Java 內部 DSL 實作：流式 API 設計圖構建範例

什麼是內部 DSL？

使用宿主語言（Java）語法構建領域專用語言，通過方法鏈與靜態導入實現接近自然語言的可讀性。

使用端效果

Graph().edge().from("a").to("b").weight(20.0)
       .edge().from("b").to("c").weight(10.0)
       .printGraph();

核心類別設計

1. 入口 GraphBuilder

public class GraphBuilder {
    private final Graph graph = new Graph();

    private GraphBuilder() {}

    public static GraphBuilder Graph() { return new GraphBuilder(); }

    public EdgeBuilder edge() { return new EdgeBuilder(graph); }

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

2. 邊構建器 EdgeBuilder（鏈式核心）

public class EdgeBuilder {
    private final Graph graph;
    private String from, to;
    private Double weight;

    public EdgeBuilder(Graph graph) { this.graph = graph; }

    public EdgeBuilder from(String v) { this.from = v; return this; }
    public EdgeBuilder to(String v)   { this.to = v; return this; }
    public EdgeBuilder weight(double w) { this.weight = w; return this; }

    /** 終結當前邊，加入圖並回傳自身以支援連續 .edge() */
    public EdgeBuilder build() {
        graph.addEdge(from, to, weight);
        return this;
    }
}

3. 圖模型 Graph、Vertex、Edge

public class Graph {
    private final Map vertices = new HashMap<>();
    private final List edges = new ArrayList<>();

    public void addEdge(String from, String to, Double w) {
        Vertex f = vertices.computeIfAbsent(from, Vertex::new);
        Vertex t = vertices.computeIfAbsent(to, Vertex::new);
        edges.add(new Edge(f, t, w));
    }
    public void print() { edges.forEach(System.out::println); }
}

public class Vertex { private final String label; /* getter */ }
public class Edge { private Vertex from, to; private Double weight; /* getter/toString */ }

DSL 設計模式總結

進階擴展

• 頂點屬性：.vertex("a").label("Node A").color("red")
• 條件邊：.edge().from("a").to("b").when(condition)
• 輸出格式：.export(DOT).toFile("g.dot")
• 型別安全：泛型 GraphBuilder<V, E> 限制頂點/邊類型

真實專案案例

• JOOQ：SQL 建構 DSL
• Mockito：when(mock.foo()).thenReturn(bar)
• AssertJ：assertThat(list).hasSize(3).contains("a")
• Gradle：Groovy/Kotlin DSL 定義建構腳本