Swagger 是一个统一前后端用于生成文档和代码的工具，它使用 yaml / json 作为描述语言通过 OpenAPI Specification 来描述 API，最后使用 Codegen 根据不同的配置来生成各种 language、library 的 Code、Docs.

其最理想的情况则是只需一份描述文件(yaml/json) 生成后端、前端（android ios web...）的代码和文档，这样的话保证了前后端的统一，且需要升级改动也只需要修改 yaml 文件。

YAML

JSON 都已经很熟悉了，虽然 Swagger 可以使用 JSON 作为描述语言，但是因为 YAML 更为简洁直观，所以更推荐 YAML。 YAML 的基本语法并不复杂，这里介绍一些基本语法：

yaml 文件以---开始 ...结尾
同一级别的成员（如 list 成员）可以通过"- "来辨识
注释以#开头的一行
list:

fruits:
    - Apple
    - Orange
    - Strawberry
    - Mango

key / value

martin:
    name: Martin D'vloper
    job: Developer
    skill: Elite

list / map 混合使用

-  martin:
    name: Martin D'vloper
    job: Developer
    skills:
      - python
      - perl
      - pascal
-  tabitha:
    name: Tabitha Bitumen
    job: Developer
    skills:
      - lisp
      - fortran
      - erlang

list / map 的简写

martin: {name: Martin D'vloper, job: Developer, skill: Elite}
fruits: ['Apple', 'Orange', 'Strawberry', 'Mango']

boolean 值的写法没有严格限制

create_key: yes
needs_agent: no
knows_oop: True
likes_emacs: TRUE
uses_cvs: false

|使用换行 >忽略换行

include_newlines: |
            exactly as you see
            will appear these three
            lines of poetry

ignore_newlines: >
            this is really a
            single line of text
            despite appearances

OpenAPI-Specification

OpenAPI 是一套用于描述 RESTful APIs 的规范。

The OpenAPI Specification (OAS) defines a standard, language-agnostic interface to RESTful APIs which allows both humans and computers to discover and understand the capabilities of the service without access to source code, documentation, or through network traffic inspection. When properly defined, a consumer can understand and interact with the remote service with a minimal amount of implementation logic.

An OpenAPI definition can then be used by documentation generation tools to display the API, code generation tools to generate servers and clients in various programming languages, testing tools, and many other use cases.

只要符合 OpenAPI 规范的都可以生成各类代码、文档、工具。目前 OpenAPI 最新 3.0.0 ,对比 2.0 对 API 结构进行了调整。
文档内容繁多请参考 OpenAPI-Specification 3.0.0

下面是 Uber API 的 example

# this is an example of the Uber API
# as a demonstration of an API spec in YAML
openapi: "3.0.0"
info:
  title: Uber API
  description: Move your app forward with the Uber API
  version: "1.0.0"
servers:
  - url: https://api.uber.com/v1
paths:
  /products:
    get:
      summary: Product Types
      description: The Products endpoint returns information about the Uber products offered at a given location. The response includes the display name and other details about each product, and lists the products in the proper display order.
      parameters:
        - name: latitude
          in: query
          description: Latitude component of location.
          required: true
          schema:
            type: number
            format: double
        - name: longitude
          in: query
          description: Longitude component of location.
          required: true
          schema:
            type: number
            format: double
      security: 
        - apikey: []
      tags: 
        - Products
      responses:  
        '200':
          description: An array of products
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/ProductList"
        default:
          description: Unexpected error
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Error"
  /estimates/price:
    get:
      summary: Price Estimates
      description: The Price Estimates endpoint returns an estimated price range for each product offered at a given location. The price estimate is provided as a formatted string with the full price range and the localized currency symbol.<br><br>The response also includes low and high estimates, and the [ISO 4217](http://en.wikipedia.org/wiki/ISO_4217) currency code for situations requiring currency conversion. When surge is active for a particular product, its surge_multiplier will be greater than 1, but the price estimate already factors in this multiplier.
      parameters:
        - name: start_latitude
          in: query
          description: Latitude component of start location.
          required: true
          schema:
            type: number
            format: double
        - name: start_longitude
          in: query
          description: Longitude component of start location.
          required: true
          schema:
            type: number
            format: double
        - name: end_latitude
          in: query
          description: Latitude component of end location.
          required: true
          schema:
            type: number
            format: double
        - name: end_longitude
          in: query
          description: Longitude component of end location.
          required: true
          schema:
            type: number
            format: double
      tags: 
        - Estimates
      responses:  
        '200':
          description: An array of price estimates by product
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: "#/components/schemas/PriceEstimate"
        default:
          description: Unexpected error
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Error"
  /estimates/time:
    get:
      summary: Time Estimates
      description: The Time Estimates endpoint returns ETAs for all products offered at a given location, with the responses expressed as integers in seconds. We recommend that this endpoint be called every minute to provide the most accurate, up-to-date ETAs.
      parameters:
        - name: start_latitude
          in: query
          description: Latitude component of start location.
          required: true
          schema:
            type: number
            format: double
        - name: start_longitude
          in: query
          description: Longitude component of start location.
          required: true
          schema:
            type: number
            format: double
        - name: customer_uuid
          in: query
          schema:
            type: string
            format: uuid
          description: Unique customer identifier to be used for experience customization.
        - name: product_id
          in: query
          schema:
            type: string
          description: Unique identifier representing a specific product for a given latitude & longitude.
      tags: 
        - Estimates
      responses:  
        '200':
          description: An array of products
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: "#/components/schemas/Product"
        default:
          description: Unexpected error
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Error"
  /me:
    get:
      summary: User Profile
      description: The User Profile endpoint returns information about the Uber user that has authorized with the application.
      tags: 
        - User
      responses:
        '200':
          description: Profile information for a user
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Profile"
        default:
          description: Unexpected error
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Error"
  /history:
    get:
      summary: User Activity
      description: The User Activity endpoint returns data about a user's lifetime activity with Uber. The response will include pickup locations and times, dropoff locations and times, the distance of past requests, and information about which products were requested.<br><br>The history array in the response will have a maximum length based on the limit parameter. The response value count may exceed limit, therefore subsequent API requests may be necessary.
      parameters:
        - name: offset
          in: query
          schema:
            type: integer
            format: int32
          description: Offset the list of returned results by this amount. Default is zero.
        - name: limit
          in: query
          schema:
            type: integer
            format: int32 
          description: Number of items to retrieve. Default is 5, maximum is 100.
      tags: 
        - User
      responses:
        '200':
          description: History information for the given user
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Activities"
        default:
          description: Unexpected error
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Error"
components:
  securitySchemes:
    apikey:
      type: apiKey
      name: server_token
      in: query
  schemas:
    Product:
      properties:
        product_id:
          type: string
          description: Unique identifier representing a specific product for a given latitude & longitude. For example, uberX in San Francisco will have a different product_id than uberX in Los Angeles.
        description:
          type: string
          description: Description of product.
        display_name:
          type: string
          description: Display name of product.
        capacity:
          type: integer
          description: Capacity of product. For example, 4 people.
        image:
          type: string
          description: Image URL representing the product.
    ProductList:
      properties:
        products:
          description: Contains the list of products
          type: array
          items: 
            $ref: "#/components/schemas/Product"
    PriceEstimate:
      properties:
        product_id:
          type: string
          description: Unique identifier representing a specific product for a given latitude & longitude. For example, uberX in San Francisco will have a different product_id than uberX in Los Angeles
        currency_code:
          type: string
          description: "[ISO 4217](http://en.wikipedia.org/wiki/ISO_4217) currency code."
        display_name:
          type: string
          description: Display name of product.
        estimate: 
          type: string
          description: Formatted string of estimate in local currency of the start location. Estimate could be a range, a single number (flat rate) or "Metered" for TAXI.
        low_estimate:
          type: number
          description: Lower bound of the estimated price.
        high_estimate:
          type: number
          description: Upper bound of the estimated price.
        surge_multiplier:
          type: number
          description: Expected surge multiplier. Surge is active if surge_multiplier is greater than 1. Price estimate already factors in the surge multiplier.
    Profile:
      properties:
        first_name:
          type: string
          description: First name of the Uber user.
        last_name:
          type: string
          description: Last name of the Uber user.
        email:
          type: string
          description: Email address of the Uber user
        picture:
          type: string
          description: Image URL of the Uber user.
        promo_code:
          type: string
          description: Promo code of the Uber user.   
    Activity:
      properties:
        uuid:
          type: string
          description: Unique identifier for the activity
    Activities:
      properties:
        offset:
          type: integer
          format: int32
          description: Position in pagination.
        limit:
          type: integer
          format: int32
          description: Number of items to retrieve (100 max).
        count:
          type: integer
          format: int32
          description: Total number of items available.
        history:
          type: array
          items:
            $ref: "#/components/schemas/Activity"
    Error:
      properties:
        code:
          type: string
        message:
          type: string
        fields:
          type: string

这里将 OpenAPI 3.0 规范中把重点使用的 api 整理了一张思维导图（还不完善，会持续更新）

Swagger

Swagger 实际上包含了一系列的工具 Editor Codegen UI ...

Editor 用于使用 OpenAPI 编辑 yaml
Codegen 用于生成不同的 language library 的代码
UI 用于生成文档

下面介绍一下 Codegen 的使用：

首先下载 swagger-codegen-cli.jar
确保装好了 maven
准备 swagger.yaml

编写 config.json 配置文件
因为之前提到 Swagger 可以生成各种 lang lib 的代码，所以这里便是进行此类配置:
查看配置 help

java -jar swagger-codegen-cli.jar config-help -l java

CONFIG OPTIONS
     sortParamsByRequiredFlag
         Sort method arguments to place required parameters before optional parameters. (Default: true)
     ensureUniqueParams
         Whether to ensure parameter names are unique in an operation (rename parameters that are not). (Default: true)
     allowUnicodeIdentifiers
         boolean, toggles whether unicode identifiers are allowed in names or not, default is false (Default: false)
     modelPackage
         package for generated models
     apiPackage
         package for generated api classes
     invokerPackage
         root package for generated code
     groupId
         groupId in generated pom.xml
     artifactId
         artifactId in generated pom.xml
     artifactVersion
         artifact version in generated pom.xml
     artifactUrl
         artifact URL in generated pom.xml
     artifactDescription
         artifact description in generated pom.xml
     scmConnection
         SCM connection in generated pom.xml
     scmDeveloperConnection
         SCM developer connection in generated pom.xml
     scmUrl
         SCM URL in generated pom.xml
     developerName
         developer name in generated pom.xml
     developerEmail
         developer email in generated pom.xml
     developerOrganization
         developer organization in generated pom.xml
     developerOrganizationUrl
         developer organization URL in generated pom.xml
     licenseName
         The name of the license
     licenseUrl
         The URL of the license
     sourceFolder
         source folder for generated code
     localVariablePrefix
         prefix for generated code members and local variables
     serializableModel
         boolean - toggle "implements Serializable" for generated models (Default: false)
     bigDecimalAsString
         Treat BigDecimal values as Strings to avoid precision loss. (Default: false)
     fullJavaUtil
         whether to use fully qualified name for classes under java.util. This option only works for Java API client (Default: false)
     hideGenerationTimestamp
         hides the timestamp when files were generated
     withXml
         whether to include support for application/xml content type. This option only works for Java API client (resttemplate) (Default: false)
     dateLibrary
         Option. Date library to use
             joda - Joda (for legacy app only)
             legacy - Legacy java.util.Date (if you really have a good reason not to use threetenbp
             java8-localdatetime - Java 8 using LocalDateTime (for legacy app only)
             java8 - Java 8 native JSR310 (preferred for jdk 1.8+) - note: this also sets "java8" to true
             threetenbp - Backport of JSR310 (preferred for jdk < 1.8)
     java8
         Option. Use Java8 classes instead of third party equivalents
             true - Use Java 8 classes such as Base64
             false - Various third party libraries as needed
     useRxJava
         Whether to use the RxJava adapter with the retrofit2 library. (Default: false)
     useRxJava2
         Whether to use the RxJava2 adapter with the retrofit2 library. (Default: false)
     parcelableModel
         Whether to generate models for Android that implement Parcelable with the okhttp-gson library. (Default: false)
     usePlay24WS
         Use Play! 2.4 Async HTTP client (Play WS API) (Default: false)
     supportJava6
         Whether to support Java6 with the Jersey1 library. (Default: false)
     useBeanValidation
         Use BeanValidation API annotations (Default: false)
     performBeanValidation
         Perform BeanValidation (Default: false)
     useGzipFeature
         Send gzip-encoded requests (Default: false)
     useRuntimeException
         Use RuntimeException instead of Exception (Default: false)
     library
         library template (sub-template) to use (Default: okhttp-gson)
             jersey1 - HTTP client: Jersey client 1.19.4. JSON processing: Jackson 2.8.9. Enable Java6 support using '-DsupportJava6=true'. Enable gzip request encoding using '-DuseGzipFeature=true'.
             feign - HTTP client: OpenFeign 9.4.0. JSON processing: Jackson 2.8.9
             jersey2 - HTTP client: Jersey client 2.25.1. JSON processing: Jackson 2.8.9
             okhttp-gson - HTTP client: OkHttp 2.7.5. JSON processing: Gson 2.8.1. Enable Parcelable models on Android using '-DparcelableModel=true'. Enable gzip request encoding using '-DuseGzipFeature=true'.
             retrofit - HTTP client: OkHttp 2.7.5. JSON processing: Gson 2.3.1 (Retrofit 1.9.0). IMPORTANT NOTE: retrofit1.x is no longer actively maintained so please upgrade to 'retrofit2' instead.
             retrofit2 - HTTP client: OkHttp 3.8.0. JSON processing: Gson 2.6.1 (Retrofit 2.3.0). Enable the RxJava adapter using '-DuseRxJava[2]=true'. (RxJava 1.x or 2.x)
             resttemplate - HTTP client: Spring RestTemplate 4.3.9-RELEASE. JSON processing: Jackson 2.8.9
             resteasy - HTTP client: Resteasy client 3.1.3.Final. JSON processing: Jackson 2.8.9

几个主要的配置参数：

library，生成的代码支付的类，有jersey1、jersey2、okhttp-gson、resttemplate、resteasy、feign、retrofit、retrofit2等几种类型，我们选择的retrofit2
developerName，开发者名字，会出现在代码文件里
developerEmail，开发者邮箱，会出现在代码文件里
developrOrganization，开发者组织，会出现在代码里
invokerPackage，项目的包名
apiPackage，生成的***Api.java文件的包名
modelPackage，生成的数据模型java文件包名
dateLibrary，时间使用的类开
useRxJava，是否使用rxjava生成api接口
useRxJava2，是否使用rxjava2的方式调用接口

generate 生成代码首先打印参数信息

java -jar swagger-codegen-cli.jar generate help

NAME
     swagger-codegen-cli generate - Generate code with chosen lang
 SYNOPSIS
     swagger-codegen-cli generate
             [(-a <authorization> | --auth <authorization>)]
             [--additional-properties <additional properties>...]
             [--api-package <api package>] [--artifact-id <artifact id>]
             [--artifact-version <artifact version>]
             [(-c <configuration file> | --config <configuration file>)]
             [-D <system properties>...] [--git-repo-id <git repo id>]
             [--git-user-id <git user id>] [--group-id <group id>]
             [--http-user-agent <http user agent>]
             (-i <spec file> | --input-spec <spec file>)
             [--ignore-file-override <ignore file override location>]
             [--import-mappings <import mappings>...]
             [--instantiation-types <instantiation types>...]
             [--invoker-package <invoker package>]
             (-l <language> | --lang <language>)
             [--language-specific-primitives <language specific primitives>...]
             [--library <library>] [--model-name-prefix <model name prefix>]
             [--model-name-suffix <model name suffix>]
             [--model-package <model package>]
             [(-o <output directory> | --output <output directory>)]
             [--release-note <release note>] [--remove-operation-id-prefix]
             [--reserved-words-mappings <reserved word mappings>...]
             [(-s | --skip-overwrite)]
             [(-t <template directory> | --template-dir <template directory>)]
             [--type-mappings <type mappings>...] [(-v | --verbose)]
OPTIONS
     -a <authorization>, --auth <authorization>
         adds authorization headers when fetching the swagger definitions
         remotely. Pass in a URL-encoded string of name:header with a comma
         separating multiple values
     --additional-properties <additional properties>
         sets additional properties that can be referenced by the mustache
         templates in the format of name=value,name=value. You can also have
         multiple occurrences of this option.
     --api-package <api package>
         package for generated api classes
     --artifact-id <artifact id>
         artifactId in generated pom.xml
     --artifact-version <artifact version>
         artifact version in generated pom.xml
     -c <configuration file>, --config <configuration file>
         Path to json configuration file. File content should be in a json
         format {"optionKey":"optionValue", "optionKey1":"optionValue1"...}
         Supported options can be different for each language. Run
         config-help -l {lang} command for language specific config options.
     -D <system properties>
         sets specified system properties in the format of
         name=value,name=value (or multiple options, each with name=value)
     --git-repo-id <git repo id>
         Git repo ID, e.g. swagger-codegen.
     --git-user-id <git user id>
         Git user ID, e.g. swagger-api.
     --group-id <group id>
         groupId in generated pom.xml
     --http-user-agent <http user agent>
         HTTP user agent, e.g. codegen_csharp_api_client, default to
         'Swagger-Codegen/{packageVersion}}/{language}'
     -i <spec file>, --input-spec <spec file>
         location of the swagger spec, as URL or file (required)
     --ignore-file-override <ignore file override location>
         Specifies an override location for the .swagger-codegen-ignore file.
         Most useful on initial generation.
     --import-mappings <import mappings>
         specifies mappings between a given class and the import that should
         be used for that class in the format of type=import,type=import. You
         can also have multiple occurrences of this option.
     --instantiation-types <instantiation types>
         sets instantiation type mappings in the format of
         type=instantiatedType,type=instantiatedType.For example (in Java):
         array=ArrayList,map=HashMap. In other words array types will get
         instantiated as ArrayList in generated code. You can also have
         multiple occurrences of this option.
     --invoker-package <invoker package>
         root package for generated code
     -l <language>, --lang <language>
         client language to generate (maybe class name in classpath,
         required)
     --language-specific-primitives <language specific primitives>
         specifies additional language specific primitive types in the format
         of type1,type2,type3,type3. For example:
         String,boolean,Boolean,Double. You can also have multiple
         occurrences of this option.
     --library <library>
         library template (sub-template)
     --model-name-prefix <model name prefix>
         Prefix that will be prepended to all model names. Default is the
         empty string.
     --model-name-suffix <model name suffix>
         Suffix that will be appended to all model names. Default is the
         empty string.
     --model-package <model package>
         package for generated models
     -o <output directory>, --output <output directory>
         where to write the generated files (current dir by default)
     --release-note <release note>
         Release note, default to 'Minor update'.
     --remove-operation-id-prefix
         Remove prefix of operationId, e.g. config_getId => getId
     --reserved-words-mappings <reserved word mappings>
         specifies how a reserved name should be escaped to. Otherwise, the
         default _<name> is used. For example id=identifier. You can also
         have multiple occurrences of this option.
     -s, --skip-overwrite
         specifies if the existing files should be overwritten during the
         generation.
     -t <template directory>, --template-dir <template directory>
         folder containing the template files
     --type-mappings <type mappings>
         sets mappings between swagger spec types and generated code types in
         the format of swaggerType=generatedType,swaggerType=generatedType.
         For example: array=List,map=Map,string=String. You can also have
         multiple occurrences of this option.
     -v, --verbose
         verbose mode

几个主要参数：

-i 表示输入的文件，editor生成的设计文件路径，如：-i ~/Desktop/swagger.yaml
-o 代码生成目录，swagger codegen 把代码生成到什么地方，如：-o ~/Desktop
-l 生成代码语言，我们是生成java，如：-l java
-c 配置文件，配制文件路径，如：-c ~/Desktop/config.json

最后生成代码

java -jar swagger-codegen-cli.jar generate -i swagger.yaml -o client -l java -c config.json

成功在 ~/Desktop 下生成了相应的 code 和 doc

参考资料以及推荐阅读的资料
http://docs.ansible.com/ansible/latest/YAMLSyntax.html https://swagger.io/docs/specification/about/
http://www.jianshu.com/p/c178c18aaf43
https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.0.md
https://github.com/swagger-api https://www.gitbook.com/book/huangwenchao/swagger/details