SLF4J: Failed toString() invocation on an object of type [org.eclipse.jetty.websocket.server......，slf4jinvocation

SLF4J: Failed toString() invocation on an object of type [org.eclipse.jetty.websocket.server......，slf4jinvocation

在配置SpringBoot的log时，遇到如下错误：

SLF4J: Failed toString() invocation on an object of type [org.eclipse.jetty.websocket.server.WebSocketServerFactory]
Reported exception:
java.lang.NullPointerException
    at org.eclipse.jetty.websocket.server.WebSocketServerFactory.toString(WebSocketServerFactory.java:641)
    at org.slf4j.helpers.MessageFormatter.safeObjectAppend(MessageFormatter.java:299)
    at org.slf4j.helpers.MessageFormatter.deeplyAppendParameter(MessageFormatter.java:271)
    at org.slf4j.helpers.MessageFormatter.arrayFormat(MessageFormatter.java:233)
    at org.slf4j.helpers.MessageFormatter.arrayFormat(MessageFormatter.java:173)
    at org.eclipse.jetty.util.log.JettyAwareLogger.log(JettyAwareLogger.java:619)
    at org.eclipse.jetty.util.log.JettyAwareLogger.debug(JettyAwareLogger.java:206)
    at org.eclipse.jetty.util.log.Slf4jLog.debug(Slf4jLog.java:89)
    at org.eclipse.jetty.util.component.ContainerLifeCycle.addBean(ContainerLifeCycle.java:322)
    at org.eclipse.jetty.util.component.ContainerLifeCycle.addBean(ContainerLifeCycle.java:226)
    at org.eclipse.jetty.websocket.server.WebSocketServerFactory.<init>(WebSocketServerFactory.java:149)
    at org.eclipse.jetty.websocket.server.WebSocketServerFactory.<init>(WebSocketServerFactory.java:126)
    at org.eclipse.jetty.websocket.server.WebSocketServerFactory.<init>(WebSocketServerFactory.java:105)
    at org.eclipse.jetty.websocket.server.NativeWebSocketConfiguration.<init>(NativeWebSocketConfiguration.java:51)
    at org.eclipse.jetty.websocket.server.NativeWebSocketServletContainerInitializer.getDefaultFrom(NativeWebSocketServletContainerInitializer.java:36)
    at org.eclipse.jetty.websocket.jsr356.server.deploy.WebSocketServerContainerInitializer.configureContext(WebSocketServerContainerInitializer.java:139)
    at org.springframework.boot.autoconfigure.websocket.JettyWebSocketContainerCustomizer$1.configure(JettyWebSocketContainerCustomizer.java:45)
    at org.eclipse.jetty.webapp.WebAppContext.configure(WebAppContext.java:517)
    at org.eclipse.jetty.webapp.WebAppContext.startContext(WebAppContext.java:1458)
    at org.eclipse.jetty.server.handler.ContextHandler.doStart(ContextHandler.java:785)
    at org.eclipse.jetty.servlet.ServletContextHandler.doStart(ServletContextHandler.java:261)
    at org.eclipse.jetty.webapp.WebAppContext.doStart(WebAppContext.java:545)
    at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:68)
    at org.eclipse.jetty.util.component.ContainerLifeCycle.start(ContainerLifeCycle.java:131)
    at org.eclipse.jetty.server.Server.start(Server.java:452)
    at org.eclipse.jetty.util.component.ContainerLifeCycle.doStart(ContainerLifeCycle.java:105)
    at org.eclipse.jetty.server.handler.AbstractHandler.doStart(AbstractHandler.java:113)
    at org.eclipse.jetty.server.Server.doStart(Server.java:419)
    at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:68)
    at org.springframework.boot.context.embedded.jetty.JettyEmbeddedServletContainer.initialize(JettyEmbeddedServletContainer.java:104)
    at org.springframework.boot.context.embedded.jetty.JettyEmbeddedServletContainer.<init>(JettyEmbeddedServletContainer.java:82)
    at org.springframework.boot.context.embedded.jetty.JettyEmbeddedServletContainerFactory.getJettyEmbeddedServletContainer(JettyEmbeddedServletContainerFactory.java:561)
    at org.springframework.boot.context.embedded.jetty.JettyEmbeddedServletContainerFactory.getEmbeddedServletContainer(JettyEmbeddedServletContainerFactory.java:190)
    at org.springframework.boot.context.embedded.EmbeddedWebApplicationContext.createEmbeddedServletContainer(EmbeddedWebApplicationContext.java:164)
    at org.springframework.boot.context.embedded.EmbeddedWebApplicationContext.onRefresh(EmbeddedWebApplicationContext.java:134)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:537)
    at org.springframework.boot.context.embedded.EmbeddedWebApplicationContext.refresh(EmbeddedWebApplicationContext.java:122)
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:693)
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:360)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:303)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1118)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1107)
    at com.baidu.hetu.Application.main(Application.java:17)

查了很多资料，应该是jetty的一个bug，jetty版本jetty-9.4.6.v20170531。
https://stackoverflow.com/questions/26416072/slf4j-failed-tostring-invocation-on-an-object-of-type-com-datastax-driver-co
原因还不是特别清楚，但是通过错误能够看出来与SLF4J有关，而且通过查资料能够看出和日志级别有关，原logback.xml 如下

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <layout class="ch.qos.logback.classic.PatternLayout">
            <Pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg\n</Pattern>
        </layout>

        <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
            <level>INFO</level>
        </filter>
    </appender>

    <root level="DEBUG">
        <appender-ref ref="STDOUT" />
    </root>
</configuration>

将<root level="DEBUG">改为<root level="INFO">可解决上面问题。具体原因有待验证。