I debugged for two hours and didn't change it, but it finally changed
After running tomcat, Artifact Example: Web exploded: Error during artifact deployment is reported, and the others are garbled, so they will not be posted.
Only provide my method
<url-pattern>hello</url-pattern> to <url-pattern>/hello</url-pattern>
It is caused by a slash
Then I explored why I should start with a forward slash,
It seems to be mainly a mapping specification, because
<url-pattern> is the root directory of the web program, which is the mapping of the servlet, and it needs to comply with the specification