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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.guice;

import com.google.common.base.Supplier;
import com.google.common.base.Suppliers;
import com.google.inject.Binder;
import com.google.inject.Inject;
import com.google.inject.Key;
import com.google.inject.Provider;
import com.google.inject.util.Types;
import org.apache.druid.guice.annotations.PublicApi;

import javax.annotation.Nullable;
import java.lang.annotation.Annotation;
import java.lang.reflect.ParameterizedType;
import java.util.Properties;


/**
 * Provides a singleton value of type {@code } from {@code Properties} bound in guice.
 * 

 * Usage
 * To install this provider, bind it in your guice module, like below.
 *
 *  * JsonConfigProvider.bind(binder, "druid.server", DruidServerConfig.class);
 * 
 * 

 * In the above case, {@code druid.server} should be a key found in the {@code Properties} bound elsewhere.
 * The value of that key should directly relate to the fields in {@code DruidServerConfig.class}.
 *
 * Implementation
 * 

 * The state of {@code } is defined by the value of the property {@code propertyBase}.
 * This value is a json structure, decoded via {@link JsonConfigurator#configurate(Properties, String, Class)}.
 * 

 *
 * An example might be if DruidServerConfig.class were
 *
 *  *   public class DruidServerConfig
 *   {
 *     @JsonProperty @NotNull public String hostname = null;
 *     @JsonProperty @Min(1025) public int port = 8080;
 *   }
 * 
 *
 * And your Properties object had in it
 *
 *  *   druid.server.hostname=0.0.0.0
 *   druid.server.port=3333
 * 
 *
 * Then this would bind a singleton instance of a DruidServerConfig object with hostname = "0.0.0.0" and port = 3333.
 *
 * If the port weren't set in the properties, then the default of 8080 would be taken.  Essentially, it is the same as
 * subtracting the "druid.server" prefix from the properties and building a Map which is then passed into
 * ObjectMapper.convertValue()
 *
 * @param  type of config object to provide.
 */
@PublicApi
public class JsonConfigProvider implements Provider
{
  @SuppressWarnings("unchecked")
  public static  void bind(Binder binder, String propertyBase, Class classToProvide)
  {
    bind(
        binder,
        propertyBase,
        classToProvide,
        Key.get(classToProvide),
        (Key) Key.get(Types.newParameterizedType(Supplier.class, classToProvide))
    );
  }

  @SuppressWarnings("unchecked")
  public static  void bindWithDefault(
      Binder binder,
      String propertyBase,
      Class classToProvide,
      Class defaultClass
  )
  {
    bind(
        binder,
        propertyBase,
        classToProvide,
        defaultClass,
        Key.get(classToProvide),
        (Key) Key.get(Types.newParameterizedType(Supplier.class, classToProvide))
    );
  }

  @SuppressWarnings("unchecked")
  public static  void bind(Binder binder, String propertyBase, Class classToProvide, Annotation annotation)
  {
    bind(
        binder,
        propertyBase,
        classToProvide,
        Key.get(classToProvide, annotation),
        (Key) Key.get(Types.newParameterizedType(Supplier.class, classToProvide), annotation)
    );
  }

  @SuppressWarnings("unchecked")
  public static  void bind(
      Binder binder,
      String propertyBase,
      Class classToProvide,
      Class annotation
  )
  {
    bind(
        binder,
        propertyBase,
        classToProvide,
        Key.get(classToProvide, annotation),
        (Key) Key.get(Types.newParameterizedType(Supplier.class, classToProvide), annotation)
    );
  }

  public static  void bind(
      Binder binder,
      String propertyBase,
      Class clazz,
      Key instanceKey,
      Key> supplierKey
  )
  {
    binder.bind(instanceKey).toProvider(new JsonConfigProvider<>(propertyBase, clazz, null)).in(LazySingleton.class);
    binder.bind(supplierKey).toProvider(new ProviderBasedGoogleSupplierProvider<>(instanceKey)).in(LazySingleton.class);
  }

  public static  void bind(
      Binder binder,
      String propertyBase,
      Class clazz,
      Class defaultClass,
      Key instanceKey,
      Key> supplierKey
  )
  {
    binder.bind(instanceKey)
          .toProvider(new JsonConfigProvider<>(propertyBase, clazz, defaultClass))
          .in(LazySingleton.class);
    binder.bind(supplierKey).toProvider(new ProviderBasedGoogleSupplierProvider<>(instanceKey)).in(LazySingleton.class);
  }

  @SuppressWarnings("unchecked")
  public static  void bindInstance(
      Binder binder,
      Key bindKey,
      T instance
  )
  {
    binder.bind(bindKey).toInstance(instance);

    final ParameterizedType supType = Types.newParameterizedType(Supplier.class, bindKey.getTypeLiteral().getType());
    final Key supplierKey;

    if (bindKey.getAnnotationType() != null) {
      supplierKey = Key.get(supType, bindKey.getAnnotationType());
    } else if (bindKey.getAnnotation() != null) {
      supplierKey = Key.get(supType, bindKey.getAnnotation());
    } else {
      supplierKey = Key.get(supType);
    }

    binder.bind(supplierKey).toInstance(Suppliers.ofInstance(instance));
  }

  public static  JsonConfigProvider of(String propertyBase, Class classToProvide)
  {
    return of(propertyBase, classToProvide, null);
  }

  public static  JsonConfigProvider of(
      String propertyBase,
      Class classToProvide,
      Class defaultClass
  )
  {
    return new JsonConfigProvider<>(propertyBase, classToProvide, defaultClass);
  }

  private final String propertyBase;
  private final Class classToProvide;
  private final Class defaultClass;

  private Properties props;
  private JsonConfigurator configurator;

  private T retVal = null;

  public JsonConfigProvider(
      String propertyBase,
      Class classToProvide,
      @Nullable Class defaultClass
  )
  {
    this.propertyBase = propertyBase;
    this.classToProvide = classToProvide;
    this.defaultClass = defaultClass;
  }

  @Inject
  public void inject(
      Properties props,
      JsonConfigurator configurator
  )
  {
    this.props = props;
    this.configurator = configurator;
  }

  @Override
  public T get()
  {
    if (retVal == null) {
      retVal = configurator.configurate(props, propertyBase, classToProvide, defaultClass);
    }
    return retVal;
  }
}