Java8采用Martin Fowler的方法创建内部DSL

我最近在读Martin Flower写的一本非常棒的关于DSLs（Domain Specific Languages)的书。围绕DSLs及语言的内容使得我们可以很方便地创建DSLs，DSLs的使用让我对DSLs的概念更加好奇，这本书让人印象深刻。在Martin Fowler一书的开始是这样定义DSLs的：

Domain-specific language (noun): 一种专注于某一领域，仅针对部分表达方式的计算机编程语言。（译者注：求专不求全。）

DSL不是什么新鲜玩意，很久以前，人们就将XML作为一种DSL的一种形式来使用了。将XML作为DSL来使用非常便捷，因为我们可用来检查DSL的XSD，有解释DSL的解释器，还有能将DSL转换成其他语言的XSLT。并且，多数的语言都对解释XML和获取该语言领域中模型对象的内容提供了很好的支持。像是Ruby，Groovy等等这些语言的出现使得DSL被更广泛的接受。比如Rails，一个使用Ruby写的Web框架，广泛地采用了DSLs。

Martin Fowler在他的书中将DSLs分为三类：内部的DSL，外部的DSL，和语言工作平台的DSL。当我读到内部DSL概念时，使用Java作为宿主语言，用我自己简单的DSL小试牛刀了一下。内部DSLs驻于宿主语言中，并且遵守宿主语言的语法。尽管使用Java作为宿主语言没有让我非常清楚的了解DSL，但却有效地让我以一种合适的方式来了解DSL。

我打算写一个能产生图表的DSL。在Java中有两种接收输入并产生表格的方法：Adjacency List和Adjacency Matrix。然而我发现，在没有将矩阵作为“一等公民”提供支持的语言（尤其是Java）实现是非常困难的，所以，我就尝试着在Java中写一个内部DSL来实现对表格的操作。

在他的书中，Martin Fowler强调需要保持语义模型不同于DSL，并且引入了一个中间表达式构造器来从DSL中产生语义模型。所以为了遵守上述内容，我通过写不同的DSL语法和表达式构造器实现了三种不同的DSLs形式，同时又使用了相同的语义模型。

理解语义模型

在这种情景下，语义模型就是包含了Edge实例数组的Graph类，每一个Edge对象又存放了从Vertex到Vertex的数据和一个weight。下面看一下代码吧。

import java.util.ArrayList;
import java.util.List;
import java.util.Set;
import java.util.TreeSet;

public class Graph {

  private List edges;
  private Set vertices;

  public Graph() {
    edges = new ArrayList<>();
    vertices = new TreeSet<>();
  }
  public void addEdge(Edge edge){
    getEdges().add(edge);
  }

  public void addVertice(Vertex v){
    getVertices().add(v);
  }

  public List getEdges() {
    return edges;
  }

  public Set getVertices() {
    return vertices;
  }

  public static void printGraph(Graph g){
    System.out.println("Vertices...");
    for (Vertex v : g.getVertices()) {
      System.out.print(v.getLabel() + " ");
    }
    System.out.println("");
    System.out.println("Edges...");
    for (Edge e : g.getEdges()) {
      System.out.println(e);
    }
  }
}

import java.util.ArrayList;

import java.util.List;

import java.util.Set;

import java.util.TreeSet;

public class Graph {

private List edges;

private Set vertices;

public Graph() {

edges = new ArrayList<>();

vertices = new TreeSet<>();

}

public void addEdge(Edge edge){

getEdges().add(edge);

}

public void addVertice(Vertex v){

getVertices().add(v);

}

public List getEdges() {

return edges;

}

public Set getVertices() {

return vertices;

}

public static void printGraph(Graph g){

System.out.println("Vertices...");

for (Vertex v : g.getVertices()) {

System.out.print(v.getLabel() + " ");

}

System.out.println("");

System.out.println("Edges...");

for (Edge e : g.getEdges()) {

System.out.println(e);

}

public class Edge {
  private Vertex fromVertex;
  private Vertex toVertex;
  private Double weight;

  public Edge() {
  }

  public Edge(Vertex fromVertex, Vertex toVertex, Double weight) {
    this.fromVertex = fromVertex;
    this.toVertex = toVertex;
    this.weight = weight;
  }

  @Override
  public String toString() {
      return fromVertex.getLabel()+" to "+
             toVertex.getLabel()+" with weight "+ 
             getWeight();
  }

  public Vertex getFromVertex() {
    return fromVertex;
  }

  public void setFromVertex(Vertex fromVertex) {
    this.fromVertex = fromVertex;
  }

  public Vertex getToVertex() {
    return toVertex;
  }

  public void setToVertex(Vertex toVertex) {
    this.toVertex = toVertex;
  }

  public Double getWeight() {
    return weight;
  }

  public void setWeight(Double weight) {
    this.weight = weight;
  }
}

public class Edge {

private Vertex fromVertex;

private Vertex toVertex;

private Double weight;

public Edge() {

}

public Edge(Vertex fromVertex, Vertex toVertex, Double weight) {

this.fromVertex = fromVertex;

this.toVertex = toVertex;

this.weight = weight;

}

@Override

public String toString() {

return fromVertex.getLabel()+" to "+

toVertex.getLabel()+" with weight "+

getWeight();

}

public Vertex getFromVertex() {

return fromVertex;

}

public void setFromVertex(Vertex fromVertex) {

this.fromVertex = fromVertex;

}

public Vertex getToVertex() {

return toVertex;

}

public void setToVertex(Vertex toVertex) {

this.toVertex = toVertex;

}

public Double getWeight() {

return weight;

}

public void setWeight(Double weight) {

this.weight = weight;

}

public class Vertex implements Comparable<Vertex> {
  private String label;

  public Vertex(String label) {
    this.label = label.toUpperCase();
  }

  @Override
  public int compareTo(Vertex o) {
    return (this.getLabel().compareTo(o.getLabel()));
  }

  public String getLabel() {
    return label;
  }

  public void setLabel(String label) {
    this.label = label;
  }
}

public class Vertex implements Comparable<Vertex> {

private String label;

public Vertex(String label) {

this.label = label.toUpperCase();

}

@Override

public int compareTo(Vertex o) {

return (this.getLabel().compareTo(o.getLabel()));

}

public String getLabel() {

return label;

}

public void setLabel(String label) {

this.label = label;

}

好了，既然语义模型已经到位，我们就开始做DSLs吧。你应该已经注意到了吧，我不打算修改我的语义模型。没有硬性规定语义模型不可以修改，相反，随着新的可以读取和修改数据的API的加入，语义模型可以不断被完善。但是将语义模型和DSL绑定的太死并不可取。保持它们相对分离可以独立地测试语义模型和DSL。

Martin Fowler陈述的创建内部DSLs 的方法是：

方法链
功能序列
嵌套函数
Lambda表达式/闭包

除了功能序列之外，我在这篇文章中图文并茂的介绍了其中的3种。但是我也在Lambda表达式/闭包中使用了功能序列的方法。

用方法链创建的DSL

我幻想出一种像这样子的DSL:

Graph()
  .edge()
    .from("a")
    .to("b")
    .weight(12.3)
  .edge()
    .from("b")
    .to("c")
    .weight(10.5)

Graph()

.edge()

.from("a")

.to("b")

.weight(12.3)

.edge()

.from("b")

.to("c")

.weight(10.5)

为了能够实现这样的DSL，我们要写一个表达式构造器，能够产生语义模型，提供能产生DSL的流接口。

我写了来年改革表达式构造器——一个用来完成图表，另一个用来建立每一个的边界。这些图表和边界建立的时候，这些表达式构造器就持有中间的图表和边界对象。以上的语法可以在表达式构造器的静态方法中实现，然后用静态导入，就可以在DSL中使用了。

Graph()方法开始生成Graph模型，同时edge()和一系列方法，也就是from(),to(),weight()产生Edge模型，edge()同时也产生Graph模型。

让我们来看一下GraphBuilder（生成Graph模型的表达式构造器）吧：

public class GraphBuilder {

  private Graph graph;

  public GraphBuilder() {
    graph = new Graph();
  }

  //Start the Graph DSL with this method.
  public static GraphBuilder Graph(){
    return new GraphBuilder();
  }

  //Start the edge building with this method.
  public EdgeBuilder edge(){
    EdgeBuilder builder = new EdgeBuilder(this);

    getGraph().addEdge(builder.edge);

    return builder;
  }

  public Graph getGraph() {
    return graph;
  }

  public void printGraph(){
    Graph.printGraph(graph);
  }
}

public class GraphBuilder {

private Graph graph;

public GraphBuilder() {

graph = new Graph();

}

//Start the Graph DSL with this method.

public static GraphBuilder Graph(){

return new GraphBuilder();

}

//Start the edge building with this method.

public EdgeBuilder edge(){

EdgeBuilder builder = new EdgeBuilder(this);

getGraph().addEdge(builder.edge);

return builder;

}

public Graph getGraph() {

return graph;

}

public void printGraph(){

Graph.printGraph(graph);

}

接下来是EdgeBuilder（生成Edge模型的表达式构造器）：

public class EdgeBuilder {

  Edge edge;

  //Keep a back reference to the Graph Builder.
  GraphBuilder gBuilder;

  public EdgeBuilder(GraphBuilder gBuilder) {
    this.gBuilder = gBuilder;
    edge = new Edge();
  }

  public EdgeBuilder from(String lbl){
    Vertex v = new Vertex(lbl);
    edge.setFromVertex(v);
    gBuilder.getGraph().addVertice(v);
    return this;
  }
  public EdgeBuilder to(String lbl){
    Vertex v = new Vertex(lbl);
    edge.setToVertex(v);
    gBuilder.getGraph().addVertice(v);
    return this;
  }

  public GraphBuilder weight(Double d){
    edge.setWeight(d);
    return gBuilder;
  }

}

public class EdgeBuilder {

Edge edge;

//Keep a back reference to the Graph Builder.

GraphBuilder gBuilder;

public EdgeBuilder(GraphBuilder gBuilder) {

this.gBuilder = gBuilder;

edge = new Edge();

}

public EdgeBuilder from(String lbl){

Vertex v = new Vertex(lbl);

edge.setFromVertex(v);

gBuilder.getGraph().addVertice(v);

return this;

}

public EdgeBuilder to(String lbl){

Vertex v = new Vertex(lbl);

edge.setToVertex(v);

gBuilder.getGraph().addVertice(v);

return this;

}

public GraphBuilder weight(Double d){

edge.setWeight(d);

return gBuilder;

}

让我们来试一下这个DSL吧：

public class GraphDslSample {

  public static void main(String[] args) {

    Graph()
      .edge()
        .from("a")
        .to("b")
        .weight(40.0)
      .edge()
        .from("b")
        .to("c")
        .weight(20.0)
      .edge()
        .from("d")
        .to("e")
        .weight(50.5)
      .printGraph();

    Graph()
      .edge()
        .from("w")
        .to("y")
        .weight(23.0)
      .edge()
        .from("d")
        .to("e")
        .weight(34.5)
      .edge()
        .from("e")
        .to("y")
        .weight(50.5)
      .printGraph();

  }
}

public class GraphDslSample {

public static void main(String[] args) {

Graph()

.edge()

.from("a")

.to("b")

.weight(40.0)

.edge()

.from("b")

.to("c")

.weight(20.0)

.edge()

.from("d")

.to("e")

.weight(50.5)

.printGraph();

Graph()

.edge()

.from("w")

.to("y")

.weight(23.0)

.edge()

.from("d")

.to("e")

.weight(34.5)

.edge()

.from("e")

.to("y")

.weight(50.5)

.printGraph();

}

输出结果是：

Vertices...
A B C D E
Edges...
A to B with weight 40.0
B to C with weight 20.0
D to E with weight 50.5
Vertices...
D E W Y
Edges...
W to Y with weight 23.0
D to E with weight 34.5
E to Y with weight 50.5

Vertices...

A B C D E

Edges...

A to B with weight 40.0

B to C with weight 20.0

D to E with weight 50.5

Vertices...

D E W Y

Edges...

W to Y with weight 23.0

D to E with weight 34.5

E to Y with weight 50.5

这个方法不是比Adjacency List或者Adjacency Matrix方法更具有可读性吗？这个方法链和我之前写的Train Wreck pattern很像。

用嵌套函数创建的DSL

在DSL中使用嵌套函数的风格会有所不同。在这中方法中，我将会在函数之中嵌套函数，来写我的语义模型，像下面这样：

Graph(
  edge(from("a"), to("b"), weight(12.3),
  edge(from("b"), to("c"), weight(10.5)
);

Graph(

edge(from("a"), to("b"), weight(12.3),

edge(from("b"), to("c"), weight(10.5)

);

这种方法的好处是，它的层次天生juice不像访法链那样必须用另一种格式来写代码。而且，这种方法不需要在表达式构造器中提供中间变量，也就是说，当DSL被解析或者执行的时候，表达式构造器不需要持有Graph和Edge对象。语义模型和上文中谈到的相同。

以下是DSL的表达式构造器。

//Populates the Graph model.
public class NestedGraphBuilder {

  public static Graph Graph(Edge... edges){
    Graph g = new Graph();
    for(Edge e : edges){
      g.addEdge(e);
      g.addVertice(e.getFromVertex());
      g.addVertice(e.getToVertex());
    }
    return g;
  }

}

//Populates the Graph model.

public class NestedGraphBuilder {

public static Graph Graph(Edge... edges){

Graph g = new Graph();

for(Edge e : edges){

g.addEdge(e);

g.addVertice(e.getFromVertex());

g.addVertice(e.getToVertex());

}

return g;

}

//Populates the Edge model.
public class NestedEdgeBuilder {

  public static Edge edge(Vertex from, Vertex to, 
                          Double weight){
    return new Edge(from, to, weight);
  }

  public static Double weight(Double value){
    return value;
  }

}

//Populates the Edge model.

public class NestedEdgeBuilder {

public static Edge edge(Vertex from, Vertex to,

Double weight){

return new Edge(from, to, weight);

}

public static Double weight(Double value){

return value;

}

//Populates the Vertex model.
public class NestedVertexBuilder {
  public static Vertex from(String lbl){
    return new Vertex(lbl);
  }

  public static Vertex to(String lbl){
    return new Vertex(lbl);
  }
}

//Populates the Vertex model.

public class NestedVertexBuilder {

public static Vertex from(String lbl){

return new Vertex(lbl);

}

public static Vertex to(String lbl){

return new Vertex(lbl);

}

如果你想严格遵守规则，让所有表达式构造器定义在静态上，我们可以使用静态导入的方法创建一个DSL。

注意：表达式构造器、语义模型和DSL分别在不同的包中，所以请根据您的包名更新一下import。

//Update this according to the package name of your builder
import static nestedfunction.NestedEdgeBuilder.*;
import static nestedfunction.NestedGraphBuilder.*;
import static nestedfunction.NestedVertexBuilder.*;

/**
 *
 * @author msanaull
 */
public class NestedGraphDsl {

  public static void main(String[] args) {
    Graph.printGraph(
      Graph(
        edge(from("a"), to("b"), weight(23.4)),
        edge(from("b"), to("c"), weight(56.7)),
        edge(from("d"), to("e"), weight(10.4)),
        edge(from("e"), to("a"), weight(45.9))
      )
    );

  }
}

//Update this according to the package name of your builder

import static nestedfunction.NestedEdgeBuilder.*;

import static nestedfunction.NestedGraphBuilder.*;

import static nestedfunction.NestedVertexBuilder.*;

/**

* @author msanaull

public class NestedGraphDsl {

public static void main(String[] args) {

Graph.printGraph(

Graph(

edge(from("a"), to("b"), weight(23.4)),

edge(from("b"), to("c"), weight(56.7)),

edge(from("d"), to("e"), weight(10.4)),

edge(from("e"), to("a"), weight(45.9))

)

);

}

输出如下：

Vertices...
A B C D E
Edges...
A to B with weight 23.4
B to C with weight 56.7
D to E with weight 10.4
E to A with weight 45.9

Vertices...

A B C D E

Edges...

A to B with weight 23.4

B to C with weight 56.7

D to E with weight 10.4

E to A with weight 45.9

有趣的部分来了：我们如何利用DSL支持的lambda表达式呢？

在内部DSL中使用lambda表达式

如果你还不知道lambda表达式在Java中能做什么的话，请先阅读本文有关语义模型的部分。

在这个例子中我们会继续使用上面描述的语义模型。这个DSL使用了支持lambda表达式的功能序列。让我们看一下最后的DSL是什么样子的吧：

Graph(g -> {
    g.edge( e -> {
      e.from("a");
      e.to("b");
      e.weight(12.3);
    });

    g.edge( e -> {
      e.from("b");
      e.to("c");
      e.weight(10.5);
    });

  }
)

Graph(g -> {

g.edge( e -> {

e.from("a");

e.to("b");

e.weight(12.3);

});

g.edge( e -> {

e.from("b");

e.to("c");

e.weight(10.5);

});

}

)

是的，我知道上面的这个DSL重载了操作符，但是我们不得不这样做。如果你不喜欢，那么可以选择另一种语言。

在这个方法中，我们的表达式构造器应该接受lambda表达式/closure/block，然后在lambda表达式/closure/block的基础上创建语义模型。这样实现的语义模型保留了Graph和Edge对象这样的中间值，就想我们在方法链中做的那样。

看一下我们的表达式构造器吧：

//Populates the Graph model.
public class GraphBuilder {

  Graph g;
  public GraphBuilder() {
    g = new Graph();
  }

  public static Graph Graph(Consumer<GraphBuilder> gConsumer){
    GraphBuilder gBuilder = new GraphBuilder();
    gConsumer.accept(gBuilder);
    return gBuilder.g;
  }

  public void edge(Consumer<EdgeBuilder> eConsumer){
    EdgeBuilder eBuilder = new EdgeBuilder();
    eConsumer.accept(eBuilder);
    Edge e = eBuilder.edge();
    g.addEdge(e);
    g.addVertice(e.getFromVertex());
    g.addVertice(e.getToVertex());
  }
}

//Populates the Graph model.

public class GraphBuilder {

Graph g;

public GraphBuilder() {

g = new Graph();

}

public static Graph Graph(Consumer<GraphBuilder> gConsumer){

GraphBuilder gBuilder = new GraphBuilder();

gConsumer.accept(gBuilder);

return gBuilder.g;

}

public void edge(Consumer<EdgeBuilder> eConsumer){

EdgeBuilder eBuilder = new EdgeBuilder();

eConsumer.accept(eBuilder);

Edge e = eBuilder.edge();

g.addEdge(e);

g.addVertice(e.getFromVertex());

g.addVertice(e.getToVertex());

}

//Populates the Edge model.
public class EdgeBuilder {
  private Edge e;
  public EdgeBuilder() {
    e = new Edge();
  }

  public Edge edge(){
    return e;
  }

  public void from(String lbl){
    e.setFromVertex(new Vertex(lbl));
  }
  public void to(String lbl){
    e.setToVertex(new Vertex(lbl));
  }
  public void weight(Double w){
    e.setWeight(w);
  }

}

//Populates the Edge model.

public class EdgeBuilder {

private Edge e;

public EdgeBuilder() {

e = new Edge();

}

public Edge edge(){

return e;

}

public void from(String lbl){

e.setFromVertex(new Vertex(lbl));

}

public void to(String lbl){

e.setToVertex(new Vertex(lbl));

}

public void weight(Double w){

e.setWeight(w);

}

在GraphBuilder中有两行高亮的代码。这个地方用了Java 8引入的功能接口，Consumer.

下面我们使用上面的表达式构造器来创建我们的DSL。

//Update the package names with the ones you have given
import graph.Graph;
import static builder.GraphBuilder.*;

public class LambdaDslDemo {
  public static void main(String[] args) {
    Graph g1 = Graph( g -> {
      g.edge( e -> {
        e.from("a");
        e.to("b");
        e.weight(12.4);
      });

      g.edge( e -> {
        e.from("c");
        e.to("d");
        e.weight(13.4);
      });
    });

    Graph.printGraph(g1);
  }
}

//Update the package names with the ones you have given

import graph.Graph;

import static builder.GraphBuilder.*;

public class LambdaDslDemo {

public static void main(String[] args) {

Graph g1 = Graph( g -> {

g.edge( e -> {

e.from("a");

e.to("b");

e.weight(12.4);

});

g.edge( e -> {

e.from("c");

e.to("d");

e.weight(13.4);

});

Graph.printGraph(g1);

}

输出如下：

Vertices...
A B C D
Edges...
A to B with weight 12.4
C to D with weight 13.4

Vertices...

A B C D

Edges...

A to B with weight 12.4

C to D with weight 13.4

当我要结束这篇长长的文章的时候，我知道你可能想让我把它分成3个部分每部分介绍一种DSL的实现来发布。我坚持发布在一篇文章中是因为这样可以比较一下这3种方法。

概括：

这篇文章中我们讨论了Martin Fowler的Domain Specific Languages一书中涉及的DSL，内部DSL。
分别介绍了三种背部DSL的实现方法
- 方法链
- 嵌套函数
- 支持功能序列的Lambda表达式

英文原文由Mohamed Sanaulla发表在javacodegeeks，赖信涛翻译，Importnew校对。

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Java8采用Martin Fowler的方法创建内部DSL

理解语义模型

用方法链创建的DSL

用嵌套函数创建的DSL

在内部DSL中使用lambda表达式

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