Java Language Tutorial => What the classpath means: how searches work

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The purpose of the classpath is to tell a JVM where to find classes and other resources. The meaning of the classpath and the search process are intertwined.

The classpath is a form of search path which specifies a sequence of locations to look for resources. In a standard classpath, these places are either, a directory in the host file system, a JAR file or a ZIP file. In each cases, the location is the root of a namespace that will be searched.

The standard procedure for searching for a class on the classpath is as follows:

Map the class name to a relative classfile pathname RP. The mapping for class names to class filenames is described elsewhere.
For each entry E in the classpath:
- If the entry is a filesystem directory:
  - Resolve RP relative to E to give an absolute pathname AP.
  - Test if AP is a path for an existing file.
  - If yes, load the class from that file
- If the entry is a JAR or ZIP file:
  - Lookup RP in the JAR / ZIP file index.
  - If the corresponding JAR / ZIP file entry exists, load the class from that entry.

The procedure for searching for a resource on the classpath depends on whether the resource path is absolute or relative. For an absolute resource path, the procedure is as above. For a relative resource path resolved using Class.getResource or Class.getResourceAsStream, the path for the classes package is prepended prior to searching.

(Note these are the procedures implemented by the standard Java classloaders. A custom classloader might perform the search differently.)

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Java Language The Classpath What the classpath means: how searches work

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