println("Could not obtain server certificate chain");            return;        }        BufferedReader reader = new BufferedReader(new InputStreamReader(                System.in));        System.out.println();        System.out.println("Server sent " + chain.length + " certificate(s):");        System.out.println();        MessageDigest sha1 = MessageDigest.getInstance("SHA1");        MessageDigest md5 = MessageDigest.getInstance("MD5");        for (int i = 0; i < chain.length; i++) {            X509Certificate cert = chain[i];            System.out.println(" " + (i + 1) + " Subject "                    + cert.getSubjectDN());            System.out.println("   Issuer  " + cert.getIssuerDN());            sha1.update(cert.getEncoded());            System.out.println("   sha1    " + toHexString(sha1.digest()));            md5.update(cert.getEncoded());            System.out.println("   md5     " + toHexString(md5.digest()));            System.out.println();        }        System.out                .println("Enter certificate to add to trusted keystore or 'q' to quit: [1]");        String line = reader.readLine().trim();        int k;        try {            k = (line.length() == 0) ? 0 : Integer.parseInt(line) - 1;        } catch (NumberFormatException e) {            System.out.println("KeyStore not changed");            return;        }        X509Certificate cert = chain[k];        String alias = host + "-" + (k + 1);        ks.setCertificateEntry(alias, cert);        OutputStream out = new FileOutputStream("jssecacerts");        ks.store(out, passphrase);        out.close();        System.out.println();        System.out.println(cert);        System.out.println();        System.out                .println("Added certificate to keystore 'jssecacerts' using alias '"                        + alias + "'");    }    private static final char[] HEXDIGITS = "0123456789abcdef".toCharArray();    private static String toHexString(byte[] bytes) {        StringBuilder sb = new StringBuilder(bytes.length * 3);        for (int b : bytes) {            b &= 0xff;            sb.append(HEXDIGITS[b >> 4]);            sb.append(HEXDIGITS[b & 15]);            sb.append(' ');        }        return sb.toString();    }    private static class SavingTrustManager implements X509TrustManager {        private final X509TrustManager tm;        private X509Certificate[] chain;        SavingTrustManager(X509TrustManager tm) {            this.tm = tm;        }        public X509Certificate[] getAcceptedIssuers() {            throw new UnsupportedOperationException();        }        public void checkClientTrusted(X509Certificate[] chain, String authType)                throws CertificateException {            throw new UnsupportedOperationException();        }        public void checkServerTrusted(X509Certificate[] chain, String authType)                throws CertificateException {            this.chain = chain;            tm.checkServerTrusted(chain, authType);        }    }}

将自己要访问的网站的域名（如 www.baidu.com）作为参数传给main函数，根据提示输入1 然后回车，会在当前工程目录下生成一个名为 jssecacerts的证书，将该证书拷贝到 $JAVA_HOME/jre/lib/security 目录下，或者通过
System.setProperty("javax.net.ssl.trustStore", "your jssecacerts path");让证书生效。