resource : Java Glossary

Use

Class.getResource is similar, giving you the URL of the resource instead of an InputStream to read it. Note that getResource is an instance method. Class.getResource makes these changes to the resource name: if the resource name starts with /, it is unchanged; otherwise, the package name is prepended to the resource name after converting . to /. This allows you to use either dots or slashes to separate the components of the name. So normally your resource member name includes the package name, but you don’t specify the package name to getResource. Alternatively, but not recommended, you can specify the fully qualified name of the resource by using a lead / on the name you feed to getResource. Never use \ in resource names. So you can access by either: /com/mindprod/entities/entitytable.ser or the short form entitytable.ser. The short form only works from classes in the com.mindprod.entities package.

Recipe

1. Using jar.exe, pack the resource in the jar under the package name and resource name. For example, a resource for use by com.mindprod.entities. Entities would be stored in the jar under the name com/mindprod/entities/entitytable.ser. Note that the class name does not appear, just the package and resource name. See jar.exe for how. Use WinZip or similar ZIP utility to verify the package name and resource name are correct inside the jar, including case.

2. // accessing a resource with getResource to give you its URL
   
   URL url = Entities.class.getResource( "entitytable.ser" );
   out.println( url );
   
   // Note that getResource is an instance method of Class,
   // so You CANNOT say:
   URL url = Class.getResource( "entitytable.ser" );

   Note the lack of dots, slashes, package name or class name in the resource.

Tips

• You can also put a resource in the root without a package name, but then you have to be careful not to look for it with an implicit package name.
• For large resources, I pre-digest them, error-check them, parse them, convert them to binary or serialized streams, then gzip them then let ANT (A Neat Tool) include them in the jar. This keeps the code that reads the resource as light as possible. Unlike zips, jars are not compressed. So unless you gzip individual resources, they will appear in the jar in uncompressed form.
• Dump out the URL that getResource gives you so you can double check where it found the resource. Resources can live in jars, the web, or local hard disk.
• Use JarLook or WinZip to make sure the resource got into your jar under the expected name.
• Look at the Image entry to see what sorts of things you can do with the URL or InputStream to get at the resource contents.

Gotchas

• Though you specify a Class when loading a resource, getResource looks not under the class name in the jar where the class is, but under that class’s package name in the hierarchy. The advantage of this scheme, rather than specifying the package name is the class precisely specifies which jar you mean, even if you don’t know the name of the jar. Think of getResource as meaning Look for this resource where the resources for this class are stored, along with the *.class files and resources for this package.
• When using Applets or Java Web Start, sometimes Thread. currentThread(). getContextClassLoader(). getResource() works better than this. getClass(). getClassLoader(). getResource(). This may bypass a bug in  Java version 1.4 and 1.5 now fixed in 1.6+.

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