使用在线工具jsonschema2pojo根据json生成java对象

如果你还在自己手动写model对象，那你就out了。

场景：使用retrofit请求github的api，要使返回的数据自动解析到java对象中你必须准备好一个和json完全对应的java对象。而github的一个Repository有很多字段，如这个请求：

https://api.github.com/repos/octocat/Hello-World

你可以直接打开这个链接查看返回的json数据。可以看到里面有很多数据项，如果我们手动写这个java对象将非常耗费时间。

所幸现在有了转换的工具。

jsonschema2pojo

jsonschema2pojo 是一个根据json转换成java对象的开源项目，只要把你的json字符串复制到相应的输入框中就能自动将其转换成java对象。它的强大之处在于，能解析列表式的json数据，把嵌套在内层的对象也解析出来。

先以上面的api请求为例，得到的json如下，这是一个单一的Repository数据：

{    "id": 1296269,    "name": "Hello-World",    "full_name": "octocat/Hello-World",    "owner": {      "login": "octocat",      "id": 583231,      "avatar_url": "https://avatars.githubusercontent.com/u/583231?v=3",      "gravatar_id": "",      "url": "https://api.github.com/users/octocat",      "html_url": "https://github.com/octocat",      "followers_url": "https://api.github.com/users/octocat/followers",      "following_url": "https://api.github.com/users/octocat/following{/other_user}",      "gists_url": "https://api.github.com/users/octocat/gists{/gist_id}",      "starred_url": "https://api.github.com/users/octocat/starred{/owner}{/repo}",      "subscriptions_url": "https://api.github.com/users/octocat/subscriptions",      "organizations_url": "https://api.github.com/users/octocat/orgs",      "repos_url": "https://api.github.com/users/octocat/repos",      "events_url": "https://api.github.com/users/octocat/events{/privacy}",      "received_events_url": "https://api.github.com/users/octocat/received_events",      "type": "User",      "site_admin": false    },    "private": false,    "html_url": "https://github.com/octocat/Hello-World",    "description": "This your first repo!",    "fork": false,    "url": "https://api.github.com/repos/octocat/Hello-World",    "forks_url": "https://api.github.com/repos/octocat/Hello-World/forks",    "keys_url": "https://api.github.com/repos/octocat/Hello-World/keys{/key_id}",    "collaborators_url": "https://api.github.com/repos/octocat/Hello-World/collaborators{/collaborator}",    "teams_url": "https://api.github.com/repos/octocat/Hello-World/teams",    "hooks_url": "https://api.github.com/repos/octocat/Hello-World/hooks",    "issue_events_url": "https://api.github.com/repos/octocat/Hello-World/issues/events{/number}",    "events_url": "https://api.github.com/repos/octocat/Hello-World/events",    "assignees_url": "https://api.github.com/repos/octocat/Hello-World/assignees{/user}",    "branches_url": "https://api.github.com/repos/octocat/Hello-World/branches{/branch}",    "tags_url": "https://api.github.com/repos/octocat/Hello-World/tags",    "blobs_url": "https://api.github.com/repos/octocat/Hello-World/git/blobs{/sha}",    "git_tags_url": "https://api.github.com/repos/octocat/Hello-World/git/tags{/sha}",    "git_refs_url": "https://api.github.com/repos/octocat/Hello-World/git/refs{/sha}",    "trees_url": "https://api.github.com/repos/octocat/Hello-World/git/trees{/sha}",    "statuses_url": "https://api.github.com/repos/octocat/Hello-World/statuses/{sha}",    "languages_url": "https://api.github.com/repos/octocat/Hello-World/languages",    "stargazers_url": "https://api.github.com/repos/octocat/Hello-World/stargazers",    "contributors_url": "https://api.github.com/repos/octocat/Hello-World/contributors",    "subscribers_url": "https://api.github.com/repos/octocat/Hello-World/subscribers",    "subscription_url": "https://api.github.com/repos/octocat/Hello-World/subscription",    "commits_url": "https://api.github.com/repos/octocat/Hello-World/commits{/sha}",    "git_commits_url": "https://api.github.com/repos/octocat/Hello-World/git/commits{/sha}",    "comments_url": "https://api.github.com/repos/octocat/Hello-World/comments{/number}",    "issue_comment_url": "https://api.github.com/repos/octocat/Hello-World/issues/comments{/number}",    "contents_url": "https://api.github.com/repos/octocat/Hello-World/contents/{+path}",    "compare_url": "https://api.github.com/repos/octocat/Hello-World/compare/{base}...{head}",    "merges_url": "https://api.github.com/repos/octocat/Hello-World/merges",    "archive_url": "https://api.github.com/repos/octocat/Hello-World/{archive_format}{/ref}",    "downloads_url": "https://api.github.com/repos/octocat/Hello-World/downloads",    "issues_url": "https://api.github.com/repos/octocat/Hello-World/issues{/number}",    "pulls_url": "https://api.github.com/repos/octocat/Hello-World/pulls{/number}",    "milestones_url": "https://api.github.com/repos/octocat/Hello-World/milestones{/number}",    "notifications_url": "https://api.github.com/repos/octocat/Hello-World/notifications{?since,all,participating}",    "labels_url": "https://api.github.com/repos/octocat/Hello-World/labels{/name}",    "releases_url": "https://api.github.com/repos/octocat/Hello-World/releases{/id}",    "created_at": "2011-01-26T19:01:12Z",    "updated_at": "2015-10-08T07:23:02Z",    "pushed_at": "2015-10-02T01:04:21Z",    "git_url": "git://github.com/octocat/Hello-World.git",    "ssh_url": "git@github.com:octocat/Hello-World.git",    "clone_url": "https://github.com/octocat/Hello-World.git",    "svn_url": "https://github.com/octocat/Hello-World",    "homepage": "",    "size": 566,    "stargazers_count": 1402,    "watchers_count": 1402,    "language": null,    "has_issues": true,    "has_downloads": true,    "has_wiki": true,    "has_pages": false,    "forks_count": 1110,    "mirror_url": null,    "open_issues_count": 139,    "forks": 1110,    "open_issues": 139,    "watchers": 1402,    "default_branch": "master",    "network_count": 1110,    "subscribers_count": 1747  }

其中有一个数据项比较特殊，那就是owner（在第5行），它的值也是一个对象。我们把这个json放在 http://www.jsonschema2pojo.org/ 网站左边的输入框中，右边的设置请按照图里的来。 

其中右边设置的Class name一项默认是Example，表示生成的对象名（最外层）就是Example，因为它无法从json中得知这到底是什么对象。我们这里期望得到的对 象名是Repository，可以改成Repository就可以了，或者这里不改，在生成之后的java对象里改。

注意里面还有个owner，它本身也是对象，jsonschema2pojo能把这个对象也解析出来，而这个对象的类名耶就是 Owner 。

我们点击Preview按钮，在弹出的对话框中生成了两个类：

第一个就是Owner，第二个截图没有显示出来，是Repository类（我把Class name改成了Repository）。

而这里的 @Generated ( "org.jsonschema2pojo" )可以直接删掉。

可以想象如此之多的变量如果自己写要花费多长时间。

不过需要注意的是jsonschema2pojo对json字符串的总长度有要求。这样如果你像比如github请求的是一个列表数据比如，那么你得到的json很可能超过长度限制。

比如我们使用github的搜索api发出一个这样的请求：

https://api.github.com/search/repositories?q=tetris+language:assembly&sort=starsℴ=desc

返回的json数据首先打印的是列表长度total_count，这里是297。这是搜索结果的总长度，但是其实这次请求返回的只有30 条，因为这是第一页的数据。但是30条已经很长了，超过了长度限制。

要把这样的json转换成对象必须得找个搜索结果比较短的，我们换成

https://api.github.com/search/repositories?q=codebox+language:java&sort=starsℴ=desc

这个返回的结果只有9项，虽然也很长但是还是没有超过限制。

直接把生成的结果复制到jsonschema2pojo的左侧输入框，把类名改成Repositories，表示这是一个数据集合。

生成的类结构从前面部分就能看出来了，分别是Repositories，items以及Owner，其中items对应的就是刚刚我们的Repository类，这里它只能根据json猜测这个类名是items。然而Class name设置的只是最外层的类名。

我们可以生成之后把items改成Repository就行了。或者有个技巧，把json的items直接改成Repository。

我之所以再举一个搜索结果为列表的例子是为了验证一个列表式的json是否可以直接转换成java类。

总结

可以看到jsonschema2pojo的实用性非常高。