Java's URL.equals() Performs DNS Resolution
source link: https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/net/URL.html
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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:
Types of URL
In general, a URL can be broken into several parts. Consider the following example:
http://www.example.com/docs/resource1.html
The URL above indicates that the protocol to use is http
(HyperText Transfer Protocol) and that the
information resides on a host machine named www.example.com
. The information on that host
machine is named /docs/resource1.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.example.com:1080/docs/resource1.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 describedhere.
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 usingand
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.
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