Velocity is a template engine written for Java. It allows us to take files (or strings) with embedded Velocity Template Language (VTL) code and transform the files (or strings) based upon supplied values.<\/p>\n
Let’s create a simple template file (by default we use the extension .vm and a good convention is to append it to the original extension file type, so you know what type of file you’re transforming) then we’ll use Velocity to transform the file.<\/p>\n
We’ll create a standard Maven layout project<\/p>\n
My template.vm looks like this<\/p>\n
\r\nHello $name,\r\n\r\nThis is a $template_name template.\r\n<\/pre>\nAs you’ve probably guessed the $ prefix denotes a variable which we will supply to Velocity.<\/p>\n
The first thing we need to do is create the Velocity engine and then set it up to allow us to load our files from the resources folder, here’s the code for this<\/p>\n
\r\nVelocityEngine velocityEngine = new VelocityEngine();\r\nvelocityEngine.setProperty(\r\n "resource.loader", \r\n "file");\r\nvelocityEngine.setProperty(\r\n "file.resource.loader.class", \r\n "org.apache.velocity.runtime.resource.loader.FileResourceLoader");\r\nvelocityEngine.setProperty(\r\n "file.resource.loader.path", \r\n "src\/test\/resources");\r\n<\/pre>\nNext up we create a VelocityContext which we use to supply our mappings to the variable names used within the template file. We do not include the $ when supplying the variable name. We simply supply key value pairs, where the key is a String which represents the variable name and the value is what we want to replace it with (this is an Object which can be more complex than just a String). For this example our value will just be a String, so we have the following<\/p>\n
\r\nVelocityContext velocityContext = new VelocityContext();\r\nvelocityContext.put("name", "PutridParrot");\r\nvelocityContext.put("template_name", "MyPutridParrotTemplate");\r\n<\/pre>\nNote: If we do not add a context mapping then the result will be the original $variable_name, unchanged.<\/em><\/p>\n
Finally, we want to actually “merge” (as Velocity calls the process) our context against the template and at the same time we’re going to convert the resultant “merged” template into a string<\/p>\n
\r\nStringWriter writer = new StringWriter();\r\nTemplate template = velocityEngine.getTemplate("templates\/template.txt.vm");\r\ntemplate.merge(velocityContext, writer);\r\n\r\n\/\/ to prove it worked...\r\nSystem.out.println(writer.toString());\r\n<\/pre>\nRunning the above code we will get the output (as you would expect)<\/p>\n
\r\nHello PutridParrot,\r\n\r\nThis is a MyPutridParrotTemplate template.\r\n<\/pre>\nTaking things a little further<\/strong><\/p>\n
As mentioned previously the value stored within the context is an Object, so we can store an array of values if we want.<\/p>\n
Let’s assume we have the following template file<\/p>\n
\r\nHello $name,\r\n\r\nMy templates\r\n\r\n#foreach( $t in $template_name)\r\n * $t\r\n#end\r\n<\/pre>\nAs you can see the VTL supports foreach loops, so if we change the code that assigns the template_name to the context to look like this<\/p>\n
\r\nArrayList<String> list = new ArrayList<>();\r\nlist.add("Putrid");\r\nlist.add("Parrot");\r\n\r\nvelocityContext.put("template_name", list);\r\n<\/pre>\nOur output will now be<\/p>\n
\r\nHello PutridParrot,\r\n\r\nMy templates\r\n\r\n * Putrid\r\n * Parrot\r\n<\/pre>\n