Eclipse does not compile the project phenomenon

The following actions will resolve most of the situation:
Open Project->build automatically try not to, or do not compile.
Open Project--build automatically another clean will probably compile and the results will not work.
Eclipse is automatically compiled by default. Even if you cancel the automatic compilation, the manual build project will not compile.

There are three general reasons why this happens:
1. If the project led to an unused jar package, and the package is deleted by you, it will appear without error, but how to compile the case of the class file, you can delete all the packages, and then one by one to introduce (need), do not bring all the package all at once, useless may cause undesirable consequences.
2. Want to delete a class file regeneration, after deleting the class file, but the folder under the Classes directory is opened by other programs, such as Total Commander. At this point the compilation will not pass, under problems may prompt "Con ' t delete classes ...". Turn off other programs to recompile.

3. There is also a situation where the JRE System Library is removed and re-imported to compile. But what causes it is unclear. (I have met several times)

Eclipse does not compile the project phenomenon