先贴一下代码吧。

1、html中获取captcha

<span class="input-group-addon" style="padding: 0px;">
    <img title="点击刷新" class="captcha" src="/admin/captcha" onclick="refreshCaptcha();" style="height:35px; ">
</span>

2、controller中的方法

public void captcha() {
    renderCaptcha();
}

3、异常信息

2017-02-24 17:18:13.287:WARN:oejs.ServletHandler:Error for /admin/captcha
java.lang.IllegalAccessError: class sun.reflect.GeneratedConstructorAccessor3 cannot access its superclass sun.reflect.ConstructorAccessorImpl
	at sun.misc.Unsafe.defineClass(Native Method)
	at sun.reflect.ClassDefiner.defineClass(ClassDefiner.java:63)
	at sun.reflect.MethodAccessorGenerator$1.run(MethodAccessorGenerator.java:399)
	at sun.reflect.MethodAccessorGenerator$1.run(MethodAccessorGenerator.java:396)
	at java.security.AccessController.doPrivileged(Native Method)
	at sun.reflect.MethodAccessorGenerator.generate(MethodAccessorGenerator.java:395)
	at sun.reflect.MethodAccessorGenerator.generateConstructor(MethodAccessorGenerator.java:94)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:48)
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at java.lang.reflect.Constructor.newInstance(Constructor.java:526)
	at java.lang.Class.newInstance(Class.java:379)
	at com.jfinal.core.ActionHandler.handle(ActionHandler.java:69)
	at com.jfinal.core.JFinalFilter.doFilter(JFinalFilter.java:74)
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1476)
	at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:501)
	at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:137)
	at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:557)
	at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:231)
	at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1086)
	at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:429)
	at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:193)
	at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1020)
	at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:135)
	at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:116)
	at org.eclipse.jetty.server.Server.handle(Server.java:370)
	at org.eclipse.jetty.server.AbstractHttpConnection.handleRequest(AbstractHttpConnection.java:494)
	at org.eclipse.jetty.server.AbstractHttpConnection.headerComplete(AbstractHttpConnection.java:971)
	at org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.headerComplete(AbstractHttpConnection.java:1033)
	at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:644)
	at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:235)
	at org.eclipse.jetty.server.AsyncHttpConnection.handle(AsyncHttpConnection.java:82)
	at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:696)
	at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:53)
	at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:608)
	at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:543)
	at java.lang.Thread.run(Thread.java:745)

当我用jfinal自带的jetty在eclipse中运行没有任何问题，在idea中运行只要在浏览器练习刷新几次就会出现以上错误。