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A module that is everything required to understands Druid Segments
There is a newer version: 30.0.1
/*
 * 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.druid.query.extraction;

import com.fasterxml.jackson.annotation.JacksonInject;
import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.google.common.base.Function;
import com.google.common.base.Preconditions;
import org.apache.druid.common.config.NullHandling;
import org.apache.druid.java.util.common.StringUtils;
import org.apache.druid.js.JavaScriptConfig;
import org.checkerframework.checker.nullness.qual.EnsuresNonNull;
import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
import org.mozilla.javascript.Context;
import org.mozilla.javascript.ContextFactory;
import org.mozilla.javascript.ScriptableObject;

import javax.annotation.Nullable;
import java.nio.ByteBuffer;

public class JavaScriptExtractionFn implements ExtractionFn
{
  private static Function compile(String function)
  {
    final ContextFactory contextFactory = ContextFactory.getGlobal();
    final Context context = contextFactory.enterContext();
    context.setOptimizationLevel(JavaScriptConfig.DEFAULT_OPTIMIZATION_LEVEL);

    final ScriptableObject scope = context.initStandardObjects();

    final org.mozilla.javascript.Function fn = context.compileFunction(scope, function, "fn", 1, null);
    Context.exit();


    return new Function()
    {
      @Override
      public String apply(Object input)
      {
        // ideally we need a close() function to discard the context once it is not used anymore
        Context cx = Context.getCurrentContext();
        if (cx == null) {
          cx = contextFactory.enterContext();
        }

        final Object res = fn.call(cx, scope, scope, new Object[]{input});
        return res != null ? Context.toString(res) : null;
      }
    };
  }

  private final String function;
  private final boolean injective;
  private final JavaScriptConfig config;

  /**
   * The field is declared volatile in order to ensure safe publication of the object
   * in {@link #compile(String)} without worrying about final modifiers
   * on the fields of the created object
   *
   * @see 
   *     https://github.com/apache/druid/pull/6662#discussion_r237013157
   */
  @MonotonicNonNull
  private volatile Function fn;

  @JsonCreator
  public JavaScriptExtractionFn(
      @JsonProperty("function") String function,
      @JsonProperty("injective") boolean injective,
      @JacksonInject JavaScriptConfig config
  )
  {
    Preconditions.checkNotNull(function, "function must not be null");

    this.function = function;
    this.injective = injective;
    this.config = config;
  }

  @JsonProperty
  public String getFunction()
  {
    return function;
  }

  @JsonProperty
  public boolean isInjective()
  {
    return this.injective;
  }

  @Override
  public byte[] getCacheKey()
  {
    byte[] bytes = StringUtils.toUtf8(function);
    return ByteBuffer.allocate(1 + bytes.length)
                     .put(ExtractionCacheHelper.CACHE_TYPE_ID_JAVASCRIPT)
                     .put(bytes)
                     .array();
  }

  @Override
  @Nullable
  public String apply(@Nullable Object value)
  {
    Function fn = getCompiledScript();
    return NullHandling.emptyToNullIfNeeded(fn.apply(value));
  }

  /**
   * {@link #apply(Object)} can be called by multiple threads, so this function should be thread-safe to avoid extra
   * script compilation.
   */
  @EnsuresNonNull("fn")
  private Function getCompiledScript()
  {
    // JavaScript configuration should be checked when it's actually used because someone might still want Druid
    // nodes to be able to deserialize JavaScript-based objects even though JavaScript is disabled.
    Preconditions.checkState(config.isEnabled(), "JavaScript is disabled");

    Function syncedFn = fn;
    if (syncedFn == null) {
      synchronized (config) {
        syncedFn = fn;
        if (syncedFn == null) {
          syncedFn = compile(function);
          fn = syncedFn;
        }
      }
    }
    return syncedFn;
  }

  @Override
  @Nullable
  public String apply(@Nullable String value)
  {
    return this.apply((Object) NullHandling.emptyToNullIfNeeded(value));
  }

  @Override
  public String apply(long value)
  {
    return this.apply((Long) value);
  }

  @Override
  public boolean preservesOrdering()
  {
    return false;
  }

  @Override
  public ExtractionType getExtractionType()
  {
    return injective ? ExtractionType.ONE_TO_ONE : ExtractionType.MANY_TO_ONE;
  }

  @Override
  public String toString()
  {
    return "JavascriptDimExtractionFn{" +
           "function='" + function + '\'' +
           '}';
  }

  @Override
  public boolean equals(Object o)
  {
    if (this == o) {
      return true;
    }
    if (o == null || getClass() != o.getClass()) {
      return false;
    }

    JavaScriptExtractionFn that = (JavaScriptExtractionFn) o;

    if (!function.equals(that.function)) {
      return false;
    }

    return true;
  }

  @Override
  public int hashCode()
  {
    return function.hashCode();
  }
}