springboot freemarker 自定义列表循环标签 带网页标签的示例

下面是一个示例，展示如何在Spring Boot中使用FreeMarker自定义列表循环标签，并带有网页标签：

1. 创建一个自定义标签类CustomListTag，继承LoopTagSupport类，并实现doOnce方法和doEnd方法。

import java.io.IOException;
import java.util.Map;

import javax.servlet.http.HttpServletRequest;

import org.springframework.web.servlet.support.RequestContextUtils;

import freemarker.core.Environment;
import freemarker.template.TemplateDirectiveBody;
import freemarker.template.TemplateDirectiveModel;
import freemarker.template.TemplateException;
import freemarker.template.TemplateModel;
import freemarker.template.TemplateModelException;
import freemarker.template.utility.DeepUnwrap;

public class CustomListTag extends LoopTagSupport implements TemplateDirectiveModel {

    private int index = 0;

    @Override
    protected boolean process() throws TemplateModelException {
        if (index >= size()) {
            return false;
        }
        Environment env = getEnvironment();
        TemplateDirectiveBody body = getBody();
        if (body != null) {
            try {
                env.setVariable("index", getStart() + index);
                env.setVariable("item", DeepUnwrap.unwrap(get(index)));
                body.render(env.getOut());
            } catch (IOException | TemplateException e) {
                throw new RuntimeException(e);
            }
        }
        index++;
        return true;
    }

    @Override
    public void doOnce(Map params, TemplateModel[] loopVars, TemplateDirectiveBody body) throws TemplateException, IOException {
        super.doOnce(params, loopVars, body);
        HttpServletRequest request = (HttpServletRequest) getEnvironment().getCustomAttribute("request");
        String contextPath = request.getContextPath();
        getEnvironment().setVariable("contextPath", getConfiguration().getObjectWrapper().wrap(contextPath));
    }

    @Override
    public void doEnd(Environment env, Map params, TemplateModel[] loopVars, TemplateDirectiveBody body)
            throws TemplateException, IOException {
        env.setVariable("index", null);
        env.setVariable("item", null);
        env.setVariable("contextPath", null);
        super.doEnd(env, params, loopVars, body);
    }

}

2. 在Spring Boot配置类中注册自定义标签

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.view.freemarker.FreeMarkerConfigurer;

@Configuration
public class FreeMarkerConfig {

    @Bean
    public FreeMarkerConfigurer freeMarkerConfigurer() {
        FreeMarkerConfigurer configurer = new FreeMarkerConfigurer();
        configurer.setTemplateLoaderPath("classpath:/templates/");
        configurer.setDefaultEncoding("UTF-8");
        Map<String, Object> variables = new HashMap<>();
        variables.put("customList", new CustomListTag());
        configurer.setFreemarkerVariables(variables);
        return configurer;
    }

}

3. 在FreeMarker模板中使用自定义标签

<#list items as item>
  <#if customList.first>
    <ul>
  </#if>
  <li>${item}</li>
  <#if customList.last>
    </ul>
  </#if>
</#list>

在这个示例中，我们将一个列表包装在自定义标签中，用<ul>和<li>标记包装它，并设置了一个变量contextPath，它包含了上下文路径，在模板中可以方便地使用。