Home > Articles > Programming > Java

Exploring Java's Network API: URIs and URLs

URIs, URLs, and URNs are standard ways to identify, locate, and name resources on the World Wide Web. In this article, Jeff Friesen explores URIs, URLs, and URNs. Along the way, he examines the Network API's URI and URL classes (along with URL-related classes) and shows how to use those classes in your programs. You'll also discover the concept of MIME and how it relates to URLs.
Like this article? We recommend

Like this article? We recommend

In 1989, Tim Berners-Lee invented the World Wide Web (WWW). Think of the WWW as a global collection of interconnected physical and abstract resources—entities supplying information on demand—that are accessed over the Internet. Physical resources range from files to people, and abstract resources include database queries. Because resources are identified in various ways (people have nonunique names, whereas computer files can be accessed via unique pathname combinations), a uniform way to identify WWW resources was needed. To address that need, Tim Berners-Lee introduced standardized ways to identify, locate, and name resources: URIs, URLs, and URNs.


You can learn more about Tim Berners-Lee (and read a few of his WWW essays and articles) by visiting his Web page at http://www.w3.org/People/Berners-Lee/.

This article, the second in my Network API trilogy, explores URIs and URLs (and, to a lesser extent, URNs). After presenting basic concepts about those entities, the article examines the Network API's URI and URL classes (along with URL-related classes) and shows how to use those classes in your programs. Along the way, you discover the concept of MIME and how that concept relates to URLs.

This article's coverage of URIs, URLs, URNs, and MIME is based on two Request For Comments (RFC) documents. (RFC documents serve as the mechanism by which the Internet's architecture evolves.) The relevant RFC documents are listed here:

What Are URIs, URLs, and URNs?

URIs, URLs, and URNs relate to each other in a hierarchy. The URI category sits at the top of that hierarchy, while the URL and URN categories sit at the bottom. That arrangement indicates that both URL and URN are subcategories of URI, as Figure 1 illustrates.

Figure 1 URI, URL, and URN form a hierarchical relationship. URL and URN are subcategories of URI.

URI stands for uniform resource identifier, a compact string of characters that identifies a resource in a uniform (standardized) manner. That string typically begins with a scheme (an identifier that names the URI's namespace—a set of related names) and has the following syntax:

[scheme:] scheme-specific-part

The URI optionally begins with scheme and a colon character. The scheme begins with an uppercase/lowercase letter, followed by zero or more uppercase/lowercase letters, digits, plus sign characters, minus sign characters, and period characters. The colon character separates scheme from the scheme-specific-part, and the scheme-specific-part's syntax and semantics (meaning) are determined by the URI's namespace. An example of a URI is http://www.cnn.com, in which http is the scheme, //http://www.cnn.com is the scheme-specific-part, and the scheme and scheme-specific-part are separated by a colon character.

URIs can be categorized as absolute or relative. An absolute URI is a URI that begins with a scheme (followed by a colon character). The earlier http://www.cnn.com is an example of an absolute URI. Other examples include mailto:jeff@javajeff.com, news:comp.lang.java.help, and xyz://whatever. Think of an absolute URI as referring to some resource in a manner that is independent of the context in which that identifier appears. To use a file system analogy, an absolute URI is like a pathname to a file that starts from the root directory. In contrast to an absolute URI, a relative URI is a URI that does not begin with a scheme (followed by a colon character). An example is articles/articles.html. Think of a relative URI as referring to some resource in a manner that is dependent on the context in which that identifier appears. Using the file system analogy, the relative URI is like a pathname to a file that starts from the current directory.

URIs can be further categorized as opaque or hierarchical. An opaque URI is an absolute URI whose scheme-specific-part does not begin with a forward slash (/) character. Examples include news:comp.lang.java and the earlier mailto:jeff@javajeff.com. Opaque URIs are not subject to parsing (beyond identifying the scheme) because the scheme-specific-part does not need to be validated. By contrast, a hierarchical URI is either an absolute URI whose scheme-specific-part begins with a forward slash character, or a relative URI.

Unlike an opaque URI, a hierarchical URI's scheme-specific-part must be parsed into various components. What components are those? The scheme-specific-part of a common subset of hierarchical URI identifies components according to the following syntax:

[//authority] [path] [?query] [#fragment]

The optional authority component identifies the naming authority for the URI's namespace. If present, that component begins with a pair of forward slash characters, is either server-based or registry-based, and terminates with the next forward slash character, question mark character, or no more characters—the end of the URI. Registry-based authority components have scheme-specific syntaxes (and are not discussed in this article because they are not commonly used), whereas server-based authority components tend to have the following syntax:

[userinfo@] host [:port]

According to this syntax, a server-based authority component optionally begins with user information (such as a username) and an "at" (@) character, continues with the name of a host, and optionally concludes with a colon (:) character and a port. For example, jeff@x.com:90 is a server-based authority component, in which jeff comprises the user information, x.com comprises the host, and 90 comprises the port.

The optional path component identifies the location of a resource according to the authority component (if present) or the scheme (if there is no authority component). A path divides into a sequence of path segments, in which each path segment (a portion of the path) is separated from other path segments by a forward slash character. The path is considered to be absolute if the first path segment begins with a forward slash character. Otherwise, the path is considered to be relative. For example, /a/b/c constitutes a path with three path segments—a, b, and c. Furthermore, that path is absolute because a forward slash character prefixes the first path segment (a). (Despite appearances to the contrary, a URI's path and a directory's path are two different things.)

The optional query component identifies data to be passed to the resource. That resource uses the data to obtain or produce other data that passes back to the caller. For example, in http://www.somesite.net/a?x=y, x=y represents a query. According to that query, x=y is data to be passed to a resource—x names some entity and y is the value of that entity.

The final component is fragment. Although that component appears to be part of a URI, it is not. When a URI is used in some kind of retrieval action, the software that performs that action later uses fragment to focus on the part of a resource that is of interest to the software (after the software has successfully retrieved data from the resource).

To put the aforementioned component information into perspective, consider the following URI:


The previous URI identifies ftp as the scheme, george@x.com:90 as the server-based authority (in which george constitutes the user information, x.com constitutes the host, and 90 constitutes the port), /public/notes as the path, text=shakespeare as the query, and hamlet as the fragment. Essentially, a user named george wants to retrieve information on hamlet from the shakespeare text that's located, via the /public/notes path, on port 90 of server x.com. After shakespeare is successfully returned to the program, the program locates the hamlet section and presents that section to the program's user.

Some URIs contain one or more path segments consisting of single-period characters. Those path segments contribute nothing to the URIs. Other URIs contain path segments consisting of two consecutive period characters, in which each of those path segments is preceded by a path segment that does not contain those characters. As with single-period character path segments, such path segments contribute nothing to the URIs. The act of removing unnecessary single-period character path segments and unnecessary double-period character path segments (plus immediately preceding non–double-period path segments) is known as normalization.

Normalization can be understood in directory terms. Suppose that directory x exists immediately below the root directory, x contains directories a and b, b contains the file memo.txt, and a is the current directory.

To display the contents of memo.txt (under Microsoft Windows), you could specify type \x\.\b\memo.txt. However, the single-period character accomplishes nothing. You could also specify type \x\a\..\b\memo.txt. In this case, the presence of a and .. are not necessary. Neither directory path is in its simplest form. However, if you specify type \x\b\memo.txt, you are specifying the simplest path, beginning with the root directory, to access memo.txt. That \x\b\memo.txt simplest path is known as a normalized directory path. (The same idea applies to URIs.)

Resources are often accessed via base and relative URIs. A base URI is an absolute URI that uniquely identifies a resource's namespace, whereas a relative URI identifies a resource relative to the base URI. (Unlike a base URI, a relative URI might never need to change in a resource's lifetime.) Because neither the base URI nor the relative URI completely identifies the resource, it is necessary to merge both URIs through a process known as resolution. Conversely, it is possible to extract the relative URI from the merged URI through a process known as relativization (the inverse of resolution.)


Unlike other URIs, opaque URIs are not subject to normalization, resolution, or relativization.

Suppose that you have x://a/ as a base URI and b/c as a relative URI. Resolving the relative URI against the base URI yields x://a/b/c. Relativizing x://a/b/c against x://a/ yields b/c.

URIs cannot locate and read from/write to resources. That is the job of the uniform resource locator (URL). A URL is a URI whose scheme component is known as a network protocol (protocol, for short), and it combines URI components with a protocol handler (a resource locator and read/write mechanism that communicates with a resource according to strict rules that have been established for the protocol).

It is also true that URIs cannot provide persistent names for resources. That is the job of the uniform resource name (URN). A URN is a URI that is globally unique and persistent, even when a resource ceases to exist or is no longer available. (That is all I have to say about URNs in this article.)

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020