Hello Html World<\/title>\n <meta charset=\"utf-8\" \/>\n <link rel=\"stylesheet\" type=\"css\" href=\"\/my.css\" \/>\n <script type=\"text\/javascript\" src=\"\/some.js\" ><\/script>\n <\/head>\n <body>\n <div class=\"article\">\n <a href=\"https:\/\/benjiweber.com\/\">\n <span class=\"label\">Click Here<\/span>\n <img loading=\"lazy\" src=\"\/htmldsl2.png\" width=\"25\" height=\"25\" \/>\n <\/a>\n <p>\n <span>some text<\/span>\n <div>block<\/div>\n <\/p>\n <\/div>\n <\/body>\n<\/html>\n<\/pre>\n\n\n\n<p>You get nice autocompletion, and feedback if you specify inappropriate values:<\/p>\n\n\n\n<img src=\"https:\/\/benjiweber.co.uk\/linkrel.gif\" style=\"width:100%;margin-bottom:20px;\"\/>\n\n\n\n<p>You’ll also get a helping hand from the types to not put tags in inappropriate places:<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img src=\"https:\/\/benjiweber.com\/htmldsl2b.png\" alt=\"\" style=\"width:100%; margin-bottom:20px\"\/><\/figure>\n\n\n\n<h2>Generating Code<\/h2>\n\n\n\n<p>As it’s Java you can easily mix other code to generate markup dynamically:<\/p>\n\n\n\n<pre lang=\"java\">assertEquals(\n \"\"\"\n <html>\n <head>\n <meta charset=\"utf-8\" \/>\n <\/head>\n <body>\n <p>Paragraph one<\/p>\n <p>Paragraph two<\/p>\n <p>Paragraph three<\/p>\n <\/body>\n <\/html>\n \"\"\".trim(),\n html(\n head(\n meta($ -> $.charset = \"utf-8\")\n ),\n body(\n Stream.of(\"one\",\"two\",\"three\")\n .map(number -> \"Paragraph \" + number)\n .map(content -> p(content))\n )\n ).formatted()\n);<\/pre>\n\n\n\n<p>And the code can help you avoid injection attacks by escaping literal values:\u00a0<\/p>\n\n\n\n<pre lang=\"java\">assertEquals(\n \"\"\"\n <html>\n <head>\n <meta charset=\"utf-8\" \/>\n <\/head>\n <body>\n <p><script src=\"attack.js\"><\/script><\/p>\n <\/body>\n <\/html>\n \"\"\".trim(),\n html(\n head(\n meta($-> $.charset = \"utf-8\")\n ),\n body(\n p(\"<script src=\\\"attack.js\\\"><\/script>\")\n )\n ).formatted()\n);\n<\/pre>\n\n\n\n<h2>How does it work?<\/h2>\n\n\n\n<p>There’s only one “trick” here that’s particularly useful for DSLs. Using the Parameter Objects pattern from my <a href=\"https:\/\/benjiweber.co.uk\/blog\/2015\/08\/04\/lambda-type-references\/#!:~:text=Parameter%20Objects\">lambda type references<\/a> post. <\/p>\n\n\n\n<p>The lambdas used for specifying the tag attributes are “aware” of their own types. And capable of instantiating the configuration they specify.<\/p>\n\n\n\n<p>When we call <\/p>\n\n\n\n<pre lang=\"java\">meta($ -> $.charset=\"utf-8\")<\/pre>\n\n\n\n<p>We make a call to <\/p>\n\n\n\n<pre lang=\"java\">default Meta meta(Parameters<Meta> params, Tag... children) { \u2026 }\n<\/pre>\n\n\n\n<p>The lambda specifying the attribute config is structurally equivalent to the Parameters<Meta> type. This provides a get() function that instantiates an instance of Meta, and then passes the new instance to the lambda function to apply the config.<\/p>\n\n\n\n<pre lang=\"java\">public interface Parameters<T> extends NewableConsumer<T> {\n default T get() {\n T t = newInstance();\n accept(t);\n return t;\n }\n}<\/pre>\n\n\n\n<p>Under the hood the newInstance() method <a href=\"https:\/\/github.com\/benjiman\/java-html-dsl2\/blob\/97a9624d35cc3d735107f3dae920259544458f0c\/src\/main\/java\/com\/benjiweber\/typeref\/MethodFinder.java#L32\">uses reflection to examine the SerializedLambda contents<\/a> and find the type parameter (in this case “Meta”) before instantiating it.<\/p>\n\n\n\n<p>You can <a href=\"https:\/\/github.com\/benjiman\/java-html-dsl2\/blob\/main\/src\/main\/java\/com\/benjiweber\/typeref\/NewableConsumer.java\">follow the code<\/a> or see <a href=\"https:\/\/benjiweber.co.uk\/blog\/2015\/08\/04\/lambda-type-references\/#!:~:text=Parameter%20Objects\">the previous post <\/a>which explains it in a bit more detail.<\/p>\n\n\n\n<h2>Add Mixins<\/h2>\n\n\n\n<p>It’s helpful to use interfaces as mixins to avoid having to have one enormous class with all the builder definitions. <\/p>\n\n\n\n<pre lang=\"java\">public interface HtmlDsl extends\n Html.Dsl,\n Head.Dsl,\n Title.Dsl,\n Meta.Dsl,\n Link.Dsl,\n Script.Dsl,\n Body.Dsl,\n Div.Dsl,\n Span.Dsl,\n A.Dsl,\n P.Dsl,\n Img.Dsl {}<\/pre>\n\n\n\n<p>Each tag definition then contains its own builder methods. We compose them together into a single HtmlDsl interface for convenience. This saves having to import hundreds of different methods. By implementing the Dsl interface a consumer gets access to all the builder methods.<\/p>\n\n\n\n<figure class=\"wp-block-image\" style=\"height: 194px; overflow:hidden;\"><img src=\"https:\/\/benjiweber.co.uk\/htmldsl2.png\" alt=\"\" \/><\/figure>\n\n\n\n<h2>Show me the code<\/h2>\n\n\n\n<p>It’s <a href=\"https:\/\/github.com\/benjiman\/java-html-dsl2\">all on github<\/a>. I’d start from the <a href=\"https:\/\/github.com\/benjiman\/java-html-dsl2\/blob\/main\/src\/test\/java\/com\/benjiweber\/html\/HtmlExampleTest.java\">test examples<\/a>. Bear in mind that it’s merely a port of the old proof of concept to a slightly different approach. I hope it helps illustrate the technique. It’s in no way attempting to be a complete implementation.<\/p>\n\n\n\n<p>This approach can also be useful as an alternative to the builder pattern for passing a specification or configuration to a method. There’s another example on the <a href=\"https:\/\/benjiweber.co.uk\/blog\/2015\/08\/04\/lambda-type-references\/#!:~:text=Parameter%20Objects\">type references<\/a> article.<\/p>\n\n\n\n<p>What else could you use this technique for?<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Some time ago I wrote a post about creating an embedded dsl for Html in Java. Sadly, it was based on an abuse of lambda name reflection that was later removed from Java. I thought I should do a followup because a lot of people still visit the old article. While it’s no longer possible… <a href=\"https:\/\/benjiweber.co.uk\/blog\/2021\/02\/28\/revisiting-html-in-java\/\" class=\"more-link\" title=\"Read Revisiting Html in Java\">Read more »<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[8],"tags":[],"yoast_head":"\n<meta name=\"description\" content=\"A followup to my article on embedded html dsl in Java. This one works with more modern Java. 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{"id":1594,"date":"2021-02-28T16:47:55","date_gmt":"2021-02-28T15:47:55","guid":{"rendered":"https:\/\/benjiweber.co.uk\/blog\/?p=1594"},"modified":"2021-02-28T16:59:31","modified_gmt":"2021-02-28T15:59:31","slug":"revisiting-html-in-java","status":"publish","type":"post","link":"https:\/\/benjiweber.co.uk\/blog\/2021\/02\/28\/revisiting-html-in-java\/","title":{"rendered":"Revisiting Html in Java"},"content":{"rendered":"\n

Some time ago I wrote a post about creating an embedded dsl for Html in Java<\/a>. Sadly, it was based on an abuse of lambda name reflection that was later removed from Java.<\/p>\n\n\n\n

I thought I should do a followup because a lot of people still visit the old article. While it’s no longer possible to use lambda parameter names in this way, we can still get fairly close.\u00a0<\/p>\n\n\n\n

The following approach is slightly less concise. That said, it does have some benefits over the original:<\/p>\n\n\n\n

a)<\/strong> You no longer need to have parameter name reflection enabled at compile time.<\/p>\n\n\n\n

b)<\/strong> The compiler can check your attribute names are valid, and you can autocomplete them.<\/p>\n\n\n\n