Java基础练习题 （7）网络编程

（1）如何使用Socket和ServerSocket？

• Socket

部分构造方法

Socket()Socket(InetAddress address, int port)Socket(InetAddress address, int port, InetAddress localAddr, int localPort)

Socket 的构造方法中除了无参的构造方法，其他方法都会尝试与给定的服务器地址建立连接，失败会抛出异常。InetAddress 类用来表示一个 ip 地址。

部分方法

bind(SocketAddress bindpoint) //绑定 Socket 到指定的地址connect(SocketAddress endpoint) //连接至指定的服务器getInputStream()getOutputStream()

getInputStream() 跟 getOutputStream() 方法获取的输入输出流可以用来跟服务器通信，比较重要的方法。

• ServerSocket

部分构造方法

ServerSocket()ServerSocket(int port)

可以在创建 ServerSocket 的时候指定监听的端口，也可以在之后绑定。

部分方法

accept()bind(SocketAddress endpoint)

bind() 方法使 ServerSocket 绑定指定地址。
accept() 方法返回一个连接队列中的一个 Socket。

一个很简单的多线程例子

public class App {  public static void main(String[] args) {    Thread client = new Thread(new Client());    Thread server = new Thread(new Server());    client.start();    server.start();  }}

public class Client implements Runnable {  @Override  public void run() {    Socket client;    try {      client = new Socket("127.0.0.1", 3666);      System.out.println("客户端链接---");      BufferedWriter out = new BufferedWriter(new OutputStreamWriter(client.getOutputStream()));      Scanner scan = new Scanner(System.in);      while(true) {        out.write(scan.nextLine() + "\n");        out.flush();      }    } catch (UnknownHostException e) {      e.printStackTrace();    } catch (IOException e) {      e.printStackTrace();    }  }}

public class Server implements Runnable {  @Override  public void run() {    ServerSocket socket;    SocketAddress address;    try {      address = new InetSocketAddress("127.0.0.1", 3666);      socket = new ServerSocket();      socket.bind(address);      System.out.println("服务端启动服务---> ip: " + socket.getLocalSocketAddress() + " port: " + socket.getLocalPort());      Socket client = socket.accept();      BufferedReader in = new BufferedReader(new InputStreamReader(client.getInputStream()));      while(true) {        System.out.println("客户端输入---> " + in.readLine());      }    } catch (IOException e) {      e.printStackTrace();    }  }}

（2）如何通过Socket发送和接收文件？
相当于将文件用字节流来传输，跟传文本类似

服务器端代码

ServerSocket socket;SocketAddress address;String filePath = "E:\\demo\\test.txt";File file = new File(filePath);try {  BufferedInputStream in = new BufferedInputStream(new FileInputStream(file));  address = new InetSocketAddress("127.0.0.1", 3666);  socket = new ServerSocket();  socket.bind(address);  System.out.println("服务端启动服务---> ip: " + socket.getLocalSocketAddress() + " port: " + socket.getLocalPort());  Socket client = socket.accept();  System.out.println("客户端连接成功！开始传输---");  DataOutputStream out = new DataOutputStream(client.getOutputStream());  byte[] b = new byte[1024];  while (in.read(b, 0, b.length) != -1) {    out.write(b);  }  out.flush();  System.out.println("传输完成！");  in.close();  out.close();  client.close();} catch (IOException e) {  e.printStackTrace();}

客户端代码

Socket client;String filePath = "E:\\demo\\test2.txt";File file = new File(filePath);try {  client = new Socket("127.0.0.1", 3666);  System.out.println("客户端链接---");  DataInputStream in  = new DataInputStream(client.getInputStream());  BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream(file));  byte[] b = new byte[1024];  while (in.read(b, 0, b.length) != -1) {    out.write(b);  }  out.flush();  System.out.println("客户端接收完成！");  in.close();  out.close();} catch (UnknownHostException e) {  e.printStackTrace();} catch (IOException e) {  e.printStackTrace();}

（3）如何使用URLConnection下载一个文件？
java.net.URLConnection 类是一个抽象类，我们可以通过 URL 类的 openConnection 获取 URLConnection 的实现类。
URLConnection 可以设置不同的请求方式，如常见的 get、post 方法等。头部信息也有很多对应的方法设置，如

• setAllowUserInteraction
• setDoInput
• setDoOutput
• setIfModifiedSince

• setUseCaches

  还有一个通用的设置方法

• setRequestProperty

  下载文件时，我们通过 getInputStream() 方法获取输入流，将下载的数据写入文件

一个很简单的例子

public void download(String urlPath, String directory) {  URL url;  try {    url = new URL(urlPath);    HttpURLConnection conn = (HttpURLConnection) url.openConnection();    conn.connect();    int fileLength = conn.getContentLength();    String filePath = url.getPath();    String fileName = filePath.substring(filePath.lastIndexOf("/") + 1);    System.out.println("文件大小：" + fileLength);    File file = new File(directory + File.separator + fileName);    if(!file.getParentFile().exists()) {      file.getParentFile().mkdirs();    }    BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream(file));    System.out.println("下载位置：" + file.getPath());    BufferedInputStream bis = new BufferedInputStream(conn.getInputStream());    byte[] b = new byte[1024];    int size = 0;    int downloadLength = 0;    while ((size = bis.read(b)) != -1) {      bos.write(b, 0, size);      downloadLength += size;      //System.out.println("已下载：" + downloadLength * 100 / fileLength + "%"); //打印下载百分比    }    bos.flush();    bos.close();    bis.close();    conn.disconnect();  } catch (MalformedURLException e1) {    e1.printStackTrace();  } catch (IOException e) {    e.printStackTrace();  }}