-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，且多於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
| Key | Meaning |
|---|---|
"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 |