Using the habit of Eclipse, you haven't started using Maven yet, and using idea is a bit of a habit, and now record it for forgetting.
/* This step is actually not working when Tomcat uses external source **/a. Ctrl+alt+s open settings→build,execution, Deployments→compiler: Tick make Pr Oject automatically
B. Click F4 on the project to open the project configuration interface
C. projects:project compiler output modified to Yourprojectpathweb-inf\classes
d.modules→ Select the Web-inf\classes folder, click on the Exclude above it to exclude the classes from the project
E. run→debug configuration Open the Tomcat config interface, select deployments to remove Projectname:war exploded, add external source, select the directory where the project resides
After you run Tomcat, if you modify the class, you can ctrl+f9 to make the class reload effective. Changes to the JSP file will take effect directly.
If you want to modify the class can also be automatically loaded after modification, it is not possible to use the form of external source deployment, you need to use Artifats, and then in the Tomcat server interface put on ' Update ' action and on frame Deactivation action is modified to reload classes and resources. In this case, you need to specify a compiled output directory separately, using the idea of maven.
Idea Tomcat +eclipse-type deployment