EchoContentHandler 类负责处理 EchoServer 服务器发送的数据

public class EchoContentHandler extends ContentHandler {
    
    /** 读取服务器发送的一行数据，把它转换为字符串对象 */
    public Object getContent(URLConnection connection) throws IOException {
    	InputStream in = connection.getInputStream();
        BufferedReader br = new BufferedReader(new InputStreamReader(in));
        return br.readLine();
    }
    
    public Object getContent(URLConnection connection, Class[] classes) throws IOException {
        InputStream in = connection.getInputStream();
        for(int i = 0; i < classes.length; i++) {
            if(classes[i] == InputStream.class) {
                return in;
            } else if(classes[i] == String.class) {
                return getContent(connection);
            }
        }
        return null;
    }
}

第二个 getContent() 方法依次遍历 classes 参数中的元素，判断元素是否为 InputSuream 类型或 String 类型，如果是，就返回相应类型的对象，它包含了服务器发送的数据。如果 classes 参数中的元素都不是 InputStream 类型或 String 类型，就返回 null

EchoContentHandlerFactory 类的 createContentHandler() 方法负责创建一个EchoContentHandler 对象

public class EchoContentHandlerFactory implements ContentHandlerFactory {
    
    public ContentHandler createContentHandler(String mimetype) {
        if(mimetype.equals("text/plain")) {
            return new EchoContentHandler();
        } else {
            return null;
        }
    }
}

在客户程序中，可以通过以下方式设置 EchoContentHandlerFactory

URLConnection.setContentHandlerFactory(new EchoContentHandlerFactory());
URL url = new URL("echo://localhost:8000");
EchoURLConnection connection = (EchoURLConnection)url.openConnection();
...
//读取服务器返回的数据，它被包装为一个字符串对象
String echoMsg = (String)connection.getContent();

4. 在 EchoClient 类运用 ECHO 协议处理框架

public class EchoClient {
    
    public static void main(String args[]) throws IOException {
        //设置URLStreamHandlerFactory
        URL.setURLStreamHandlerFactory(new EchoURLStreamHandlerFactory());
        //设置ContentHandlerFactory
        URLConnection.setContentHandlerFactory(new EchoContentHandlerFactory());
        
        URL url = new URL("echo://localhost:8000");
        EchoURLConnection connection = (EchoURlConnection) url.openConnection();
        //允许获得输出流
        connection.setDoOutput(true);
        //获得输出流
        PrintWriter pw = new PrintWriter(connection.getOutputStream(), true);
        while(true) {
            BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
            String msg = br.readLine();
            //向服务器发送消息
            pw.println(msg);
            //读取服务器返回的消息
            String echoMsg = (String) connection.getContent();
            System.out.println(echoMsg);
            if(echoMsg.equals("echo:bye")) {
                //断开连接
                connection.disconnect();
                break;
            }
        }
    }
}