What is the difference between getDefaultInstance() and getInstance() in Session class?

问题

What is the difference between Session.getDefaultInstance(props, authenticator) and getInstance(props, authenticator)? In general, when will you choose one over the other?

I also read Java doc on getDefaultInstance(props, authenticator), but still couldn't able to make out the difference distinctly/clearly.

Hope experts can help me in understanding this better.

UPDATE: Actual reason that triggered to ask this question is: We've used Session.getDefaultInstance() method in some places within our web-based application. Sometimes, it throws java.lang.SecurityException: Access to default session denied, on quick googling, it suggested to use Session.getInstance() method instead. Hence, when one would choose one over the other?

回答1:

If you read the documentation, you will see that

getDefaultInstance Get the default Session object. If a default has not yet been setup, a new Session object is created and installed as the default.

Therefore, if one does not already exist, it call getInstance()

getInstance Get a new Session object.

So, a new session object is created, regardless of whether one already exists.

回答2:

FAQ says: https://javaee.github.io/javamail/FAQ#getdefaultinstance

Q: When should I use Session.getDefaultInstance and when should I use Session.getInstance?

A: Almost all code should use Session.getInstance. The Session.getDefaultInstance method creates a new Session the first time it's called, using the Properties that are passed. Subsequent calls will return that original Session and ignore any Properties you pass in. If you want to create different Sessions with different properties, Session.getDefaultInstance won't do that. If some other code in the same JVM (e.g., in the same app server) has already created the default Session with their properties, you may end up using their Session and your properties will be ignored. This often explains why your property settings seem to be ignored. Always use Session.getInstance to avoid this problem.

回答3:

Cause This error is raised in the getDefaultInstance method in javax.mail.Session.java. According to this source code, this error occures when the default session object is already initialized, but authenticator instance is renewed or changed, or the class loader of the default session object is different from the argument authentificator's. Maybe the java source code using the default session instance of the java mail is recompiled and reloaded, or duplicate javamail class libraries are included into the Classpath of the environment. it gives proper solution

javax.mail.Session.java file
   public static synchronized Session getDefaultInstance(Properties props,
                                       Authenticator authenticator) {
       if (defaultSession == null)
           defaultSession = new Session(props, authenticator);
       else {
           // have to check whether caller is allowed to see default session
           if (defaultSession.authenticator == authenticator)
               ;       // either same object or both null, either way OK
           else if (defaultSession.authenticator != null &&
                   authenticator != null &&
                   defaultSession.authenticator.getClass().getClassLoader() ==
                       authenticator.getClass().getClassLoader())
               ;       // both objects came from the same class loader, OK
           else
               // anything else is not allowed
               throw new SecurityException("Access to default session denied");
       }

       return defaultSession;
   }

回答4:

For me, it was very important to use getInstance() instead of getDefaultInstance().

Because after mail session properties was changed, mail session still was storing old properties.

So getDefaultInstance() - it is looks like Singleton.

As docs said:

Note also that the Properties object is used only the first time this method is called, when a new Session object is created. Subsequent calls return the Session object that was created by the first call, and ignore the passed Properties object. Use the getInstance method to get a new Session object every time the method is called.

来源：https://stackoverflow.com/questions/4184204/what-is-the-difference-between-getdefaultinstance-and-getinstance-in-session