Read-only files
If a file is read-only, it is marked with the closed lock icon 
in the status bar. If a file is writable, it is marked with the open lock icon 
.
Library files in IntelliJ IDEA are normally read-only by default as the code from them should be used as is. For such files, you will not see the lock icon in the status bar. IntelliJ IDEA applies Reader mode to library files by default.
Toggle read-only mode on and off
Open a file in the editor and click the lock icon
/ in the status bar.Alternatively, select a file in the Project tool window (Alt+1), go to in the main menu and click Make file read-only.
If a read-only status is set by a version control system, it is suggested that you use IntelliJ IDEA version control integration features. For more information, refer to Version control.