Java URL Handling

URL (Uniform Resource Locator) is also known as the Uniform Resource Locator in Chinese, and is sometimes俗称 as the web address. It represents resources on the Internet, such as web pages or FTP addresses.

In this chapter, we will introduce how Java handles URLs. URLs can be divided into the following parts.

protocol://host:port/path?query#fragment

protocol (protocol) can be HTTP, HTTPS, FTP, and File, port is the port number, and path is the file path and file name.

The following is an example of a URL for the HTTP protocol:

http://www.oldtoolbag.com/index.html?language=cn#j2se

URL Parsing:

• 协议为(protocol)：http

• 主机为(host:port)：www.oldtoolbag.com

• 端口号为(port): 80, the above URL example does not specify the port because the default port number for the HTTP protocol is 80。

• 文件路径为(path)：/index.html

• 请求参数(query)：language=cn

• 定位位置(fragment)：j2定位到网页中 id 属性为 j2the position of the HTML element .

URL Class Methods

The java.net package defines the URL class, which is used to handle content related to URLs. The creation and usage of the URL class will be introduced separately below.

java.net.URL provides various URL construction methods and can be used to retrieve resources.

The URL class contains many methods for accessing various parts of a URL, the specific methods and descriptions are as follows:

Online Example

The following example demonstrates how to obtain various parts of a URL using the URL class in java.net:

import java.net.*;
import java.io.*;
　

{
　　　public static void main(String [] args)
　　　{
　　　　　　try
　　　　　　{
　　　　　　　　　URL url = new URL("http://www.oldtoolbag.com/index.html?language=cn#j2se
　　　　　　　　　System.out.println("URL is: ")　+　url.toString());
　　　　　　　　　System.out.println("Protocol: ")　+　url.getProtocol());
　　　　　　　　　System.out.println("Verification information: ")　+　url.getAuthority());
　　　　　　　　　System.out.println("Filename and request parameters: ",　+　url.getFile());
　　　　　　　　　System.out.println("Hostname: ",　+　url.getHost());
　　　　　　　　　System.out.println("Path: ",　+　url.getPath());
　　　　　　　　　System.out.println("Port: ",　+　url.getPort());
　　　　　　　　　System.out.println("Default port: ",　+　url.getDefaultPort());
　　　　　　　　　System.out.println("Request parameters: ",　+　url.getQuery());
　　　　　　　　　System.out.println("Location: ",　+　url.getRef());
　　　　　　catch (IOException e)
　　　　　　{
　　　　　　　　　e.printStackTrace();
　　　　　　}
　　　}
}

The compiled and run results of the above examples are as follows:

URL is: http://www.oldtoolbag.com/index.html?language=cn#j2se
Protocol is: http
Verification information: www.oldtoolbag.com
Filename and request parameters:/index.html?language=cn
Hostname: www.oldtoolbag.com
Path:/index.html
Port:-1
Default port:80
Request parameters: language=cn
Location: j2se

URLConnections class methods

openConnection() returns a java.net.URLConnection.

For example:

• If the URL you connect to is an HTTP protocol URL, the openConnection() method returns an HttpURLConnection object.

• If the URL you connect to is a JAR file, the openConnection() method will return a JarURLConnection object.

• etc...

The following is a list of URLConnection methods:

Online Example

The following example uses the HTTP protocol for the URL. The openConnection method returns an HttpURLConnection object.

import java.net.*;
import java.io.*;
public class URLConnDemo
{
　　　public static void main(String [] args)
　　　{
　　　　　　try
　　　　　　{
　　　　　　　　　URL url = new URL("http://www.oldtoolbag.com");
　　　　　　　　　URLConnection urlConnection = url.openConnection();
　　　　　　　　　HttpURLConnection connection = null;
　　　　　　　　　if(urlConnection instanceof HttpURLConnection)
　　　　　　　　　{
　　　　　　　　　　　　connection = (HttpURLConnection) urlConnection;
　　　　　　　　　}
　　　　　　　　　else
　　　　　　　　　{
　　　　　　　　　　　　System.out.println("Please enter URL address");
　　　　　　　　　　　　return;
　　　　　　　　　}
　　　　　　　　　BufferedReader in = new BufferedReader(
　　　　　　　　　new InputStreamReader(connection.getInputStream()));
　　　　　　　　　String urlString = "";
　　　　　　　　　String current;
　　　　　　　　　while ((current = in.readLine()) != null)
　　　　　　　　　{
　　　　　　　　　　　　urlString　+= current;
　　　　　　　　　}
　　　　　　　　　System.out.println(urlString);
　　　　　　catch (IOException e)
　　　　　　{
　　　　　　　　　e.printStackTrace();
　　　　　　}
　　　}
}

The compilation and execution results of the above examples are as follows:

$ javac URLConnDemo.java　
$ java URLConnDemo
...Here will output the homepage of the basic tutorial website (http://www.oldtoolbag.com) HTML Content...