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What's the difference between a Resource, URI, URL, Path and File in Java?


As he says, the word "file" has totally different (practically unrelated) meanings in URL#getFile vs java.io.File - may be that's part of the confusion.

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What's the difference between a Resource, URI, URL, Path and File in Java?


URL#getFile
URL#getPath
path+query
protocol://host/path?query
  • File represents a file or directory accessible via file system
  • usually resources are files distributed with the application / library and loaded via class-loading mechanism (when they reside on class-path)

@kibibyte: I would expect the call to be blocking, to have an asynchronous implementation of hashcode and equals now that would be very unsettling. I think what you meant is that the calls will try and resolve the host to find if they are equivalent and thus could potentially make blocking network calls.

Also be aware that that URL#getFile is unrelated to java.io.File.

And is there a Resource object?

From the JRE point of view, it's just a term. Some frameworks provide you with such class (e.g. Spring's Resource).

Generally, you should minimize usage of URL unless it is absolutely needed. The reason is that the URL's equals and hashCode methods are implemented in a surprising way: they are blocking method calls.

In Java, URI is just a data structure for manipulating the generic identifier itself.

It is enough. Only if you want to pass the resource to some component which can work only with files, you need to get File from it. However not all resource URLs can be converted to Files.

Please note that I do not consider myself 100% competent to answer, but nevertheless here are some comments:

There's also java.nio.file.Path, which is basically a (Java 7+) replacement for java.io.File, as the latter API was apparently poorly thought out in the early days of Java.

URL on the other hand is really a resource locator and offers you features to actually read the resource via registered URLStreamHandlers.

URLs can lead to file-system resources and you can construct URL for every file system resource by using file:// protocol (hence File <-> URL relation).

Why do I need File object; why isn't a Resource (URL) enough?

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What's the difference between a Resource, URI, URL, Path and File in Java?


As he says, the word "file" has totally different (practically unrelated) meanings in URL#getFile vs java.io.File - may be that's part of the confusion.

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What's the difference between a Resource, URI, URL, Path and File in Java?


An abstract, generic piece of data that can be located and read.

It does not have a direct class or interface representation in Java

  • Backwards compatibility. Old applications should run on newer installations, ideally without modification. This means that an old API (with its names and terminology) needs to be maintained through all newer versions.
  • Cross-platform. The API should provide a usable abstraction of its underlying platform, whether that be an operating system or a browser.

@Christian I meant "only the name" as in: does not in any way model the contents of the file; it's merely a thin wrapper around a string. The "abstract representation" part is quoted from the API docs. ;)

An abstract representation of file and directory pathnames.

An object that may be used to locate a file in a file system. It will typically represent a system dependent file path.

Because one of Java's early design goals was to be run inside of a browser, as a sandboxed application (applets!) with very limited rights/privileges/security clearance, Java makes a clear (theoretical) difference between a file (something on the local file system) and a resource (something it needs to read). This is why reading something relative to the application (icons, class files, and so on) is done through ClassLoader.getResource and not through the File class.

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.

Class.getResource returns an URL: it is more flexible than returning File, and it has served the needs of the system as imagined in the early 1990's.

I'm totally confused right now - mostly because of the terminology, I guess. Can someone please walk me through the differences, or provide a few links to Dummy-proof material? Especially URI to URL and Resource to File? To me, it feels like they should be the same thing, respectively...

In Java, the main difference between URL and URI is that an URL carries the expectation of being resolvable, something the application might want an InputStream from; an URI is treated more like an abstract thingamajig that might point to something resolvable (and usually does), but what it means and how to reach it are more open to context and interpretation.

In tandem with the concept of a resource, the URL represents that resource the same way the File class represents a file in the host platform: as a structured string that points to a resource. URL additionally contains a scheme that hints at how to reach the resource (with "file:" being "ask the host platform"), and so allows pointing at resources through HTTP, FTP, inside a JAR, and whatnot.

Lastly - and most importantly - why do I need File object; why isn't a Resource (URL) enough?

No, there isn't. There are many things named like it, but they are not a resource in the sense of ClassLoader.getResource.

Note that File doesn't need to point to something local, just something that the host platform understands in the context of file access, e.g. a UNC path in Windows. If you mount a ZIP file as a file system in your OS, then File will read its contained entries just fine.

Now, if I have a locator that references a class or package in a jar file, will those two (i.e. path an file strings) differ?

Path takes away the reliance on the host platform's concept of a file. It could be an entry in a ZIP file, a file reachable through FTP or SSH-FS, a multi-rooted representation of the application classpath, or really anything that can be meaningfully represented through the FileSystem interface and its driver, FileSystemProvider. It brings the power of "mounting" file systems into the context of a Java application.

Represents a Uniform Resource Identifier (URI) reference.

The File class represents a resource that is reachable through the platform's native file system. It contains only the name of the file, so it is really more a path (see later) that the host platform interprets according to its own settings, rules, and syntax.

The host platform is represented through the "default file system"; when you call File.toPath, you get a Path on the default file system.

The new file API, iconified in the Path interface, allows for much greater flexibility than the File class could offer. The Path interface is an abstraction of the File class, and is part of the New IO File API. Where File necessarily points to a "file" as understood by the host platform, Path is more generic: it represents a file (resource) in an arbitrary file system.

The terminology is confusing and sometimes befuddling, and mostly born from the evolution both of Java as an API and as a platform over time. To understand how these terms came to mean what they do, it is important to recognise two things that influence Java's design:

URI is a (slight) abstraction over URL. The difference between URI and URL is conceptual and mostly academic, but URI is better defined in a formal sense, and covers a wider range of use cases. Because URL and URI are/were not the same thing, a new class was introduced to represent them, with methods URI.toURL and URL.toURI to move between one and the other.

URL might not be enough because File gives you access to housekeeping data such as permissions (readable, writable, executable), file type (am I a directory?), and the ability to search and manipulate the local file system. If these are features you need, then File or Path provide them.

Unfortunately, URLs come with their own syntax and terminology, including the use of "file" and "path". In case the URL is a file-URL, URL.getFile will return a string identical to the path string of the referenced file.

Unfortunately, because "resource" is also a useful generic term outside of this interpretation, it is also used to name very specific things (e.g. class ResourceBundle, UIResource, Resource) that are not, in this sense, a resource.

Unlikely. If the jar file is on the local file system, you should not have a query component, so URL.getPath and URL.getFile should return the same result. However, pick the one you need: file-URLs may not typically have query components, but I could sure add one anyway.

Wow, very thorough. Just going through it, but already have the first follow-up question: When you say a File "contains only the name of the file", don't you contradict your initial statement that it's "An abstract representation of file and directory pathnames" - i.e.more?

You don't need File if you have access to Path. Some older API may require File, though.

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What's the difference between a Resource, URI, URL, Path and File in Java?


An abstract, generic piece of data that can be located and read.

It does not have a direct class or interface representation in Java

  • Backwards compatibility. Old applications should run on newer installations, ideally without modification. This means that an old API (with its names and terminology) needs to be maintained through all newer versions.
  • Cross-platform. The API should provide a usable abstraction of its underlying platform, whether that be an operating system or a browser.

@Christian I meant "only the name" as in: does not in any way model the contents of the file; it's merely a thin wrapper around a string. The "abstract representation" part is quoted from the API docs. ;)

An abstract representation of file and directory pathnames.

An object that may be used to locate a file in a file system. It will typically represent a system dependent file path.

Because one of Java's early design goals was to be run inside of a browser, as a sandboxed application (applets!) with very limited rights/privileges/security clearance, Java makes a clear (theoretical) difference between a file (something on the local file system) and a resource (something it needs to read). This is why reading something relative to the application (icons, class files, and so on) is done through ClassLoader.getResource and not through the File class.

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.

Class.getResource returns an URL: it is more flexible than returning File, and it has served the needs of the system as imagined in the early 1990's.

I'm totally confused right now - mostly because of the terminology, I guess. Can someone please walk me through the differences, or provide a few links to Dummy-proof material? Especially URI to URL and Resource to File? To me, it feels like they should be the same thing, respectively...

In Java, the main difference between URL and URI is that an URL carries the expectation of being resolvable, something the application might want an InputStream from; an URI is treated more like an abstract thingamajig that might point to something resolvable (and usually does), but what it means and how to reach it are more open to context and interpretation.

In tandem with the concept of a resource, the URL represents that resource the same way the File class represents a file in the host platform: as a structured string that points to a resource. URL additionally contains a scheme that hints at how to reach the resource (with "file:" being "ask the host platform"), and so allows pointing at resources through HTTP, FTP, inside a JAR, and whatnot.

Lastly - and most importantly - why do I need File object; why isn't a Resource (URL) enough?

No, there isn't. There are many things named like it, but they are not a resource in the sense of ClassLoader.getResource.

Note that File doesn't need to point to something local, just something that the host platform understands in the context of file access, e.g. a UNC path in Windows. If you mount a ZIP file as a file system in your OS, then File will read its contained entries just fine.

Now, if I have a locator that references a class or package in a jar file, will those two (i.e. path an file strings) differ?

Path takes away the reliance on the host platform's concept of a file. It could be an entry in a ZIP file, a file reachable through FTP or SSH-FS, a multi-rooted representation of the application classpath, or really anything that can be meaningfully represented through the FileSystem interface and its driver, FileSystemProvider. It brings the power of "mounting" file systems into the context of a Java application.

Represents a Uniform Resource Identifier (URI) reference.

The File class represents a resource that is reachable through the platform's native file system. It contains only the name of the file, so it is really more a path (see later) that the host platform interprets according to its own settings, rules, and syntax.

The host platform is represented through the "default file system"; when you call File.toPath, you get a Path on the default file system.

The new file API, iconified in the Path interface, allows for much greater flexibility than the File class could offer. The Path interface is an abstraction of the File class, and is part of the New IO File API. Where File necessarily points to a "file" as understood by the host platform, Path is more generic: it represents a file (resource) in an arbitrary file system.

The terminology is confusing and sometimes befuddling, and mostly born from the evolution both of Java as an API and as a platform over time. To understand how these terms came to mean what they do, it is important to recognise two things that influence Java's design:

URI is a (slight) abstraction over URL. The difference between URI and URL is conceptual and mostly academic, but URI is better defined in a formal sense, and covers a wider range of use cases. Because URL and URI are/were not the same thing, a new class was introduced to represent them, with methods URI.toURL and URL.toURI to move between one and the other.

URL might not be enough because File gives you access to housekeeping data such as permissions (readable, writable, executable), file type (am I a directory?), and the ability to search and manipulate the local file system. If these are features you need, then File or Path provide them.

Unfortunately, URLs come with their own syntax and terminology, including the use of "file" and "path". In case the URL is a file-URL, URL.getFile will return a string identical to the path string of the referenced file.

Unfortunately, because "resource" is also a useful generic term outside of this interpretation, it is also used to name very specific things (e.g. class ResourceBundle, UIResource, Resource) that are not, in this sense, a resource.

Unlikely. If the jar file is on the local file system, you should not have a query component, so URL.getPath and URL.getFile should return the same result. However, pick the one you need: file-URLs may not typically have query components, but I could sure add one anyway.

Wow, very thorough. Just going through it, but already have the first follow-up question: When you say a File "contains only the name of the file", don't you contradict your initial statement that it's "An abstract representation of file and directory pathnames" - i.e.more?

You don't need File if you have access to Path. Some older API may require File, though.

Note
Rectangle 27 0

What's the difference between a Resource, URI, URL, Path and File in Java?


URL#getFile
URL#getPath
path+query
protocol://host/path?query
  • File represents a file or directory accessible via file system
  • usually resources are files distributed with the application / library and loaded via class-loading mechanism (when they reside on class-path)

@kibibyte: I would expect the call to be blocking, to have an asynchronous implementation of hashcode and equals now that would be very unsettling. I think what you meant is that the calls will try and resolve the host to find if they are equivalent and thus could potentially make blocking network calls.

Also be aware that that URL#getFile is unrelated to java.io.File.

And is there a Resource object?

From the JRE point of view, it's just a term. Some frameworks provide you with such class (e.g. Spring's Resource).

Generally, you should minimize usage of URL unless it is absolutely needed. The reason is that the URL's equals and hashCode methods are implemented in a surprising way: they are blocking method calls.

In Java, URI is just a data structure for manipulating the generic identifier itself.

It is enough. Only if you want to pass the resource to some component which can work only with files, you need to get File from it. However not all resource URLs can be converted to Files.

Please note that I do not consider myself 100% competent to answer, but nevertheless here are some comments:

There's also java.nio.file.Path, which is basically a (Java 7+) replacement for java.io.File, as the latter API was apparently poorly thought out in the early days of Java.

URL on the other hand is really a resource locator and offers you features to actually read the resource via registered URLStreamHandlers.

URLs can lead to file-system resources and you can construct URL for every file system resource by using file:// protocol (hence File <-> URL relation).

Why do I need File object; why isn't a Resource (URL) enough?

Note
Rectangle 27 0

What's the difference between a Resource, URI, URL, Path and File in Java?


  • File: This is a reference to a directory or file. It's used to modify files, open them etc.
  • Path: A path is basically a string. But it comes with some handy functions to concatenate multiple strings, or add files to a string. It makes sure the path you are building is valid.
  • Resource: Resources are files inside your jar. They are used to load files out of jars / containers.
  • URIs: Ever URL is an URI. But URIs can also contain things like "mailto:", so they are also, well some what of a "script" I'd say.
  • URLs: An URL is a definite location on the internt (just a normal webaddress like - stackoverflow.com)

And local: Resource, Path and Files

As I understand them, you could categorize them as following:

It would be easier if they would be merged into one class - they are really confusing :D

Note
Rectangle 27 0

What's the difference between a Resource, URI, URL, Path and File in Java?


  • File: This is a reference to a directory or file. It's used to modify files, open them etc.
  • Path: A path is basically a string. But it comes with some handy functions to concatenate multiple strings, or add files to a string. It makes sure the path you are building is valid.
  • Resource: Resources are files inside your jar. They are used to load files out of jars / containers.
  • URIs: Ever URL is an URI. But URIs can also contain things like "mailto:", so they are also, well some what of a "script" I'd say.
  • URLs: An URL is a definite location on the internt (just a normal webaddress like - stackoverflow.com)

And local: Resource, Path and Files

As I understand them, you could categorize them as following:

It would be easier if they would be merged into one class - they are really confusing :D

Note