要想实现网络传输，需要考虑哪些问题

1. 如何准确地定位到网络上的一台主机
2. 如何才能进行可靠高效的传输

Java如何实现网络通信

1. 使用IP地址定位主机，使用端口号定位主机上的应用 ---> InetAddress类
2. InetAddress类对象使用

   InetAddress inet = InetAddress.getByName(""127.0.0.1);
   inet.getLocalHost();//获取本机的InetAddress对象
   inet.getHostName();//获取域名
   inet.getHostAddress();//获取主机地址
   

3. 如何实现通信 ---> TCP/UDP

   public class TestUDP {
       @Test
       public void send(){
           DatagramSocket ds = null;
           try {
               ds = new DatagramSocket();
               byte[] b = "data to be send".getBytes();
               //创建一个数据报，每个数据报不能大于64k，每个都记录着数据信息+发送端IP及端口号+接收端IP及端口号
               DatagramPacket packet = new DatagramPacket(b,0, b.length,
                       InetAddress.getByName("127.0.0.1"), 9090);
   
               ds.send(packet);
           } catch (IOException e) {
               e.printStackTrace();
           }finally{
               if (ds != null)
                   ds.close();
           }
   
       }
   
       @Test
       public void receive(){
           DatagramSocket ds = null;
           try {
               ds = new DatagramSocket(9090);
               byte[] b = new byte[1024];
               DatagramPacket packet = new DatagramPacket(b, 0, b.length);
               ds.receive(packet);
               String str = new String(packet.getData(), 0,packet.getLength() );
               System.out.println(str);
           } catch (IOException e) {
               e.printStackTrace();
           } finally {
               ds.close();
           }
       }
   }
   

   //TCP编程例一：客户端给服务端发送信息，服务端输出此信息到控制台上
   public class TestTCP1 {
       //client
       @Test
       public void client() {
           Socket socket = null;
           OutputStream os = null;
           try {
               //1.创建一个Socket的对象，通过构造器指明服务端的IP地址+端口号
               socket = new Socket(InetAddress.getByName("127.0.0.1"), 9090);
               //2. getOutputStream()，发送数据，该方法返回一个OutputStream对象
               os = socket.getOutputStream();
               //3.具体的输出过程
               os.write("I am client".getBytes());
           } catch (IOException e) {
               e.printStackTrace();
           }finally {
               //4.关闭相应的流
               if(os != null)
                   try {
                       os.close();
                   } catch (IOException e) {
                       e.printStackTrace();
                   }
               if(socket != null)
                   try {
                       socket.close();
                   } catch (IOException e) {
                       e.printStackTrace();
                   }
           }
       }
   
       //server
       @Test
       public void server() {
           ServerSocket ss = null;
           InputStream is = null;
           Socket s = null;
           try {
               //1. 创建serversocket对象，通过构造器指明自身的端口号
               ss = new ServerSocket(9090);
               //2.调用accept方法返回一个socket对象
               s = ss.accept();
               //3.调用socket对象的getInputStream方法获取同客户端发送过来的输入流
               is = s.getInputStream();
               //4 对获取的流进行的具体操作
               byte[] b = new byte[20];
               int len;
               while((len = is.read(b)) != -1){
                   String str = new String(b,0,len);
                   System.out.print(str);
               }
               System.out.println("\n receive message from " + s.getInetAddress().getHostAddress());
           } catch (IOException e) {
               e.printStackTrace();
           }finally {
               //5.关闭相应的流、socket、serversocket的对象
               if(is != null)
                   try {
                       is.close();
                   } catch (IOException e) {
                       e.printStackTrace();
                   }
               if(s != null)
                   try {
                       s.close();
                   } catch (IOException e) {
                       e.printStackTrace();
                   }
               if(ss != null)
                   try {
                       ss.close();
                   } catch (IOException e) {
                       e.printStackTrace();
                   }
           }
       }
   }
   

4. URL编程

   public class TestURL {
       public static void main(String[] args) {
           InputStream is = null;
           URL url = null;
           try {
               url = new URL("https://www.runoob.com/java/java-collections.html");//类似于File类的初始化
   //        url.getProtocol();//获取该URL的协议名
   //        url.getHost();//获取该URL的主机名
   //        url.getPort();//获取该URL的端口号
   //        url.getPath();//获取该URL的文件路径
   //        url.getFile();//获取该URL的文件名
   //        url.getRef();//获取该URL在文件中的相对位置
   //        url.getQuery();//获取该URL的查名
               //如何将服务端数据读入:openStream()
               is = url.openStream();
               byte[] b = new byte[20];
               int len;
               while ((len = is.read(b)) != -1) {
                   String str = new String(b, 0, len);
                   System.out.println(str);
               }
           } catch (IOException e) {
               e.printStackTrace();
           } finally {
               try {
                   if (is != null)
                       is.close();
               } catch (IOException e) {
                   e.printStackTrace();
               }
           }
   
           //如果既有数据输入又有数据输出，考虑使用URLConnection
           InputStream is1 = null;
           FileOutputStream fos = null;
           try {
               URLConnection urlConnection = url.openConnection();
               is1 = urlConnection.getInputStream();
               fos = new FileOutputStream(new File("abc.txt"));
               byte[] b = new byte[20];
               int len1;
               while((len1 = is1.read(b))!=-1){
                   fos.write(b,0,len1);
               }
           } catch (IOException e) {
               e.printStackTrace();
           }finally {
               try {
                   if(fos != null)
                       fos.close();
                   if(is1 != null)
                       is1.close();
               } catch (IOException e) {
                   e.printStackTrace();
               }
           }
   
       }
   }