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From ahgittin <...@git.apache.org>
Subject [GitHub] brooklyn-server pull request #718: improvement to OSGi serialization strateg...
Date Wed, 07 Jun 2017 22:53:05 GMT
Github user ahgittin commented on a diff in the pull request:

    https://github.com/apache/brooklyn-server/pull/718#discussion_r120764791
  
    --- Diff: core/src/main/java/org/apache/brooklyn/core/mgmt/classloading/ClassLoaderFromStackOfBrooklynClassLoadingContext.java
---
    @@ -0,0 +1,137 @@
    +/*
    + * Licensed to the Apache Software Foundation (ASF) under one
    + * or more contributor license agreements.  See the NOTICE file
    + * distributed with this work for additional information
    + * regarding copyright ownership.  The ASF licenses this file
    + * to you 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 org.apache.brooklyn.core.mgmt.classloading;
    +
    +import static com.google.common.base.Preconditions.checkNotNull;
    +
    +import java.util.Stack;
    +import java.util.concurrent.atomic.AtomicReference;
    +
    +import org.apache.brooklyn.api.mgmt.ManagementContext;
    +import org.apache.brooklyn.api.mgmt.classloading.BrooklynClassLoadingContext;
    +import org.apache.brooklyn.core.mgmt.classloading.BrooklynClassLoadingContextSequential;
    +import org.apache.brooklyn.core.mgmt.classloading.ClassLoaderFromBrooklynClassLoadingContext;
    +import org.apache.brooklyn.core.mgmt.classloading.JavaBrooklynClassLoadingContext;
    +import org.apache.brooklyn.util.core.ClassLoaderUtils;
    +import org.apache.brooklyn.util.exceptions.Exceptions;
    +import org.apache.brooklyn.util.javalang.ClassLoadingContext;
    +
    +/** Provides a stack where {@link ClassLoadingContext} instances can be pushed and popped,
    + * with the most recently pushed one in effect at any given time.
    + * <p>
    + * This is useful when traversing a tree some of whose elements may bring custom search
paths,
    + * and the worker wants to have a simple view of the loader to use at any point in time.
    + * For example XStream keeps a class-loader in effect but when deserializing things 
    + * some of those things may define bundles to use. */
    +public class ClassLoaderFromStackOfBrooklynClassLoadingContext extends ClassLoader {
    +    
    +    private final Stack<BrooklynClassLoadingContext> contexts = new Stack<BrooklynClassLoadingContext>();
    +    private final Stack<ClassLoader> cls = new Stack<ClassLoader>();
    +    private final AtomicReference<Thread> lockOwner = new AtomicReference<Thread>();
    +    private ManagementContext mgmt;
    +    private ClassLoader currentClassLoader;
    +    private AtomicReference<ClassLoaderUtils> currentLoader = new AtomicReference<>();
    +    private int lockCount;
    +    
    +    public ClassLoaderFromStackOfBrooklynClassLoadingContext(ClassLoader classLoader)
{
    +        setCurrentClassLoader(classLoader);
    +    }
    +    
    +    public void setManagementContext(ManagementContext mgmt) {
    +        this.mgmt = checkNotNull(mgmt, "mgmt");
    +        currentLoader.set(new ClassLoaderUtils(currentClassLoader, mgmt));
    +    }
    +
    +    @Override
    +    protected Class<?> findClass(String name) throws ClassNotFoundException {
    +        return currentLoader.get().loadClass(name);
    +    }
    +    
    +    @Override
    +    protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException
{
    +        return findClass(name);
    +    }
    +
    +    /** Must be accompanied by a corresponding {@link #popClassLoadingContext()} when
finished. */
    +    @SuppressWarnings("deprecation")
    +    public void pushClassLoadingContext(BrooklynClassLoadingContext clcNew) {
    +        acquireLock();
    +        BrooklynClassLoadingContext oldClc;
    +        if (!contexts.isEmpty()) {
    +            oldClc = contexts.peek();
    +        } else {
    +            // TODO xml serializers using this should take a BCLC instead of a CL
    +            oldClc = JavaBrooklynClassLoadingContext.create(mgmt, getCurrentClassLoader());
    +        }
    +        BrooklynClassLoadingContextSequential clcMerged = new BrooklynClassLoadingContextSequential(mgmt,
oldClc, clcNew);
    +        ClassLoader newCL = ClassLoaderFromBrooklynClassLoadingContext.of(clcMerged);
    +        contexts.push(clcMerged);
    +        cls.push(getCurrentClassLoader());
    +        setCurrentClassLoader(newCL);
    +    }
    +
    +    public void popClassLoadingContext() {
    +        synchronized (lockOwner) {
    +            releaseXstreamLock();
    +            setCurrentClassLoader(cls.pop());
    +            contexts.pop();
    +        }
    +    }
    +    
    +    private ClassLoader getCurrentClassLoader() {
    +        return currentClassLoader;
    +    }
    +    
    +    private void setCurrentClassLoader(ClassLoader classLoader) {
    +        currentClassLoader = checkNotNull(classLoader);
    +        currentLoader.set(new ClassLoaderUtils(currentClassLoader, mgmt));
    +    }
    +    
    +    protected void acquireLock() {
    +        synchronized (lockOwner) {
    --- End diff --
    
    I thought I took `xstream` out of the names but I missed at least one!
    
    I'm really not sure what the threading guarantees/requirements are.  It was made just
for xstream to support osgi and follows whatever was needed there.  i was amused to discover
despite its name it had nothing to do with osgi or xstream!  i thought i needed to refactor
it for tests though i ended up doing the tests differents; still the rename felt worth keeping.
    
    my 2p it's good to be reminded of this, there might well be other places such a thing
might be useful, and if there is then we'll better think through threading and locking.


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