URL

  • The URL class in Java represents a Uniform Resource Locator (URL) and is used to locate resources on the web, encompassing various components like protocol, host, port, path, and query.

  • It offers multiple constructors for creating URL objects with different levels of specificity, along with methods to access individual URL components and open connections to the resource.

  • While the URL class doesn't handle URL encoding/decoding or format validation directly, it provides methods for retrieving content, opening connections, and interacting with stream protocol handlers.

  • For tasks involving URL encoding and decoding, it's recommended to utilize the URI class and convert between URL and URI objects using their respective methods (toURI(), URI.toURL()).

  • Several key methods are available, including openStream() for reading data, sameFile() for comparing URLs, and toURI() for converting a URL to a URI object, each with potential exceptions and considerations.
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:

http://www.socs.uts.edu.au/MosaicDocs-old/url-primer.html

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 Transfer Protocol) and that the information resides on a host machine named www.socs.uts.edu.au. The information on that host machine is named /MosaicDocs-old/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 path component.

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: 

     http://www.socs.uts.edu.au:80/MosaicDocs-old/url-primer.html

The syntax of URL is defined by RFC 2396: Uniform Resource Identifiers (URI): Generic Syntax, amended by RFC 2732: Format for Literal IPv6 Addresses in URLs. The Literal IPv6 address format also supports scope_ids. The syntax and usage of scope_ids is described here.

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

     http://java.sun.com/index.html#chapter1

This fragment 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: 

     http://java.sun.com/index.html
contained within it the relative URL: 
     FAQ.html
it would be a shorthand for: 
     http://java.sun.com/FAQ.html

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 fragment is not inherited.

The URL class does not itself encode or decode any URL components according to the escaping mechanism defined in RFC2396. It is the responsibility of the caller to encode any fields, which need to be escaped prior to calling URL, and also to decode any escaped fields, that are returned from URL. Furthermore, because URL has no knowledge of URL escaping, it does not recognise equivalence between the encoded or decoded form of the same URL. For example, the two URLs:

    http://foo.com/hello world/ and http://foo.com/hello%20world
 would be considered not equal to each other.

Note, the URI class does perform escaping of its component fields in certain circumstances. The recommended way to manage the encoding and decoding of URLs is to use URI, and to convert between these two classes using toURI() and URI.toURL().

The URLEncoder and URLDecoder classes can also be used, but only for HTML form encoding, which is not the same as the encoding scheme defined in RFC2396.

Public Constructor Summary

URL(String protocol, String host, int port, String file)
Creates a URL object from the specified protocol, host, port number, and file.
URL(String protocol, String host, String file)
Creates a URL from the specified protocol name, host name, and file name.
URL(String protocol, String host, int port, String file, Object handler)
Creates a URL object from the specified protocol, host, port number, file, and handler.
URL(String spec)
Creates a URL object from the String representation.
URL(URL context, String spec)
Creates a URL by parsing the given spec within a specified context.
URL(URL context, String spec, Object handler)
Creates a URL by parsing the given spec with the specified handler within a specified context.

Public Method Summary

boolean
equals(Object obj)
Compares this URL for equality with another object.
String
getAuthority()
Gets the authority part of this URL.
Object
getContent()
Gets the contents of this URL.
Object
getContent(Class[]<?> classes)
Gets the contents of this URL.
int
getDefaultPort()
Gets the default port number of the protocol associated with this URL.
String
getFile()
Gets the file name of this URL.
String
getHost()
Gets the host name of this URL, if applicable.
String
getPath()
Gets the path part of this URL.
int
getPort()
Gets the port number of this URL.
String
getProtocol()
Gets the protocol name of this URL.
String
getQuery()
Gets the query part of this URL.
String
getRef()
Gets the anchor (also known as the "reference") of this URL.
String
getUserInfo()
Gets the userInfo part of this URL.
synchronized int
hashCode()
Creates an integer suitable for hash table indexing.
URLConnection
openConnection()
Returns a URLConnection instance that represents a connection to the remote object referred to by the URL.
URLConnection
openConnection(Object proxy)
Same 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.
InputStream
openStream()
Opens a connection to this URL and returns an InputStream for reading from that connection.
boolean
sameFile(URL other)
Compares two URLs, excluding the fragment component.
static void
setURLStreamHandlerFactory(URLStreamHandlerFactory fac)
String
toExternalForm()
Constructs a string representation of this URL.
String
toString()
Constructs a string representation of this URL.
URI
toURI()
Returns a URI equivalent to this URL.

Protected Method Summary

void
set(String protocol, String host, int port, String authority, String userInfo, String path, String query, String ref)
Sets the specified 8 fields of the URL.
void
set(String protocol, String host, int port, String file, String ref)
Sets the fields of the URL.

Inherited Method Summary

Public Constructors

public URL (String protocol, String host, int port, String file)

Creates a URL object from the specified protocol, host, port number, and file.

host can be expressed as a host name or a literal IP address. If IPv6 literal address is used, it should be enclosed in square brackets ('[' and ']'), as specified by RFC 2732; However, the literal IPv6 address format defined in RFC 2373: IP Version 6 Addressing Architecture is also accepted.

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.protocol.handler.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 from a system default package. 
             <system default package>.<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.

Protocol handlers for the following protocols are guaranteed to exist on the search path :- 

     http, https, ftp, file, and jar
Protocol handlers for additional protocols may also be available.

No validation of the inputs is performed by this constructor.

Parameters
protocol the name of the protocol to use.
host the name of the host.
port the port number on the host.
file the file on the host
Throws
MalformedURLException if an unknown protocol is specified.
See Also

public URL (String protocol, String host, String file)

Creates a 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. No validation of the inputs is performed by this constructor.

Parameters
protocol the name of the protocol to use.
host the name of the host.
file the file on the host.
Throws
MalformedURLException if an unknown protocol is specified.
See Also

public URL (String protocol, String host, int port, String file, Object handler)

Creates a URL object from the specified protocol, host, port number, file, and handler. Specifying a port number of -1 indicates that the URL should use the default port for the protocol. Specifying a handler of null indicates that the URL should use a default stream handler for the protocol, as outlined for: java.net.URL#URL(java.lang.String, java.lang.String, int, java.lang.String)

If the handler is not null and there is a security manager, the security manager's checkPermission method is called with a NetPermission("specifyStreamHandler") permission. This may result in a SecurityException. No validation of the inputs is performed by this constructor.

Parameters
protocol the name of the protocol to use.
host the name of the host.
port the port number on the host.
file the file on the host
handler the stream handler for the URL.
Throws
MalformedURLException if an unknown protocol is specified.
SecurityException if a security manager exists and its checkPermission method doesn't allow specifying a stream handler explicitly.
See Also

public URL (String spec)

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 no protocol is specified, or an unknown protocol is found, or spec is null.
See Also

public URL (URL context, String spec)

Creates a URL by parsing the given spec within a specified context. The new URL is created from the given context URL and the spec argument as described in RFC2396 "Uniform Resource Identifiers : Generic * Syntax" : 

          <scheme>://<authority><path>?<query>#<fragment>
The reference is parsed into the scheme, authority, path, query and fragment parts. If the path component is empty and the scheme, authority, and query components are undefined, then the new URL is a reference to the current document. Otherwise, the fragment and query parts present in the spec are used in the new URL.

If the scheme component is defined in the given spec and does not match the scheme of the context, then the new URL is created as an absolute URL based on the spec alone. Otherwise the scheme component is inherited from the context URL.

If the authority component is present in the spec then the spec is treated as absolute and the spec authority and path will replace the context authority and path. If the authority component is absent in the spec then the authority of the new URL will be inherited from the context.

If the spec's path component begins with a slash character "/" then the path is treated as absolute and the spec path replaces the context path.

Otherwise, the path is treated as a relative path and is appended to the context path, as described in RFC2396. Also, in this case, the path is canonicalized through the removal of directory changes made by occurences of ".." and ".".

For a more detailed description of URL parsing, refer to RFC2396.

Parameters
context the context in which to parse the specification.
spec the String to parse as a URL.
Throws
MalformedURLException if no protocol is specified, or an unknown protocol is found, or spec is null.
See Also

public URL (URL context, String spec, Object handler)

Creates a URL by parsing the given spec with the specified handler within a specified context. If the handler is null, the parsing occurs as with the two argument constructor.

Parameters
context the context in which to parse the specification.
spec the String to parse as a URL.
handler the stream handler for the URL.
Throws
MalformedURLException if no protocol is specified, or an unknown protocol is found, or spec is null.
SecurityException if a security manager exists and its checkPermission method doesn't allow specifying a stream handler.
See Also

Public Methods

public boolean equals (Object obj)

Compares this URL for equality with another object.

If the given object is not a URL then this method immediately returns false.

Two URL objects are equal if they have the same protocol, reference equivalent hosts, have the same port number on the host, and the same file and fragment of the file.

Returns true if this URL equals o. URLs are equal if they have the same protocol, host, port, file, and reference.

Network I/O Warning

Some implementations of URL.equals() resolve host names over the network. This is problematic:

  • The network may be slow. Many classes, including core collections like Map and Set expect that equals and hashCode will return quickly. By violating this assumption, this method posed potential performance problems.
  • Equal IP addresses do not imply equal content. Virtual hosting permits unrelated sites to share an IP address. This method could report two otherwise unrelated URLs to be equal because they're hosted on the same server.
  • The network may not be available. Two URLs could be equal when a network is available and unequal otherwise.
  • The network may change. The IP address for a given host name varies by network and over time. This is problematic for mobile devices. Two URLs could be equal on some networks and unequal on others.

This problem is fixed in Android 4.0 (Ice Cream Sandwich). In that release, URLs are only equal if their host names are equal (ignoring case).

Parameters
obj the URL to compare against.
Returns
  • true if the objects are the same; false otherwise.

public String getAuthority ()

Gets the authority part of this URL.

Returns
  • the authority part of this URL

public Object getContent ()

Gets 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

public Object getContent (Class[]<?> classes)

Gets the contents of this URL. This method is a shorthand for: 

     openConnection().getContent(Class[])

Parameters
classes an array of Java types
Returns
  • the content object of this URL that is the first match of the types specified in the classes array. null if none of the requested types are supported.
Throws
IOException if an I/O exception occurs.
See Also

public int getDefaultPort ()

Gets the default port number of the protocol associated with this URL. If the URL scheme or the URLStreamHandler for the URL do not define a default port number, then -1 is returned.

Returns
  • the port number

public String getFile ()

Gets the file name of this URL. The returned file portion will be the same as getPath(), plus the concatenation of the value of getQuery(), if any. If there is no query portion, this method and getPath() will return identical results.

Returns
  • the file name of this URL, or an empty string if one does not exist

public String getHost ()

Gets the host name of this URL, if applicable. The format of the host conforms to RFC 2732, i.e. for a literal IPv6 address, this method will return the IPv6 address enclosed in square brackets ('[' and ']').

Returns
  • the host name of this URL.

public String getPath ()

Gets the path part of this URL.

Returns
  • the path part of this URL, or an empty string if one does not exist

public int getPort ()

Gets the port number of this URL.

Returns
  • the port number, or -1 if the port is not set

public String getProtocol ()

Gets the protocol name of this URL.

Returns
  • the protocol of this URL.

public String getQuery ()

Gets the query part of this URL.

Returns
  • the query part of this URL, or null if one does not exist

public String getRef ()

Gets the anchor (also known as the "reference") of this URL.

Returns
  • the anchor (also known as the "reference") of this URL, or null if one does not exist

public String getUserInfo ()

Gets the userInfo part of this URL.

Returns
  • the userInfo part of this URL, or null if one does not exist

public synchronized int hashCode ()

Creates an integer suitable for hash table indexing.

The hash code is based upon all the URL components relevant for URL comparison. As such, this operation is a blocking operation.

Returns
  • a hash code for this URL.

public URLConnection openConnection ()

Returns a URLConnection instance that represents a connection to the remote object referred to by the URL.

A new instance of {@linkplain java.net.URLConnection URLConnection} is created every time when invoking the {@linkplain java.net.URLStreamHandler#openConnection(URL) URLStreamHandler.openConnection(URL)} method of the protocol handler for this URL.

It should be noted that a URLConnection instance does not establish the actual network connection on creation. This will happen only when calling {@linkplain java.net.URLConnection#connect() URLConnection.connect()}.

If for the URL's protocol (such as HTTP or JAR), there exists a public, specialized URLConnection subclass belonging to one of the following packages or one of their subpackages: java.lang, java.io, java.util, java.net, the connection returned will be of that subclass. For example, for HTTP an HttpURLConnection will be returned, and for JAR a JarURLConnection will be returned.

Returns
Throws
IOException if an I/O exception occurs.
See Also

public URLConnection openConnection (Object proxy)

Same 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. Invoking this method preempts the system's default ProxySelector settings.

Parameters
proxy the Proxy through which this connection will be made. If direct connection is desired, Proxy.NO_PROXY should be specified.
Returns
  • a URLConnection to the URL.
Throws
IOException if an I/O exception occurs.
SecurityException if a security manager is present and the caller doesn't have permission to connect to the proxy.
IllegalArgumentException will be thrown if proxy is null, or proxy has the wrong type
UnsupportedOperationException if the subclass that implements the protocol handler doesn't support this method.
See Also

public InputStream openStream ()

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.
See Also

public boolean sameFile (URL other)

Compares two URLs, excluding the fragment component.

Returns true if this URL and the other argument are equal without taking the fragment component into consideration.

Parameters
other the URL to compare against.
Returns
  • true if they reference the same remote object; false otherwise.

public static void setURLStreamHandlerFactory (URLStreamHandlerFactory fac)

Parameters
fac

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

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.
See Also

public URI toURI ()

Returns a URI equivalent to this URL. This method functions in the same way as new URI (this.toString()).

Note, any URL instance that complies with RFC 2396 can be converted to a URI. However, some URLs that are not strictly in compliance can not be converted to a URI.

Returns
  • a URI instance equivalent to this URL.
Throws
URISyntaxException if this URL is not formatted strictly according to to RFC2396 and cannot be converted to a URI.

Protected Methods

protected void set (String protocol, String host, int port, String authority, String userInfo, String path, String query, String ref)

Sets the specified 8 fields of the URL. This is not a public method so that only URLStreamHandlers can modify URL fields. URLs are otherwise constant.

Parameters
protocol the name of the protocol to use
host the name of the host
port the port number on the host
authority the authority part for the url
userInfo the username and password
path the file on the host
query the query part of this URL
ref the internal reference in the URL

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.

Parameters
protocol the name of the protocol to use
host the name of the host
port the port number on the host
file the file on the host
ref the internal reference in the URL