最近在開發java web application的時候，因為很多原因，無法對自己開發的項目在本地進行調試，常常需要進行遠程調試，之前一直通過打logger的方式進行，每次都要重新部署，相當的痛苦，今天下午研究了以下，如果進行遠程調試。
開發的web application是部署在tomcat上面的，那么問題就轉化為如何調試tomcat。其實調試tomcat，本質上就是調試JVM。JVM的強大，從J2SE1.4.2開始，就實現了JPDA （Java Platform Debug Architecture）。
tomcat默認情況下，是沒有啟用jpda的，如果要啟用，需要傳入參數
-Xdebug -Xrunjdwp:transport=dt_socket, address=8000,server=y,suspend=y
那么問題是這些參數如何在tomcat啟動的使用傳入呢？這是時候，需要了解tomcat的啟動腳本，在TOMCAT_HOME/bin
目錄下，有三個腳本catalina.sh, startup.sh, 和 shutdown.sh。如果查看startup.sh和shutdown.sh，都是通過catalina.sh來啟動的。腳本如下：
EXECUTABLE=catalina.sh
exec "$PRGDIR"/"$EXECUTABLE" start "$@"
于是，我們可以查看下catalina.sh的腳本是如何實現的。
# JPDA_TRANSPORT (Optional) JPDA transport used when the "jpda start"
# command is executed. The default is "dt_socket".
#
# JPDA_ADDRESS (Optional) Java runtime options used when the "jpda start"
# command is executed. The default is 8000.
#
# JPDA_SUSPEND (Optional) Java runtime options used when the "jpda start"
# command is executed. Specifies whether JVM should suspend
# execution immediately after startup. Default is "n".
#
# JPDA_OPTS (Optional) Java runtime options used when the "jpda start"
# command is executed. If used, JPDA_TRANSPORT, JPDA_ADDRESS,
# and JPDA_SUSPEND are ignored. Thus, all required jpda
# options MUST be specified. The default is:
#
# -agentlib:jdwp=transport=$JPDA_TRANSPORT,
# address=$JPDA_ADDRESS,server=y,suspend=$JPDA_SUSPEND
if [ "$1" = "jpda" ] ; then
if [ -z "$JPDA_TRANSPORT" ]; then
JPDA_TRANSPORT="dt_socket"
fi
if [ -z "$JPDA_ADDRESS" ]; then
JPDA_ADDRESS="8000"
fi
if [ -z "$JPDA_SUSPEND" ]; then
JPDA_SUSPEND="n"
fi
if [ -z "$JPDA_OPTS" ]; then
JPDA_OPTS="-agentlib:jdwp=transport=$JPDA_TRANSPORT,address=$JPDA_ADDRESS,server=y,suspend=$JPDA_SUSPEND"
fi
CATALINA_OPTS="$CATALINA_OPTS $JPDA_OPTS"
shift
fi
通過這個代碼，我們可以看出，其實要啟動jpda, 最主要的是要對JPDA_SUSPEND的值進行設置，由N改為Y。
借鑒start.sh的啟動，在linux下，我們可以自己創建一個jpda.sh的腳本，用來啟動開啟debug模式的tomcat，具體腳本如下，黑體為修改部分。
os400=false
darwin=false
case "`uname`" in
CYGWIN*) cygwin=true;;
OS400*) os400=true;;
Darwin*) darwin=true;;
esac
# resolve links - $0 may be a softlink
PRG="$0"
while [ -h "$PRG" ] ; do
ls=`ls -ld "$PRG"`
link=`expr "$ls" : '.*-> \(.*\)$'`
if expr "$link" : '/.*' > /dev/null; then
PRG="$link"
else
PRG=`dirname "$PRG"`/"$link"
fi
done
PRGDIR=`dirname "$PRG"`
EXECUTABLE=catalina.sh
# Check that target executable exists
if $os400; then
# -x will Only work on the os400 if the files are:
# 1. owned by the user
# 2. owned by the PRIMARY group of the user
# this will not work if the user belongs in secondary groups
eval
else
if [ ! -x "$PRGDIR"/"$EXECUTABLE" ]; then
echo "Cannot find $PRGDIR/$EXECUTABLE"
echo "The file is absent or does not have execute permission"
echo "This file is needed to run this program"
exit 1
fi
fi
export JPDA_SUSPEND=y
exec "$PRGDIR"/"$EXECUTABLE" jpda start "$@"
在Eclipse中遠程調試Tomcat
首先將Tomcat 5.5.26的源代碼分為container connectors jasper servletapi build五個項目，導入到Eclipse中。啟動相關的代碼主要在container中，就以它為當前項目，打開”Debug Configurations“對話框。
然后創建一個”Remote Java Application“，Connection Type選擇”Standard (Socket Attach)“，Host填寫localhost（Tomcat所在的主機地址），Port填寫8000。最后點擊”Apply“保存。
首先確保已經執行了jpda.bat，Tomcat正在等待調試器連接；然后執行上述的Debug Configuration，Eclipse就可以連上Tomcat。
Tomcat的啟動是從Bootstrap的main方法開始，我在第一行代碼處設置了斷點，Tomcat的啟動就停在了這一行：
接著，讓Tomcat繼續執行，我們可以看到，控制臺輸出了啟動信息。
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