Using FreeMarker for Web template engine processing in Java API development

With the rapid development of the Internet, the development of Web applications has become more and more common. To make web applications more readable and maintainable, developers often use web template engines for view rendering. In Java development, there are many popular web template engines, and FreeMarker is one of them.

This article will introduce the FreeMarker Web template engine and its use in Java API development, including its core features, configuration and its application in actual combat.

1. What is FreeMarker

FreeMarker is an open source Java template engine that uses a template-based method to generate static text or dynamic web pages. Its feature is the separation of templates and program code, which clarifies the boundaries between performance and logic, separates page rendering and business logic, and improves the readability and maintainability of the code. FreeMarker supports multiple template types such as text templates, XML templates, HTML templates, JSP tag libraries, etc., and can be integrated with a variety of web frameworks, such as Struts2, Spring MVC, etc.

FreeMarker has the following features:

1. Separation of concerns: The template file only defines the display effect of the page and the interactive behavior of front-end users, without embedded business logic code.
2. Strong type support: Compared with JSP, FreeMarker has strong type support capabilities, which can help us check type errors at compile time and reduce runtime errors.
3. Various template file formats: FreeMarker supports the processing of multiple template file formats, including, but not limited to, HTML, XML, and JSON.
4. Compatibility: FreeMarker can be integrated into various web frameworks and is widely used in SpringMVC, Struts2 and other frameworks.

2. Use of FreeMarker API

FreeMarker provides many APIs to use it to generate templates. Our initial call involves configuring FreeMarker to emit templates. Next, we show how to set up and use the FreeMarker API.

1. Introducing FreeMarker’s dependency package

First, we need to add FreeMarker’s dependency in the project’s pom.xml file:

    <dependency>
        <groupId>org.freemarker</groupId>
        <artifactId>freemarker</artifactId>
        <version>2.3.31</version>
    </dependency>

2. Configuring FreeMarker

To create FreeMarker or you need a configuration to tell it how to load the template, refer to the following code example:

Configuration configuration = new Configuration(Configuration.VERSION_2_3_28);
configuration.setClassForTemplateLoading(YourClass.class, "templates");

Among them, VERSION_2_3_28 is the FreeMarker version number, setClassForTemplateLoading() Method sets the path for FreeMarker to load templates.

3. Set the data model

Next, you need to set the input data of the template. In FreeMarker, this background is a Map, and this Map needs to contain all the data we want to use in the template. We can use the SimpleHash type to create this Map:

Map<String, Object> input = new HashMap<String, Object>();
input.put("title", "FreeMarker Example");

In this example, we added "title" as the key and "FreeMarker Example" as the value to the input.

4. Load and render the template

Finally, we need to load the template and render the input data into the template, refer to the following code example:

Template template = configuration.getTemplate("example.ftl");
Writer out = new OutputStreamWriter(System.out);
template.process(input, out);
out.flush();

In this example, "example.ftl" is a template file. We use the configuration.getTemplate() method to load it, and the template.process() method to render the data in the input into the template, and finally output it through out.

5. FreeMarker template syntax

FreeMarker template syntax defines template tags, built-in formats, and methods. Template markers are directives in templates, consisting of FreeMarker template code in a pair of ${} or <% %> tags.

The following are some FreeMarker markers:

1. ${...}: FreeMarker expressions can contain any legal Java expression.
2. <% ... %>: Contains original template tags.
3. <@...>..: Indicates an aggregate template fragment.
4. #...#list..#assign...#recover...#stop: The original tag for operating and controlling template instances.
5. <#macro...>...<#nested>: Define a reusable template block.

3. Application examples of FreeMarker

Below we will demonstrate how to use FreeMarker to write templates in Java API development.

1. Write template file

First, we need to write a FreeMarker template file, for example, test.ftl:

<html>
<head>
<title>${title}</title>
</head>
<body>
<h1>${title}</h1>
<ul>
<#list users as user>
<li>${user.name} (${user.email})</li>
</#list>
</ul>
</body>
</html>

In this example, we Use the ${...} tag to reference the data in the input, and use the <#list ...> tag to loop through users and get the name and email attributes from each user.

2. Set the data model and load the template

Then, we need to set the data model, refer to the following code example:

Map<String, Object> input = new HashMap<String, Object>();
input.put("title", "FreeMarker Example");
List userList = new ArrayList();
userList.add(new User("Tom", "tom@example.com"));
userList.add(new User("Jerry", "jerry@example.com"));
input.put("users", userList);

Configuration configuration = new Configuration(Configuration.VERSION_2_3_28);
configuration.setClassForTemplateLoading(YourClass.class, "/templates");

Template template = configuration.getTemplate("test.ftl");
Writer out = new OutputStreamWriter(System.out);
template.process(input, out);
out.flush();

In this example, we A JavaBean class named User is created. When creating the Map, we use userList as the key and the List reference as the value, and add it to the input.

3. Summary

This article introduces the FreeMarker Web template engine and its use in Java API development. FreeMarker makes web application development easier while improving code readability and maintainability. By explaining the core features, configuration and application of FreeMarker in practice, we hope to help readers better understand and apply FreeMarker.

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