When I try to output javadoc from eclipse, I sometimes get the error "javadoc error --input length = 1 cannot be read", but the solution did not come out even after searching, so the method I solved Is described here.
In my case, I specify the javadoc command when outputting javadoc, but the Java version of the javadoc command and the Java version of the project are different. Modifying this to specify the Java version of the project's javadoc command no longer causes the error.
This may be too obvious, but when I tried to fix an old Java version of a Java project with the latest eclipse, it was a blind spot because the javadoc command was set in the latest JDK by default.