This chapter covers how Spring handles resources and how you can work with resources in Spring. It includes the following topics:
Java’s standard java.net.URL class and standard handlers for various URL prefixes,
unfortunately, are not quite adequate enough for all access to low-level resources. For
example, there is no standardized URL implementation that may be used to access a
resource that needs to be obtained from the classpath or relative to a
ServletContext. While it is possible to register new handlers for specialized URL
prefixes (similar to existing handlers for prefixes such as http:), this is generally
quite complicated, and the URL interface still lacks some desirable functionality,
such as a method to check for the existence of the resource being pointed to.
Spring’s Resource interface is meant to be a more capable interface for abstracting
access to low-level resources. The following listing shows the Resource interface
definition:
public interface Resource extends InputStreamSource {
boolean exists();
boolean isOpen();
URL getURL() throws IOException;
File getFile() throws IOException;
Resource createRelative(String relativePath) throws IOException;
String getFilename();
String getDescription();
}As the definition of the Resource interface shows, it extends the InputStreamSource
interface. The following listing shows the definition of the InputStreamSource
interface:
public interface InputStreamSource {
InputStream getInputStream() throws IOException;
}Some of the most important methods from the Resource interface are:
-
getInputStream(): Locates and opens the resource, returning anInputStreamfor reading from the resource. It is expected that each invocation returns a freshInputStream. It is the responsibility of the caller to close the stream. -
exists(): Returns abooleanindicating whether this resource actually exists in physical form. -
isOpen(): Returns abooleanindicating whether this resource represents a handle with an open stream. Iftrue, theInputStreamcannot be read multiple times and must be read once only and then closed to avoid resource leaks. Returnsfalsefor all usual resource implementations, with the exception ofInputStreamResource. -
getDescription(): Returns a description for this resource, to be used for error output when working with the resource. This is often the fully qualified file name or the actual URL of the resource.
Other methods let you obtain an actual URL or File object representing the
resource (if the underlying implementation is compatible and supports that
functionality).
Spring itself uses the Resource abstraction extensively, as an argument type in
many method signatures when a resource is needed. Other methods in some Spring APIs
(such as the constructors to various ApplicationContext implementations) take a
String which in unadorned or simple form is used to create a Resource appropriate to
that context implementation or, via special prefixes on the String path, let the
caller specify that a specific Resource implementation must be created and used.
While the Resource interface is used a lot with Spring and by Spring, it is actually
very useful to use as a general utility class by itself in your own code, for access to
resources, even when your code does not know or care about any other parts of Spring.
While this couples your code to Spring, it really only couples it to this small set of
utility classes, which serve as a more capable replacement for URL and can be
considered equivalent to any other library you would use for this purpose.
|
Note
|
The Resource abstraction does not replace functionality.
It wraps it where possible. For example, a UrlResource wraps a URL and uses the
wrapped URL to do its work.
|
Spring includes the following Resource implementations:
UrlResource wraps a java.net.URL and can be used to access any object that is
normally accessible with a URL, such as files, an HTTP target, an FTP target, and others. All
URLs have a standardized String representation, such that appropriate standardized
prefixes are used to indicate one URL type from another. This includes file: for
accessing filesystem paths, http: for accessing resources through the HTTP protocol,
ftp: for accessing resources through FTP, and others.
A UrlResource is created by Java code by explicitly using the UrlResource constructor
but is often created implicitly when you call an API method that takes a String
argument meant to represent a path. For the latter case, a JavaBeans
PropertyEditor ultimately decides which type of Resource to create. If the path
string contains well-known (to it, that is) prefix (such as classpath:), it
creates an appropriate specialized Resource for that prefix. However, if it does not
recognize the prefix, it assume the string is a standard URL string and
creates a UrlResource.
This class represents a resource that should be obtained from the classpath. It uses either the thread context class loader, a given class loader, or a given class for loading resources.
This Resource implementation supports resolution as java.io.File if the class path
resource resides in the file system but not for classpath resources that reside in a
jar and have not been expanded (by the servlet engine or whatever the environment is)
to the filesystem. To address this, the various Resource implementations always support
resolution as a java.net.URL.
A ClassPathResource is created by Java code by explicitly using the ClassPathResource
constructor but is often created implicitly when you call an API method that takes a
String argument meant to represent a path. For the latter case, a JavaBeans
PropertyEditor recognizes the special prefix, classpath:, on the string path and
creates a ClassPathResource in that case.
This is a Resource implementation for java.io.File and java.nio.file.Path handles.
It supports resolution as a File and as a URL.
This is a Resource implementation for ServletContext resources that interprets
relative paths within the relevant web application’s root directory.
It always supports stream access and URL access but allows java.io.File access only
when the web application archive is expanded and the resource is physically on the
filesystem. Whether or not it is expanded and on the filesystem or accessed
directly from the JAR or somewhere else like a database (which is conceivable) is actually
dependent on the Servlet container.
An InputStreamResource is a Resource implementation for a given InputStream. It should be used only if no
specific Resource implementation is applicable. In particular, prefer
ByteArrayResource or any of the file-based Resource implementations where possible.
In contrast to other Resource implementations, this is a descriptor for an already-opened
resource. Therefore, it returns true from isOpen(). Do not use it if you need
to keep the resource descriptor somewhere or if you need to read a stream multiple
times.
The ResourceLoader interface is meant to be implemented by objects that can return
(that is, load) Resource instances. The following listing shows the ResourceLoader
interface definition:
public interface ResourceLoader {
Resource getResource(String location);
}All application contexts implement the ResourceLoader interface. Therefore, all
application contexts may be used to obtain Resource instances.
When you call getResource() on a specific application context, and the location path
specified doesn’t have a specific prefix, you get back a Resource type that is
appropriate to that particular application context. For example, assume the following
snippet of code was executed against a ClassPathXmlApplicationContext instance:
Resource template = ctx.getResource("some/resource/path/myTemplate.txt");Against a ClassPathXmlApplicationContext, that code returns a ClassPathResource. If the same method were executed
against a FileSystemXmlApplicationContext instance, it would return a
FileSystemResource. For a WebApplicationContext, it would return a
ServletContextResource. It would similarly return appropriate objects for each context.
As a result, you can load resources in a fashion appropriate to the particular application context.
On the other hand, you may also force ClassPathResource to be used, regardless of the
application context type, by specifying the special classpath: prefix, as the following
example shows:
Resource template = ctx.getResource("classpath:some/resource/path/myTemplate.txt");Similarly, you can force a UrlResource to be used by specifying any of the standard
java.net.URL prefixes. The following pair of examples use the file and http
prefixes:
Resource template = ctx.getResource("file:///some/resource/path/myTemplate.txt");Resource template = ctx.getResource("http://myhost.com/resource/path/myTemplate.txt");The following table summarizes the strategy for converting String objects to Resource objects:
| Prefix | Example | Explanation |
|---|---|---|
classpath: |
|
Loaded from the classpath. |
file: |
|
Loaded as a |
http: |
Loaded as a |
|
(none) |
|
Depends on the underlying |
The ResourceLoaderAware interface is a special callback interface which identifies
components that expect to be provided with a ResourceLoader reference. The following
listing shows the definition of the ResourceLoaderAware interface:
public interface ResourceLoaderAware {
void setResourceLoader(ResourceLoader resourceLoader);
}When a class implements ResourceLoaderAware and is deployed into an application context
(as a Spring-managed bean), it is recognized as ResourceLoaderAware by the application
context. The application context then invokes setResourceLoader(ResourceLoader),
supplying itself as the argument (remember, all application contexts in Spring implement
the ResourceLoader interface).
Since an ApplicationContext is a ResourceLoader, the bean could also implement the
ApplicationContextAware interface and use the supplied application context directly to
load resources. However, in general, it is better to use the specialized ResourceLoader
interface if that is all you need. The code would be coupled only to the resource loading
interface (which can be considered a utility interface) and not to the whole Spring
ApplicationContext interface.
In application components, you may also rely upon autowiring of the ResourceLoader as
an alternative to implementing the ResourceLoaderAware interface. The “traditional”
constructor and byType autowiring modes (as described in [beans-factory-autowire])
are capable of providing a ResourceLoader for either a constructor argument or a
setter method parameter, respectively. For more flexibility (including the ability to
autowire fields and multiple parameter methods), consider using the annotation-based
autowiring features. In that case, the ResourceLoader is autowired into a field,
constructor argument, or method parameter that expects the ResourceLoader type as long
as the field, constructor, or method in question carries the @Autowired annotation.
For more information, see [beans-autowired-annotation].
If the bean itself is going to determine and supply the resource path through some sort
of dynamic process, it probably makes sense for the bean to use the ResourceLoader
interface to load resources. For example, consider the loading of a template of some
sort, where the specific resource that is needed depends on the role of the user. If the
resources are static, it makes sense to eliminate the use of the ResourceLoader
interface completely, have the bean expose the Resource properties it needs,
and expect them to be injected into it.
What makes it trivial to then inject these properties is that all application contexts
register and use a special JavaBeans PropertyEditor, which can convert String paths
to Resource objects. So, if myBean has a template property of type Resource, it can
be configured with a simple string for that resource, as the following example shows:
<bean id="myBean" class="...">
<property name="template" value="some/resource/path/myTemplate.txt"/>
</bean>Note that the resource path has no prefix. Consequently, because the application context itself is
going to be used as the ResourceLoader, the resource itself is loaded through a
ClassPathResource, a FileSystemResource, or a ServletContextResource,
depending on the exact type of the context.
If you need to force a specific Resource type to be used, you can use a prefix.
The following two examples show how to force a ClassPathResource and a
UrlResource (the latter being used to access a filesystem file):
<property name="template" value="classpath:some/resource/path/myTemplate.txt"><property name="template" value="file:///some/resource/path/myTemplate.txt"/>This section covers how to create application contexts with resources, including shortcuts that work with XML, how to use wildcards, and other details.
An application context constructor (for a specific application context type) generally takes a string or array of strings as the location paths of the resources, such as XML files that make up the definition of the context.
When such a location path does not have a prefix, the specific Resource type built from
that path and used to load the bean definitions depends on and is appropriate to the
specific application context. For example, consider the following example, which creates a
ClassPathXmlApplicationContext:
ApplicationContext ctx = new ClassPathXmlApplicationContext("conf/appContext.xml");The bean definitions are loaded from the classpath, because a ClassPathResource is
used. However, consider the following example, which creates a FileSystemXmlApplicationContext:
ApplicationContext ctx =
new FileSystemXmlApplicationContext("conf/appContext.xml");Now the bean definition is loaded from a filesystem location (in this case, relative to the current working directory).
Note that the use of the special classpath prefix or a standard URL prefix on the
location path overrides the default type of Resource created to load the
definition. Consider the following example:
ApplicationContext ctx =
new FileSystemXmlApplicationContext("classpath:conf/appContext.xml");Using FileSystemXmlApplicationContext loads the bean definitions from the classpath. However, it is still a
FileSystemXmlApplicationContext. If it is subsequently used as a ResourceLoader, any
unprefixed paths are still treated as filesystem paths.
The ClassPathXmlApplicationContext exposes a number of constructors to enable
convenient instantiation. The basic idea is that you can supply merely a string array
that contains only the filenames of the XML files themselves (without the leading path
information) and also supplies a Class. The ClassPathXmlApplicationContext
then derives the path information from the supplied class.
Consider the following directory layout:
com/
foo/
services.xml
daos.xml
MessengerService.class
The following example shows how a ClassPathXmlApplicationContext instance composed of the beans defined in
files named services.xml and daos.xml (which are on the classpath) can be instantiated:
ApplicationContext ctx = new ClassPathXmlApplicationContext(
new String[] {"services.xml", "daos.xml"}, MessengerService.class);See the {api-spring-framework}/jca/context/SpringContextResourceAdapter.html[ClassPathXmlApplicationContext]
javadoc for details on the various constructors.
The resource paths in application context constructor values may be simple paths (as
shown earlier), each of which has a one-to-one mapping to a target Resource or, alternately, may
contain the special "classpath*:" prefix or internal Ant-style regular expressions
(matched by using Spring’s PathMatcher utility). Both of the latter are effectively
wildcards.
One use for this mechanism is when you need to do component-style application assembly. All
components can 'publish' context definition fragments to a well-known location path, and,
when the final application context is created using the same path prefixed with
classpath*:, all component fragments are automatically picked up.
Note that this wildcarding is specific to the use of resource paths in application context
constructors (or when you use the PathMatcher utility class hierarchy directly) and is
resolved at construction time. It has nothing to do with the Resource type itself.
You cannot use the classpath*: prefix to construct an actual Resource, as
a resource points to just one resource at a time.
Path locations can contain Ant-style patterns, as the following example shows:
/WEB-INF/*-context.xml com/mycompany/**/applicationContext.xml file:C:/some/path/*-context.xml classpath:com/mycompany/**/applicationContext.xml
When the path location contains an Ant-style pattern, the resolver follows a more complex procedure to try to resolve the
wildcard. It produces a Resource for the path up to the last non-wildcard segment and
obtains a URL from it. If this URL is not a jar: URL or container-specific variant
(such as zip: in WebLogic, wsjar in WebSphere, and so on), a java.io.File is
obtained from it and used to resolve the wildcard by traversing the filesystem. In the
case of a jar URL, the resolver either gets a java.net.JarURLConnection from it or
manually parses the jar URL and then traverses the contents of the jar file to resolve
the wildcards.
If the specified path is already a file URL (either implicitly because the base
ResourceLoader is a filesystem one or explicitly), wildcarding is guaranteed to
work in a completely portable fashion.
If the specified path is a classpath location, the resolver must obtain the last
non-wildcard path segment URL by making a Classloader.getResource() call. Since this
is just a node of the path (not the file at the end), it is actually undefined (in the
ClassLoader javadoc) exactly what sort of a URL is returned in this case. In practice,
it is always a java.io.File representing the directory (where the classpath resource
resolves to a filesystem location) or a jar URL of some sort (where the classpath resource
resolves to a jar location). Still, there is a portability concern on this operation.
If a jar URL is obtained for the last non-wildcard segment, the resolver must be able to
get a java.net.JarURLConnection from it or manually parse the jar URL, to be able to
walk the contents of the jar and resolve the wildcard. This does work in most environments
but fails in others, and we strongly recommend that the wildcard resolution of resources
coming from jars be thoroughly tested in your specific environment before you rely on it.
When constructing an XML-based application context, a location string may use the
special classpath*: prefix, as the following example shows:
ApplicationContext ctx =
new ClassPathXmlApplicationContext("classpath*:conf/appContext.xml");This special prefix specifies that all classpath resources that match the given name
must be obtained (internally, this essentially happens through a call to
ClassLoader.getResources(…)) and then merged to form the final application
context definition.
|
Note
|
The wildcard classpath relies on the getResources() method of the underlying
classloader. As most application servers nowadays supply their own classloader
implementation, the behavior might differ, especially when dealing with jar files. A
simple test to check if classpath* works is to use the classloader to load a file from
within a jar on the classpath:
getClass().getClassLoader().getResources("<someFileInsideTheJar>"). Try this test with
files that have the same name but are placed inside two different locations. In case an
inappropriate result is returned, check the application server documentation for
settings that might affect the classloader behavior.
|
You can also combine the classpath*: prefix with a PathMatcher pattern in the
rest of the location path (for example, classpath*:META-INF/*-beans.xml). In this
case, the resolution strategy is fairly simple: A ClassLoader.getResources() call is
used on the last non-wildcard path segment to get all the matching resources in the
class loader hierarchy and then, off each resource, the same PathMatcher resolution
strategy described earlier is used for the wildcard subpath.
Note that classpath*:, when combined with Ant-style patterns, only works
reliably with at least one root directory before the pattern starts, unless the actual
target files reside in the file system. This means that a pattern such as
classpath*:*.xml might not retrieve files from the root of jar files but rather only
from the root of expanded directories.
Spring’s ability to retrieve classpath entries originates from the JDK’s
ClassLoader.getResources() method, which only returns file system locations for an
empty string (indicating potential roots to search). Spring evaluates
URLClassLoader runtime configuration and the java.class.path manifest in jar files
as well, but this is not guaranteed to lead to portable behavior.
|
Note
|
The scanning of classpath packages requires the presence of corresponding directory entries in the classpath. When you build JARs with Ant, do not activate the files-only switch of the JAR task. Also, classpath directories may not get exposed based on security policies in some environments — for example, stand-alone applications on JDK 1.7.0_45 and higher (which requires 'Trusted-Library' to be set up in your manifests. See http://stackoverflow.com/questions/19394570/java-jre-7u45-breaks-classloader-getresources). On JDK 9’s module path (Jigsaw), Spring’s classpath scanning generally works as expected. Putting resources into a dedicated directory is highly recommendable here as well, avoiding the aforementioned portability problems with searching the jar file root level. |
Ant-style patterns with classpath: resources are not guaranteed to find matching
resources if the root package to search is available in multiple class path locations.
Consider the following example of a resource location:
com/mycompany/package1/service-context.xml
Now consider an Ant-style path that someone might use to try to find that file:
classpath:com/mycompany/**/service-context.xml
Such a resource may be in only one location, but when a path such as the preceding example
is used to try to resolve it, the resolver works off the (first) URL returned by
getResource("com/mycompany");. If this base package node exists in multiple
classloader locations, the actual end resource may not be there. Therefore, in such a case
you should prefer using classpath*: with the same Ant-style pattern, which
searches all class path locations that contain the root package.
A FileSystemResource that is not attached to a FileSystemApplicationContext (that
is, when a FileSystemApplicationContext is not the actual ResourceLoader) treats
absolute and relative paths as you would expect. Relative paths are relative to the
current working directory, while absolute paths are relative to the root of the
filesystem.
For backwards compatibility (historical) reasons however, this changes when the
FileSystemApplicationContext is the ResourceLoader. The
FileSystemApplicationContext forces all attached FileSystemResource instances
to treat all location paths as relative, whether they start with a leading slash or not.
In practice, this means the following examples are equivalent:
ApplicationContext ctx =
new FileSystemXmlApplicationContext("conf/context.xml");ApplicationContext ctx =
new FileSystemXmlApplicationContext("/conf/context.xml");The following examples are also equivalent (even though it would make sense for them to be different, as one case is relative and the other absolute):
FileSystemXmlApplicationContext ctx = ...;
ctx.getResource("some/resource/path/myTemplate.txt");FileSystemXmlApplicationContext ctx = ...;
ctx.getResource("/some/resource/path/myTemplate.txt");In practice, if you need true absolute filesystem paths, you should avoid using
absolute paths with FileSystemResource or FileSystemXmlApplicationContext and
force the use of a UrlResource by using the file: URL prefix. The following examples
show how to do so:
// actual context type doesn't matter, the Resource will always be UrlResource
ctx.getResource("file:///some/resource/path/myTemplate.txt");// force this FileSystemXmlApplicationContext to load its definition via a UrlResource
ApplicationContext ctx =
new FileSystemXmlApplicationContext("file:///conf/context.xml");