为什么JavaMail使用System.getProperties（）？

Question

Why do we need to add the properties like

Properties props = System.getProperties();
    props.put("mail.smtp.starttls.enable", "true"); // added this line
    props.put("mail.smtp.host", host);
    props.put("mail.smtp.user", from);
    props.put("mail.smtp.password", pass);
    props.put("mail.smtp.port", "587");


Session session = Session.getDefaultInstance(props, null);

to the system properties to send a mail. Why should it be specifically system properties?

Answer 1

You don't actually need to add them to the system properties.

If you create a new Properties instance and populate it with your attributes it will still work just the same.

Answer 2

As others have said, they don't need to be system properties. But, the following may be the reason why many examples show it this way: The Java Mail package supports a large number of settings/debug options. For example, https://javamail.java.net/nonav/docs/api/com/sun/mail/smtp/package-summary.html lists 50 different settings for the SMTP provider alone.

Suppose you want to set this option: "mail.smtp.ssl.checkserveridentity". If you use the System properties as your starting point, then you can restart your Java process with

-Dmail.smtp.ssl.checkserveridentity=true 

to change the option. If you build up your Properties object yourself from scratch, then you might need a code change to set the option.

Answer 3

They do NOT need to be System Properties. They can be java.util.Properties .