[GitHub] incubator-brooklyn pull request: Compiler independent persistence

[aledsage <gi...@git.apache.org>]

Github user aledsage commented on a diff in the pull request:

    https://github.com/apache/incubator-brooklyn/pull/520#discussion_r25696529
  
    --- Diff: core/src/main/java/brooklyn/util/xstream/CompilerIndependentOuterClassFieldMapper.java ---
    @@ -0,0 +1,167 @@
    +/*
    + * 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 brooklyn.util.xstream;
    +
    +import java.lang.reflect.Field;
    +import java.util.ArrayList;
    +import java.util.Collection;
    +import java.util.Collections;
    +import java.util.HashSet;
    +import java.util.Map;
    +import java.util.Set;
    +import java.util.concurrent.ConcurrentHashMap;
    +
    +import org.slf4j.Logger;
    +import org.slf4j.LoggerFactory;
    +
    +import com.thoughtworks.xstream.core.Caching;
    +import com.thoughtworks.xstream.mapper.Mapper;
    +import com.thoughtworks.xstream.mapper.MapperWrapper;
    +
    +/**
    + * <p>Compiler independent outer class field mapper.</p>
    + * <p>Different compilers generate different indexes for the names of outer class reference
    + *    field (this$N) leading to deserialization errors.</p>
    + * <ul>
    + *   <li> eclipse-[groovy-]compiler counts all outer static classes
    + *   <li> OpenJDK/Oracle/IBM compiler starts at 0, regardless of the nesting level
    + * </ul>
    + * <p>The mapper will be able to update field names for instances with a single this$N
    + *    field only (including those from parent classes).</p>
    + * <p>For difference between generated field names compare
    + *    {@code src/test/java/brooklyn/util/xstream/compiler_compatibility_eclipse.xml} and
    + *    {@code src/test/java/brooklyn/util/xstream/compiler_compatibility_oracle.xml},
    + *    generated from {@code brooklyn.util.xstream.CompilerCompatibilityTest}</p>
    + * <p>JLS 1.1 relevant section, copied verbatim for a lack of reliable URL:</p>
    + * <blockquote>
    + *  <p>Java 1.1 compilers are strongly encouraged, though not required, to use the
    + *     following naming conventions when implementing inner classes. Compilers may
    + *     not use synthetic names of the forms defined here for any other purposes.</p>
    + *  <p>A synthetic field pointing to the outermost enclosing instance is named this$0.
    + *     The next-outermost enclosing instance is this$1, and so forth. (At most one such
    + *     field is necessary in any given inner class.) A synthetic field containing a copy
    + *     of a constant v is named val$v. These fields are final.</p>
    + * </blockquote>
    + * <p>Currently available at
    + *    http://web.archive.org/web/20000830111107/http://java.sun.com/products/jdk/1.1/docs/guide/innerclasses/spec/innerclasses.doc10.html</p>
    + */
    +public class CompilerIndependentOuterClassFieldMapper extends MapperWrapper implements Caching {
    +    public static final Logger LOG = LoggerFactory.getLogger(CompilerIndependentOuterClassFieldMapper.class);
    +
    +    private static final String OUTER_CLASS_FIELD_PREFIX = "this$";
    +
    +    private final Map<String, Collection<String>> classOuterFields = new ConcurrentHashMap<String, Collection<String>>();
    +
    +    public CompilerIndependentOuterClassFieldMapper(Mapper wrapped) {
    +        super(wrapped);
    +        classOuterFields.put(Object.class.getName(), Collections.<String>emptyList());
    +    }
    +
    +    @Override
    +    public String realMember(@SuppressWarnings("rawtypes") Class type, String serialized) {
    +        // Let com.thoughtworks.xstream.mapper.OuterClassMapper also run on the input.
    +        String serializedFieldName = super.realMember(type, serialized);
    +
    +        if (serializedFieldName.startsWith(OUTER_CLASS_FIELD_PREFIX)) {
    +            Collection<String> compiledFieldNames = findOuterClassFieldNames(type);
    +            if (compiledFieldNames.size() == 0) {
    +                throw new IllegalStateException("Unable to find any outer class fields in " + type + ", searching specifically for " + serializedFieldName);
    +            }
    +
    +            Set<String> uniqueFieldNames = new HashSet<String>(compiledFieldNames);
    +            String deepestField = compiledFieldNames.iterator().next();
    +            String deserializeFieldName;
    +            if (!compiledFieldNames.contains(serializedFieldName)) {
    +                String msg =
    +                        "Unable to find outer class field " + serializedFieldName + " in class " + type + ". " +
    +                        "This could be caused by " +
    +                        "1) changing the class (or one of its parents) to a static or " +
    +                        "2) moving the class to a different lexical level (enclosing classes) or " +
    +                        "3) using a different compiler (i.e eclipse vs oracle) at the time the object was serialized. ";
    +                if (uniqueFieldNames.size() == 1) {
    +                    // Try to fix the field naming only for the case with a single field or 
    +                    // multiple fields with the same name, in which case XStream puts defined-in
    +                    // for the field declared in super.
    +                    //
    +                    // We don't have access to the XML elements from here to check for same name
    +                    // so we check the target class instead. This should work most of the time, but
    +                    // if code is recompiled in such a way that the new instance has fields with
    +                    // different names, where only the field of the extending class is renamed and
    +                    // the super field is not, then the instance will be deserialized incorrectly -
    +                    // the super field will be assigned both times. If the field type is incompatible
    +                    // then a casting exception will be thrown, if it's the same then only the warning
    +                    // below will indicate of a possible problem down the line - most probably NPE on
    +                    // the this$N field.
    +                    LOG.warn(msg + "Will use the field " + deepestField + " instead.");
    +                    deserializeFieldName = deepestField;
    --- End diff --
    
    I'd personally delete the `deepestField` variable (which is only used here), and just assign `deserializeFieldName = Iterables.getOnlyElement(uniqueFieldNames)l`. The name "deepestField" threw me at first.


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