2.4/dev/core/src/com/google/gwt/core/ext/linker/Artifact.java

/*
 * 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();
}