可替换系统（alternatives system）的应用浅议
[root@BlueSky tv2.0]# javaUsage: gij [OPTION] … CLASS [ARGS] … to invoke CLASS.main, or gij –jar [OPTION] … JARFILE [ARGS] … to execute a jar fileTry `gij —help‘ for more information.于是：
[root@BlueSky tv2.0]# which java/usr/bin/java[root@BlueSky software]# ls /usr/bin/java –llrwxrwxrwx 1 root root 22 01–15 21:23 /usr/bin/java -> /etc/alternatives/java[root@BlueSky software]# ll /etc/alternatives/javalrwxrwxrwx 1 root root 35 05–10 13:48 /etc/alternatives/java -> /usr/lib/jvm/jre–1.4.2–gcj/bin/java这说明java是受fc下同一软件的版本控制工具alternatives控制的，下面就是用这个工具将系统自带的GNU的java换成Sun的java版本；
查看alternatives的帮助；
[root@BlueSky software]# alternatives —help替换选择(alternatives)的版本 1.3.30 – 版权 (C) 1997–2000 Red Hat, Inc.在 GNU 公共许可的条款下，本软件可以被自由发行。用法：alternatives —install <link> <name> <path> <priority> [—initscript <service>] [—slave <link> <name> <path>]* alternatives —remove <name> <path> alternatives —auto <name> alternatives —config <name> alternatives —display <name> alternatives —set <name> <path>公用选项：—verbose —test —help —usage —version —altdir <directory> —admindir <directory>并通过实验得到，install是安装同一软件的不同版本，remove则是移出某个版本，config则是选择当前系统使用那个版本，而display则是显示当前使用的版本，auto则是更改当前的状态为自动判断，也就是可以恢复GNU版本java的一个参数；
下面先安装jre1.5.0_09，此版本可以到sun的公司网站上下得；
[root@BlueSky software]# rpm –ivh jre–1_5_0_09–linux–i586.rpm Preparing… ########################################### [100%] 1:jre ########################################### [100%]检查是否安装成功：
[root@BlueSky software]# /usr/java/jre1.5.0_09/bin/javaUsage: java [–options] class [args…] (to execute a class) or java [–options] –jar jarfile [args…] (to execute a jar file)where options include: –d32 use a 32–bit data model if available –d64 use a 64–bit data model if available –client to select the “client“ VM –server to select the “server“ VM –hotspot is a synonym for the “client“ VM [deprecated] The default VM is client. –cp <class search path of directories and zip/jar files> –classpath <class search path of directories and zip/jar files> A : separated list of directories, JAR archives, and ZIP archives to search for class files. –D<name>=<value> set a system property –verbose[:class|gc|jni] enable verbose output –version print product version and exit –version:<value> require the specified version to run –showversion print product version and continue –jre–restrict–search | –jre–no–restrict–search include/exclude user private JREs in the version search -? –help print this help message –X print help on non–standard options –ea[:<packagename>…|:<classname>] –enableassertions[:<packagename>…|:<classname>] enable assertions –da[:<packagename>…|:<classname>] –disableassertions[:<packagename>…|:<classname>] disable assertions –esa | –enablesystemassertions enable system assertions –dsa | –disablesystemassertions disable system assertions –agentlib:<libname>[=<options>] load native agent library <libname>, e.g. –agentlib:hprof see also, –agentlib:jdwp=help and –agentlib:hprof=help –agentpath:<pathname>[=<options>] load native agent library by full pathname –javaagent:<jarpath>[=<options>] load Java programming language agent, see java.lang.instrument出现这些字样，表明jre安装成功了；
接着，使用alternatives工具增加这个版本；
[root@BlueSky software]# alternatives —install /usr/bin/java java /usr/java/jre1.5.0_09/bin/java 300配置：
[root@BlueSky software]# alternatives —config java共有 2 个程序提供“java”。 选择 命令———————————————–* 1 /usr/lib/jvm/jre–1.4.2–gcj/bin/java 2 /usr/java/jre1.5.0_09/bin/java按 Enter 来保存当前选择[+]，或键入选择号码：2[root@BlueSky software]# javaUsage: java [–options] class [args…] (to execute a class) or java [–options] –jar jarfile [args…] (to execute a jar file)where options include: –d32 use a 32–bit data model if available –d64 use a 64–bit data model if available –client to select the “client“ VM –server to select the “server“ VM –hotspot is a synonym for the “client“ VM [deprecated] The default VM is client. –cp <class search path of directories and zip/jar files> –classpath <class search path of directories and zip/jar files> A : separated list of directories, JAR archives, and ZIP archives to search for class files. –D<name>=<value> set a system property –verbose[:class|gc|jni] enable verbose output –version print product version and exit –version:<value> require the specified version to run –showversion print product version and continue –jre–restrict–search | –jre–no–restrict–search include/exclude user private JREs in the version search -? –help print this help message –X print help on non–standard options –ea[:<packagename>…|:<classname>] –enableassertions[:<packagename>…|:<classname>] enable assertions –da[:<packagename>…|:<classname>] –disableassertions[:<packagename>…|:<classname>] disable assertions –esa | –enablesystemassertions enable system assertions –dsa | –disablesystemassertions disable system assertions –agentlib:<libname>[=<options>] load native agent library <libname>, e.g. –agentlib:hprof see also, –agentlib:jdwp=help and –agentlib:hprof=help –agentpath:<pathname>[=<options>] load native agent library by full pathname –javaagent:<jarpath>[=<options>] load Java programming language agent, see java.lang.instrument如此，成功；
查看效果：
[root@BlueSky software]# alternatives —display javajava – 状态是手工。 链接目前指向 /usr/java/jre1.5.0_09/bin/java/usr/lib/jvm/jre–1.4.2–gcj/bin/java – 优先度 1420 从 keytool：/usr/lib/jvm/jre–1.4.2–gcj/bin/keytool 从 rmiregistry：/usr/lib/jvm/jre–1.4.2–gcj/bin/rmiregistry 从 jre_exports：/usr/lib/jvm–exports/jre–1.4.2–gcj 从 jre：/usr/lib/jvm/jre–1.4.2–gcj/usr/java/jre1.5.0_09/bin/java – 优先度 300 从 keytool：(null) 从 rmiregistry：(null) 从 jre_exports：(null) 从 jre：(null)当前“最佳”版本是 /usr/lib/jvm/jre–1.4.2–gcj/bin/java。结论，alternaties这个工具还是不错的。。。
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