問題：啓動tomcat報找不到JAVA_HOME或者JRE_HOME環境變量的問題

一、錯誤來源

在非linux系統上(即外部)啓動tomcat時顯示的錯誤

二、報錯原文

Neither the JAVA_HOME nor the JRE_HOME environment variable is defined
At least one of these environment variable is needed to run this program

三、錯誤譯文

JAVA_HOME和JRE_HOME環境變量都沒有定義
運行這個程序至少需要這些環境變量中的一個

四、錯誤思索

我明明在/etc/profile這個文件裏面添加了java和tomcat的環境變量啊，爲什麼還出錯，而在服務器上啓動tomcat就不報這個錯誤

五、粗略分析錯誤原因

在服務器上啓動tomcat不報錯是因爲啓動tomcat時區/etc/profile文件裏面找到了java和tomcat的環境變量，所以順利啓動，而在外部啓動tomcat時，tomcat程序首先調用startup.sh腳本文件，startup.sh又去調用catalina.sh這個腳本文件，而catalina.sh腳本文件又會去setclasspath.sh這個腳本文件裏面尋找tomcat變量環境，這是因爲setclasspath.sh文件就是tomcat變量環境腳本，這是因爲這個腳本默認是沒有在腳本開頭申請java和jdk的變量環境劉靜的，因此報錯，setclasspath.sh的路徑是：
/usr/local/tomcat/bin/setclasspath.sh
（因爲我的tomcat是裝在/usr/local路徑下了，所以我的是這個，你們要按照你們的安裝路徑在查找）

六、查看setclasspath.sh腳本文件，精確分析原因

setclasspath.sh腳本原文以及中文解釋如下:

#!/bin/sh
# Licensed to the Apache Software Foundation (ASF) under one or more
# contributor license agreements.  See the NOTICE file distributed with
# this work for additional information regarding copyright ownership.
# The ASF licenses this file to You under the Apache License, Version 2.0
# (the "License"); you may not use this file except in compliance with
# the License.  You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
# -----------------------------------------------------------------------------
#  Set CLASSPATH and Java options
#
#  $Id: setclasspath.sh 795037 2009-07-17 10:52:16Z markt $
# -----------------------------------------------------------------------------
# Make sure prerequisite environment variables are set
#因爲setclasspath.sh腳本是被catalina.sh調用，所以可以繼承catalina.sh中的變量申明
if [ -z "$JAVA_HOME" -a -z "$JRE_HOME" ]; then
#判斷用戶有沒有提前做$JAVA_HOME和$JRE_HOME全局變量聲明，如果都沒進行申明
  # Bugzilla 37284 (reviewed).
  if $darwin; then
  #要理解這個判斷，先看下startup.sh和shutdown.sh就會明白
  #這個是win仿真unix不用管下面兩個語句
    if [ -d "/System/Library/Frameworks/JavaVM.framework/Versions/CurrentJDK/Home" ]; then
      export JAVA_HOME="/System/Library/Frameworks/JavaVM.framework/Versions/CurrentJDK/Home"
    fi
  else
  #其他環境沒有申明，那麼系統自己想辦法找這兩個變量的路徑
    JAVA_PATH=`which java 2>/dev/null`
#此語句可以把java命令位置找出來
    if [ "x$JAVA_PATH" != "x" ]; then
#如果能找出java路徑，則可以定位到java命令的路徑，經過作者驗證不是java的裝路徑
#所以通過此處就可以看出，老鳥們爲什麼都要自己指定這兩個變量了
      JAVA_PATH=`dirname $JAVA_PATH 2>/dev/null`
      JRE_HOME=`dirname $JAVA_PATH 2>/dev/null`
    fi
    if [ "x$JRE_HOME" = "x" ]; then
#如果找不到java路徑，那麼就看有沒有/usr/bin/java這個執行文件，有的話就它了，沒有就算了
      # XXX: Should we try other locations?
      if [ -x /usr/bin/java ]; then
        JRE_HOME=/usr
      fi
    fi
  fi

  if [ -z "$JAVA_HOME" -a -z "$JRE_HOME" ]; then
  #再驗證一邊，有沒有這兩個變量，沒有不好意思，我不執行了，退出
  #這個exit 1 不但是結束setclasspath.sh，會徹底退出catalina.sh腳本的
  #對於在腳本中引用腳本的童鞋們，就需要注意了，小心使用exit。 
    echo "Neither the JAVA_HOME nor the JRE_HOME environment variable is defined"
    echo "At least one of these environment variable is needed to run this program"
    exit 1
  fi
fi
if [ -z "$JAVA_HOME" -a "$1" = "debug" ]; then
  echo "JAVA_HOME should point to a JDK in order to run in debug mode."
  exit 1
fi
if [ -z "$JRE_HOME" ]; then
  JRE_HOME="$JAVA_HOME"
fi
# If we're running under jdb, we need a full jdk.
if [ "$1" = "debug" ] ; then
  if [ "$os400" = "true" ]; then
    if [ ! -x "$JAVA_HOME"/bin/java -o ! -x "$JAVA_HOME"/bin/javac ]; then
      echo "The JAVA_HOME environment variable is not defined correctly"
      echo "This environment variable is needed to run this program"
      echo "NB: JAVA_HOME should point to a JDK not a JRE"
      exit 1
    fi
  else
    if [ ! -x "$JAVA_HOME"/bin/java -o ! -x "$JAVA_HOME"/bin/jdb -o ! -x "$JAVA_HOME"/bin/javac ]; then
      echo "The JAVA_HOME environment variable is not defined correctly"
      echo "This environment variable is needed to run this program"
      echo "NB: JAVA_HOME should point to a JDK not a JRE"
      exit 1
    fi
  fi
fi
#上段的代碼都是在確認$JAVA_HOME和$JRE_HOME變量的申明情況及後續的解決過程
if [ -z "$BASEDIR" ]; then
#對"$BASEDIR變量的檢查，木有的話就退出
  echo "The BASEDIR environment variable is not defined"
  echo "This environment variable is needed to run this program"
  exit 1
fi
if [ ! -x "$BASEDIR"/bin/setclasspath.sh ]; then
#確認"$BASEDIR"/bin/setclasspath.sh有木有，木有還是退出
  if $os400; then
    # -x will Only work on the os400 if the files are:
    # 1. owned by the user
    # 2. owned by the PRIMARY group of the user
    # this will not work if the user belongs in secondary groups
    eval
#eval不清楚嘛意思
  else 
    echo "The BASEDIR environment variable is not defined correctly"
    echo "This environment variable is needed to run this program"
    exit 1
  fi
fi
# Don't override the endorsed dir if the user has set it previously
#這個是確認JAVA_ENDORSED_DIRS的位置
if [ -z "$JAVA_ENDORSED_DIRS" ]; then
  # Set the default -Djava.endorsed.dirs argument
  JAVA_ENDORSED_DIRS="$BASEDIR"/endorsed
fi
# OSX hack to CLASSPATH
JIKESPATH=
if [ `uname -s` = "Darwin" ]; then
  OSXHACK="/System/Library/Frameworks/JavaVM.framework/Versions/CurrentJDK/Classes"
  if [ -d "$OSXHACK" ]; then
    for i in "$OSXHACK"/*.jar; do
      JIKESPATH="$JIKESPATH":"$i"
    done
  fi
fi
# Set standard commands for invoking Java.
#這句是響噹噹的重要，確定了$_RUNJAVA的值
_RUNJAVA="$JRE_HOME"/bin/java
if [ "$os400" != "true" ]; then
  _RUNJDB="$JAVA_HOME"/bin/jdb
fi

說明：通過對這個腳本的分析，我們可以看到，這個腳本就做了一件事情，檢查各種變量是否賦值，驗證tomcat啓動停止需要涉及到的文件，保障tomcat順利啓動停止。而我們報錯就是因爲它在全文搜索了半天結果一直找不到JRE_HOME和JAVA_HOME這兩個變量到底在哪。

七、解決方案
在setclasspath.sh腳本文件的所有沒有註釋的行的開頭加入截圖中的兩行即可：

最後重啓tomcat就解決了問題。