模板渲染 - Velocity

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模板引擎除了去做后端应用的页面渲染，很多时候也可以用来生成一些代码，例如利用sql生成pojo对象，生成模板页面等
这里简单记录一下Velocity渲染模板时最基本的代码

  1. 引入Velocity Maven信息

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    <dependency>
        <groupId>org.apache.velocity</groupId>
        <artifactId>velocity</artifactId>
        <version>1.7</version>
    </dependency>
    <dependency>
        <groupId>org.apache.velocity</groupId>
        <artifactId>velocity-tools</artifactId>
        <version>2.0</version>
    </dependency>

  2. 实现代码

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    import com.google.common.base.Charsets;
    import com.google.common.collect.Maps;
    import com.github.pdf.tools.exception.VelocityException;
    import net.sf.cglib.beans.BeanMap;
    import org.apache.velocity.Template;
    import org.apache.velocity.VelocityContext;
    import org.apache.velocity.app.Velocity;
    import org.apache.velocity.app.VelocityEngine;

    import java.io.StringWriter;
    import java.util.Collections;
    import java.util.Map;
    import java.util.Objects;
    import java.util.Properties;
    import java.util.concurrent.ConcurrentHashMap;

    /**
     * @author wangdongbo
     * @since 2019/7/3.
     */
    public class VelocityTemplate extends AbstractTemplate {

        private static final Map<String, VelocityEngine> ENGINE_MAP = new ConcurrentHashMap<>(16);

        private static VelocityEngine getEngine(String templatePath) {
            if (ENGINE_MAP.containsKey(templatePath)) {
                return ENGINE_MAP.get(templatePath);
            }
            Properties properties = new Properties();
            properties.setProperty(Velocity.OUTPUT_ENCODING, Charsets.UTF_8.toString());
            properties.setProperty(Velocity.INPUT_ENCODING, Charsets.UTF_8.toString());
            properties.setProperty(Velocity.FILE_RESOURCE_LOADER_PATH, templatePath);
            VelocityEngine velocityEngine = new VelocityEngine(properties);
            ENGINE_MAP.put(templatePath, velocityEngine);
            return velocityEngine;
        }

        @Override
        protected String getContent(String templatePath, String templateName, Object data) {
            try {
                VelocityContext velocityContext = new VelocityContext(Objects.nonNull(data) ? Maps.newHashMap(BeanMap.create(data)) : Collections.emptyMap());
                Template template = getEngine(templatePath).getTemplate(templateName, Charsets.UTF_8.toString());
                StringWriter writer = new StringWriter();
                template.merge(velocityContext, writer);
                writer.flush();
                return writer.toString();
            } catch (Exception e) {
                throw new VelocityException("Velocity exception", e);
            }
        }
    }