Java 简明教程

Java - URL Processing

URL 代表统一资源定位符，表示万维网上的资源，例如网页或 FTP 目录。

URL stands for Uniform Resource Locator and represents a resource on the World Wide Web, such as a Web page or FTP directory.

此部分向您展示如何编写与 URL 通信的 Java 程序。URL 可以分解为以下部分：

This section shows you how to write Java programs that communicate with a URL. A URL can be broken down into parts, as follows −

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

协议示例包括 HTTP、 HTTPS、FTP 和 File。路径也称为文件名，主机也称为权限。

Examples of protocols include HTTP, HTTPS, FTP, and File. The path is also referred to as the filename, and the host is also called the authority.

以下是一个协议为 HTTP 的网页的 URL：

The following is a URL to a web page whose protocol is HTTP −

https://www.amrood.com/index.htm?language=en#j2se

请注意此 URL 未指定端口，在这种情况下将使用该协议的默认端口。对于 HTTP，默认端口为 80。

Notice that this URL does not specify a port, in which case the default port for the protocol is used. With HTTP, the default port is 80.

Constructors

java.net.URL 类表示一个 URL，并具有用于在 Java 中处理 URL 的一组完整方法。

The java.net.URL class represents a URL and has a complete set of methods to manipulate URL in Java.

URL 类具有多个构造方法，用于创建 URL，包括以下方法：

The URL class has several constructors for creating URLs, including the following −

Sr.No.	Constructors & Description
1	public URL(String protocol, String host, int port, String file) throws MalformedURLException Creates a URL by putting together the given parts.
2	public URL(String protocol, String host, String file) throws MalformedURLException Identical to the previous constructor, except that the default port for the given protocol is used.
3	public URL(String url) throws MalformedURLException Creates a URL from the given String.
4	public URL(URL context, String url) throws MalformedURLException Creates a URL by parsing together the URL and String arguments.

URL 类包含许多用于访问所表示 URL 的各个部分的方法。URL 类中的一些方法包括以下方法：

The URL class contains many methods for accessing the various parts of the URL being represented. Some of the methods in the URL class include the following −

Sr.No.	Method & Description
1	public equals(Object obj) This method compares this URL for equality with another object.
2	public String getAuthority() This method returns the authority of the URL.
3	public Object getContent() This method returns the contents of this URL.
4	public Object getContent(Class<?>[] classes) This method returns the contents of this URL.
5	public int getDefaultPort() This method returns the default port for the protocol of the URL.
6	public String getFile() This method returns the filename of the URL.
7	public String getHost() This method returns the host of the URL.
8	public String getPath() This method returns the path of the URL.
9	public int getPort() This method returns the port of the URL.
10	public String getProtocol() This method returns the protocol of the URL.
11	public String getQuery() This method returns the query part of the URL.
12	public String getRef() This method returns the reference part of the URL.
13	public String getUserInfo() This method returns the userInfo part of the URL.
14	public int hashCode() This method creates and return an integer suitable for hash table indexing.
15	public URLConnection openConnection() This method returns a URLConnection instance that represents a connection to the remote object referred to by the URL.
16	public URLConnection openConnection(Proxy proxy) This method acts as openConnection(), except that the connection will be made through the specified proxy; Protocol handlers that do not support proxing will ignore the proxy parameter and make a normal connection.
17	public InputStream openStream() This method opens a connection to this URL and returns an InputStream for reading from that connection.
18	public boolean sameFile(URL other) This method compares two URLs, excluding the fragment component.
19	public static void setURLStreamHandlerFactory(URLStreamHandlerFactory fac) This method sets an application’s URLStreamHandlerFactory.
20	public String toExternalForm() This method constructs and return a string representation of this URL.
21	public String toString() This method constructs and return a string representation of this URL.
22	public String toURI() This method returns a URI equivalent to this URL.

Example

以下 URLDemo 程序演示了 URL 的各个部分。URL 输入在命令行上，URLDemo 程序输出给定 URL 的每个部分。

The following URLDemo program demonstrates the various parts of a URL. A URL is entered on the command line, and the URLDemo program outputs each part of the given URL.

// File Name : URLDemo.java
import java.io.IOException;
import java.net.URL;

public class URLDemo {

   public static void main(String [] args) {
      try {
         URL url = new URL("https://www.tutorialspoint.com/index.htm?language=en#j2se");

         System.out.println("URL is " + url.toString());
         System.out.println("protocol is " + url.getProtocol());
         System.out.println("authority is " + url.getAuthority());
         System.out.println("file name is " + url.getFile());
         System.out.println("host is " + url.getHost());
         System.out.println("path is " + url.getPath());
         System.out.println("port is " + url.getPort());
         System.out.println("default port is " + url.getDefaultPort());
         System.out.println("query is " + url.getQuery());
         System.out.println("ref is " + url.getRef());
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

此程序的示例运行将生成以下结果 −

A sample run of the this program will produce the following result −

Output

URL is https://www.tutorialspoint.com/index.htm?language=en#j2se
protocol is https
authority is www.tutorialspoint.com
file name is /index.htm?language=en
host is www.tutorialspoint.com
path is /index.htm
port is -1
default port is 443
query is language=en
ref is j2se

URLConnections Class Methods

openConnection() 方法返回 java.net.URLConnection，其子类代表各种类型的 URL 连接的抽象类。

The openConnection() method returns a java.net.URLConnection, an abstract class whose subclasses represent the various types of URL connections.

例如 -

For example −

If you connect to a URL whose protocol is HTTP, the openConnection() method returns an HttpURLConnection object.
If you connect to a URL that represents a JAR file, the openConnection() method returns a JarURLConnection object, etc.

URLConnection 类具有用于设置或确定有关连接信息的许多方法，包括以下方法 −

The URLConnection class has many methods for setting or determining information about the connection, including the following −

Sr.No.	Method & Description
1	void addRequestProperty(String key, String value) Adds a general request property specified by a key-value pair.
2	boolean getAllowUserInteraction() Returns the value of the allowUserInteraction field for this object.
3	int getConnectTimeout() Returns setting for connect timeout.
4	Object getContent() Retrieves the contents of this URL connection.
5	Object getContent(Class[] classes) Retrieves the contents of this URL connection.
6	String getContentEncoding() Returns the value of the content-encoding header field.
7	int getContentLength() Returns the value of the content-length header field.
8	long getContentLengthLong() Returns the value of the content-length header field as long.
9	String getContentType() Returns the value of the content-type header field.
10	long getDate() Returns the value of the date header field.
11	static boolean getDefaultAllowUserInteraction() Returns the default value of the allowUserInteraction field.
12	boolean getDefaultUseCaches() Returns the default value of a URLConnection’s useCaches flag.
13	static boolean getDefaultUseCaches(String protocol) Returns the default value of the useCaches flag for the given protocol.
14	boolean getDoInput() Returns the value of this URLConnection’s doInput flag.
15	boolean getDoOutput() Returns the value of this URLConnection’s doOutput flag.
16	long getExpiration() Returns the value of the expires header field.
17	static FileNameMap getFileNameMap() Loads filename map (a mimetable) from a data file.
18	String getHeaderField(int n) Returns the value for the nth header field.
19	String getHeaderField(String name) Returns the value of the named header field.
20	long getHeaderFieldDate(String name, long Default) Returns the value of the named field parsed as date.
21	int getHeaderFieldInt(String name, int Default) Returns the value of the named field parsed as a number.
22	String getHeaderFieldKey(int n) Returns the key for the nth header field.
23	long getHeaderFieldLong(String name, long Default) Returns the value of the named field parsed as a number.
24	Map<String,List<String>> getHeaderFields() Returns an unmodifiable Map of the header fields.
25	long getIfModifiedSince() Returns the value of this object’s ifModifiedSince field.
26	InputStream getInputStream() Returns an input stream that reads from this open connection.
27	int getLastModified() Returns the value of the last-modified header field.
28	OutputStream getOutputStream() Returns an output stream that writes to this connection.
29	Permission getPermission() Returns a permission object representing the permission necessary to make the connection represented by this object.
30	int getReadTimeout() Returns setting for read timeout. 0 return implies that the option is disabled (i.e., timeout of infinity).
31	Map<String,List<String>> getRequestProperties() Returns an unmodifiable Map of general request properties for this connection.
32	String getRequestProperty(String key) Returns the value of the named general request property for this connection.
33	URL getURL() Returns the value of this URLConnection’s URL field.
34	boolean getUseCaches() Returns the value of this URLConnection’s useCaches field.
35	static String guessContentTypeFromName(String fname) Tries to determine the content type of an object, based on the specified "file" component of a URL.
36	static String guessContentTypeFromStream(InputStream is) Tries to determine the type of an input stream based on the characters at the beginning of the input stream.
37	void setAllowUserInteraction(boolean allowuserinteraction) Set the value of the allowUserInteraction field of this URLConnection.
38	void setConnectTimeout(int timeout) Sets a specified timeout value, in milliseconds, to be used when opening a communications link to the resource referenced by this URLConnection.
39	static void setContentHandlerFactory(ContentHandlerFactory fac) Sets the ContentHandlerFactory of an application.
40	static void setDefaultAllowUserInteraction(boolean defaultallowuserinteraction) Sets the default value of the allowUserInteraction field for all future URLConnection objects to the specified value.
41	void setDefaultUseCaches(boolean defaultusecaches) Sets the default value of the useCaches field to the specified value.
42	static void setDefaultUseCaches(String protocol, boolean defaultVal) Sets the default value of the useCaches field for the named protocol to the given value.
43	void setDoInput(boolean doinput) Sets the value of the doInput field for this URLConnection to the specified value.
44	void setDoOutput(boolean dooutput) Sets the value of the doOutput field for this URLConnection to the specified value.
45	static void setFileNameMap(FileNameMap map) Sets the FileNameMap.
46	void setIfModifiedSince(long ifmodifiedsince) Sets the value of the ifModifiedSince field of this URLConnection to the specified value.
47	void setReadTimeout(int timeout) Sets the read timeout to a specified timeout, in milliseconds.
48	void setRequestProperty(String key, String value) Sets the general request property.
49	void setUseCaches(boolean usecaches) Sets the value of the useCaches field of this URLConnection to the specified value.
50	String toString() Returns a String representation of this URL connection.

Example

以下 URLConnectionDemo 程序连接到从命令行输入的 URL。

The following URLConnectionDemo program connects to a URL entered from the command line.

如果 URL 表示 HTTP 资源，则连接会被强制转换为 HttpURLConnection，并且资源中的数据会被逐行读取。

If the URL represents an HTTP resource, the connection is cast to HttpURLConnection, and the data in the resource is read one line at a time.

package com.tutorialspoint;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLConnection;

public class URLConnDemo {
   public static void main(String [] args) {
      try {
         URL url = new URL("https://www.tutorialspoint.com");
         URLConnection urlConnection = url.openConnection();
         HttpURLConnection connection = null;
         if(urlConnection instanceof HttpURLConnection) {
            connection = (HttpURLConnection) urlConnection;
         }else {
            System.out.println("Please enter an HTTP URL.");
            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();
      }
   }
}

此程序的示例运行将产生以下结果：

A sample run of this program will produce the following result −

Output

$ java URLConnDemo

.....a complete HTML content of home page of tutorialspoint.com.....