Elasticsearch 设计模式—建造者模式

Elasticsearch 设计模式—建造者模式

建造者模式

构造者模式,或称建造者模式(Builder Pattern).

概念

将一个复杂的构建与其表示相分离，使得同样的构建过程可以创建不同的表示。 [构建与表示分离，同构建不同表示]

UML 图

一般来说, 构造者模式会涉及到四个角色:

• 抽象建造者（Builder）角色

    给 出一个抽象接口，以规范产品对象的各个组成成分的建造。一般而言，此接口独立于应用程序的商业逻辑。模式中直接创建产品对象的是具体建造者 (ConcreteBuilder)角色
  

• 具体建造者（ConcreteBuilder）角色

    担任这个角色的是与应用程序紧密相关的一些类，它们在应用程序调用下创建产品的实例。这个角色要完成的任务包括：1.实现抽象建造者Builder所声明的接口，给出一步一步地完成创建产品实例的操作。2.在建造过程完成后，提供产品的实例
  

• 导演者（Director）角色

    担任这个角色的类调用具体建造者角色以创建产品对象。应当指出的是，导演者角色并没有产品类的具体知识，真正拥有产品类的具体知识的是具体建造者角色
  

• 产品（Product）角色

    产品便是建造中的复杂对象。一般来说，一个系统中会有多于一个的产品类，而且这些产品类并不一定有共同的接口，而完全可以是不相关联的。
  

Elasticsearch 对于建造者模式的应用

众所周知,Elasticsearch 提供了及其丰富的查询 API,而这些查询 API 的构建核心就是利用建造者模式来完成的,可以说 Elasticsearch 是我见过将建造者模式应用的最好
和最丰富最完美的一个架构.接下来我们就看看 Elasticsearch 是如何在自己的系统里使用建造者模式.

抽象建造者（Builder）角色

QueryBuilder 是构建所有查询的入口,源码如下:

public interface QueryBuilder extends NamedWriteable, ToXContentObject {

    /**
     * Converts this QueryBuilder to a lucene {@link Query}.
     * Returns <tt>null</tt> if this query should be ignored in the context of
     * parent queries.
     *
     * @param context additional information needed to construct the queries
     * @return the {@link Query} or <tt>null</tt> if this query should be ignored upstream
     */
    Query toQuery(QueryShardContext context) throws IOException;

    /**
     * Converts this QueryBuilder to an unscored lucene {@link Query} that acts as a filter.
     * Returns <tt>null</tt> if this query should be ignored in the context of
     * parent queries.
     *
     * @param context additional information needed to construct the queries
     * @return the {@link Query} or <tt>null</tt> if this query should be ignored upstream
     */
    Query toFilter(QueryShardContext context) throws IOException;

    /**
     * Sets the arbitrary name to be assigned to the query (see named queries).
     * Implementers should return the concrete type of the
     * {@link QueryBuilder} so that calls can be chained. This is done
     * automatically when extending {@link AbstractQueryBuilder}.
     */
    QueryBuilder queryName(String queryName);

    /**
     * Returns the arbitrary name assigned to the query (see named queries).
     */
    String queryName();

    /**
     * Returns the boost for this query.
     */
    float boost();

    /**
     * Sets the boost for this query.  Documents matching this query will (in addition to the normal
     * weightings) have their score multiplied by the boost provided.
     * Implementers should return the concrete type of the
     * {@link QueryBuilder} so that calls can be chained. This is done
     * automatically when extending {@link AbstractQueryBuilder}.
     */
    QueryBuilder boost(float boost);

    /**
     * Returns the name that identifies uniquely the query
     */
    String getName();

    /**
     * Rewrites this query builder into its primitive form. By default this method return the builder itself. If the builder
     * did not change the identity reference must be returned otherwise the builder will be rewritten infinitely.
     */
    default QueryBuilder rewrite(QueryRewriteContext queryShardContext) throws IOException {
        return this;
    }

    /**
     * Rewrites the given query into its primitive form. Queries that for instance fetch resources from remote hosts or
     * can simplify / optimize itself should do their heavy lifting during {@link #rewrite(QueryRewriteContext)}. This method
     * rewrites the query until it doesn't change anymore.
     * @throws IOException if an {@link IOException} occurs
     */
    static QueryBuilder rewriteQuery(QueryBuilder original, QueryRewriteContext context) throws IOException {
        QueryBuilder builder = original;
        for (QueryBuilder rewrittenBuilder = builder.rewrite(context); rewrittenBuilder != builder;
             rewrittenBuilder = builder.rewrite(context)) {
            builder = rewrittenBuilder;
        }
        return builder;
    }

}

该接口用来规范定义想要构造一个查询构造器的标准条件,是一个最顶层的抽象构造器接口：

• boost
• queryName

而其子类 AbstractQueryBuilder 则是默认实现了接口中的一些规范,两者共同充当建造者模式中的抽象建造者（Builder）角色.

具体建造者（ConcreteBuilder）角色

Elasticsearch 中拥有大量的具体的建造者角色,这些具体的建造者角色就是 Elasticsearch 根据不同的查询场景的构造者,比如:BoolQueryBuilder,MatchPhraseQueryBuilder 等

这里只拿 BoolQueryBuilder 进行说明,部分源码如下:

public BoolQueryBuilder must(QueryBuilder queryBuilder) {
        if (queryBuilder == null) {
            throw new IllegalArgumentException("inner bool query clause cannot be null");
        }
        mustClauses.add(queryBuilder);
        return this;
    }
     public BoolQueryBuilder filter(QueryBuilder queryBuilder) {
            if (queryBuilder == null) {
                throw new IllegalArgumentException("inner bool query clause cannot be null");
            }
            filterClauses.add(queryBuilder);
            return this;
        }
         public BoolQueryBuilder mustNot(QueryBuilder queryBuilder) {
                if (queryBuilder == null) {
                    throw new IllegalArgumentException("inner bool query clause cannot be null");
                }
                mustNotClauses.add(queryBuilder);
                return this;
            }
            public BoolQueryBuilder should(QueryBuilder queryBuilder) {
                    if (queryBuilder == null) {
                        throw new IllegalArgumentException("inner bool query clause cannot be null");
                    }
                    shouldClauses.add(queryBuilder);
                    return this;
                }

从源码中可以看到, 针对不同的条件,可以使用不同的方法进行构造布尔查询。

导演者（Director）角色

Elasticsearch 中充当导演者角色的类是 SearchRequestBuilder,其自身也是一个构造器,同时也是一个导演者（Director）角色,内部持有具体的构造器来完成
具体的构建过程.

产品（Director）角色

Elasticsearch 中充当导演者角色的就是这些构造器本身

参考

• 《JAVA 与模式》之建造模式