最近升級了MAC OS，裝了JDK7 for mac，在這裡下載JDK7 for mac，裝完之後發現在預設的路徑下找不到JDK7的HOME，如下所示：
$ which java
/usr/bin/java
$ ls -l /usr/bin/java
lrwxr-xr-x 1 root wheel 74 Jan 4 20:24 /usr/bin/java -> /System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/java
$ ls -l /System/Library/Frameworks/JavaVM.framework/Versions
total 64
lrwxr-xr-x 1 root wheel 10 Jan 4 20:24 1.4 -> CurrentJDK
lrwxr-xr-x 1 root wheel 10 Jan 4 20:24 1.4.2 -> CurrentJDK
lrwxr-xr-x 1 root wheel 10 Jan 4 20:24 1.5 -> CurrentJDK
lrwxr-xr-x 1 root wheel 10 Jan 4 20:24 1.5.0 -> CurrentJDK
lrwxr-xr-x 1 root wheel 10 Jan 4 20:24 1.6 -> CurrentJDK
lrwxr-xr-x 1 root wheel 10 Jan 4 20:24 1.6.0 -> CurrentJDK
drwxr-xr-x 7 root wheel 238 Jan 4 20:25 A
lrwxr-xr-x 1 root wheel 1 Jan 4 20:24 Current -> A
lrwxr-xr-x 1 root wheel 59 Jan 4 20:24 CurrentJDK -> /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents
可是檢視java的版本確實是1.7，如下：
$ java -version
java version "1.7.0_45"
Java(TM) SE Runtime Environment (build 1.7.0_45-b18)
Java HotSpot(TM) 64-Bit Server VM (build 24.45-b08, mixed mode)
google了一下，發現了這篇文章Important Java Directories on Mac OS X，可以使用工具命令“/usr/libexec/java_home”來定位JAVA_HOME，這下爽了：
$ cat ~/.profile
JAVA_HOME=`/usr/libexec/java_home`
export JAVA_HOME
$ echo $JAVA_HOME
/Library/Java/JavaVirtualMachines/jdk1.7.0_45.jdk/Contents/Home
另外可以使用-V選項列出所有版本的JAVA_HOME：
$ /usr/libexec/java_home -V
Matching Java Virtual Machines (3):
1.7.0_45, x86_64: "Java SE 7" /Library/Java/JavaVirtualMachines/jdk1.7.0_45.jdk/Contents/Home
1.6.0_65-b14-462, x86_64: "Java SE 6" /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home
1.6.0_65-b14-462, i386: "Java SE 6" /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home
/Library/Java/JavaVirtualMachines/jdk1.7.0_45.jdk/Contents/Home