third-party/java/emma-2.0.5312/core/java12/com/vladium/util/ClassLoaderResolver.java - buck - Git at Google

 /* Copyright (C) 2003 Vladimir Roubtsov. All rights reserved.
  *
  * This program and the accompanying materials are made available under
  * the terms of the Common Public License v1.0 which accompanies this distribution,
  * and is available at http://www.eclipse.org/legal/cpl-v10.html
  *
  * $Id: ClassLoaderResolver.java,v 1.1.1.1.2.2 2004/07/10 03:34:53 vlad_r Exp $
  */
 package com.vladium.util;

 // ----------------------------------------------------------------------------
 /**
  * This non-instantiable non-subclassable class acts as the global point for
  * choosing a ClassLoader for dynamic class/resource loading at any point
  * in an application.
  *
  * @see ResourceLoader
  * @see IClassLoadStrategy
  * @see DefaultClassLoadStrategy
  *
  * @author Vlad Roubtsov, (C) 2003
  */
 public
 abstract class ClassLoaderResolver
 {
     // public: ................................................................

     // NOTE: don't use Logger in this class to avoid infinite recursion

     /**
      * This method selects the "best" classloader instance to be used for
      * class/resource loading by whoever calls this method. The decision
      * typically involves choosing between the caller's current, thread context,
      * system, and other classloaders in the JVM and is made by the {@link IClassLoadStrategy}
      * instance established by the last call to {@link #setStrategy}.<P>
      *
      * This method does not throw.
      *
      * @param caller [null input eliminates the caller's current classloader
      * from consideration]
      *
      * @return classloader to be used by the caller ['null' indicates the
      * primordial loader]
      */
     public static synchronized ClassLoader getClassLoader (final Class caller)
     {
         final ClassLoadContext ctx = new ClassLoadContext (caller);

         return s_strategy.getClassLoader (ctx);
     }

     /**
      * This method selects the "best" classloader instance to be used for
      * class/resource loading by whoever calls this method. The decision
      * typically involves choosing between the caller's current, thread context,
      * system, and other classloaders in the JVM and is made by the {@link IClassLoadStrategy}
      * instance established by the last call to {@link #setStrategy}.<P>
      *
      * This method uses its own caller to set the call context. To be able to
      * override this decision explicitly, use {@link #getClassLoader(Class)}.<P>
      *
      * This method does not throw.
      *
      * @return classloader to be used by the caller ['null' indicates the
      * primordial loader]
      */
     public static synchronized ClassLoader getClassLoader ()
     {
         final Class caller = getCallerClass (1); // 'caller' can be set to null
         final ClassLoadContext ctx = new ClassLoadContext (caller);

         return s_strategy.getClassLoader (ctx);
     }

     /*
      * Indexes into the current method call context with a given offset. Offset 0
      * corresponds to the immediate caller, offset 1 corresponds to its caller,
      * etc.<P>
      *
      * Invariant: getCallerClass(0) == class containing code that performs this call
      */
     public static Class getCallerClass (final int callerOffset)
     {
         if (CALLER_RESOLVER == null) return null; // only happens if <clinit> failed

         return CALLER_RESOLVER.getClassContext () [CALL_CONTEXT_OFFSET + callerOffset];
     }

     /**
      * Returns 'true' if 'loader2' is a delegation child of 'loader1' [or if
      * 'loader1'=='loader2']. Of course, this works only for classloaders that
      * set their parent pointers correctly. 'null' is interpreted as the
      * primordial loader [i.e., everybody's parent].
      */
     public static boolean isChild (final ClassLoader loader1, ClassLoader loader2)
     {
         if (loader1 == loader2) return true;
         if (loader2 == null) return false;
         if (loader1 == null) return true;

         for ( ; loader2 != null; loader2 = loader2.getParent ())
         {
             if (loader2 == loader1) return true;
         }

         return false;
     }

     /**
      * Gets the current classloader selection strategy setting.
      */
     public static synchronized IClassLoadStrategy getStrategy ()
     {
         return s_strategy;
     }

     /**
      * Sets the classloader selection strategy to be used by subsequent calls
      * to {@link #getClassLoader()}. An instance of {@link ClassLoaderResolver.DefaultClassLoadStrategy}
      * is in effect if this method is never called.
      *
      * @param strategy new strategy [may not be null]
      * @return previous setting
      */
     public static synchronized IClassLoadStrategy setStrategy (final IClassLoadStrategy strategy)
     {
         if (strategy == null) throw new IllegalArgumentException ("null input: strategy");

         final IClassLoadStrategy old = s_strategy;
         s_strategy = strategy;

         return old;
     }

     // protected: .............................................................

     // package: ...............................................................

     // private: ...............................................................


     private static final class DefaultClassLoadStrategy implements IClassLoadStrategy
     {
         public ClassLoader getClassLoader (final ClassLoadContext ctx)
         {
             if (ctx == null) throw new IllegalArgumentException ("null input: ctx");

             final Class caller = ctx.getCallerClass ();
             final ClassLoader contextLoader = Thread.currentThread ().getContextClassLoader ();

             ClassLoader result;

             if (caller == null)
                 result = contextLoader;
             else
             {
                 final ClassLoader callerLoader = caller.getClassLoader ();

                 // if 'callerLoader' and 'contextLoader' are in a parent-child
                 // relationship, always choose the child:

                 // SF BUG 975080: change the sibling case to use 'callerLoader'
                 // to work around ANT 1.6.x incorrect classloading model:

                 if (isChild (callerLoader, contextLoader))
                     result = contextLoader;
                 else
                     result = callerLoader;
             }

             final ClassLoader systemLoader = ClassLoader.getSystemClassLoader ();

             // precaution for when deployed as a bootstrap or extension class:
             if (isChild (result, systemLoader))
                 result = systemLoader;

             return result;
         }

     } // end of nested class


     /**
      * A helper class to get the call context. It subclasses SecurityManager
      * to make getClassContext() accessible. An instance of CallerResolver
      * only needs to be created, not installed as an actual security
      * manager.
      */
     private static final class CallerResolver extends SecurityManager
     {
         protected Class [] getClassContext ()
         {
             return super.getClassContext ();
         }

     } // end of nested class


     private ClassLoaderResolver () {} // prevent subclassing


     private static IClassLoadStrategy s_strategy; // initialized in <clinit>

     private static final int CALL_CONTEXT_OFFSET = 2; // may need to change if this class is redesigned
     private static final CallerResolver CALLER_RESOLVER; // set in <clinit>
     //private static Throwable CLINIT_FAILURE;

     static
     {
         CallerResolver temp = null;
         try
         {
             // this can fail if the current SecurityManager does not allow
             // RuntimePermission ("createSecurityManager"):

             temp = new CallerResolver ();
         }
         catch (Throwable t)
         {
             //CLINIT_FAILURE = t;
         }
         CALLER_RESOLVER = temp;

         s_strategy = new DefaultClassLoadStrategy ();
     }

 } // end of class
 // ----------------------------------------------------------------------------
	/* Copyright (C) 2003 Vladimir Roubtsov. All rights reserved.
	*
	* This program and the accompanying materials are made available under
	* the terms of the Common Public License v1.0 which accompanies this distribution,
	* and is available at http://www.eclipse.org/legal/cpl-v10.html
	*
	* $Id: ClassLoaderResolver.java,v 1.1.1.1.2.2 2004/07/10 03:34:53 vlad_r Exp $
	*/
	package com.vladium.util;

	// ----------------------------------------------------------------------------
	/**
	* This non-instantiable non-subclassable class acts as the global point for
	* choosing a ClassLoader for dynamic class/resource loading at any point
	* in an application.
	*
	* @see ResourceLoader
	* @see IClassLoadStrategy
	* @see DefaultClassLoadStrategy
	*
	* @author Vlad Roubtsov, (C) 2003
	*/
	public
	abstract class ClassLoaderResolver
	{
	// public: ................................................................

	// NOTE: don't use Logger in this class to avoid infinite recursion

	/**
	* This method selects the "best" classloader instance to be used for
	* class/resource loading by whoever calls this method. The decision
	* typically involves choosing between the caller's current, thread context,
	* system, and other classloaders in the JVM and is made by the {@link IClassLoadStrategy}
	* instance established by the last call to {@link #setStrategy}.<P>
	*
	* This method does not throw.
	*
	* @param caller [null input eliminates the caller's current classloader
	* from consideration]
	*
	* @return classloader to be used by the caller ['null' indicates the
	* primordial loader]
	*/
	public static synchronized ClassLoader getClassLoader (final Class caller)
	{
	final ClassLoadContext ctx = new ClassLoadContext (caller);

	return s_strategy.getClassLoader (ctx);
	}

	/**
	* This method selects the "best" classloader instance to be used for
	* class/resource loading by whoever calls this method. The decision
	* typically involves choosing between the caller's current, thread context,
	* system, and other classloaders in the JVM and is made by the {@link IClassLoadStrategy}
	* instance established by the last call to {@link #setStrategy}.<P>
	*
	* This method uses its own caller to set the call context. To be able to
	* override this decision explicitly, use {@link #getClassLoader(Class)}.<P>
	*
	* This method does not throw.
	*
	* @return classloader to be used by the caller ['null' indicates the
	* primordial loader]
	*/
	public static synchronized ClassLoader getClassLoader ()
	{
	final Class caller = getCallerClass (1); // 'caller' can be set to null
	final ClassLoadContext ctx = new ClassLoadContext (caller);

	return s_strategy.getClassLoader (ctx);
	}

	/*
	* Indexes into the current method call context with a given offset. Offset 0
	* corresponds to the immediate caller, offset 1 corresponds to its caller,
	* etc.<P>
	*
	* Invariant: getCallerClass(0) == class containing code that performs this call
	*/
	public static Class getCallerClass (final int callerOffset)
	{
	if (CALLER_RESOLVER == null) return null; // only happens if <clinit> failed

	return CALLER_RESOLVER.getClassContext () [CALL_CONTEXT_OFFSET + callerOffset];
	}

	/**
	* Returns 'true' if 'loader2' is a delegation child of 'loader1' [or if
	* 'loader1'=='loader2']. Of course, this works only for classloaders that
	* set their parent pointers correctly. 'null' is interpreted as the
	* primordial loader [i.e., everybody's parent].
	*/
	public static boolean isChild (final ClassLoader loader1, ClassLoader loader2)
	{
	if (loader1 == loader2) return true;
	if (loader2 == null) return false;
	if (loader1 == null) return true;

	for ( ; loader2 != null; loader2 = loader2.getParent ())
	{
	if (loader2 == loader1) return true;
	}

	return false;
	}

	/**
	* Gets the current classloader selection strategy setting.
	*/
	public static synchronized IClassLoadStrategy getStrategy ()
	{
	return s_strategy;
	}

	/**
	* Sets the classloader selection strategy to be used by subsequent calls
	* to {@link #getClassLoader()}. An instance of {@link ClassLoaderResolver.DefaultClassLoadStrategy}
	* is in effect if this method is never called.
	*
	* @param strategy new strategy [may not be null]
	* @return previous setting
	*/
	public static synchronized IClassLoadStrategy setStrategy (final IClassLoadStrategy strategy)
	{
	if (strategy == null) throw new IllegalArgumentException ("null input: strategy");

	final IClassLoadStrategy old = s_strategy;
	s_strategy = strategy;

	return old;
	}

	// protected: .............................................................

	// package: ...............................................................

	// private: ...............................................................


	private static final class DefaultClassLoadStrategy implements IClassLoadStrategy
	{
	public ClassLoader getClassLoader (final ClassLoadContext ctx)
	{
	if (ctx == null) throw new IllegalArgumentException ("null input: ctx");

	final Class caller = ctx.getCallerClass ();
	final ClassLoader contextLoader = Thread.currentThread ().getContextClassLoader ();

	ClassLoader result;

	if (caller == null)
	result = contextLoader;
	else
	{
	final ClassLoader callerLoader = caller.getClassLoader ();

	// if 'callerLoader' and 'contextLoader' are in a parent-child
	// relationship, always choose the child:

	// SF BUG 975080: change the sibling case to use 'callerLoader'
	// to work around ANT 1.6.x incorrect classloading model:

	if (isChild (callerLoader, contextLoader))
	result = contextLoader;
	else
	result = callerLoader;
	}

	final ClassLoader systemLoader = ClassLoader.getSystemClassLoader ();

	// precaution for when deployed as a bootstrap or extension class:
	if (isChild (result, systemLoader))
	result = systemLoader;

	return result;
	}

	} // end of nested class


	/**
	* A helper class to get the call context. It subclasses SecurityManager
	* to make getClassContext() accessible. An instance of CallerResolver
	* only needs to be created, not installed as an actual security
	* manager.
	*/
	private static final class CallerResolver extends SecurityManager
	{
	protected Class [] getClassContext ()
	{
	return super.getClassContext ();
	}

	} // end of nested class


	private ClassLoaderResolver () {} // prevent subclassing


	private static IClassLoadStrategy s_strategy; // initialized in <clinit>

	private static final int CALL_CONTEXT_OFFSET = 2; // may need to change if this class is redesigned
	private static final CallerResolver CALLER_RESOLVER; // set in <clinit>
	//private static Throwable CLINIT_FAILURE;

	static
	{
	CallerResolver temp = null;
	try
	{
	// this can fail if the current SecurityManager does not allow
	// RuntimePermission ("createSecurityManager"):

	temp = new CallerResolver ();
	}
	catch (Throwable t)
	{
	//CLINIT_FAILURE = t;
	}
	CALLER_RESOLVER = temp;

	s_strategy = new DefaultClassLoadStrategy ();
	}

	} // end of class
	// ----------------------------------------------------------------------------