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Java URL使用指南

Java中使用URL(Uniform Resource Locator)可以访问不同协议的资源。常见的协议有HTTP、FTP、Telnet等,在这里我们着重讲解HTTP协议。

一、URL的构成

URL由三部分组成:协议、主机和路径。例如:http://www.example.com/index.html。其中,http是协议,www.example.com是主机,index.html是路径。除此之外,URL还可以有查询参数和锚点。

下面我们通过代码实现获取URL的各个部分:

import java.net.URL;

public class URLDemo {
    public static void main(String[] args) throws Exception {
        URL url = new URL("http://www.example.com/index.html?name=yiibai#example");
        System.out.println("协议:" + url.getProtocol());
        System.out.println("主机:" + url.getHost());
        System.out.println("路径:" + url.getPath());
        System.out.println("查询参数:" + url.getQuery());
        System.out.println("锚点:" + url.getRef());
    }
}

二、URL的解析

在实际应用中,我们常常需要对URL进行解析,获取其中的参数。URL的解析可以使用java.net.URLDecoder类。下面通过代码演示解析URL:

import java.net.URLDecoder;
import java.util.HashMap;
import java.util.Map;

public class URLParser {
    public static Map parse(String url) throws Exception {
        Map
    params = new HashMap<>();
        int index = url.indexOf('?');
        if (index != -1) {
            String query = url.substring(index + 1);
            String[] pairs = query.split("&");
            for (String pair : pairs) {
                int pos = pair.indexOf('=');
                if (pos != -1) {
                    params.put(URLDecoder.decode(pair.substring(0, pos), "UTF-8"),
                            URLDecoder.decode(pair.substring(pos + 1), "UTF-8"));
                }
            }
        }
        return params;
    }

    public static void main(String[] args) throws Exception {
        String url = "http://www.example.com/login?username=yiibai&password=123456";
        Map
     params = URLParser.parse(url);
        System.out.println("用户名:" + params.get("username"));
        System.out.println("密码:" + params.get("password"));
    }
}

    
   
  

三、URL的请求

使用Java可以方便地实现对URL的请求,可以使用HttpURLConnection类。下面通过代码演示对URL进行GET请求:

import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.Scanner;

public class HttpDemo {
    public static String getRequest(String urlString) throws IOException {
        URL url = new URL(urlString);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("GET");
        conn.setConnectTimeout(5000);
        InputStream is = conn.getInputStream();
        Scanner scanner = new Scanner(is, "UTF-8");
        StringBuilder sb = new StringBuilder();
        while (scanner.hasNextLine()) {
            sb.append(scanner.nextLine());
        }
        scanner.close();
        conn.disconnect();
        return sb.toString();
    }

    public static void main(String[] args) throws IOException {
        String url = "http://www.example.com/index.html";
        String content = getRequest(url);
        System.out.println(content);
    }
}

如果需要发送POST请求,可以调用HttpURLConnection的setRequestMethod方法,并且需要设置Content-Type和Content-Length:

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.nio.charset.StandardCharsets;
import java.util.Scanner;

public class HttpDemo {
    public static String postRequest(String urlString, String body) throws IOException {
        URL url = new URL(urlString);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("POST");
        conn.setConnectTimeout(5000);
        conn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
        conn.setRequestProperty("Content-Length", Integer.toString(body.getBytes(StandardCharsets.UTF_8).length));
        conn.setDoOutput(true);
        OutputStream os = conn.getOutputStream();
        os.write(body.getBytes(StandardCharsets.UTF_8));
        os.close();
        InputStream is = conn.getInputStream();
        Scanner scanner = new Scanner(is, "UTF-8");
        StringBuilder sb = new StringBuilder();
        while (scanner.hasNextLine()) {
            sb.append(scanner.nextLine());
        }
        scanner.close();
        conn.disconnect();
        return sb.toString();
    }

    public static void main(String[] args) throws IOException {
        String url = "http://www.example.com/login";
        String body = "username=yiibai&password=123456";
        String content = postRequest(url, body);
        System.out.println(content);
    }
}

四、URL的拼接

有时候我们需要动态生成URL,可以使用StringBuilder来拼接URL。下面通过代码演示:

public class URLBuilder {
    public static String build(String baseUrl, Map params) {
        StringBuilder sb = new StringBuilder();
        sb.append(baseUrl);
        sb.append('?');
        for (Map.Entry
    entry : params.entrySet()) {
            sb.append(entry.getKey());
            sb.append('=');
            sb.append(entry.getValue());
            sb.append('&');
        }
        sb.deleteCharAt(sb.length() - 1);
        return sb.toString();
    }

    public static void main(String[] args) {
        String baseUrl = "http://www.example.com/search";
        Map
     params = new HashMap<>();
        params.put("q", "Java");
        params.put("page", "1");
        String url = URLBuilder.build(baseUrl, params);
        System.out.println(url);
    }
}

    
   
  

五、URL的编码

URL中的参数需要进行编码,避免出现特殊字符。Java内置了URLEncoder和URLDecoder类,可以用于URL的编码和解码。下面通过代码演示:

import java.io.UnsupportedEncodingException;
import java.net.URLEncoder;

public class URLEncodeDemo {
    public static void main(String[] args) throws UnsupportedEncodingException {
        String url = "http://www.example.com/search?q=Java 编程";
        String encodedUrl = URLEncoder.encode(url, "UTF-8");
        System.out.println(encodedUrl);
    }
}

需要注意的是,对于URL中的路径部分,只需要对特殊字符进行编码,而对于查询参数部分,需要对整个参数字符串进行编码。