Micronaut集成Thymeleaf模板时#request对象为空问题咨询

问题背景 #

我正在开发一个使用Micronaut 2.57、Thymeleaf 3.0.12、micronaut-views-thymeleaf 2.21的应用，用于渲染视图。根据Thymeleaf官方文档说明，模板中应该可以直接使用#request对象，但实际使用时该对象始终为null。

复现代码 #

模板代码 #

<!DOCTYPE html>
<html lang="en" xmlns:th="http://www.thymeleaf.org">
<head>
    <title>Home</title>
</head>
<body>
<h3>#request.contextPath</h3>
<span th:utext="${#request.contextPath}"></span>

<h3>#request.requestURI</h3>
<span th:utext="${#request.requestURI}"></span>

<h3>#request.requestURL</h3>
<span th:utext="${#request.requestURL}"></span>
</body>
</html>

控制器代码 #

import io.micronaut.http.HttpResponse;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import io.micronaut.views.View;

@Controller
public class HomeController {

    @View("home2")
    @Get("/")
    public HttpResponse<Object> showHome() {
        return HttpResponse.ok();
    }
}

报错信息 #

启动应用后访问localhost:8080，出现如下堆栈报错，提示#request对象不存在：

17:52:57.326 [main] INFO  io.micronaut.runtime.Micronaut - Startup completed in 2231ms. Server Running: http://localhost:8080
17:53:06.644 [io-executor-thread-1] ERROR org.thymeleaf.TemplateEngine - [THYMELEAF][io-executor-thread-1] Exception processing template "home2": Exception evaluating OGNL expression: "#request.contextPath" (template: "home2" - line 8, col 7)
org.thymeleaf.exceptions.TemplateProcessingException: Exception evaluating OGNL expression: "#request.contextPath" (template: "home2" - line 8, col 7)
    at org.thymeleaf.standard.expression.OGNLVariableExpressionEvaluator.evaluate(OGNLVariableExpressionEvaluator.java:191)
    at org.thymeleaf.standard.expression.OGNLVariableExpressionEvaluator.evaluate(OGNLVariableExpressionEvaluator.java:95)
    at org.thymeleaf.standard.expression.VariableExpression.executeVariableExpression(VariableExpression.java:166)
    at org.thymeleaf.standard.expression.SimpleExpression.executeSimple(SimpleExpression.java:66)
    at org.thymeleaf.standard.expression.Expression.execute(Expression.java:109)
    at org.thymeleaf.standard.expression.Expression.execute(Expression.java:138)
    at org.thymeleaf.standard.processor.StandardUtextTagProcessor.doProcess(StandardUtextTagProcessor.java:87)
    at org.thymeleaf.processor.element.AbstractAttributeTagProcessor.doProcess(AbstractAttributeTagProcessor.java:74)
    at org.thymeleaf.processor.element.AbstractElementTagProcessor.process(AbstractElementTagProcessor.java:95)
    at org.thymeleaf.util.ProcessorConfigurationUtils$ElementTagProcessorWrapper.process(ProcessorConfigurationUtils.java:633)
    at org.thymeleaf.engine.ProcessorTemplateHandler.handleOpenElement(ProcessorTemplateHandler.java:1314)
    at org.thymeleaf.engine.OpenElementTag.beHandled(OpenElementTag.java:205)
    at org.thymeleaf.engine.TemplateModel.process(TemplateModel.java:136)
    at org.thymeleaf.engine.TemplateManager.parseAndProcess(TemplateManager.java:661)
    at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1098)
    at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1067)
    at io.micronaut.views.thymeleaf.ThymeleafViewsRenderer.render(ThymeleafViewsRenderer.java:109)
    at io.micronaut.views.thymeleaf.ThymeleafViewsRenderer.lambda$render$1(ThymeleafViewsRenderer.java:96)
    at io.micronaut.core.io.Writable.writeTo(Writable.java:77)
    at io.micronaut.http.server.netty.RoutingInBoundHandler.lambda$encodeHttpResponse$7(RoutingInBoundHandler.java:1683)
    at io.micronaut.scheduling.instrument.InvocationInstrumenterWrappedRunnable.run(InvocationInstrumenterWrappedRunnable.java:47)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
    at java.base/java.lang.Thread.run(Thread.java:829)
Caused by: ognl.OgnlException: source is null for getProperty(null, "contextPath")
    at ognl.OgnlRuntime.getProperty(OgnlRuntime.java:3229)
    at ognl.ASTProperty.getValueBody(ASTProperty.java:114)
    at ognl.SimpleNode.evaluateGetValueBody(SimpleNode.java:212)
    at ognl.SimpleNode.getValue(SimpleNode.java:258)
    at ognl.ASTChain.getValueBody(ASTChain.java:141)
    at ognl.SimpleNode.evaluateGetValueBody(SimpleNode.java:212)
    at ognl.SimpleNode.getValue(SimpleNode.java:258)
    at ognl.Ognl.getValue(Ognl.java:537)
    at ognl.Ognl.getValue(Ognl.java:501)
    at org.thymeleaf.standard.expression.OGNLVariableExpressionEvaluator.executeExpression(OGNLVariableExpressionEvaluator.java:328)
    at org.thymeleaf.standard.expression.OGNLVariableExpressionEvaluator.evaluate(OGNLVariableExpressionEvaluator.java:170)
    ... 23 common frames omitted

其他内置变量测试结果 #

• #locale - 正常可用
• #ctx - 正常可用
• #request - 始终为null
• #response - 始终为null
• #session - 始终为null
• #servletContext - 始终为null

排查结论 #

已知io.micronaut.views.thymeleaf.ThymeleafViewsRenderer会给渲染引擎传递WebContext，调试发现contextVariableMap中包含的key列表为[ctx, root, vars, object, locale, request, response, session, servletContext, conversions, uris, calendars, dates, bools, numbers, objects, strings, arrays, lists, sets, maps, aggregates, messages, ids, execInfo, httpServletRequest, httpSession]，但其中的servletContext、httpServletRequest、request取值均为null。

进一步排查org.thymeleaf.standard.expression.StandardExpressionObjectFactory类的逻辑：

if (REQUEST_EXPRESSION_OBJECT_NAME.equals(expressionObjectName)) {
    if (context instanceof IWebContext) {
        return ((IWebContext) context).getRequest();
    }
    return null;
}

发现传入的上下文类型为WebEngineContext，但instanceof判断IWebContext结果为false。最终定位到根本原因：Micronaut中创建的WebEngineContext属于io.micronaut.views.thymeleaf.WebEngineContext类，继承自EngineContext，和Thymeleaf原生实现了IWebContext接口的org.thymeleaf.context.WebEngineContext不是同一个类，因此无法被Thymeleaf的内置对象工厂识别，无法正确注入request等对象。

解决方案 #

方案1：升级依赖版本 #

该问题属于Micronaut Thymeleaf集成模块的历史缺陷，在micronaut-views-thymeleaf 3.0及以上版本中已经修复，将Micronaut框架及对应视图模块升级到3.x最新稳定版即可原生支持#request等内置对象。

方案2：适配内置对象调用方式 #

Micronaut自定义的WebEngineContext本身提供了getRequest公共方法，无需额外配置，直接在模板中使用${#ctx.getRequest()}即可获取request对象，对应属性可以通过${#ctx.getRequest().getContextPath()}的方式读取。

方案3：手动注入request属性 #

临时解决方案，可以在控制器中注入HttpRequest对象，放入返回的模型数据中，模板中直接读取对应属性即可：

import io.micronaut.http.HttpRequest;
import io.micronaut.http.HttpResponse;
import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import io.micronaut.views.View;
import java.util.Map;

@Controller
public class HomeController {

    @View("home2")
    @Get("/")
    public HttpResponse<Map<String, Object>> showHome(HttpRequest<?> request) {
        return HttpResponse.ok(Map.of(
            "contextPath", request.getContextPath(),
            "requestURI", request.getUri(),
            "requestURL", request.getServerAddress() + request.getUri()
        ));
    }
}

模板中直接用${contextPath}等对应key取值即可。

内容的提问来源于stack exchange，提问作者Jeffrey Devloo