给出一个java客户端代码例子，这个客户端程序向REST服务器发送HTTPS请求，客户端和服务器端实现TLS双向认证。

import java.util.Map;
import java.util.HashMap;
import java.util.Map.Entry;
import java.util.Set;

import java.net.HttpURLConnection;
import java.net.URL;
import java.security.KeyStore;
import java.security.cert.CertificateException;
import java.security.cert.X509Certificate;
import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.StringReader;
import java.io.InputStreamReader;
import java.io.InputStream;

import javax.net.ssl.HttpsURLConnection;
import javax.net.ssl.KeyManagerFactory;
import javax.net.ssl.SSLContext;
import javax.net.ssl.SSLSocketFactory;
import javax.net.ssl.TrustManager;
import javax.net.ssl.TrustManagerFactory;
import javax.net.ssl.X509TrustManager;

public class Test {
    private String serverUrl = "https://10.20.xxx.xxx:8080/api/v1.1/service/test";
    private SSLSocketFactory sslFactory = null;

    public void run() {
        try {
            String requestBody ="{\"instance\":\"instance\",\"list\":[{\"id\":\"id\",\"status\":\"OK\",\"action\":\"UPDATE\"}]}";
            HttpURLConnection connection = doHttpRequest(serverUrl, "PUT", requestBody, null);
            int responseCode = getResponseCode(connection);
            String responseBody = getResponseBodyAsString(connection);
            connection.disconnect();
            System.out.println("response code=" + responseCode + ", body=[" + responseBody + "]");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        Test test = new Test();
        test.run();
    }

    private synchronized SSLSocketFactory getSSLFactory() throws Exception {
        if (sslFactory == null) {
            SSLContext sslContext = SSLContext.getInstance("SSL");
            TrustManager[] tm = { new MyX509TrustManager() };

            KeyStore truststore = KeyStore.getInstance("JKS");
            truststore.load(new FileInputStream("cert.jks"), "123456".toCharArray());
            KeyManagerFactory kmf = KeyManagerFactory.getInstance("SunX509");
            kmf.init(truststore, "123456".toCharArray());

            sslContext.init(kmf.getKeyManagers(), tm, new java.security.SecureRandom());
            sslFactory = sslContext.getSocketFactory();
        }
        return sslFactory;
    }

    private HttpURLConnection doHttpRequest(String requestUrl, String method, String body, Map<String, String> header) throws Exception {
        HttpURLConnection conn;

        if (method == null || method.length() == 0 ) {
            method = "GET";
        }
        if ("GET".equals(method) && body != null && ! body.isEmpty()) {
            requestUrl = requestUrl + "?" + body;
        }

        URL url = new URL(requestUrl);
        conn = (HttpURLConnection) url.openConnection();

        conn.setDoOutput(true);
        conn.setDoInput(true);
        conn.setUseCaches(false);
        conn.setInstanceFollowRedirects(true);
        conn.setRequestMethod(method);

        if (requestUrl.matches("^(https?)://.*$")){
            ((HttpsURLConnection) conn).setSSLSocketFactory(this.getSSLFactory());
        }

        if (header != null) {
            for (String key : header.keySet()) {
                conn.setRequestProperty(key, header.get(key));
            }
        }

        if (body != null && ! body.isEmpty()) {
            if (!method.equals("GET") ) {
                OutputStreamWriter wr = new OutputStreamWriter(conn.getOutputStream());
                wr.write(body);
                wr.close();
            }
        }
        conn.connect();
        return conn;
    }

    public int getResponseCode(HttpURLConnection connection) throws IOException {
        return connection.getResponseCode();
    }

    public String getResponseBodyAsString(HttpURLConnection connection) throws Exception {
        BufferedReader reader = null;
        if (connection.getResponseCode() == 200) {
            reader = new BufferedReader(new InputStreamReader(connection.getInputStream()));
        } else {
            reader = new BufferedReader(new InputStreamReader(connection.getErrorStream()));
        }

        StringBuffer buffer = new StringBuffer();
        String line=null;
        while ((line = reader.readLine()) != null) {
            buffer.append(line);
        }
        return buffer.toString();
    }

    class MyX509TrustManager implements X509TrustManager {
        private X509TrustManager sunJSSEX509TrustManager;

        MyX509TrustManager() throws Exception {
            // create a "default" JSSE X509TrustManager.
            KeyStore ks = KeyStore.getInstance("JKS");
            ks.load(new FileInputStream("ca.jks"), "123456".toCharArray());
            TrustManagerFactory tmf = TrustManagerFactory.getInstance("SunX509", "SunJSSE");
            tmf.init(ks);
            TrustManager tms[] = tmf.getTrustManagers();

            /*
             * Iterate over the returned trustmanagers, look for an instance of
             * X509TrustManager. If found, use that as our "default" trust manager.
             */
            for (int i = 0; i < tms.length; i++) {
                if (tms[i] instanceof X509TrustManager) {
                    sunJSSEX509TrustManager = (X509TrustManager) tms[i];
                    return;
                }
            }
            throw new Exception("Couldn't initialize");
        }

        @Override
        public void checkClientTrusted(X509Certificate[] chain, String authType) throws CertificateException {
            try {
                sunJSSEX509TrustManager.checkClientTrusted(chain, authType);
            } catch (CertificateException excep) {
            }
        }

        @Override
        public void checkServerTrusted(X509Certificate[] chain, String authType) throws CertificateException {
            try {
                sunJSSEX509TrustManager.checkServerTrusted(chain, authType);
            } catch (CertificateException excep) {
            }
        }

        @Override
        public X509Certificate[] getAcceptedIssuers() {
            return sunJSSEX509TrustManager.getAcceptedIssuers();
        }
    }
}

准备客户端证书

使用openssl生成PEM格式证书

包含三个文件：
ca.pem // CA证书
cert.pem // 客户端证书
key.pem // 客户端私钥

使用keytool生成JKS格式证书

使用JDK自带的keytool工具从PEM格式证书生成JKS格式证书

$ keytool -import -noprompt -file ca.pem -keystore ca.jks -storepass 123456
$ openssl pkcs12 -export -in cert.pem -inkey key.pem -out cert.p12 -passout pass:123456
$ keytool -importkeystore -srckeystore cert.p12 -srcstoretype PKCS12 -srcstorepass 123456 -deststorepass 123456} -destkeystore cert.jks

工具keytool 一般在${JDK_HOME}/bin目录下面；"123456"是随便取的密码。

最终生成两个需要的JKS格式证书。
ca.jks # CA证书
cert.jks # 客户端证书

其实是我一直没有找到java如何直接加载PEM格式证书；如果能加载PEM格式证书，就不需要转换成JKS格式。

编译运行

javac Test.java && java Test