在jsp页面调用bean报错和介绍些解决debug的书

[henson2007]

我知道是null值问题，但我不知怎样改
一下是Myeclipse中的debug:
Thread [http-80-1] (Suspended (exception NullPointerException))
JspServletWrapper.handleJspException(Exception) line: 436
this JspServletWrapper (id=46)
available 0
compileException JasperException (id=53)
config StandardWrapperFacade (id=67)
ctxt JspCompilationContext (id=70)
firstTime false
isTagFile false
jspUri "/hellob.jsp"
lastModificationTest 1208145472265
log Jdk14Logger (id=79)
options EmbeddedServletOptions (id=82)
reload true
servletClass null
servletClassLastModifiedTime 0
tagHandlerClass null
theServlet null
tripCount 0
realException null

ex JasperException (id=53)
cause JasperException (id=53)
detailMessage "/hellob.jsp(4,0) The value for the useBean class attribute TestFont is invalid."
rootCause null
stackTrace null

///////////////////////////////////////////////


JspServletWrapper.service(HttpServletRequest, HttpServletResponse, boolean) line: 371

this JspServletWrapper (id=46)
available 0
compileException JasperException (id=53)
config StandardWrapperFacade (id=67)
ctxt JspCompilationContext (id=70)
firstTime false
isTagFile false
jspUri "/hellob.jsp"
lastModificationTest 1208145472265
log Jdk14Logger (id=79)
options EmbeddedServletOptions (id=82)
reload true
servletClass null
servletClassLastModifiedTime 0
tagHandlerClass null
theServlet null
tripCount 0
ex JasperException (id=53)
cause JasperException (id=53)
detailMessage "/hellob.jsp(4,0) The value for the useBean class attribute TestFont is invalid."
rootCause null
stackTrace null

request RequestFacade (id=59)
request Request (id=76)

response ResponseFacade (id=63)
response Response (id=109)

precompile false
///////////////////////////////////////////////////////
JspServlet.serviceJspFile(HttpServletRequest, HttpServletResponse, String, Throwable, boolean) line: 315
this JspServlet (id=47)
config StandardWrapperFacade (id=67)
config StandardWrapperFacade (id=67)
context ApplicationContextFacade (id=92)
log Jdk14Logger (id=91)
options EmbeddedServletOptions (id=82)
rctxt JspRuntimeContext (id=95)
request RequestFacade (id=59)
request Request (id=76)
response ResponseFacade (id=63)
response Response (id=109)
jspUri "/hellob.jsp"
count 11
hash -103288680
offset 0
value char[11] (id=219)
exception null
precompile false
wrapper JspServletWrapper (id=46)
available 0
compileException JasperException (id=53)
config StandardWrapperFacade (id=67)
ctxt JspCompilationContext (id=70)
firstTime false
isTagFile false
jspUri "/hellob.jsp"
lastModificationTest 1208145472265
log Jdk14Logger (id=79)
options EmbeddedServletOptions (id=82)
reload true
servletClass null
servletClassLastModifiedTime 0
tagHandlerClass null
theServlet null
tripCount 0


//////////////////////////////////////////////////////////////////////
JspServlet.service(HttpServletRequest, HttpServletResponse) line: 265
JspServlet(HttpServlet).service(ServletRequest, ServletResponse) line: 803
ApplicationFilterChain.internalDoFilter(ServletRequest, ServletResponse) line: 269
ApplicationFilterChain.doFilter(ServletRequest, ServletResponse) line: 188
StandardWrapperValve.invoke(Request, Response) line: 210
StandardContextValve.invoke(Request, Response) line: 174
StandardHostValve.invoke(Request, Response) line: 127
ErrorReportValve.invoke(Request, Response) line: 117
StandardEngineValve.invoke(Request, Response) line: 108
CoyoteAdapter.service(Request, Response) line: 151
Http11AprProcessor.process(long) line: 834
Http11AprProtocol$Http11ConnectionHandler.process(long) line: 640
AprEndpoint$Worker.run() line: 1286
ThreadWithAttributes(Thread).run() line: 595 [local variables unavailable]

[bruni]

/hellob.jsp(4,0) The value for the useBean class attribute TestFont is invalid."
=> bean的属性TestFont有问题

[Kit_LO]

看这句JspServletWrapper.handleJspException(Exception) line: 436
说明是jsp页面有问题
这句更确切detailMessage "/hellob.jsp(4,0) The value for the useBean class attribute TestFont is invalid."
意思是hellob.jsp中的第四行