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A resource is data (images, audio, text, and so on) that a
program needs to access in a way that is independent of the
location of the program code. Java programs can use two mechanisms
to access resources: Applets use Applet.getCodeBase()
to get the base URL for the applet code and then extend the base
URL with a relative path to load the desired resource, for example
with Applet.getAudioClip(url). Applications use "well
known locations" such as
System.getProperty("user.home") or
System.getProperty("java.home"), then add
"/lib/resource", and open that file.
Methods in the classes Class and
ClassLoader provide a location-independent way to
locate resources. For example, they enable locating resources
for:
These methods do not provide specific support for locating localized resources. Localized resources are supported by the internationalization facilities.
A resource is identified by a string consisting of a sequence of
substrings, delimited by slashes (/), followed by a resource name.
Each substring must be a valid Java identifier. The resource name
is of the form shortName or
shortName.extension. Both
shortName and extension
must be Java identifiers.
The name of a resource is independent of the Java implementation; in particular, the path separator is always a slash (/). However, the Java implementation controls the details of how the contents of the resource are mapped into a file, database, or other object containing the actual resource.
The interpretation of a resource name is relative to a class
loader instance. Methods implemented by the
ClassLoader class do this interpretation.
A system resource is a resource that is either built-in to the
system, or kept by the host implementation in, for example, a local
file system. Programs access system resources through the
ClassLoader methods getSystemResource and
getSystemResourceAsStream.
For example, in a particular implementation, locating a system
resource may involve searching the entries in the CLASSPATH. The
ClassLoader methods search each directory, ZIP file,
or JAR file entry in the CLASSPATH for the resource file, and, if
found, returns either an InputStream, or the resource
name. If not found, the methods return null. A resource may be
found in a different entry in the CLASSPATH than the location where
the class file was loaded.
The implementation of getResource on a class loader
depends on the details of the ClassLoader class. For
example, AppletClassLoader:
All class loaders will search for a resource first as a system resource, in a manner analogous to searcing for class files. This search rule permits overwriting locally any resource. Clients should choose a resource name that will be unique (using the company or package name as a prefix, for instance).
A common convention for the name of a resource used by a class
is to use the fully qualified name of the package of the class, but
convert all periods (.) to slashes (/), and add a resource name of
the form name.extension. To support this, and
to simplify handling the details of system classes (for which
getClassLoader returns null), the class
Class provides two convenience methods that call the
appropriate methods in ClassLoader.
The resource name given to a Class method may have
an initial starting "/" that identifies it as an "absolute" name.
Resource names that do not start with a "/" are "relative".
Absolute names are stripped of their starting "/" and are
passed, without any further modification, to the appropriate
ClassLoader method to locate the resource. Relative
names are modified according to the convention described previously
and then are passed to a ClassLoader method.
The Class class implements several methods for
loading resources.
The method getResource() returns a URL for the
resource. The URL (and its representation) is specific to the
implementation and the JVM (that is, the URL obtained in one
runtime instance may not work in another). Its protocol is usually
specific to the ClassLoader loading the resource. If
the resource does not exist or is not visible due to security
considerations, the methods return null.
If the client code wants to read the contents of the resource as
an InputStream, it can apply the
openStream() method on the URL. This is common enough
to justify adding getResourceAsStream() to
Class and ClassLoader.
getResourceAsStream() the same as calling
getResource().openStream(), except that
getResourceAsStream() catches IO exceptions returns a
null InputStream.
Client code code can also request the contents of the resource
as an object by applying the java.net.URL.getContent()
method on the URL. This is useful when the resource contains the
data for an image, for instance. In the case of an image, the
result is an awt.image.ImageProducer object, not an
Image object.
The getResource and
getResourceAsStream methods find a resource with a
given name. They return null if they do not find a resource with
the specified name. The rules for searching for resources
associated with a given class are implemented by the class's
ClassLoader. The Class methods delegate to
ClassLoader methods, after applying a naming
convention: if the resource name starts with "/", it is used as is.
Otherwise, the name of the package is prepended, after converting
all periods (.) to slashes (/).
public InputStream getResourceAsStream(String name) {
name = resolveName(name);
ClassLoader cl = getClassLoader();
if (cl==null) {
return ClassLoader.getSystemResourceAsStream(name); // A system class.
}
return cl.getResourceAsStream(name);
}
public java.net.URL getResource(String name) {
name = resolveName(name);
ClassLoader cl = getClassLoader();
if (cl==null) {
return ClassLoader.getSystemResource(name); // A system class.
}
return cl.getResource(name);
}
The resolveName method adds a package name prefix
if the name is not absolute, and removes any leading "/" if the
name is absolute. It is possible, though uncommon, to have classes
in diffent packages sharing the same resource.
private String resolveName(String name) {
if (name == null) {
return name;
}
if (!name.startsWith("/")) {
Class c = this;
while (c.isArray()) {
c = c.getComponentType();
}
String baseName = c.getName();
int index = baseName.lastIndexOf('.');
if (index != -1) {
name = baseName.substring(0, index).replace('.', '/') + "/" + name;
}
} else {
name = name.substring(1);
}
return name;
}
The ClassLoader class has two sets of methods to
access resources. One set returns an InputStream for
the resource. The other set returns a URL. The methods that return
an InputStream are easier to use and will satisfy many
needs, while the methods that return URLs provide access to more
complex information, such as an Image and an AudioClip.
The ClassLoadermanges resources similarly to the
way it manages classes. A ClassLoader controls how to
map the name of a resource to its content. ClassLoader
also provides methods for accessing system resources,
analogous to the system classes. The Class
class provides some convenience methods that delegate functionality
to the ClassLoader methods.
Many Java programs will access these methods indirectly through
the I18N (localization) APIs. Others will access it through methods
in Class. A few will directly invoke the
ClassLoader methods.
The methods in ClassLoader use the given String as
the name of the resource without applying any absolute/relative
transformation (see the methods in Class). The name should not have
a leading "/".
System resources are those that are handled by the host implemenation directly. For example, they may be located in the CLASSPATH.
The name of a resource is a "/"-separated sequence of
identifiers. The Class class provides convenience
methods for accessing resources; the methods implement a convention
where the package name is prefixed to the short name of the
resource.
Resources can be accessed as an InputStream, or a
URL.
The getSystemResourceAsStream method returns an
InputStream for the specified system resource or null if it does
not find the resource. The resource name may be any system
resource.
The getSystemResource method finds a system
resource with the specified name. It returns a URL to the resource
or null if it does not find the resource. Calling
java.net.URL.getContent() with the URL will return an
object such as ImageProducer, AudioClip,
or InputStream.
The getResourceAsStream method returns an
InputStream for the specified resource or null if it
does not find the resource.
The getResource method finds a resource with the
specified name. It returns a URL to the resource or null if it does
not find the resource. Calling
java.net.URL.getContent() with the URL will return an
object such as ImageProducer, AudioClip,
or InputStream.
Since getResource() provides access to information,
it must have well-defined and well-founded security rules. If
security considerations do not allow a resource to be visible in
some security context, the getResource() method will
fail (return null) as if the resource were not present at all, this
addresses existence attacks.
Class loaders may not provide access to the contents of a .class file for both security and performance reasons. Whether it is possible to obtain a URL for a .class file depends on the specifics, as shown below.
There are no specified security issues or restrictions regarding
resources that are found by a non-system class loader.
AppletClassLoader provides access to information that
is loaded from a source location, either individually, or in a
group through a JAR file; thus AppletClassLoader
should apply the same checkConnect() rules when
dealing with URLs through getResource().
The system ClassLoader provides access to
information in the CLASSPATH. A CLASSPATH may contain directories
and JAR files. Since a JAR file is created intentionally, it has a
different significance than a directory where things may end up in
a more casual manner. In particular, we are more strict on getting
information out of a directory than out from a JAR file.
If a resource is in a directory:
getResource() invocations will use
File.exists() to determine whether to make the
corresponding file visible to the user. Recall that
File.exists() uses the checkRead() method
in the security manager.getResourceAsStream().If the resource is in a JAR file:
getResource() invocations will succeed for all
files, regardless of whether the invocation is done from within a
system or a non-system class.getResourceAsStream() invocations will succeed for
non .class resources, and so will for
java.net.URL.getContent() on corresponding URLs.This section provides two examples of client code. The first
example uses "absolute resource" names and traditional mechanisms
to get a Class object.
package pkg;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintStream;
class Test {
private static final String absName = "/pkg/mumble.baf";
public static void test1() {
Class c=null;
try {
c = Class.forName("pkg.Test");
} catch (Exception ex) {
// This should not happen.
}
InputStream s = c.getResourceAsStream(absName);
// do something with it.
}
public void test2() {
InputStream s = this.getClass().getResourceAsStream(absName);
// do something with it.
}
}
This example uses "relative resource" names and the mechanism
available from the compiler through the -experimental
flag, to get a Class object.
package pkg;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintStream;
class Test {
private static final String relName = "mumble.baf";
public static void test1() {
InputStream s = Test.class.getResourceAsStream(relName);
// do something with it.
}
public void test2() {
InputStream s = Test.class.getResourceAsStream(relName);
// do something with it.
}
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