URL to load resources from the classpath in Java

Question

In Java  you can load all kinds of resources using the same API but with different URL protocols   file    tmp txt http   127 0 0 1 8080 a properties jar http   www foo com bar baz jar  COM foo Quux class   This nicely decouples the actual loading of the resource from the application that needs the resource  and since a URL is just a String  resource loading is also very easily configurable   Is there a protocol to load resources using the current classloader  This is similar to the Jar protocol  except that I do not need to know which jar file or class folder the resource is coming from   I can do that using Class getResourceAsStream  a xml    of course  but that would require me to use a different API  and hence changes to existing code  I want to be able to use this in all places where I can specify a URL for the resource already  by just updating a property file

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Similar to Azder s answer  but a slightly different tact    I don t believe there is a predefined protocol handler for content from the classpath   The so-called classpath  protocol    However  Java does allow you to add your own protocols  This is done through providing concrete implementations java net URLStreamHandler and java net URLConnection   This article describes how a custom stream handler can be implemented  http   java sun com developer onlineTraining protocolhandlers

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I think this is worth its own answer - if you re using Spring  you already have this with  Resource firstResource       context getResource  http   www google fi     Resource anotherResource       context getResource  classpath some resource path myTemplate txt      Like explained in the spring documentation and pointed out in the comments by skaffman

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Inspire by  Stephen https   stackoverflow com a 1769454 980442 and http   docstore mik ua orelly java exp ch09 06 htm  To use   new URL  classpath org my package resource extension   openConnection     just create this class into sun net www protocol classpath package and  run it into Oracle JVM implementation to work like a charm   package sun net www protocol classpath   import java io IOException  import java net URL  import java net URLConnection  import java net URLStreamHandler   public class Handler extends URLStreamHandler         Override     protected URLConnection openConnection URL u  throws IOException           return Thread currentThread   getContextClassLoader   getResource u getPath    openConnection              In case you are using another JVM implementation set the java protocol handler pkgs sun net www protocol system property   FYI   http   docs oracle com javase 7 docs api java net URL html URL java lang String  20java lang String  20int  20java lang String

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If you have tomcat on the classpath  it s as simple as   TomcatURLStreamHandlerFactory register      This will register handlers for  war  and  classpath  protocols

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In a Spring Boot app  I used the following to get the file URL   Thread currentThread   getContextClassLoader   getResource  PromotionalOfferIdServiceV2 wsdl

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I ve created a class which helps to reduce errors in setting up custom handlers and takes advantage of the system property so there are no issues with calling a method first or not being in the right container   There s  also an exception class if you get things wrong   CustomURLScheme java        The CustomURLScheme class has a static method for adding cutom protocol    handlers without getting bogged down with other class loaders and having to    call setURLStreamHandlerFactory before the next guy        package com cybernostics lib net customurl   import java net URLStreamHandler  import java util regex Matcher  import java util regex Pattern          Allows you to add your own URL handler without running into problems    of race conditions with setURLStream handler         To add your custom protocol eg myprot   blahblah         1  Create a new protocol package which ends in myprot eg com myfirm protocols myprot    2  Create a subclass of URLStreamHandler called Handler in this package    3  Before you use the protocol  call CustomURLScheme add com myfirm protocols myprot Handler class       author jasonw     public class CustomURLScheme           this is the package name required to implelent a Handler class     private static Pattern packagePattern   Pattern compile         protocols                                Call this method with your handlerclass         param handlerClass         throws Exception              public static void add  Class lt   extends URLStreamHandler gt  handlerClass   throws Exception               if   handlerClass getSimpleName   equals   Handler                            String pkgName   handlerClass getPackage   getName                Matcher m   packagePattern matcher  pkgName                 if   m matches                                   String protocolPackage   m group  1                    add  protocolPackage                              else                               throw new CustomURLHandlerException   Your Handler class package must end in  protocols yourprotocolname  eg com somefirm blah protocols yourprotocol                                      else                       throw new CustomURLHandlerException   Your handler class must be called  Handler                           private static void add  String handlerPackage                    this property controls where java looks for            stream handlers - always uses current value          final String key    java protocol handler pkgs            String newValue   handlerPackage          if   System getProperty  key      null                         final String previousValue   System getProperty  key                newValue          previousValue                    System setProperty  key  newValue              CustomURLHandlerException java        Exception if you get things mixed up creating a custom url protocol     package com cybernostics lib net customurl              author jasonw     public class CustomURLHandlerException extends Exception        public CustomURLHandlerException String msg                 super  msg

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I dont know if there is one already  but you can make it yourself easilly   That different protocols example looks to me like a facade pattern  You have a common interface when there are different implementations for each case   You could use the same principle  make a ResourceLoader class which takes the string from your properties file  and checks for a custom protocol of ours  myprotocol a xml myprotocol file    tmp txt myprotocol http   127 0 0 1 8080 a properties myprotocol jar http   www foo com bar baz jar  COM foo Quux class   strips the myprotocol  from the start of the string and then makes a decision of which way to load the resource  and just gives you the resource

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I try to avoid the URL class and instead rely on URI  Thus for things that need URL where I would like to do Spring Resource like lookup with out Spring I do the following   public static URL toURL URI u  ClassLoader loader  throws MalformedURLException       if   classpath  equals u getScheme               String path   u getPath            if  path startsWith                    path   path substring     length                       return loader getResource path             else if  u getScheme      null  amp  amp  u getPath      null              Assume that its a file          return new File u getPath    toURI   toURL              else           return u toURL              To create a URI you can use URI create      This way is also better because you control the ClassLoader that will do the resource lookup   I noticed some other answers trying to parse the URL as a String to detect the scheme  I think its better to pass around URI and use it to parse instead   I have actually filed an issue a while ago with Spring Source begging them to separate out their Resource code from core so that you don t need all the other Spring stuff

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From Java 9  and up  you can define a new URLStreamHandlerProvider  The URL class uses the service loader framework to load it at run time   Create a provider   package org example   import java io IOException  import java net URL  import java net URLConnection  import java net URLStreamHandler  import java net spi URLStreamHandlerProvider   public class ClasspathURLStreamHandlerProvider extends URLStreamHandlerProvider         Override     public URLStreamHandler createURLStreamHandler String protocol            if   classpath  equals protocol                 return new URLStreamHandler                      Override                 protected URLConnection openConnection URL u  throws IOException                       return ClassLoader getSystemClassLoader   getResource u getPath    openConnection                                                       return null             Create a file called java net spi URLStreamHandlerProvider in the META-INF services directory with the contents   org example ClasspathURLStreamHandlerProvider   Now the URL class will use the provider when it sees something like   URL url   new URL  classpath myfile txt

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An extension to Dilums s answer   Without changing code  you likely need pursue custom implementations of URL related interfaces as Dilum recommends   To simplify things for you  I can recommend looking at the source for Spring Framework s Resources   While the code is not in the form of a stream handler  it has been designed to do exactly what you are looking to do and is under the ASL 2 0 license  making it friendly enough for re-use in your code with due credit

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You can also set the property programmatically during startup   final String key    java protocol handler pkgs   String newValue    org my protocols   if  System getProperty key     null        final String previousValue   System getProperty key       newValue          previousValue    System setProperty key  newValue     Using this class   package org my protocols classpath   import java io IOException  import java net URL  import java net URLConnection  import java net URLStreamHandler   public class Handler extends URLStreamHandler         Override     protected URLConnection openConnection final URL u  throws IOException           final URL resourceUrl   ClassLoader getSystemClassLoader   getResource u getPath             return resourceUrl openConnection              Thus you get the least intrusive way to do this     java net URL will always use the current value from the system properties

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Solution with registering URLStreamHandlers is most correct  of course  but sometimes the simplest solution is needed  So  I use the following method for that          Opens a local file or remote resource represented by given path     Supports protocols      lt ul gt      lt li gt  file   file    path to file in filesystem lt  li gt      lt li gt  http  or  https   http   host path to resource - gzipped resources are supported also lt  li gt      lt li gt  classpath   classpath path to resource lt  li gt      lt  ul gt         param path An URI-formatted path that points to resource to be loaded     return Appropriate implementation of   link InputStream      throws IOException in any case is stream cannot be opened     public static InputStream getInputStreamFromPath String path  throws IOException       InputStream is      String protocol   path replaceFirst      w           1   toLowerCase        switch  protocol            case  http           case  https               HttpURLConnection connection    HttpURLConnection  new URL path  openConnection                int code   connection getResponseCode                if  code  gt   400  throw new IOException  Server returned error code      code               is   connection getInputStream                String contentEncoding   connection getContentEncoding                if  contentEncoding    null  amp  amp  contentEncoding equalsIgnoreCase  gzip                    is   new GZIPInputStream is               break          case  file               is   new URL path  openStream                break          case  classpath               is   Thread currentThread   getContextClassLoader   getResourceAsStream path replaceFirst     w                       break          default              throw new IOException  Missed or unsupported protocol in path      path                   return is

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URL url   getClass   getClassLoader   getResource  someresource xxx      That should do it

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Intro and basic Implementation   First up  you re going to need at least a URLStreamHandler   This will actually open the connection to a given URL   Notice that this is simply called Handler  this allows you to specify java -Djava protocol handler pkgs org my protocols and it will automatically be picked up  using the  simple  package name as the supported protocol  in this case  classpath     Usage  new URL  classpath org my package resource extension   openConnection      Code  package org my protocols classpath   import java io IOException  import java net URL  import java net URLConnection  import java net URLStreamHandler       A   link URLStreamHandler  that handles resources on the classpath     public class Handler extends URLStreamHandler           The classloader to find resources from         private final ClassLoader classLoader       public Handler             this classLoader   getClass   getClassLoader               public Handler ClassLoader classLoader            this classLoader   classLoader              Override     protected URLConnection openConnection URL u  throws IOException           final URL resourceUrl   classLoader getResource u getPath             return resourceUrl openConnection              Launch issuesIf you re anything like me  you don t want to rely on a property being set in the launch to get you somewhere  in my case  I like to keep my options open like Java WebStart - which is why I need all this    Workarounds Enhancements  Manual code Handler specification  If you control the code  you can do  new URL null   classpath some package resource extension   new org my protocols classpath Handler ClassLoader getSystemClassLoader       and this will use your handler to open the connection   But again  this is less than satisfactory  as you don t need a URL to do this - you want to do this because some lib you can t  or don t want to  control wants urls     JVM Handler registration  The ultimate option is to register a URLStreamHandlerFactory that will handle all urls across the jvm   package my org url   import java net URLStreamHandler  import java net URLStreamHandlerFactory  import java util HashMap  import java util Map   class ConfigurableStreamHandlerFactory implements URLStreamHandlerFactory       private final Map lt String  URLStreamHandler gt  protocolHandlers       public ConfigurableStreamHandlerFactory String protocol  URLStreamHandler urlHandler            protocolHandlers   new HashMap lt String  URLStreamHandler gt             addHandler protocol  urlHandler              public void addHandler String protocol  URLStreamHandler urlHandler            protocolHandlers put protocol  urlHandler              public URLStreamHandler createURLStreamHandler String protocol            return protocolHandlers get protocol             To register the handler  call URL setURLStreamHandlerFactory   with your configured factory   Then do new URL  classpath org my package resource extension   like the first example and away you go   JVM Handler Registration Issue  Note that this method may only be called once per JVM  and note well that Tomcat will use this method to register a JNDI handler  AFAIK    Try Jetty  I will be   at worst  you can use the method first and then it has to work around you   License  I release this to the public domain  and ask that if you wish to modify that you start a OSS project somewhere and comment here with the details   A better implementation would be to have a URLStreamHandlerFactory that uses ThreadLocals to store URLStreamHandlers for each Thread currentThread   getContextClassLoader    I ll even give you my modifications and test classes

[java] URL to load resources from the classpath in Java

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