JavaEE Training Note - Day 16

Today’s topic is about some debugging methods and toolkit used to solve problems. One is the tcpdump and other terminal tools for remote package sniffer and traffic analysis. Another is some wired errors occurred in the development progress of project.

Remote Package Sniffer

For the remote deployed server, the most convenient method to connect , monitor and control it is the remote shell. By using the ssh secured remote shell connection, the remote server can be accessed just as a local machine. However, when some error occurred, the terminal tools is the only choice to solve that.

When debugging the interface of a web service, logging it in-ward and out-word packages can is a useful approach to separate the application issues(the server), network issues(the connection), and the request source issues(the client). The following tools are used to handle these problems.

The tools are tested on a ubuntu 16.04 server based host.

port usage and service status

First thing is to ensure the remote application is executing correctly. The initial step is to print process list and check the target service/application is running.

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ps aux | grep tomcat

Then make sure the specified remote tcp port is allocated to target program.

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netstats -ltupn
# l: local address (no dns reverse resolving)
# t: TCP connection
# u: UDP connection
# p: port
# n: numeric representation of port
#    (no known protocol name replacing)

use tcpdump to sniffer package

tcpdump is a network tool to dump the traffic on a network. For a http protocol based service running on port 80, all of its traffic can be dumped by the following setting:

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sudo tcpdump -q -s 0 -A 'tcp port 80'
# A: pring each package in ASCII mode
# q: reduce the print of protocol related information
# s: snarf the package of setted size, 0 to disable this feature
# 'tcp port 80': package filter to dump package of 80 port on TCP stack

use tshare to dump package for further analysis

wireshark is an advanced toolkit for network package analysis. The tshark is the terminal tool in this program packages. With the tshark program, the package can be used to dump package to standard output or save the package to standard pcap (package capture) format then download the package for further analysis. Besides, the package filter can also use lua based scripts to process the output or filter.

SQL keyword conflict

In JPA/Hibernate based application, database table can be created by the annotated based table declaration. However, when some SQL keyword are used in name of the table, the generate SQL statement will contain an un-resolvable structure thus a error will occur.

This will generally print a stacktrace at the start up procedure of web application. However, this problem will not prevent the application to load in further progress, until a database operation related to that table is performed.