This section provides a tutorial example on how to find resource files using the ClassLoader.getSystemResource() method. A resource file can an image file, a class file, or any file.
java.lang.ClassLoader also offers methods to help you find resource files:
URL getResource(String name) - Finds the first resource with the given name with this ClassLoader
Enumeration<URL> getResources(String name) - Finds all the resources with the given name with this ClassLoader.
static URL getSystemResource(String name) - Finds the first resource of the given name with the System ClassLoader.
static Enumeration<URL> getSystemResources(String name) - Finds all resources of the given name with the System ClassLoader.
These get resource methods are usually used to locate a file of images, audio or video data.
But they can also be used to locate class files.
When a ClassLoader is called to find a resource file, it will call its parent ClassLoader to
find the resource file first.
When the System ClassLoader is called to find a resource file, it will search for the file
in all folders, ZIP files, or JAR files specified in the "classpath".
When you specify the resource name to a get resource method, you should use the relative path name
like logo.jpg, Hello.class, or java/lang/String.class.
Here is tutorial example program showing you how to find a reource file:
/**
* ClassLoaderGetSystemResource.java
* Copyright (c) 2010 by Dr. Herong Yang, herongyang.com
*/
class ClassLoaderGetSystemResource {
public static void main(String[] a) {
java.io.PrintStream out = System.out;
java.net.URL u = null;
out.println("");
out.println("Find the location of logo.jpg...");
u = ClassLoader.getSystemResource("logo.jpg");
out.println("URL: "+u);
out.println("");
out.println("Find the location of Hello.class...");
u = ClassLoader.getSystemResource("Hello.class");
out.println("URL: "+u);
out.println("");
out.println("Find the location of sun/security/pkcs11/P11Key.class...");
u = ClassLoader.getSystemResource("sun/security/pkcs11/P11Key.class");
out.println("URL: "+u);
out.println("");
out.println("Find the location of java/lang/String.class");
u = ClassLoader.getSystemResource("java/lang/String.class");
out.println("URL: "+u);
}
}
Before running this tutorial program, I prepared these files in the current folder:
Created logo.jpg file.
Created Hello.class file.
Copied logo.jpg into MyZIP.zip file.
Moved Hello.class into MyZIP.zip file.
First let's run ClassLoaderGetSystemResource.java with the default "classpath" setting:
C:\herong\jvm>java ClassLoaderGetSystemResource
Find the location of logo.jpg...
URL: file:/C:/herong/jvm/logo.jpg
Find the location of Hello.class...
URL: null
Find the location of sun/security/pkcs11/P11Key.class...
URL: jar:file:/C:/Program%20Files/Java/jdk/jre/lib/ext/sunpkcs11.jar
!/sun/security/pkcs11/P11Key.class
Find the location of java/lang/String.class
URL: jar:file:/C:/Program%20Files/Java/jdk/jre/lib/rt.jar
!/java/lang/String.class
The test result tells me that:
The System ClassLoader found logo.jpg in the default "classpath" - the current folder.
The System ClassLoader did not find Hello.class.
The parent ClassLoader, Extensions ClassLoader, of the System ClassLoader found
sun/security/pkcs11/P11Key.class in a JAR file in the default extensions folder: <JAVA_HOME>/lib/ext.
The grand parent ClassLoader, Bootstrap ClassLoader, of the System ClassLoader found
java/lang/String.class in a JAR file in the default system folder: <JAVA_HOME>/lib.
Now run ClassLoaderGetSystemResource.java with a differnt "classpath" setting:
C:\herong\jvm>java -classpath MyZIP.zip;. ClassLoaderGetSystemResource
Find the location of logo.jpg...
URL: jar:file:/C:/herong/jvm_20080000/cod/MyZIP.zip!/logo.jpg
Find the location of Hello.class...
URL: jar:file:/C:/herong/jvm_20080000/cod/MyZIP.zip!/Hello.class
Find the location of sun/security/pkcs11/P11Key.class...
URL: jar:file:/C:/Program%20Files/Java/jdk/jre/lib/ext/sunpkcs11.jar
!/sun/security/pkcs11/P11Key.class
Find the location of java/lang/String.class
URL: jar:file:/C:/Program%20Files/Java/jdk/jre/lib/rt.jar
!/java/lang/String.class
This result is different than the previous one:
The System ClassLoader found logo.jpg in MyZIP.zip specified in "classpath" instead of the current folder,
because MyZIP.zip is listed before the current folder in the "classpath".
The System ClassLoader found Hello.class in MyZIP.zip specified in "classpath".
