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Class java.net.URL - JDK 5 Documentation v1.1.8, Java 2 SDK 英文文档

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Class java.net.URL 

java.lang.Object
   |
   +----java.net.URL
public final class URL
extends Object
implements Serializable
Class URL represents a Uniform Resource Locator, a pointer to a "resource" on the World Wide Web. A resource can be something as simple as a file or a directory, or it can be a reference to a more complicated object, such as a query to a database or to a search engine. More information on the types of URLs and their formats can be found at:

In general, a URL can be broken into several parts. The previous example of a URL indicates that the protocol to use is http (HyperText Transport Protocol) and that the information resides on a host machine named www.ncsa.uiuc.edu. The information on that host machine is named demoweb/url-primer.html. The exact meaning of this name on the host machine is both protocol dependent and host dependent. The information normally resides in a file, but it could be generated on the fly. This component of the URL is called the file component, even though the information is not necessarily in a file.

A URL can optionally specify a "port", which is the port number to which the TCP connection is made on the remote host machine. If the port is not specified, the default port for the protocol is used instead. For example, the default port for http is 80. An alternative port could be specified as:

A URL may have appended to it an "anchor", also known as a "ref" or a "reference". The anchor is indicated by the sharp sign character "#" followed by more characters. For example,

This anchor is not technically part of the URL. Rather, it indicates that after the specified resource is retrieved, the application is specifically interested in that part of the document that has the tag chapter1 attached to it. The meaning of a tag is resource specific.

An application can also specify a "relative URL", which contains only enough information to reach the resource relative to another URL. Relative URLs are frequently used within HTML pages. For example, if the contents of the URL:

contained within it the relative URL: it would be a shorthand for:

The relative URL need not specify all the components of a URL. If the protocol, host name, or port number is missing, the value is inherited from the fully specified URL. The file component must be specified. The optional anchor is not inherited.

Constructor Index

o URL(String)
Creates a URL object from the String representation.
o URL(String, String, int, String)
Creates a URL object from the specified protocol, host, port number, and file.
o URL(String, String, String)
Creates an absolute URL from the specified protocol name, host name, and file name.
o URL(URL, String)
Creates a URL by parsing the specification spec within a specified context.

Method Index

o equals(Object)
Compares two URLs.
o getContent()
Returns the contents of this URL.
o getFile()
Returns the file name of this URL.
o getHost()
Returns the host name of this URL, if applicable.
o getPort()
Returns the port number of this URL.
o getProtocol()
Returns the protocol name this URL.
o getRef()
Returns the anchor (also known as the "reference") of this URL.
o hashCode()
Creates an integer suitable for hash table indexing.
o openConnection()
Returns a URLConnection object that represents a connection to the remote object referred to by the URL.
o openStream()
Opens a connection to this URL and returns an InputStream for reading from that connection.
o sameFile(URL)
Compares two URLs, excluding the "ref" fields.
o set(String, String, int, String, String)
Sets the fields of the URL.
o setURLStreamHandlerFactory(URLStreamHandlerFactory)
Sets an application's URLStreamHandlerFactory.
o toExternalForm()
Constructs a string representation of this URL.
o toString()
Constructs a string representation of this URL.

Constructors

o URL 
 public URL(String protocol,
            String host,
            int port,
            String file) throws MalformedURLException
Creates a URL object from the specified protocol, host, port number, and file. Specifying a port number of -1 indicates that the URL should use the default port for the protocol.

If this is the first URL object being created with the specified protocol, a stream protocol handler object, an instance of class URLStreamHandler, is created for that protocol:

  1. If the application has previously set up an instance of URLStreamHandlerFactory as the stream handler factory, then the createURLStreamHandler method of that instance is called with the protocol string as an argument to create the stream protocol handler.
  2. If no URLStreamHandlerFactory has yet been set up, or if the factory's createURLStreamHandler method returns null, then the constructor finds the value of the system property:
      java.handler.protol.pkgs
    If the value of that system property is not null, it is interpreted as a list of packages separated by a vertical slash character '|'. The constructor tries to load the class named:
      <package>.<protocol>.Handler
    where <package> is replaced by the name of the package and <protocol> is replaced by the name of the protocol. If this class does not exist, or if the class exists but it is not a subclass of URLStreamHandler, then the next package in the list is tried.
  3. If the previous step fails to find a protocol handler, then the constructor tries to load the class named:
      sun.net.www.protocol.<protocol>.Handler
    If this class does not exist, or if the class exists but it is not a subclass of URLStreamHandler, then a MalformedURLException is thrown.

Parameters:
protocol - the name of the protocol.
host - the name of the host.
port - the port number.
file - the host file.
Throws: MalformedURLException
if an unknown protocol is specified.
See Also:
getProperty, setURLStreamHandlerFactory, URLStreamHandler, createURLStreamHandler
o URL 
 public URL(String protocol,
            String host,
            String file) throws MalformedURLException
Creates an absolute URL from the specified protocol name, host name, and file name. The default port for the specified protocol is used.

This method is equivalent to calling the four-argument constructor with the arguments being protocol, host, -1, and file.

Parameters:
protocol - the protocol to use.
host - the host to connect to.
file - the file on that host.
Throws: MalformedURLException
if an unknown protocol is specified.
See Also:
URL
o URL 
 public URL(String spec) throws MalformedURLException
Creates a URL object from the String representation.

This constructor is equivalent to a call to the two-argument constructor with a null first argument.

Parameters:
spec - the String to parse as a URL.
Throws: MalformedURLException
If the string specifies an unknown protocol.
See Also:
URL
o URL 
 public URL(URL context,
            String spec) throws MalformedURLException
Creates a URL by parsing the specification spec within a specified context. If the context argument is not null and the spec argument is a partial URL specification, then any of the strings missing components are inherited from the context argument.

The specification given by the String argument is parsed to determine if it specifies a protocol. If the String contains an ASCII colon ':' character before the first occurrence of an ASCII slash character '/', then the characters before the colon comprise the protocol.

  • If the spec argument does not specify a protocol:
    • If the context argument is not null, then the protocol is copied from the context argument.
    • If the context argument is null, then a MalformedURLException is thrown.
  • If the spec argument does specify a protocol:
    • If the context argument is null, or specifies a different protocol than the specification argument, the context argument is ignored.
    • If the context argument is not null and specifies the same protocol as the specification, the host, port number, and file are copied from the context argument into the newly created URL.

The constructor then searches for an appropriate stream protocol handler of type URLStreamHandler as outlined for:

    java.net.URL#URL(java.lang.String, java.lang.String, int, java.lang.String)
The stream protocol handler's parseURL method is called to parse the remaining fields of the specification that override any defaults set by the context argument.

Parameters:
context - the context in which to parse the specification.
spec - a String representation of a URL.
Throws: MalformedURLException
if no protocol is specified, or an unknown protocol is found.
See Also:
URL, URLStreamHandler, parseURL

Methods

o set 
 protected void set(String protocol,
                    String host,
                    int port,
                    String file,
                    String ref)
Sets the fields of the URL. This is not a public method so that only URLStreamHandlers can modify URL fields. URLs are otherwise constant. REMIND: this method will be moved to URLStreamHandler

Parameters:
protocol - the protocol to use
host - the host name to connecto to
port - the protocol port to connect to
file - the specified file name on that host
ref - the reference
o getPort 
 public int getPort()
Returns the port number of this URL. Returns -1 if the port is not set.

Returns:
the port number
o getProtocol 
 public String getProtocol()
Returns the protocol name this URL.

Returns:
the protocol of this URL.
o getHost 
 public String getHost()
Returns the host name of this URL, if applicable. For "file" protocol, this is an empty string.

Returns:
the host name of this URL.
o getFile 
 public String getFile()
Returns the file name of this URL.

Returns:
the file name of this URL.
o getRef 
 public String getRef()
Returns the anchor (also known as the "reference") of this URL.

Returns:
the anchor (also known as the "reference") of this URL.
o equals 
 public boolean equals(Object obj)
Compares two URLs. The result is true if and only if the argument is not null and is a URL object that represents the same URL as this object. Two URL objects are equal if they have the same protocol and reference the same host, the same port number on the host, and the same file on the host. The anchors of the URL objects are not compared.

This method is equivalent to:

    (obj instanceof URL) && sameFile((URL)obj)

Parameters:
obj - the URL to compare against.
Returns:
true if the objects are the same; false otherwise.
Overrides:
equals in class Object
o hashCode 
 public int hashCode()
Creates an integer suitable for hash table indexing.

Returns:
a hash code for this URL.
Overrides:
hashCode in class Object
o sameFile 
 public boolean sameFile(URL other)
Compares two URLs, excluding the "ref" fields. Returns true if this URL and the other argument both refer to the same resource. The two URLs might not both contain the same anchor.

Parameters:
other - the URL to compare against.
Returns:
true if they reference the same remote object; false otherwise.
o toString 
 public String toString()
Constructs a string representation of this URL. The string is created by calling the toExternalForm method of the stream protocol handler for this object.

Returns:
a string representation of this object.
Overrides:
toString in class Object
See Also:
URL, toExternalForm
o toExternalForm 
 public String toExternalForm()
Constructs a string representation of this URL. The string is created by calling the toExternalForm method of the stream protocol handler for this object.

Returns:
a string representation of this object.
See Also:
URL, toExternalForm
o openConnection 
 public URLConnection openConnection() throws IOException
Returns a URLConnection object that represents a connection to the remote object referred to by the URL.

If there is not already an open connection, the connection is opened by calling the openConnection method of the protocol handler for this URL.

Returns:
a URLConnection to the URL.
Throws: IOException
if an I/O exception occurs.
See Also:
URL, URLConnection, openConnection
o openStream 
 public final InputStream openStream() throws IOException
Opens a connection to this URL and returns an InputStream for reading from that connection. This method is a shorthand for:
    openConnection().getInputStream()

Returns:
an input stream for reading from the URL connection.
Throws: IOException
if an I/O exception occurs.
o getContent 
 public final Object getContent() throws IOException
Returns the contents of this URL. This method is a shorthand for:
    openConnection().getContent()

Returns:
the contents of this URL.
Throws: IOException
if an I/O exception occurs.
See Also:
getContent
o setURLStreamHandlerFactory 
 public static synchronized void setURLStreamHandlerFactory(URLStreamHandlerFactory fac)
Sets an application's URLStreamHandlerFactory. This method can be called at most once by an application.

The URLStreamHandlerFactory instance is used to construct a stream protocol handler from a protocol name.

Parameters:
fac - the desired factory.
Throws: Error
if the application has already set a factory.
See Also:
URL, URLStreamHandlerFactory 
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