spring boot 之 错误：SpelEvaluationException: EL1008E: Property or field 'timestamp' cannot be found on object of type 'java.util.HashMap'

这个错误我也见过很多次了，今天终于理解了其出现的原因。

错误是这样的：

2017-11-23 18:05:39.504 ERROR 4092 --- [nio-8080-exec-3] o.a.c.c.C.[Tomcat].[localhost]           : Exception Processing ErrorPage[errorCode=0, location=/error]

    org.springframework.web.util.NestedServletException: Request processing failed; nested exception is org.springframework.expression.spel.SpelEvaluationException: EL1008E: Property or field 'timestamp' cannot be found on object of type 'java.util.HashMap' - maybe not public?
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:982) ~[spring-webmvc-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861) ~[spring-webmvc-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:635) ~[tomcat-embed-core-8.5.23.jar:8.5.23]
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846) ~[spring-webmvc-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:742) ~[tomcat-embed-core-8.5.23.jar:8.5.23]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-8.5.23.jar:8.5.23]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.23.jar:8.5.23]
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:728) ~[tomcat-embed-core-8.5.23.jar:8.5.23]
    at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:469) ~[tomcat-embed-core-8.5.23.jar:8.5.23]
    at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:392) ~[tomcat-embed-core-8.5.23.jar:8.5.23]
    at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:311) ~[tomcat-embed-core-8.5.23.jar:8.5.23]
    at org.apache.catalina.core.StandardHostValve.custom(StandardHostValve.java:395) [tomcat-embed-core-8.5.23.jar:8.5.23]
    at org.apache.catalina.core.StandardHostValve.status(StandardHostValve.java:254) [tomcat-embed-core-8.5.23.jar:8.5.23]
    at org.apache.catalina.core.StandardHostValve.throwable(StandardHostValve.java:349) [tomcat-embed-core-8.5.23.jar:8.5.23]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:175) [tomcat-embed-core-8.5.23.jar:8.5.23]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81) [tomcat-embed-core-8.5.23.jar:8.5.23]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [tomcat-embed-core-8.5.23.jar:8.5.23]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) [tomcat-embed-core-8.5.23.jar:8.5.23]
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:803) [tomcat-embed-core-8.5.23.jar:8.5.23]
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-8.5.23.jar:8.5.23]
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868) [tomcat-embed-core-8.5.23.jar:8.5.23]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1459) [tomcat-embed-core-8.5.23.jar:8.5.23]
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-8.5.23.jar:8.5.23]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_121]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_121]
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.5.23.jar:8.5.23]
    at java.lang.Thread.run(Thread.java:745) [na:1.8.0_121]
    Caused by: org.springframework.expression.spel.SpelEvaluationException: EL1008E: Property or field 'timestamp' cannot be found on object of type 'java.util.HashMap' - maybe not public?
    at org.springframework.expression.spel.ast.PropertyOrFieldReference.readProperty(PropertyOrFieldReference.java:226) ~[spring-expression-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    at org.springframework.expression.spel.ast.PropertyOrFieldReference.getValueInternal(PropertyOrFieldReference.java:94) ~[spring-expression-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    at org.springframework.expression.spel.ast.PropertyOrFieldReference.getValueInternal(PropertyOrFieldReference.java:81) ~[spring-expression-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    at org.springframework.expression.spel.ast.SpelNodeImpl.getValue(SpelNodeImpl.java:121) ~[spring-expression-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    at org.springframework.expression.spel.standard.SpelExpression.getValue(SpelExpression.java:262) ~[spring-expression-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    at org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration$ExpressionResolver.resolvePlaceholder(ErrorMvcAutoConfiguration.java:288) ~[spring-boot-autoconfigure-1.5.8.RELEASE.jar:1.5.8.RELEASE]
    at org.springframework.boot.autoconfigure.web.NonRecursivePropertyPlaceholderHelper$NonRecursivePlaceholderResolver.resolvePlaceholder(NonRecursivePropertyPlaceholderHelper.java:56) ~[spring-boot-autoconfigure-1.5.8.RELEASE.jar:1.5.8.RELEASE]
    at org.springframework.util.PropertyPlaceholderHelper.parseStringValue(PropertyPlaceholderHelper.java:147) ~[spring-core-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    at org.springframework.boot.autoconfigure.web.NonRecursivePropertyPlaceholderHelper.parseStringValue(NonRecursivePropertyPlaceholderHelper.java:38) ~[spring-boot-autoconfigure-1.5.8.RELEASE.jar:1.5.8.RELEASE]
    at org.springframework.util.PropertyPlaceholderHelper.replacePlaceholders(PropertyPlaceholderHelper.java:126) ~[spring-core-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    at org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration$SpelView.render(ErrorMvcAutoConfiguration.java:226) ~[spring-boot-autoconfigure-1.5.8.RELEASE.jar:1.5.8.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1286) ~[spring-webmvc-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1041) ~[spring-webmvc-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:984) ~[spring-webmvc-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:901) ~[spring-webmvc-4.3.12.RELEASE.jar:4.3.12.RELEASE]
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970) ~[spring-webmvc-4.3.12.RELEASE.jar:4.3.12.RELEASE]
            ... 26 common frames omitted

其实仔细看就会发现原因： SpelView 需要  timestamp 这么一个变量，以进行渲染， 它其实是来自于 DefaultErrorAttributes 以配合SpelView 使用的。但是呢， 由于我自定义了boot 的Error viewController， 但是没有自定义errorview， 故还是使用的是 SpelView，也就是白标。

@Controller
public class ErrControlller implements ErrorController {

    @RequestMapping("/error")
    public String error() {
        System.out.println("ErrControlller.error");
        return "error";
    }

//    @Override
    public String getErrorPath() {
        return "error";
    }
}

但是呢， 当我自定义ErrorController 之后，下面的BasicErrorController 便不再会生效，也就是DefaultErrorAttributes 虽然注册成了bean，但是不会实际起作用。 

    @Bean
    @ConditionalOnMissingBean(
        value = {ErrorAttributes.class},
        search = SearchStrategy.CURRENT
    )
    public DefaultErrorAttributes errorAttributes() {
        return new DefaultErrorAttributes();
    }

    @Bean
    @ConditionalOnMissingBean(
        value = {ErrorController.class},
        search = SearchStrategy.CURRENT
    )
    public BasicErrorController basicErrorController(ErrorAttributes errorAttributes) {
        return new BasicErrorController(errorAttributes, this.serverProperties.getError(), this.errorViewResolvers);
    }

解决方案呢，我目前想到一个，那就是@RequestMapping("/error") 方法 不要再返回 "error"了， 不然， boot 又会去使用 SpelView来渲染：

    @RequestMapping("/error")
    public String error() {
        System.out.println("ErrControlller.error");
        return "err"; // 不使用/error， 可以返回其他的， 但是要保证这个视图存在， 比如存在 public/err.html 
    }