src/main/java/com/google/devtools/common/options/OptionsBase.java

// Copyright 2014 The Bazel Authors. All rights reserved.
//
// 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.devtools.common.options;

import com.google.common.escape.CharEscaperBuilder;
import com.google.common.escape.Escaper;
import java.lang.reflect.Field;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;

/**
 * Base class for all options classes. Extend this class, adding public instance fields annotated
 * with {@link Option}. Then you can create instances either programmatically:
 *
 * <pre>
 *   X x = Options.getDefaults(X.class);
 *   x.host = "localhost";
 *   x.port = 80;
 * </pre>
 *
 * or from an array of command-line arguments:
 *
 * <pre>
 *   OptionsParser parser = OptionsParser.newOptionsParser(X.class);
 *   parser.parse("--host", "localhost", "--port", "80");
 *   X x = parser.getOptions(X.class);
 * </pre>
 *
 * <p>Subclasses of {@code OptionsBase} <i>must</i> be constructed reflectively, i.e. using not
 * {@code new MyOptions()}, but one of the above methods instead. (Direct construction creates an
 * empty instance, not containing default values. This leads to surprising behavior and often {@code
 * NullPointerExceptions}, etc.)
 */
public abstract class OptionsBase {

  private static final Escaper ESCAPER = new CharEscaperBuilder()
      .addEscape('\\', "\\\\").addEscape('"', "\\\"").toEscaper();

  /**
   * Subclasses must provide a default (no argument) constructor.
   */
  protected OptionsBase() {
    // There used to be a sanity check here that checks the stack trace of this constructor
    // invocation; unfortunately, that makes the options construction about 10x slower. So be
    // careful with how you construct options classes.
  }

  /**
   * Returns a mapping from option names to values, for each option on this object, including
   * inherited ones. The mapping is a copy, so subsequent mutations to it or to this object are
   * independent. Entries are sorted alphabetically.
   */
  public final <O extends OptionsBase> Map<String, Object> asMap() {
    // Generic O is needed to tell the type system that the toMap() call is safe.
    // The casts are safe because "this" is an instance of "getClass()"
    // which subclasses OptionsBase.
    @SuppressWarnings("unchecked")
    O castThis = (O) this;
    @SuppressWarnings("unchecked")
    Class<O> castClass = (Class<O>) getClass();

    Map<String, Object> map = new LinkedHashMap<>();
    for (Map.Entry<Field, Object> entry : OptionsParser.toMap(castClass, castThis).entrySet()) {
      String name = entry.getKey().getAnnotation(Option.class).name();
      map.put(name, entry.getValue());
    }
    return map;
  }

  @Override
  public final String toString() {
    return getClass().getName() + asMap();
  }

  /**
   * Returns a string that uniquely identifies the options. This value is
   * intended for analysis caching.
   */
  public final String cacheKey() {
    StringBuilder result = new StringBuilder(getClass().getName()).append("{");

    for (Entry<String, Object> entry : asMap().entrySet()) {
      result.append(entry.getKey()).append("=");

      Object value = entry.getValue();
      // This special case is needed because List.toString() prints the same
      // ("[]") for an empty list and for a list with a single empty string.
      if (value instanceof List<?> && ((List<?>) value).isEmpty()) {
        result.append("EMPTY");
      } else if (value == null) {
        result.append("NULL");
      } else {
        result
            .append('"')
            .append(ESCAPER.escape(value.toString()))
            .append('"');
      }
      result.append(", ");
    }

    return result.append("}").toString();
  }

  @Override
  public final boolean equals(Object that) {
    return that != null &&
        this.getClass() == that.getClass() &&
        this.asMap().equals(((OptionsBase) that).asMap());
  }

  @Override
  public final int hashCode() {
    return this.getClass().hashCode() + asMap().hashCode();
  }
}