Packages and Reflection in an Applet


R

Richard Maher

Hi,

I have an unsigned Applet that needs to obtain a Username/Password from the
user (via generic dialog boxes outside of the web-page) and optionally, on
successful login, display some "welcome" info from the server. To that end,
I have come up with a rudimentary AWT interface that pops-up a couple of
modal dialogue boxes. (There is also a "status" window that has bytes
received/sent etc).

So, I have the default, fallback, rough-as-guts option for obtaining user
credentials but would like to allow the consumers of my Applet to override
the default and supply their own much slicker +/- localized UI. So I have
the simple interface below that each prospective UI class must implement but
I don't know what part packages/protection play in this scenario as well as
reflection. If there are any people here with direct experience of the
following conditions/questions I would be very grateful for their advice: -

1) Can a class loaded via Reflection (eg: myInter localGUI =
(myInter)(class.forName("appletParameter").newInstance())) be in the same
Package, and enjoy the benefits/protection of same, as the caller? Ideally
here, the methods/variables/properties of the newly loaded class (albeit
under the restriction of being loaded from the same codebase) being package
private and not public? Again ideally, I do not want the methods in this GUI
to be accessible from JavaScript but obviously accessible from my applet.

2) I'd expect the "tier3Client" implementing class (and any additional
classes) to be supplied in a separate archive/JAR file and this will be
located via the "archive=a,b,c" specifier (with "codebase_lookup" set to
false). My "understanding" is that class.forName() will use the same
classloader as the Applet and therefore I'm hoping that it will happily
search through all specified JAR files looking for the requested class?

3) Are there any other restrictions/requirements or wil this just not work?

Cheers Richard Maher

/**
* Copyright Tier3 (c) Software. All rights reserved.
*
* @author Richard Maher
* @version 1.0
*
*/

package tier3Client;

interface Tier3UserDialog {

boolean doCredentials();

String getUsername();

String getPassword();

boolean isWelcomeRequired();

void doWelcome(byte[] t3IdBuf);

void doConsole(byte[] t3IdBuf,
Tier3Logoff sourceSession,
String codeHost,
int portNum);

void updateRcvd(int bytes);

void updateSent(int bytes);

void showRefCnt(int pages);

void appendConsoleMsg(String msg, int color);

void appendConsoleMsg(String msg);

void closeConsole();
}
 
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A

Arne Vajhøj

I have an unsigned Applet that needs to obtain a Username/Password from the
user (via generic dialog boxes outside of the web-page) and optionally, on
successful login, display some "welcome" info from the server. To that end,
I have come up with a rudimentary AWT interface that pops-up a couple of
modal dialogue boxes. (There is also a "status" window that has bytes
received/sent etc).

Why not use Swing instead of AWT?

Or maybe even do it the Java 8 way with JavaFX?
So, I have the default, fallback, rough-as-guts option for obtaining user
credentials but would like to allow the consumers of my Applet to override
the default and supply their own much slicker +/- localized UI. So I have
the simple interface below that each prospective UI class must implement but
I don't know what part packages/protection play in this scenario as well as
reflection. If there are any people here with direct experience of the
following conditions/questions I would be very grateful for their advice: -

1) Can a class loaded via Reflection (eg: myInter localGUI =
(myInter)(class.forName("appletParameter").newInstance())) be in the same
Package, and enjoy the benefits/protection of same, as the caller? Ideally
here, the methods/variables/properties of the newly loaded class (albeit
under the restriction of being loaded from the same codebase) being package
private and not public? Again ideally, I do not want the methods in this GUI
to be accessible from JavaScript but obviously accessible from my applet.

The loaded class can certainly be in the same package as the caller.
2) I'd expect the "tier3Client" implementing class (and any additional
classes) to be supplied in a separate archive/JAR file and this will be
located via the "archive=a,b,c" specifier (with "codebase_lookup" set to
false). My "understanding" is that class.forName() will use the same
classloader as the Applet and therefore I'm hoping that it will happily
search through all specified JAR files looking for the requested class?

If you do not specify a classloader in the forName call (there is an
overload for that), then you will get the current classes classloader.
3) Are there any other restrictions/requirements or wil this just not work?

Will their jar and your jar be loaded from the same location?

Arne
 
M

markspace

classes) to be supplied in a separate archive/JAR file and this will be
located via the "archive=a,b,c" specifier (with "codebase_lookup" set to
false). My "understanding" is that class.forName() will use the same
classloader as the Applet and therefore I'm hoping that it will happily
search through all specified JAR files looking for the requested class?


There *are* restrictions when running an applet regarding classloaders
and class loading. Basically the plug-in won't let you do anything that
*might* be a threat to Java applet security. I don't know if this
applies to your current situation however. I'd just make certain to
have a backup plan in case this one ends up not working as well as you'd
hoped.
 
R

Richard Maher

Hi Arne,

Thanks as always for your replies.

Arne Vajhøj said:
Why not use Swing instead of AWT?

That's the whole point. If someone wants to use Swing then I'm happy to
plug-in the results. (Or a localized/foreign-language version, or a.n.other)
Or maybe even do it the Java 8 way with JavaFX?

I'm sure JavaFX is appropriate for those who see fit to use it but it's a
non-issue here.
The loaded class can certainly be in the same package as the caller.

Cool. So they just point their classpath at my interface bearing JAR an it
should all be good/doable.

I guess I'll have some Manifest issues getting the/my original JAR to look
for the new gui.JAR or with the Multi-JAR codebase/archive Object attributes
sort the classpath out automagically?
If you do not specify a classloader in the forName call (there is an
overload for that), then you will get the current classes classloader.


Will their jar and your jar be loaded from the same location?
Yep.


Arne

Cheers Richard Maher
 
R

Richard Maher

Hi Mark,

markspace said:
There *are* restrictions when running an applet regarding classloaders and
class loading. Basically the plug-in won't let you do anything that
*might* be a threat to Java applet security. I don't know if this applies
to your current situation however.

I'm not asking for another/different classloader and the loadee class is at
the same codebase so I think/dream I'm good to go.
I'd just make certain to have a backup plan in case this one ends up not
working as well as you'd hoped.

Yeah, it's probably a version 2 nice to have. The fallback plan is it all
works with a vanilla AWT interface right now. I'm guessing ProGuard
configuration could also be an issue? "-keep theInterfaces" and --keep only
th emethods that implement the interfaces in a munged class? (The 3rd party
would implement an interface that callsback into other class instance
methods in the Applet. (eg: Tier3Logoff from a console button)

Anyway, I'm too happy about the threading/multiplexing model working now to
worry about customization/localization.
Cheers Richard Maher
 
L

Lew

Richard said:
Hi Mark,

"markspace"<[email protected]> wrote ...

His name isn't "Mark".

His handle is "markspace". Not "Mark Space".

Some posters use handles instead of names.

In such cases it's not effective to use only part of the handle, because it
isn't a name. Furthermore, if "markspace" were his name, you still have to
make a leap to assume that "Mark" is a valid intimate form. It'd be like
saying, "Hi, Le" to me.

Regardless, in markspace's particular case, his name isn't "Mark", so it's
better to greet him with "Hi, markspace."

(FYI, conventional English grammar rules mandate a comma to set off a vocative.)

FWIW, "markspace" is a quintessentially geekish handle, referring as it does
to the old days of teletypes. Full props.
 
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M

markspace

FWIW, "markspace" is a quintessentially geekish handle, referring as it
does to the old days of teletypes. Full props.


As far as I know, it's much older than that. In telecommunications it
originally referred to Morse code, literally the marks and spaces on a
telegraph tape.

More recently it was still used in various signaling to refer to encoded
1's and 0' in electronic transmissions like the old phone modem encoding
(300 baud, 1200 baud, etc.). I'm not sure what the current state of the
art is, since I haven't done anything that low-level since my course
work 20 years ago.
 
G

Gene Wirchenko

As far as I know, it's much older than that. In telecommunications it
originally referred to Morse code, literally the marks and spaces on a
telegraph tape.

More recently it was still used in various signaling to refer to encoded
1's and 0' in electronic transmissions like the old phone modem encoding
(300 baud, 1200 baud, etc.). I'm not sure what the current state of the
art is, since I haven't done anything that low-level since my course
work 20 years ago.

It was 20 milliamp.

Sincerely,

Gene Wirchenko
 
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A

Arne Vajhøj

Cool. So they just point their classpath at my interface bearing JAR an it
should all be good/doable.

I guess I'll have some Manifest issues getting the/my original JAR to look
for the new gui.JAR or with the Multi-JAR codebase/archive Object attributes
sort the classpath out automagically?

I think specifying multi jar in the HTML is the way to go.

That avoids some potential issues.

Arne
 

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