When to Use Servlet only, EJB only and, both Servlet And EJB???

I have a question, If I want to build a Web application. Sometime i found that there is no need to use EJB as Servlet already provide that functionality.
Can anyone tell me when do I use Pure Servlet? When do I use Pure EJB and, When do I use Both EJB and Servlet???
I am very interesting in it.
Thanks.
Jack

EJB and servlets are two different things. Trying to compare them doesn't really make sense. A web application is always going to need some way to process Http requests and this in Java is done accomplished through Servlets (or jsp pages which in turn also use servlets).
EJBs are the basis of distributed cross-platform objects. They are handled through name services and stored in databases. An Entity Bean can be "brought back to life" as long as it's info is stored in a database. Regardless of a server being restarted. All this database support behind an Entity Bean (transactions, queries etc.) are very transparent to the programmer. They are provided by the EJB vendor.
The class properties of a Servlet (or class-level member variables) are shared among all requests (running thread instances of the service method). That's why they should be used with a lot of caution. This feature can be very useful but if used improperly can also be very vulnerable to security and synchronization issues.
I'm not an EJB expert but this explanation should give you some idea of what EJBs are all about.

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06/11/2014 22:22:03,507
QuickTime Player[1723]
postExceptionEvent caught non-fatal NSInvalidArgumentException '*** -[NSDictionary initWithObjects:forKeys:]: count of objects (0) differs from count of keys (4)' with backtrace (
0   CoreFoundation                      0x00007fff8ac6564c __exceptionPreprocess + 172
1   libobjc.A.dylib                     0x00007fff8924f6de objc_exception_throw + 43
2   CoreFoundation                      0x00007fff8ab98dcd -[NSDictionary initWithObjects:forKeys:] + 941
3   Foundation                          0x00007fff8a6a8bcc -[NSDictionary(NSDictionary) initWithCoder:] + 245
4   Foundation                          0x00007fff8a686150 _decodeObjectBinary + 2872
5   Foundation                          0x00007fff8a6854a6 _decodeObject + 278
6   Foundation                          0x00007fff8a6a9921 -[NSKeyedUnarchiver decodeObjectOfClasses:forKey:] + 173
7   Foundation                          0x00007fff8a75f2b4 -[NSError initWithCoder:] + 253
8   Foundation                          0x00007fff8a686150 _decodeObjectBinary + 2872
9   Foundation                          0x00007fff8a6854a6 _decodeObject + 278
10 Foundation                          0x00007fff8a6ab0c9 +[NSKeyedUnarchiver unarchiveObjectWithData:] + 89
11 QuickTime Player                    0x000000010dcd8253 QuickTime Player + 758355
12 QuickTime Player                    0x000000010dc8981f QuickTime Player + 436255
13 QuickTime Player                    0x000000010dc894fb QuickTime Player + 435451
14 QuickTime Player                    0x000000010dc6ce48 QuickTime Player + 319048
15 AppKit                              0x00007fff93c5d75a __85-[NSDocument runModalSavePanelForSaveOperation:delegate:didSaveSelector:contextInfo:]_block_ invoke1714 + 113
16 AppKit                              0x00007fff93c4efca -[NSDocument continueActivityUsingBlock:] + 323
17 AppKit                              0x00007fff93c5d5cb __85-[NSDocument runModalSavePanelForSaveOperation:delegate:didSaveSelector:contextInfo:]_block_ invoke_21706 + 458
18 AppKit                              0x00007fff93c5d0ec __85-[NSDocument runModalSavePanelForSaveOperation:delegate:didSaveSelector:contextInfo:]_block_ invoke1690 + 1365
19 AppKit                              0x00007fff940903b6 -[NSVBSavePanel didEndPanelWithReturnCode:] + 68
20 AppKit                              0x00007fff9409323d -[NSVBSavePanel didEndSheet:returnCode:contextInfo:] + 154
21 AppKit                              0x00007fff93f718d9 -[NSWindow _endWindowBlockingModalSession:returnCode:] + 315
22 AppKit                              0x00007fff9409307c -[NSVBSavePanel observeValueForKeyPath:ofObject:change:context:] + 560
23 Foundation                          0x00007fff8a677c73 NSKeyValueNotifyObserver + 382
24 Foundation                          0x00007fff8a676e48 NSKeyValueDidChange + 463
25 Foundation                          0x00007fff8a67b9ad -[NSObject(NSKeyValueObserverNotification) didChangeValueForKey:] + 118
26 ViewBridge                          0x00007fff959cbbd7 -[NSViewBridge nonLocalChangeInProgress:block:] + 382
27 ViewBridge                          0x00007fff95a15766 -[NSRemoteViewMarshal exceptionSafeSetRemoteObject:forKey:withReply:] + 275
28 ViewBridge                          0x00007fff95a158f1 -[NSRemoteViewMarshal setRemoteObject:forKey:withReply:] + 56
29 CoreFoundation                      0x00007fff8ab3e33c __invoking___ + 140
30 CoreFoundation                      0x00007fff8ab3e192 -[NSInvocation invoke] + 290
31 ViewBridge                          0x00007fff95a1bce3 __deferNSXPCInvocationOntoMainThread_block_invoke + 85
32 ViewBridge                          0x00007fff95a1b91e __deferBlockOntoMainThread_block_invoke_2 + 390
33 CoreFoundation                      0x00007fff8ab8354c __CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__ + 12
34 CoreFoundation                      0x00007fff8ab75655 __CFRunLoopDoBlocks + 341
35 CoreFoundation                      0x00007fff8ab75196 __CFRunLoopRun + 1814
36 CoreFoundation                      0x00007fff8ab74838 CFRunLoopRunSpecific + 296
37 HIToolbox                           0x00007fff8cb6f43f RunCurrentEventLoopInMode + 235
38 HIToolbox                           0x00007fff8cb6f1ba ReceiveNextEventCommon + 431
39 HIToolbox                           0x00007fff8cb6effb _BlockUntilNextEventMatchingListInModeWithFilter + 71
40 AppKit                              0x00007fff9383b821 _DPSNextEvent + 964
41 AppKit                              0x00007fff9383afd0 -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] + 194
42 AppKit                              0x00007fff9382ef73 -[NSApplication run] + 594
43 AppKit                              0x00007fff9381a424 NSApplicationMain + 1832
44 libdyld.dylib                       0x00007fff8ec845c9 start + 1
timestamp: 22:22:03.507 Thursday 06 November 2014
process/thread/queue: QuickTime Player (1723) / 0x7fff78cd0300 / com.apple.main-thread
code: line 844 of /SourceCache/ViewBridge/ViewBridge-99/ViewBridgeUtilities.m in logCaughtException
#exceptions
06/11/2014 22:22:03,525
com.apple.appkit.xpc.openAndSavePanelService[1748]
remote view failed to set bridge object {
    URL = "file:///Users/pascalgros/Documents/Videos%20Maurice%202013/Exports/M2U01020.m4 v";
    URLs =     (
        "file:///Users/pascalgros/Documents/Videos%20Maurice%202013/Exports/M2U01020.m4 v"
    index = 1;
    result = 1;
} for key most-recent-completion due to Error Domain=com.apple.ViewBridge Code=8 "L’opération n’a pas pu s’achever. (com.apple.ViewBridge erreur 8)." UserInfo=0x61000006ebc0 {com.apple.ViewBridge.error.hint=most-recent-completion, com.apple.ViewBridge.error.description=NSViewBridgeExceptionError}; service call stack was (
0   ViewBridge                          0x00007fff959e8b30 -[NSViewServiceMarshal setRemoteObject:forKey:] + 341
1   Foundation                          0x00007fff8a677c73 NSKeyValueNotifyObserver + 382
2   Foundation                          0x00007fff8a676e48 NSKeyValueDidChange + 463
3   Foundation                          0x00007fff8a67b9ad -[NSObject(NSKeyValueObserverNotification) didChangeValueForKey:] + 118
4   com.apple.appkit.xpc.openAndSavePanelService 0x000000010a661407 com.apple.appkit.xpc.openAndSavePanelService + 9223
5   AppKit                              0x00007fff93dead5a -[NSSavePanel _dismissModeless:] + 109
6   com.apple.appkit.xpc.openAndSavePanelService 0x000000010a66191c com.apple.appkit.xpc.openAndSavePanelService + 10524
7   AppKit                              0x00007fff93ded580 -[NSSavePanel ok:] + 449
8   com.apple.appkit.xpc.openAndSavePanelService 0x000000010a661952 com.apple.appkit.xpc.openAndSavePanelService + 10578
9   libsystem_trace.dylib               0x00007fff8d3ebcd7 _os_activity_initiate + 75
10 AppKit                              0x00007fff93a0a5e7 -[NSApplication sendAction:to:from:] + 410
11 AppKit                              0x00007fff93a0a410 -[NSControl sendAction:to:] + 86
12 AppKit                              0x00007fff93bdedaf __26-[NSCell _sendActionFrom:]_block_invoke + 131
13 libsystem_trace.dylib               0x00007fff8d3ebcd7 _os_activity_initiate + 75
14 AppKit                              0x00007fff93a52f2c -[NSCell _sendActionFrom:] + 144
15 AppKit                              0x00007fff93bc5ab2 -[NSButtonCell _sendActionFrom:] + 39
16 libsystem_trace.dylib               0x00007fff8d3ebcd7 _os_activity_initiate + 75
17 AppKit                              0x00007fff93a6da66 -[NSCell trackMouse:inRect:ofView:untilMouseUp:] + 2731
18 AppKit                              0x00007fff93a6ccc1 -[NSButtonCell trackMouse:inRect:ofView:untilMouseUp:] + 491
19 AppKit                              0x00007fff93a6c289 -[NSControl mouseDown:] + 714
20 AppKit                              0x00007fff93f65fef -[NSWindow _reallySendEvent:] + 12827
21 AppKit                              0x00007fff939f065c -[NSWindow sendEvent:] + 368
22 AppKit                              0x00007fff939a21e6 -[NSApplication sendEvent:] + 2238
23 ViewBridge                          0x00007fff959d18ab -[NSViewServiceApplication sendEventWithoutCatch:withForwarding:] + 652
24 ViewBridge                          0x00007fff959d1fbd -[NSViewServiceApplication sendEvent:withForwarding:] + 29
25 AppKit                              0x00007fff9382efe8 -[NSApplication run] + 711
26 AppKit                              0x00007fff9381a424 NSApplicationMain + 1832
27 libxpc.dylib                        0x00007fff88837ef2 _xpc_objc_main + 793
28 libxpc.dylib                        0x00007fff88839a9d _parse_sysctl_once + 0
29 ViewBridge                          0x00007fff95a20165 xpc_connection_handler + 0
30 ViewBridge                          0x00007fff959d4ff7 NSViewServiceApplicationMain + 1892
31 com.apple.appkit.xpc.openAndSavePanelService 0x000000010a665035 main + 208
32 libdyld.dylib                       0x00007fff8ec845c9 start + 1
33 ???                                 0x0000000000000001 0x0 + 1
timestamp: 22:22:03.525 Thursday 06 November 2014
process/thread/queue: com.apple.appkit.xpc.openAndSavePanelService (1748) / 0x7fff78cd0300 / com.apple.main-thread
code: line 3847 of /SourceCache/ViewBridge/ViewBridge-99/NSViewServiceMarshal.m in __47-[NSViewServiceMarshal setRemoteObject:forKey:]_block_invoke_2
#communications_failure

Hi,
have you tried to connect on the command line (instead of using R3trans) ?
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Best regards,
Jens

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