Java URL使用指南

Java中使用URL（Uniform Resource Locator）可以訪問不同協議的資源。常見的協議有HTTP、FTP、Telnet等，在這裡我們着重講解HTTP協議。

一、URL的構成

URL由三部分組成：協議、主機和路徑。例如：http://www.example.com/index.html。其中，http是協議，www.example.com是主機，index.html是路徑。除此之外，URL還可以有查詢參數和錨點。

下面我們通過代碼實現獲取URL的各個部分：

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("協議：" + url.getProtocol());
        System.out.println("主機：" + url.getHost());
        System.out.println("路徑：" + url.getPath());
        System.out.println("查詢參數：" + url.getQuery());
        System.out.println("錨點：" + url.getRef());
    }
}

二、URL的解析

在實際應用中，我們常常需要對URL進行解析，獲取其中的參數。URL的解析可以使用java.net.URLDecoder類。下面通過代碼演示解析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("用戶名：" + params.get("username"));
        System.out.println("密碼：" + params.get("password"));
    }
}

三、URL的請求

使用Java可以方便地實現對URL的請求，可以使用HttpURLConnection類。下面通過代碼演示對URL進行GET請求：

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);
    }
}

如果需要發送POST請求，可以調用HttpURLConnection的setRequestMethod方法，並且需要設置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

有時候我們需要動態生成URL，可以使用StringBuilder來拼接URL。下面通過代碼演示：

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的編碼

URL中的參數需要進行編碼，避免出現特殊字符。Java內置了URLEncoder和URLDecoder類，可以用於URL的編碼和解碼。下面通過代碼演示：

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 編程";
        String encodedUrl = URLEncoder.encode(url, "UTF-8");
        System.out.println(encodedUrl);
    }
}

需要注意的是，對於URL中的路徑部分，只需要對特殊字符進行編碼，而對於查詢參數部分，需要對整個參數字符串進行編碼。