com.galeoconsulting.leonardinius.api.impl.ChainingClassLoader.java Source code

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Here is the source code for com.galeoconsulting.leonardinius.api.impl.ChainingClassLoader.java

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/*
 * Copyright 2011 Leonid Maslov<leonidms@gmail.com>
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.galeoconsulting.leonardinius.api.impl;

/**
 * User: 23059892
 * Date: 4/11/11
 * Time: 12:21 PM
 */

import com.google.common.collect.ImmutableList;
import org.apache.commons.lang.Validate;

import java.io.IOException;
import java.io.InputStream;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.URL;
import java.util.*;

/**
 * A class loader that delegates to a list of class loaders. The order is important as classes and resources will be
 * loaded from the first classloader that can load them.
 */
class ChainingClassLoader extends ClassLoader {
    // ------------------------------ FIELDS ------------------------------

    //private static final Logger log = LoggerFactory.getLogger(ChainingClassLoader.class);

    /**
     * The list of classloader to delegate to.
     */
    private final List<ClassLoader> classLoaders;

    // --------------------------- CONSTRUCTORS ---------------------------

    public ChainingClassLoader(ClassLoader... classLoaders) {
        Validate.noNullElements(classLoaders, "ClassLoader arguments cannot be null");
        this.classLoaders = ImmutableList
                .of(new LinkedHashSet<ClassLoader>(Arrays.asList(classLoaders)).toArray(new ClassLoader[0]));
    }

    // -------------------------- OTHER METHODS --------------------------

    @Override
    public synchronized void clearAssertionStatus() {
        for (ClassLoader classloader : classLoaders) {
            classloader.clearAssertionStatus();
        }
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        for (ClassLoader classloader : classLoaders) {
            try {
                Class<?> classInstance = callFindClass(classloader, name);
                if (classInstance != null) {
                    return classInstance;
                }
            } catch (ClassNotFoundException e) {
                // ignoring until we reach the end of the list since we are chaining
            }
        }

        throw new ClassNotFoundException(name);
    }

    private Class<?> callFindClass(ClassLoader classloader, String name) throws ClassNotFoundException {
        try {
            return (Class<?>) MethodsHolder.findClassMethod.invoke(classloader, name);
        } catch (IllegalAccessException e) {
            throw new AssertionError(e); //unexpected
        } catch (InvocationTargetException e) {
            if (e.getTargetException() instanceof ClassNotFoundException) {
                throw (ClassNotFoundException) e.getTargetException();
            }
            throw new AssertionError(e); // unexpected
        }
    }

    @Override
    public URL getResource(String name) {
        for (ClassLoader classloader : classLoaders) {
            final URL url = classloader.getResource(name);
            if (url != null) {
                return url;
            }
        }
        return null;
    }

    @Override
    public InputStream getResourceAsStream(String name) {
        for (ClassLoader classloader : classLoaders) {
            final InputStream inputStream = classloader.getResourceAsStream(name);
            if (inputStream != null) {
                return inputStream;
            }
        }

        return null;
    }

    @Override
    public Enumeration<URL> getResources(String name) throws IOException {
        return new ResourcesEnumeration(name, classLoaders);
    }

    @Override
    public Class<?> loadClass(String name) throws ClassNotFoundException {
        for (ClassLoader classloader : classLoaders) {
            try {
                return classloader.loadClass(name);
            } catch (ClassNotFoundException e) {
                // ignoring until we reach the end of the list since we are chaining
            }
        }
        throw new ClassNotFoundException(name);
    }

    @Override
    public synchronized void setClassAssertionStatus(String className, boolean enabled) {
        for (ClassLoader classloader : classLoaders) {
            classloader.setClassAssertionStatus(className, enabled);
        }
    }

    @Override
    public synchronized void setDefaultAssertionStatus(boolean enabled) {
        for (ClassLoader classloader : classLoaders) {
            classloader.setDefaultAssertionStatus(enabled);
        }
    }

    @Override
    public synchronized void setPackageAssertionStatus(String packageName, boolean enabled) {
        for (ClassLoader classloader : classLoaders) {
            classloader.setPackageAssertionStatus(packageName, enabled);
        }
    }

    // -------------------------- INNER CLASSES --------------------------

    private static final class MethodsHolder {
        private static final Method findClassMethod;

        static {
            // single check idiom 
            // it's safe to recalculate same method several times (on concurrent access)
            try {
                findClassMethod = ClassLoader.class.getDeclaredMethod("findClass", String.class);
                findClassMethod.setAccessible(true);
            } catch (NoSuchMethodException e) {
                throw new AssertionError(e); //unexpected
            }
        }
    }

    private static final class ResourcesEnumeration implements Enumeration<URL> {
        private final List<Enumeration<URL>> resources;
        private final String resourceName;

        ResourcesEnumeration(String resourceName, List<ClassLoader> classLoaders) throws IOException {
            this.resourceName = resourceName;
            this.resources = new LinkedList<Enumeration<URL>>();
            for (ClassLoader classLoader : classLoaders) {
                resources.add(classLoader.getResources(resourceName));
            }
        }

        public boolean hasMoreElements() {
            for (Enumeration<URL> resource : resources) {
                if (resource.hasMoreElements()) {
                    return true;
                }
            }

            return false;
        }

        public URL nextElement() {
            for (Enumeration<URL> resource : resources) {
                if (resource.hasMoreElements()) {
                    return resource.nextElement();
                }
            }
            throw new NoSuchElementException(resourceName);
        }
    }
}