java -jar xxx.jar params 的形式用了這麼多年，都沒在意原來java的執行命令可以多元的。
這兩天剛好在看rocketmq，順便看了下它的shell腳本，然後才發現原來我這麼多年的java -jar有多麼的無知。
先來一段命令：
java -Djava.ext.dirs=$ROCKETMQ_HOME/lib -cp $ROCKETMQ_HOME/lib/rocketmq-example-3.5.8.jar com.alibaba.rocketmq.example.simple.Producer
這段命令是爲了測試rocketmq的消息產生類產生消息。
於是回頭再看一下java -help，瞬間知道了。那我們許多的工程寫法，其實都不用糾結java打包的啓動類是否在打包時候已經打入，也不用糾結一個jar包有N多個入口。
2012年時候我在某個項目做一個二級中心的時候，那時候爲了一個工程中的三個入口都被調用到，同一套代碼，配置了三個META-INF，裏面配置了入口類，同時打了三個jar，如果改動代碼再打包，費時費力。
現在回想，如果現在來做，就是打一個jar包，編三個shell腳本，瞬間解決一切。那些創造這些事務的人真是天才，使用的人可能就需要無盡的學習。
[rocketmq@cent65-1 ~]$ java -help
Usage: 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
-server to select the "server" VM
The default VM is server,
because you are running on a server-class machine.
-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>
Warning: this feature is deprecated and will be removed
in a future release.
require the specified version to run
-showversion print product version and continue
-jre-restrict-search | -no-jre-restrict-search
Warning: this feature is deprecated and will be removed
in a future release.
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 with specified granularity
-da[:<packagename>...|:<classname>]
-disableassertions[:<packagename>...|:<classname>]
disable assertions with specified granularity
-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
-splash:<imagepath>
show splash screen with specified image
See http://www.oracle.com/technetwork/java/javase/documentation/index.html for more details.