package-info类解读

类不能带有public、private访问权限。package-info.java再怎么特殊，也是一个类文件，也会被编译成package-info.class，但是在package-info.java中只能声明默认访问权限的类，也就是友好类。

Javac中与package-info相关的命令有：

-Xpkginfo:[always,legacy,nonempty]

Control whether javac generates package-info.class files from package-info.java files. Possible mode arguments for this option include the following.

（1）always

Always generate a package-info.class file for every package-info.java file. This option may be useful if you use a build system such as Ant, which checks that each .java file has a corresponding .class file.

（2）legacy

Generate a package-info.class file only if package-info.java contains annotations. Don't generate a package-info.class file if package-info.java only contains comments.

Note: A package-info.class file might be generated but be empty if all the annotations in the package-info.java file have RetentionPolicy.SOURCE.

（3）nonempty

Generate a package-info.class file only if package-info.java contains annotations with RetentionPolicy.CLASS or RetentionPolicy.RUNTIME.