keytool -genkey -dname "CN=demo, OU=softDept, O=company, L=puddong,S=shanghai, C=cn" -alias demo -keyalg RSA -keysize 1024 -keystore demoKeystore -validity 3650 -storepass storePwd -keypass demoPwd
生成保存公钥和私钥的密钥仓库，保存在demoKeystore文件中。这里storepass 和 keypass 不要有java 正则表达式中的特殊字符，否则程序里要转义麻烦。

keytool -export -alias demo -keystore demoKeystore -rfc -file demo.cer //从密钥仓库中导出保存公钥的证书
输入keypass 即demoPwd 


  try{     
   //密钥仓库
   KeyStore ks = KeyStore.getInstance("JKS");
//读取密钥仓库
   FileInputStream ksfis = new FileInputStream("demoKeystore");
   BufferedInputStream ksbufin = new BufferedInputStream(ksfis);
   char[] storePwd = "storePwd".toCharArray();
   ks.load(ksbufin, storePwd);
   ksbufin.close();
   char[] keyPwd = "demoPwd".toCharArray();
//从密钥仓库得到私钥
   PrivateKey priK = (PrivateKey) ks.getKey("demo", keyPwd);  
//生成cipher
   Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding",new org.bouncycastle.jce.provider.BouncyCastleProvider());
//用私钥初始化cipher
   cipher.init(Cipher.ENCRYPT_MODE, priK);
   byte[] plain = "This is plain text".getBytes("UTF-8");
   
   //因为用的1024位rsa算法，一次只能加密1024/8-11字节数据，分开加密
   byte[] code = new byte[(((plain.length-1)/117+1))*128];  
            int ixplain = 0;
            int ixcode = 0;
            while((plain.length - ixplain) > 117) {//每117字节做一次加密
                ixcode += cipher.doFinal(plain, ixplain, 117, code, ixcode);
                ixplain += 117;
            }
            cipher.doFinal(plain, ixplain, plain.length - ixplain, code, ixcode);
            //加密后的code
            System.out.println(Arrays.toString(code));
            //通常会用base64编码
           String base64 = encoder.encode(code);

   CertificateFactory certificatefactory = CertificateFactory
     .getInstance("X.509");
   //读取证书
   FileInputStream fin = new FileInputStream("demo.cer");
   X509Certificate certificate = (X509Certificate) certificatefactory
     .generateCertificate(fin);
   fin.close();
   //得到公钥
   PublicKey pubK = certificate.getPublicKey();
         //初始化cipher
            cipher.init(Cipher.DECRYPT_MODE, pubK);
      //base64解码
            code = decoder.decodeBuffer(base64);
            System.out.println(Arrays.toString(code));
            byte[] plain2 = new byte[code.length];
            int ixplain2 = 0;
            int ixcode2 = 0;
            while((code.length - ixcode2) > 128) {//每128字节做一次解密
                ixplain2 += cipher.doFinal(code, ixcode2, 128, plain2, ixplain2);
                ixcode2 += 128;
            }
            ixplain2 += cipher.doFinal(code, ixcode2, code.length - ixcode2, plain2, ixplain2);
            String s2 = new String(plain2, 0, ixplain2, "UTF-8");
            System.out.println(s2);
   
  }catch(Exception ex){
   ex.printStackTrace();
  }

keytool使用方法可以参考jdk文档
Java keytool工具的作用及使用方法