com.google.gwt.core.ext.linker.Artifact.java Source code

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Here is the source code for com.google.gwt.core.ext.linker.Artifact.java

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/*
 * Copyright 2008 Google Inc.
 * 
 * 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.google.gwt.core.ext.linker;

import com.google.gwt.core.ext.Linker;

import java.io.Serializable;

/**
 * A base type for all artifacts relating to the link process. In order to
 * ensure stable output between runs of the compiler, Artifact types must
 * implement a stable comparison between instances of a relevant base type (the
 * exact comparison order is irrelevant).
 * 
 * @param <C> The type of Artifact interface that the Artifact can be compared
 *          to.
 */
public abstract class Artifact<C extends Artifact<C>> implements Comparable<Artifact<?>>, Serializable {
    private transient Class<? extends Linker> linker;
    private final String linkerName;

    /**
     * Constructor.
     * 
     * @param linker the type of Linker that instantiated the Artifact.
     */
    protected Artifact(Class<? extends Linker> linker) {
        assert linker != null;
        this.linkerName = linker.getName();
        this.linker = linker;
    }

    public final int compareTo(Artifact<?> o) {
        if (getComparableArtifactType().equals(o.getComparableArtifactType())) {
            return compareToComparableArtifact(getComparableArtifactType().cast(o));
        } else {
            return getComparableArtifactType().getName().compareTo(o.getComparableArtifactType().getName());
        }
    }

    /**
     * Delegates to {@link #compareTo(Artifact)}.
     */
    @Override
    public final boolean equals(Object obj) {
        if (obj instanceof Artifact<?>) {
            return compareTo((Artifact<?>) obj) == 0;
        } else {
            return false;
        }
    }

    /**
     * Returns the Linker that created the Artifact.
     */
    public final Class<? extends Linker> getLinker() {
        // linker is null when deserialized.
        if (linker == null) {
            try {
                Class<?> clazz = Class.forName(linkerName, false, Thread.currentThread().getContextClassLoader());
                linker = clazz.asSubclass(Linker.class);
            } catch (ClassNotFoundException e) {
                // The class may not be available.
                linker = Linker.class;
            }
        }
        return linker;
    }

    /**
     * The class which is returned from {@link #getComparableArtifactType()} must
     * declare a final implementation which returns the same hash code for objects
     * for which {@link #compareToComparableArtifact(Artifact)} returns 0.
     */
    @Override
    public abstract int hashCode();

    /**
     * Returns whether the {@link Transferable} annotation is present on this
     * class. See {@link Transferable} for the implications.
     */
    public final boolean isTransferableFromShards() {
        return getClass().isAnnotationPresent(Transferable.class);
    }

    @Override
    public String toString() {
        return getClass().getName() + " created by " + getLinker().getName();
    }

    /**
     * Performs comparison with an artifact of a compatible base type. Objects
     * which compare to 0 are assumed equal, and must return the same
     * {@link #hashCode()}.
     */
    protected abstract int compareToComparableArtifact(C o);

    /**
     * Returns the base type to use for comparisons between Artifacts. All
     * concrete implementations of this methods must be final.
     */
    protected abstract Class<C> getComparableArtifactType();
}