如何使用字符串作为速度模板？

Question

从字符串创建速度模板的最佳方法是什么？

我知道Velocity.evaluate方法，我可以在其中传递 String 或 StringReader，但我很好奇是否有更好的方法来做到这一点（例如，创建模板实例的任何优势）。

Answer 1

有一些开销解析模板。 如果您的模板很大并且您反复使用它，您可能会通过预解析模板看到一些性能提升。 你可以做这样的事情，

RuntimeServices runtimeServices = RuntimeSingleton.getRuntimeServices();
StringReader reader = new StringReader(bufferForYourTemplate);
Template template = new Template();
template.setRuntimeServices(runtimeServices);

/*
 * The following line works for Velocity version up to 1.7
 * For version 2, replace "Template name" with the variable, template
 */
template.setData(runtimeServices.parse(reader, "Template name")));

template.initDocument();

然后你可以一遍又一遍地调用template.merge()而无需每次都解析它。

顺便说一句，您可以将 String 直接传递给Velocity.evaluate() 。

Answer 2

上面的示例代码对我有用。 它使用 Velocity 1.7 版和 log4j。

private static void velocityWithStringTemplateExample() {
    // Initialize the engine.
    VelocityEngine engine = new VelocityEngine();
    engine.setProperty(RuntimeConstants.RUNTIME_LOG_LOGSYSTEM_CLASS, "org.apache.velocity.runtime.log.Log4JLogChute");
    engine.setProperty("runtime.log.logsystem.log4j.logger", LOGGER.getName());
    engine.setProperty(Velocity.RESOURCE_LOADER, "string");
    engine.addProperty("string.resource.loader.class", StringResourceLoader.class.getName());
    engine.addProperty("string.resource.loader.repository.static", "false");
    //  engine.addProperty("string.resource.loader.modificationCheckInterval", "1");
    engine.init();

    // Initialize my template repository. You can replace the "Hello $w" with your String.
    StringResourceRepository repo = (StringResourceRepository) engine.getApplicationAttribute(StringResourceLoader.REPOSITORY_NAME_DEFAULT);
    repo.putStringResource("woogie2", "Hello $w");

    // Set parameters for my template.
    VelocityContext context = new VelocityContext();
    context.put("w", "world!");

    // Get and merge the template with my parameters.
    Template template = engine.getTemplate("woogie2");
    StringWriter writer = new StringWriter();
    template.merge(context, writer);

    // Show the result.
    System.out.println(writer.toString());
}

一个类似的问题。

Answer 3

这适用于速度 2.1

// Initialize the engine
VelocityEngine velocityEngine = new VelocityEngine();
velocityEngine.setProperty(Velocity.RESOURCE_LOADERS, "string");
velocityEngine.setProperty("resource.loader.string.class", StringResourceLoader.class.getName());
velocityEngine.setProperty("resource.loader.string.cache", true);
velocityEngine.setProperty("resource.loader.string.modification_check_interval", 60);
velocityEngine.init();

// Add template to repository
StringResourceRepository repository = StringResourceLoader.getRepository()
repository.putStringResource("hello_world", "Hello $w");

// Set parameters
VelocityContext context = new VelocityContext();
context.put("w", "world!");

// Process the template
StringWriter writer = new StringWriter();
velocityEngine.getTemplate('hello_world').merge( context, writer );

System.out.println(writer.toString());

Answer 4

Velocity 2 可以集成到 JSR223 Java 脚本语言框架中，这提供了另一种将字符串转换为模板的选项：

ScriptEngineManager manager = new ScriptEngineManager();
manager.registerEngineName("velocity", new VelocityScriptEngineFactory());
ScriptEngine engine = manager.getEngineByName("velocity");


System.setProperty(VelocityScriptEngine.VELOCITY_PROPERTIES, "path/to/velocity.properties");
String script = "Hello $world";
Writer writer = new StringWriter();
engine.getContext().setWriter(writer);
Object result = engine.eval(script);
System.out.println(writer);

Answer 5

RuntimeServices rs = RuntimeSingleton.getRuntimeServices();            
StringReader sr = new StringReader("Username is $username");
SimpleNode sn = rs.parse(sr, "User Information");

Template t = new Template();
    t.setRuntimeServices(rs);
    t.setData(sn);
    t.initDocument();

VelocityContext vc = new VelocityContext();
vc.put("username", "John");

StringWriter sw = new StringWriter();
t.merge(vc, sw);

System.out.println(sw.toString());

Answer 6

如果您只是在寻找变量替换，那么以下工作

public String velocityEvaluate(final String template, final NotificationDTO notificationDTO) {
    final Map<String, String> context = getContextMap(notificationDTO);
    final VelocityContext velocityContext = new VelocityContext(context);
    final StringWriter stringWriter = new StringWriter();
    final StringReader reader = new StringReader(template);
    Velocity.evaluate(velocityContext, stringWriter, "Velocity String Template Evaluation", reader);
    return stringWriter.toString();
}

Answer 7

如果有人希望从 json 字符串转换为 json 对象，在这种情况下，需要将 json 字符串转换为 JsonNode 并将其存储在上下文中。 例如：

String jsonDataAsString = "{"name": "Aps"}";
JsonNode nodes = new ObjectMapper().readTree(jsonDataAsString );
VelocityContext velocityContext = new VelocityContext();
velocityContext.put("root", nodes);

然后在您的模板中，您可以通过“$root.”+property 引用设置为“root”的原始数据

$root.name

希望它可以帮助某人。