The Connected Limited Device Configuration - Java 2 Micro Edition Application Development - page 26

• User-defined class loaders cannot be created, in order to prevent programmers from
overriding the class loading mechanism provided by the VM.
For PDAP, an additional security layer allows the user to grant applications access permissions
such as network and personal information management access. By default, PDAP applications are
not allowed to establish network connections or access information stored in the device address
database or calendar. However, such access can be allowed via the application manager.
Off-Device Preverification
The preverification step was discussed in Chapter 1 . This additional step is applied to Java class
files after they are generated from the corresponding Java source files. The reason for introducing
this additional step, which leads to much confusion for developers, was that the original class file
verification performed by the JVM was very expensive in terms of memory and computational
power. Basically, the preverification step enriches the class file with hints for the on-device
verifier. Thus, the final verification can be performed more efficiently.
Please note that preverification does not mean less security. If you think of the verification process
as confirming that a way exists through a labyrinth, then the preverification step marks the way.
The way can still be verified in the device, and if the way is not valid, verification will detect that;
but preverification lets you avoid the much greater effort involved in finding the way.
General Device Hardware Limitations
General hardware limitations of CLDC devices are
• Limited computing capabilities
• Limited memory
• Limited heap space
The computing power of processors used for mobile phones is usually very limited when
compared to desktop systems. Also the memory provided by CLDC devices is very limited. Even
worse, for many devices, there is a distinction between persistent (flash) memory and the heap
space available, and the heap space usually is only a small fraction of the total memory (32-
512KB). So the memory available at runtime may be even more restricted than the memory
available in the device.
CLDC 1.1
CLDC 1.1, the "next generation" of the CLDC configuration, which is used in the PDA profile,
lifts some of the original CLDC limitations. Namely, CLDC-NG no longer explicitly forbids
floating point operations, and weak references are added.
CLDC Application Design
The design rules for CLDC applications are quite simple: Keep everything as small and as simple
as possible. CLDC applications should be designed to consume as few resources as possible, and
the user should be allowed to exit at any time without losing data. Design rules concerning the
user interface will be covered in more detail in Chapters 3 , "MIDP Programming," and 4 , "PDAP
Programming."