13.2 Spring Boot启动报错：Whitelabel Error Page
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<h1>13.2 Spring Boot启动报错：Whitelabel Error Page</h1>
问题描述
Whitelabel Error Page
This application has no explicit mapping for /error, so you are seeing this as a fallback.
Tue Mar 28 22:25:43 CST 2017
There was an unexpected error (type=Internal Server Error, status=500).
Circular view path [login]: would dispatch back to the current handler URL [/login] again. Check your ViewResolver setup! (Hint: This may be the result of an unspecified view, due to default view name generation.)
原因分析
首先，这个出错页面是SpringBoot的一个默认出错页面。源码在：org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration 第151行。
这种错误一般是配置错误，或者MVC报错引起的错误。
解决方案
在application.properties 文件中正确配置模板文件的命名前后缀:
spring.mvc.view.prefix=/WEB-INF/jsp/
spring.mvc.view.suffix=.jsp
另外，在早期版本的springboot中，这个key中是不带mvc的。
spring.view.prefix: /WEB-INF/jsp/
spring.view.suffix: .jsp
这个配置类是1.1版本之后的，在org.springframework.boot.autoconfigure.web.servlet.WebMvcProperties类中。上面的注解@ConfigurationProperties(prefix = "spring.mvc")指明了这个key。
相应的，模板文件要放在正确的目录下。上面的配置，对应的目录可以是：
/src/main/webapp/WEB-INF/jsp/
/src/main/resources/META-INF/resources/WEB-INF/jsp
SpringBoot常见的模板文件默认目录，举例如下:
# THYMELEAF (ThymeleafAutoConfiguration)
spring.thymeleaf.cache=true # Enable template caching.
spring.thymeleaf.check-template=true # Check that the template exists before rendering it.
spring.thymeleaf.check-template-location=true # Check that the templates location exists.
spring.thymeleaf.enabled=true # Enable Thymeleaf view resolution for Web frameworks.
spring.thymeleaf.encoding=UTF-8 # Template files encoding.
spring.thymeleaf.excluded-view-names= # Comma-separated list of view names that should be excluded from resolution.
spring.thymeleaf.mode=HTML5 # Template mode to be applied to templates. See also StandardTemplateModeHandlers.
spring.thymeleaf.prefix=classpath:/templates/ # Prefix that gets prepended to view names when building a URL.
spring.thymeleaf.reactive.max-chunk-size= # Maximum size of data buffers used for writing to the response, in bytes.
spring.thymeleaf.reactive.media-types=text/html # Media types supported by the view technology.
spring.thymeleaf.servlet.content-type=text/html # Content-Type value written to HTTP responses.
spring.thymeleaf.suffix=.html # Suffix that gets appended to view names when building a URL.
spring.thymeleaf.template-resolver-order= # Order of the template resolver in the chain.
spring.thymeleaf.view-names= # Comma-separated list of view names that can be resolved.
# FREEMARKER (FreeMarkerAutoConfiguration)
spring.freemarker.allow-request-override=false # Set whether HttpServletRequest attributes are allowed to override (hide) controller generated model attributes of the same name.
spring.freemarker.allow-session-override=false # Set whether HttpSession attributes are allowed to override (hide) controller generated model attributes of the same name.
spring.freemarker.cache=false # Enable template caching.
spring.freemarker.charset=UTF-8 # Template encoding.
spring.freemarker.check-template-location=true # Check that the templates location exists.
spring.freemarker.content-type=text/html # Content-Type value.
spring.freemarker.enabled=true # Enable MVC view resolution for this technology.
spring.freemarker.expose-request-attributes=false # Set whether all request attributes should be added to the model prior to merging with the template.
spring.freemarker.expose-session-attributes=false # Set whether all HttpSession attributes should be added to the model prior to merging with the template.
spring.freemarker.expose-spring-macro-helpers=true # Set whether to expose a RequestContext for use by Spring's macro library, under the name "springMacroRequestContext".
spring.freemarker.prefer-file-system-access=true # Prefer file system access for template loading. File system access enables hot detection of template changes.
spring.freemarker.prefix= # Prefix that gets prepended to view names when building a URL.
spring.freemarker.request-context-attribute= # Name of the RequestContext attribute for all views.
spring.freemarker.settings.*= # Well-known FreeMarker keys which will be passed to FreeMarker's Configuration.
spring.freemarker.suffix= # Suffix that gets appended to view names when building a URL.
spring.freemarker.template-loader-path=classpath:/templates/ # Comma-separated list of template paths.
spring.freemarker.view-names= # White list of view names that can be resolved.
# GROOVY TEMPLATES (GroovyTemplateAutoConfiguration)
spring.groovy.template.allow-request-override=false # Set whether HttpServletRequest attributes are allowed to override (hide) controller generated model attributes of the same name.
spring.groovy.template.allow-session-override=false # Set whether HttpSession attributes are allowed to override (hide) controller generated model attributes of the same name.
spring.groovy.template.cache= # Enable template caching.
spring.groovy.template.charset=UTF-8 # Template encoding.
spring.groovy.template.check-template-location=true # Check that the templates location exists.
spring.groovy.template.configuration.*= # See GroovyMarkupConfigurer
spring.groovy.template.content-type=test/html # Content-Type value.
spring.groovy.template.enabled=true # Enable MVC view resolution for this technology.
spring.groovy.template.expose-request-attributes=false # Set whether all request attributes should be added to the model prior to merging with the template.
spring.groovy.template.expose-session-attributes=false # Set whether all HttpSession attributes should be added to the model prior to merging with the template.
spring.groovy.template.expose-spring-macro-helpers=true # Set whether to expose a RequestContext for use by Spring's macro library, under the name "springMacroRequestContext".
spring.groovy.template.prefix= # Prefix that gets prepended to view names when building a URL.
spring.groovy.template.request-context-attribute= # Name of the RequestContext attribute for all views.
spring.groovy.template.resource-loader-path=classpath:/templates/ # Template path.
spring.groovy.template.suffix=.tpl # Suffix that gets appended to view names when building a URL.
spring.groovy.template.view-names= # White list of view names that can be resolved.
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