运行环境：
Ubuntu 18、jdk_1.8.2u231、jfinal-undertow 2.1、jfinal 4.9
本地环境没有问题，部署服务器报错。来官网找了下解决办法，发现有同样的问题，但未解决：
传送门：【jfinal自带的登录验证码问题 http://www.jfinal.com/feedback/5761】
错误日志内容：
java.lang.RuntimeException: java.lang.NoClassDefFoundError: Could not initialize class com.jfinal.captcha.CaptchaRender at com.jfinal.aop.Invocation.invoke(Invocation.java:107) at com.jfinal.core.ActionHandler.handle(ActionHandler.java:89) at com.jfinal.plugin.druid.DruidStatViewHandler.handle(DruidStatViewHandler.java:81) at com.jfinal.core.JFinalFilter.doFilter(JFinalFilter.java:90) at io.undertow.servlet.core.ManagedFilter.doFilter(ManagedFilter.java:61) at io.undertow.servlet.handlers.FilterHandler$FilterChainImpl.doFilter(FilterHandler.java:131) at io.undertow.servlet.handlers.FilterHandler.handleRequest(FilterHandler.java:84) at io.undertow.servlet.handlers.security.ServletSecurityRoleHandler.handleRequest(ServletSecurityRoleHandler.java:62) at io.undertow.servlet.handlers.ServletChain$1.handleRequest(ServletChain.java:68) at io.undertow.servlet.handlers.ServletDispatchingHandler.handleRequest(ServletDispatchingHandler.java:36) at io.undertow.servlet.handlers.RedirectDirHandler.handleRequest(RedirectDirHandler.java:68) at io.undertow.servlet.handlers.security.SSLInformationAssociationHandler.handleRequest(SSLInformationAssociationHandler.java:132) at io.undertow.servlet.handlers.security.ServletAuthenticationCallHandler.handleRequest(ServletAuthenticationCallHandler.java:57) at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43) at io.undertow.security.handlers.AbstractConfidentialityHandler.handleRequest(AbstractConfidentialityHandler.java:46) at io.undertow.servlet.handlers.security.ServletConfidentialityConstraintHandler.handleRequest(ServletConfidentialityConstraintHandler.java:64) at io.undertow.security.handlers.AuthenticationMechanismsHandler.handleRequest(AuthenticationMechanismsHandler.java:60) at io.undertow.servlet.handlers.security.CachedAuthenticatedSessionHandler.handleRequest(CachedAuthenticatedSessionHandler.java:77) at io.undertow.security.handlers.AbstractSecurityContextAssociationHandler.handleRequest(AbstractSecurityContextAssociationHandler.java:43) at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43) at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43) at io.undertow.servlet.handlers.ServletInitialHandler.handleFirstRequest(ServletInitialHandler.java:269) at io.undertow.servlet.handlers.ServletInitialHandler.access$100(ServletInitialHandler.java:78) at io.undertow.servlet.handlers.ServletInitialHandler$2.call(ServletInitialHandler.java:133) at io.undertow.servlet.handlers.ServletInitialHandler$2.call(ServletInitialHandler.java:130) at io.undertow.servlet.core.ServletRequestContextThreadSetupAction$1.call(ServletRequestContextThreadSetupAction.java:48) at io.undertow.servlet.core.ContextClassLoaderSetupAction$1.call(ContextClassLoaderSetupAction.java:43) at io.undertow.servlet.handlers.ServletInitialHandler.dispatchRequest(ServletInitialHandler.java:249) at io.undertow.servlet.handlers.ServletInitialHandler.access$000(ServletInitialHandler.java:78) at io.undertow.servlet.handlers.ServletInitialHandler$1.handleRequest(ServletInitialHandler.java:99) at io.undertow.server.Connectors.executeRootHandler(Connectors.java:376) at io.undertow.server.HttpServerExchange$1.run(HttpServerExchange.java:830) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748) Caused by: java.lang.NoClassDefFoundError: Could not initialize class com.jfinal.captcha.CaptchaRender at com.jfinal.render.RenderFactory.getCaptchaRender(RenderFactory.java:179) at com.jfinal.core.Controller.renderCaptcha(Controller.java:1292) at com.jmult._admin.login._LoginController.captcha(_LoginController.java:76) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at com.jfinal.aop.Invocation.invoke(Invocation.java:97) ... 34 more
根据波总的回答找到了方向：
解决过程
1、安装相关字体
apt-get install ttf-dejavu
2、安装字体的依赖包
sudo apt-get install liberation-* sudo apt-get install font-manager
以上操作完成后，不报错了，但还是不行，报以下错误：
java.lang.NoClassDefFoundError: Could not initialize class sun.awt.X11GraphicsEnvironment at java.lang.Class.forName0(Native Method) at java.lang.Class.forName(Class.java:264) at java.awt.GraphicsEnvironment.createGE(GraphicsEnvironment.java:103) at java.awt.GraphicsEnvironment.getLocalGraphicsEnvironment(GraphicsEnvironment.java:82) at java.awt.image.BufferedImage.createGraphics(BufferedImage.java:1181) at com.jfinal.captcha.CaptchaRender.drawGraphic(CaptchaRender.java:142) at com.jfinal.captcha.CaptchaRender.render(CaptchaRender.java:95) at com.jfinal.core.ActionHandler.handle(ActionHandler.java:114) at com.jfinal.plugin.druid.DruidStatViewHandler.handle(DruidStatViewHandler.java:81) at com.jfinal.core.JFinalFilter.doFilter(JFinalFilter.java:90) at io.undertow.servlet.core.ManagedFilter.doFilter(ManagedFilter.java:61) at io.undertow.servlet.handlers.FilterHandler$FilterChainImpl.doFilter(FilterHandler.java:131) at io.undertow.servlet.handlers.FilterHandler.handleRequest(FilterHandler.java:84) at io.undertow.servlet.handlers.security.ServletSecurityRoleHandler.handleRequest(ServletSecurityRoleHandler.java:62) at io.undertow.servlet.handlers.ServletChain$1.handleRequest(ServletChain.java:68) at io.undertow.servlet.handlers.ServletDispatchingHandler.handleRequest(ServletDispatchingHandler.java:36) at io.undertow.servlet.handlers.RedirectDirHandler.handleRequest(RedirectDirHandler.java:68) at io.undertow.servlet.handlers.security.SSLInformationAssociationHandler.handleRequest(SSLInformationAssociationHandler.java:132) at io.undertow.servlet.handlers.security.ServletAuthenticationCallHandler.handleRequest(ServletAuthenticationCallHandler.java:57) at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43) at io.undertow.security.handlers.AbstractConfidentialityHandler.handleRequest(AbstractConfidentialityHandler.java:46) at io.undertow.servlet.handlers.security.ServletConfidentialityConstraintHandler.handleRequest(ServletConfidentialityConstraintHandler.java:64) at io.undertow.security.handlers.AuthenticationMechanismsHandler.handleRequest(AuthenticationMechanismsHandler.java:60) at io.undertow.servlet.handlers.security.CachedAuthenticatedSessionHandler.handleRequest(CachedAuthenticatedSessionHandler.java:77) at io.undertow.security.handlers.AbstractSecurityContextAssociationHandler.handleRequest(AbstractSecurityContextAssociationHandler.java:43) at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43) at io.undertow.server.handlers.PredicateHandler.handleRequest(PredicateHandler.java:43) at io.undertow.servlet.handlers.ServletInitialHandler.handleFirstRequest(ServletInitialHandler.java:269) at io.undertow.servlet.handlers.ServletInitialHandler.access$100(ServletInitialHandler.java:78) at io.undertow.servlet.handlers.ServletInitialHandler$2.call(ServletInitialHandler.java:133) at io.undertow.servlet.handlers.ServletInitialHandler$2.call(ServletInitialHandler.java:130) at io.undertow.servlet.core.ServletRequestContextThreadSetupAction$1.call(ServletRequestContextThreadSetupAction.java:48) at io.undertow.servlet.core.ContextClassLoaderSetupAction$1.call(ContextClassLoaderSetupAction.java:43) at io.undertow.servlet.handlers.ServletInitialHandler.dispatchRequest(ServletInitialHandler.java:249) at io.undertow.servlet.handlers.ServletInitialHandler.access$000(ServletInitialHandler.java:78) at io.undertow.servlet.handlers.ServletInitialHandler$1.handleRequest(ServletInitialHandler.java:99) at io.undertow.server.Connectors.executeRootHandler(Connectors.java:376) at io.undertow.server.HttpServerExchange$1.run(HttpServerExchange.java:830) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748)
验证码采用awt图形处理并内存中生成，java程序会去寻找linux上的图形界面是否启动 ，如linux缺少组件则, 缺少了显示设备、键盘或鼠标会报这个错误。
所以需要开启无头模式, Headless模式是系统的一种配置模式 , 开始激活headless模式，告诉程序，现在你要工作在Headless mode下，就不要指望硬件帮忙了，你得自力更生，依靠系统的计算能力模拟出这些特性来继续。
网上基本上都是通过设置tomcat来解决：
在tomcat/bin/catalina.sh里加入
JAVA_OPTS="$JAVA_OPTS -Djava.awt.headless=true
jfinal undertow项目解决办法，在配置文件中设置：
public void configConstant(Constants me) {
System.setProperty("java.awt.headless","true");
}完美解决：
前不久 @eova 作者也提出过要添加这个支持，我正考虑要添加到 jfinal 新版本中去：
https://blog.csdn.net/wodeyuer125/article/details/50502914
原因是windows内核集成了gui，而linux上没有启动x server
解决办法：
1.启动x server
2.在java运行参数上加-Djava.awt.headless=true
用tomcat 和 Undertow 都可能遇到这个问题, 尤其是 Centos 7
就是不知道开启这个配置有没有啥副作用：
System.setProperty("java.awt.headless","true");