Understand the working principle of Apache Velocity Engine and its role in the Java class library

Apache Velocity Engine (hereinafter referred to as Velocity) is an open source tool for building a template engine in Java applications.It provides a simple and flexible method to generate text output by merging templates and data. Velocity's working principle is very intuitive.It defines the template as a file containing static text and the Velocity command.These commands are packaged in the special grammar that starts with `$`.For example, `$ variable` quotes a variable,`#if` starts a conditional statement, `#FOREACH` is iterative iterative, and so on. The core engine of Velocity will analyze the template file and generate the final output by executing the Velocity command.Before generating output, Velocity also supports transmitting dynamic data to the template.These data exist in the form of key -value pairs and can be represented by various methods such as Java objects, MAPS, Lists.Velocity combines the data with the template according to the commands and variables defined in the template to generate the final text output. Let's explain the role of Velocity through a specific example.Suppose we have a simple template file `Hello.vm`, the content is as follows: Hello $name! We can use velocity to merge the `Hello.vm` template with a data object containing a` Name` variable.The following is a complete Java code example: import org.apache.velocity.VelocityContext; import org.apache.velocity.app.Velocity; import java.io.StringWriter; public class VelocityExample { public static void main(String[] args) { // Initialize the velocity engine Velocity.init(); // Create a velocity context VelocityContext context = new VelocityContext(); context.put("name", "World"); // Use StringWriter to capture generated output StringWriter writer = new StringWriter(); // Merge template and data Velocity.mergeTemplate("hello.vm", "UTF-8", context, writer); // Print the output generated System.out.println(writer.toString()); } } In the above examples, we first initialize the velocity engine by calling `velocity.init ()`.Then, we created a velocity context object, and used the `put ()` method to add the `name` variable to the context.Next, we use the `StringWriter` object to capture the generated output, and finally merge the template and data by calling the` velocity.mergeTemplate () method. Run the above code will output `Hello World! In summary, Apache Velocity Engine is a powerful and easy -to -use template engine to generate text output in Java applications.It merged the template and dynamic data by combining the static text with the Velocity command.Velocity is widely used in many Java libraries and frameworks, such as Apache Struts, Apache Maven, etc., providing developers with a flexible and convenient way to generate various types of dynamic text output.