Class diagrams: packages and namespaces

简介 / Introduction

Class diagrams don’t just show relationships — they also use packages or namespaces to organize model elements into a hierarchical structure. This article uses the same scenario to compare PlantUML and Mermaid’s package/namespace syntax, highlighting similarities and differences.

PlantUML 版

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package "com.example.app" {
  class MyClass
  class Helper
}

package "com.example.lib" {
  class Service
  class Dao
}

MyClass --> Service : uses
MyClass --> Dao : queries
Helper --> Service

package "com.example.util" {
  class StringUtils
}

Service ..> StringUtils : imports

PlantUML uses package "name" { ... } to group classes, supports nesting and colors/icons, and expresses dependencies with arrows.

Mermaid 版

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classDiagram
    namespace com.example.app {
        class MyClass {
            +void doWork()
        }
        class Helper {
            +void assist()
        }
    }

    namespace com.example.lib {
        class Service {
            +void process()
        }
        class Dao {
            +void fetch()
        }
    }

    namespace com.example.util {
        class StringUtils {
            +static String format()
        }
    }

    MyClass --> Service : uses
    MyClass --> Dao : queries
    Helper --> Service
    Service ..> StringUtils : imports

Mermaid uses namespace 名称 { ... } to separate classes under the same namespace. Class bodies can optionally declare member methods, and dependency arrows work the same way.

对比 / Side-by-side

小结 / Wrap-up

PlantUML’s package syntax is richer — it supports nesting, colors, and icons, making it better suited for formal modeling. Mermaid’s namespace is lighter and more readable, ideal for documentation and quick diagrams. Dependency arrow syntax is nearly identical between the two, keeping the learning curve low.