Problem solving about Eclipse not compiling project

"project" Menu

The Project menu can perform actions (build or compile) on items in the workbench.

name	function
Open Project (Project open)	displays a dialog box that lets you choose to open a closed item
Close Project (closing project)	closes the currently selected item
build All (built all)	this directive will perform incremental (incremental) compilation of all items in the workbench. That is, it compiles (compiles) all the resources that are affected by any resource changes in the workbench since the previous incremental compilation. This instruction is only available when automatic compilation is turned off.
build project (Compile project)	This instruction compiles an incremental (incremental) of the currently selected project. That is, it compiles (compiles) all the resources in the project that have been affected by any resource changes since the previous compilation. This instruction is only available when automatic compilation is turned off.
build workings Set (recompile working set)	This menu can perform incremental (incremental) compilation on the working set. That is, it compiles (compiles) all the resources in the work set that were affected by any resource changes after the previous compilation. This instruction is only available when automatic compilation is turned off.
Clean (Clear)	This instruction discards all previous compilation results. If automatic compilation is turned on, this will call the full compilation.
build automatically (automatic compilation)	Automatically compiles all items in the workspace. This instruction can toggle the automatic compilation preferences.

Eclipse has a problem with not compiling the project. How to refresh the classes directory is also empty.
The details are operational:
Open Project->build automatically after trial, or not compile.
Open the project--> build automatically another clean will probably compile, and the result is not.
Eclipse defaults to Automatic compilation. Even if you cancel the automatic compilation, the manual build works and cannot be compiled.

There are generally three reasons for this situation:
1. If the project leads to some unused jar bag, and the bag is deleted by you, there will be no error, but how can not compile the class file situation, you can remove all the packages, and then one after the introduction (need), not all of a sudden put all the bags are introduced, useless may cause undesirable consequences.
2. Want to delete a class file regeneration, remove the class file, but the Classes directory folder 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 and recompile.

3. There is also the case that the JRE System Library is removed and can be compiled by re-import. But what causes it is not clear.