How to solve the error when integrating freemarker in springboot

springboot整合freemarker踩坑

报错

2021-04-23 02:01:18.148 ERROR 9484 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is freemarker.core.InvalidReferenceException: The following has evaluated to null or missing:
==> hots  [in template "inc/right.ftl" at line 6, column 11]

----
Tip: If the failing expression is known to legally refer to something that's sometimes null or missing, either specify a default value like myOptionalVar!myDefault, or use when-presentwhen-missing#if>. (These only cover the last step of the expression; to cover the whole expression, use parenthesis: (myOptionalVar.foo)!myDefault, (myOptionalVar.foo)??
----

----
FTL stack trace ("~" means nesting-related):
    - Failed at: @hots  [in template "inc/right.ftl" at line 6, column 9]
    - Reached through: #include "/inc/right.ftl"  [in template "index.ftl" at line 52, column 13]
    ~ Reached through: #nested  [in template "inc/layout.ftl" in macro "layout" at line 25, column 5]
    ~ Reached through: @layout "首页"  [in template "index.ftl" at line 2, column 1]
----] with root cause

freemarker.core.InvalidReferenceException: The following has evaluated to null or missing:
==> hots  [in template "inc/right.ftl" at line 6, column 11]

----
Tip: If the failing expression is known to legally refer to something that's sometimes null or missing, either specify a default value like myOptionalVar!myDefault, or use when-presentwhen-missing#if>. (These only cover the last step of the expression; to cover the whole expression, use parenthesis: (myOptionalVar.foo)!myDefault, (myOptionalVar.foo)??
----

----
FTL stack trace ("~" means nesting-related):
    - Failed at: @hots  [in template "inc/right.ftl" at line 6, column 9]
    - Reached through: #include "/inc/right.ftl"  [in template "index.ftl" at line 52, column 13]
    ~ Reached through: #nested  [in template "inc/layout.ftl" in macro "layout" at line 25, column 5]
    ~ Reached through: @layout "首页"  [in template "index.ftl" at line 2, column 1]
----
    at freemarker.core.InvalidReferenceException.getInstance(InvalidReferenceException.java:134) ~[freemarker-2.3.28.jar:2.3.28]
    at freemarker.core.UnifiedCall.accept(UnifiedCall.java:106) ~[freemarker-2.3.28.jar:2.3.28]
    at freemarker.core.Environment.visit(Environment.java:330) ~[freemarker-2.3.28.jar:2.3.28]
    at freemarker.core.Environment.visit(Environment.java:336) ~[freemarker-2.3.28.jar:2.3.28]
    at freemarker.core.Environment.include(Environment.java:2582) ~[freemarker-2.3.28.jar:2.3.28]
    at freemarker.core.Include.accept(Include.java:171) ~[freemarker-2.3.28.jar:2.3.28]
    at freemarker.core.Environment.visit(Environment.java:366) ~[freemarker-2.3.28.jar:2.3.28]
    at freemarker.core.Environment.invokeNestedContent(Environment.java:618) ~[freemarker-2.3.28.jar:2.3.28]
    at freemarker.core.BodyInstruction.accept(BodyInstruction.java:60) ~[freemarker-2.3.28.jar:2.3.28]
    at freemarker.core.Environment.visit(Environment.java:366) ~[freemarker-2.3.28.jar:2.3.28]
    at freemarker.core.Environment.invoke(Environment.java:775) ~[freemarker-2.3.28.jar:2.3.28]
    at freemarker.core.UnifiedCall.accept(UnifiedCall.java:83) ~[freemarker-2.3.28.jar:2.3.28]
    at freemarker.core.Environment.visit(Environment.java:330) ~[freemarker-2.3.28.jar:2.3.28]
    at freemarker.core.Environment.visit(Environment.java:336) ~[freemarker-2.3.28.jar:2.3.28]
    at freemarker.core.Environment.process(Environment.java:309) ~[freemarker-2.3.28.jar:2.3.28]
    at freemarker.template.Template.process(Template.java:384) ~[freemarker-2.3.28.jar:2.3.28]
    at org.springframework.web.servlet.view.freemarker.FreeMarkerView.processTemplate(FreeMarkerView.java:389) ~[spring-webmvc-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.servlet.view.freemarker.FreeMarkerView.doRender(FreeMarkerView.java:302) ~[spring-webmvc-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.servlet.view.freemarker.FreeMarkerView.renderMergedTemplateModel(FreeMarkerView.java:253) ~[spring-webmvc-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.servlet.view.AbstractTemplateView.renderMergedOutputModel(AbstractTemplateView.java:178) ~[spring-webmvc-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:316) ~[spring-webmvc-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1370) ~[spring-webmvc-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1116) ~[spring-webmvc-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1055) ~[spring-webmvc-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:942) ~[spring-webmvc-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1005) ~[spring-webmvc-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:897) ~[spring-webmvc-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:634) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:882) ~[spring-webmvc-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:741) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) ~[tomcat-embed-websocket-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:92) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:93) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.1.4.RELEASE.jar:5.1.4.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:490) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:408) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:834) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1417) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) ~[na:na]
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) ~[na:na]
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) ~[tomcat-embed-core-9.0.14.jar:9.0.14]
    at java.base/java.lang.Thread.run(Thread.java:834) ~[na:na]

2021-04-23 02:01:18.177 ERROR 9484 --- [nio-8080-exec-1] s.e.ErrorMvcAutoConfiguration$StaticView : Cannot render error page for request [/] and exception [The following has evaluated to null or missing:
==> hots  [in template "inc/right.ftl" at line 6, column 11]

----
Tip: If the failing expression is known to legally refer to something that's sometimes null or missing, either specify a default value like myOptionalVar!myDefault, or use when-presentwhen-missing#if>. (These only cover the last step of the expression; to cover the whole expression, use parenthesis: (myOptionalVar.foo)!myDefault, (myOptionalVar.foo)??
----

----
FTL stack trace ("~" means nesting-related):
    - Failed at: @hots  [in template "inc/right.ftl" at line 6, column 9]
    - Reached through: #include "/inc/right.ftl"  [in template "index.ftl" at line 52, column 13]
    ~ Reached through: #nested  [in template "inc/layout.ftl" in macro "layout" at line 25, column 5]
    ~ Reached through: @layout "首页"  [in template "index.ftl" at line 2, column 1]
----] as the response has already been committed. As a result, the response may have the wrong status code.

问题原因

因为在service里试着把redis逻辑代码运行，然后再控制台上输出，结果发现结果是没有问题的，而且通过报错信息初步认为是前端的问题，hots没找到，但是跟之前的一个类似功能相比较，然后通过不断的对比，找到问题是出在配置类。

之前一直认为template类中的getname方法就能完成映射功能

其实需要在FreemarkerConfig类里面进行声明

解决方法

再FreemarkerConfig类先通过@Autowired导入template类，然后使用configuration.setSharedVariable(“hots”, hotsTemplate);进行声明，缺少这一步会使得前端对于这个标签无法识别。

springboot freemarker基础配置及使用

1.基础配置

1.1引入依赖

<!-- 引入freemarker模板引擎的依赖 -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-freemarker</artifactId>
</dependency>

1.2在application.properties配置文件中添加如下配置：

# 是否开启thymeleaf缓存,本地为false，生产建议为true
spring.freemarker.cache=false
spring.freemarker.charset=UTF-8
spring.freemarker.allow-request-override=false
spring.freemarker.check-template-location=true
#类型
spring.freemarker.content-type=text/html
spring.freemarker.expose-request-attributes=true
spring.freemarker.expose-session-attributes=true
#
#文件后缀
spring.freemarker.suffix=.ftl
#路径 .ftl文件就创建在templates下面或者子目录
spring.freemarker.template-loader-path=classpath:/templates/

2.基础使用

首先我在controller类中编写一个测试接口用来测试使用，编写内容如下

@Controller
public class FreeMarkerController {

    Map<String ,Object> map = new HashMap<>();
    List<Object> list = new ArrayList();
    @GetMapping("/freemaker/{id}/{name}")
    public String demo(@PathVariable("id")String id ,@PathVariable("name") String name,Model model){

        map.clear();
        map.put("id",id);
        map.put("name",name);
        list.add(2);
        list.add(4);
        map.put("aa",null);
        model.addAttribute("map",map);
        model.addAttribute("list",list);
        model.addAttribute("date",new Date());
        return "index";
    }
}

以下所有变量都是有上面接口提供

2.1调用map类型数据,实体类也可使用此格式调用属性

/*可直接获取id对应的v值*/
id:${map.id}
name:${map.name}

2.2集合调用，set集合也可用此格式调用相应的元素

list01:${list[0]}
list02:${list[1]}

2.3字符串拼接的两种方法

${"你好啊${map.name}"}
${"你好啊"+map.name}

2.4FreeMarker表达式中支持“ ”、“－”、“*”、“/”、“%”运算符

${list[1]+list[0]}
${list[1]-list[0]}
${list[1]*list[0]}
${list[1]/list[0]}
${list[1]%list[0]}

2.5表达式中支持的比较运算符有如下几种：

1. =（或者==）：判断两个值是否相等；
2. !=：判断两个值是否不相等；
注： =和!=可以用作字符串、数值和日期的比较，但两边的数据类型必须相同。而且FreeMarker的比较是精确比较，不会忽略大小写及空格。
3. >（或者gt）：大于
4. >=（或者gte）：大于等于
5. <（或者lt）：小于
6. <=（或者lte）：小于等于
注： 上面这些比较运算符可以用于数字和日期，但不能用于字符串。大部分时候，使用gt比>有更好的效果，因为FreeMarker会把>解释成标签的结束字符。可以使用括号来避免这种

2.6时间格式转换

${date?string("yyyy-MM-dd")}

2.7支持逻辑表达式

FreeMarker支持逻辑运算但是只支持布尔型逻辑运算两个值进行比较运算的的时候需要添加括号才能进行逻辑运算,一般用于指令中

FreeMarker支持逻辑运算但是只支持布尔型逻辑运算两个值进行比较运算的的时候需要添加括号才能进行逻辑运算,一般用于指令中
1. &&： 逻辑与；
2. ||：逻辑或；
3. !：逻辑非
逻辑运算符只能用于布尔值。

2.8FreeMarker无法理解null值

FreeMarker的变量必须赋值，否则就会抛出异常。而对于FreeMarker来说，null值和不存在的变量是完全一样的，因为FreeMarker无法理解null值。
FreeMarker提供两个运算符来避免空值：
1.!运算
1.1若是msg不存在不报异常默认空值，若是不使用！会报异常

${msg!}
1.2若是存在则输出，若是不存在则默认为abc

${map.a!"abc"}

2.？？返回布尔值一般用在指令中下面会有引用此处不解释

2.9 if指令

<#if map.aa?? >
    存在
    <#else >
    不存在
</#if>
<br>
<#if (list[0]>=3) >
    优秀
    <#elseif (list[0]>=2)>
    良好
    <#else >
    差劲
</#if>

2.10 list指令 遍历

<#list list as li>
    ${li}<br>
</#list>

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