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

Question

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

Answer 1

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;
   }