First enter the API download official website: https://www.oracle.com/java/technologies/javase-downloads.html
Then pull down and find the version you need. Click Documentation Download
as shown in the figure below. I chose the version of 8u251.
After entering the download page, choose to download according to your needs
as shown in the figure below. I entered the download page of the 8u251 version, and there are jdk reference documents and Refer to the javafx document and choose to download according to your needs.
After you click the download, the following message will pop up.
Tick agree and click Download to download successfully