D<name>=<value>? ：? set a system property??設置系統屬性。

官方解釋：

Set a system property value. If value is a string that contains spaces, you must enclose the string in double quotes:

1.java -D參數簡化加入多個jar

??????java命令引入jar時可以-cp參數，但時-cp不能用通配符(多個jar時什么煩要一個個寫,不能*.jar)，面通常的jar都在同一目錄，且多于1個。）-Djava.ext.dirs 就可以解決 多個jar的問題

如：

java -Djava.ext.dirs=lib MyClass

?

2.可以在運行前配置一些屬性，比如其他屬性

java -Dconf1="wqbin" Myclass

可以通過System.getProperty("conf1");獲得這個值

也可以從idea上看：

?

?

3.補充Standard System Properties

?

?

KeyMeaning

"file.separator"	Character that separates components of a file path. This is "/" on UNIX and "\" on Windows.
"java.class.path"	Path used to find directories and JAR archives containing class files. Elements of the class path are separated by a platform-specific character specified in the?path.separator?property.
"java.home"	Installation directory for Java Runtime Environment (JRE)
"java.vendor"	JRE vendor name
"java.vendor.url"	JRE vender URL
"java.version"	JRE version number
"line.separator"	Sequence used by operating system to separate lines in text files
"os.arch"	Operating system architecture
"os.name"	Operating system name
"os.version"	Operating system version
"path.separator"	Path separator character used in?java.class.path
"user.dir"	User working directory
"user.home"	User home directory
"user.name"	User account name ?

?

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