Package hudson.util

Class ExistenceCheckingClassLoader

java.lang.Object

java.lang.ClassLoader

hudson.util.DelegatingClassLoader

hudson.util.ExistenceCheckingClassLoader

@Restricted(org.kohsuke.accmod.restrictions.NoExternalUse.class)
public final class ExistenceCheckingClassLoader
extends DelegatingClassLoader

A ClassLoader that verifies the existence of a .class resource before attempting to load the class. Intended to sit in front of servlet container loaders we do not control.

This implementation overrides loadClass(String, boolean) and uses ClassLoader.getResource(String) to check whether the corresponding .class file is available in the classpath. If the resource is not found, a ClassNotFoundException is thrown immediately.

Parallel-capable parent loaders retain a per-class-name lock object for every load attempt, including misses. By checking getResource(name + ".class") first and throwing ClassNotFoundException on absence, we avoid calling loadClass on misses, thus preventing the parent from populating its lock map for nonexistent classes.

This class is only needed in PluginManager.UberClassLoader. It is unnecessary for plugin ClassLoaders (because URLClassLoader2 mitigates lock retention via ClassLoader.getClassLoadingLock(java.lang.String)) and redundant for delegators (because DelegatingClassLoader already avoids base locking).

Author:

Dmytro Ukhlov

See Also:

• ClassLoader
• ClassLoader.getResource(String)

Constructor Summary

Constructors

Constructor	Description
ExistenceCheckingClassLoader(ClassLoader parent)
ExistenceCheckingClassLoader(String name, ClassLoader parent)

Method Summary

Modifier and Type	Method	Description
protected Class<?>	loadClass(String name, boolean resolve)	Short-circuits misses by checking for the .class resource prior to delegation.

Methods inherited from class java.lang.ClassLoader

clearAssertionStatus, defineClass, defineClass, defineClass, defineClass, definePackage, findClass, findClass, findLibrary, findLoadedClass, findResource, findResource, findResources, findSystemClass, getClassLoadingLock, getDefinedPackage, getDefinedPackages, getName, getPackage, getPackages, getParent, getPlatformClassLoader, getResource, getResourceAsStream, getResources, getSystemClassLoader, getSystemResource, getSystemResourceAsStream, getSystemResources, getUnnamedModule, isRegisteredAsParallelCapable, loadClass, registerAsParallelCapable, resolveClass, resources, setClassAssertionStatus, setDefaultAssertionStatus, setPackageAssertionStatus, setSigners

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

ExistenceCheckingClassLoader

public ExistenceCheckingClassLoader(String name,
 ClassLoader parent)

ExistenceCheckingClassLoader

public ExistenceCheckingClassLoader(ClassLoader parent)

Method Details

loadClass

protected Class<?> loadClass(String name,
 boolean resolve)
                      throws ClassNotFoundException

Short-circuits misses by checking for the .class resource prior to delegation. Successful loads behave exactly as the parent would; misses do not touch the parent's per-name lock map.

Overrides:

loadClass in class DelegatingClassLoader

Parameters:

name - The binary name of the class

resolve - If true then resolve the class

Returns:

The resulting Class object

Throws:

ClassNotFoundException - If the class could not be found