Where storage lives (From TIJ)
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I'm pretty free these days, so I download the English version TIJ4 for a review. Lots of new understanding shall be found during the review. Let's record the first pick.
1. Registers. This is the fastest storage because it exists in a place different from that of
other storage: inside the processor. However, the number of registers is severely
limited, so registers are allocated as they are needed. You don’t have direct control,
nor do you see any evidence in your programs that registers even exist (C & C++, on
the other hand, allow you to suggest register allocation to the compiler).
2. The stack. This lives in the general random-access memory (RAM) area, but has
direct support from the processor via its stack pointer. The stack pointer is moved
down to create new memory and moved up to release that memory. This is an
extremely fast and efficient way to allocate storage, second only to registers. The Java
system must know, while it is creating the program, the exact lifetime of all the items
that are stored on the stack. This constraint places limits on the flexibility of your
programs, so while some Java storage exists on the stack—in particular, object
references—Java objects themselves are not placed on the stack.
3. The heap. This is a general-purpose pool of memory (also in the RAM area) where all
Java objects live. The nice thing about the heap is that, unlike the stack, the compiler
doesn’t need to know how long that storage must stay on the heap. Thus, there’s a
great deal of flexibility in using storage on the heap. Whenever you need an object, you
simply write the code to create it by using new, and the storage is allocated on the
heap when that code is executed. Of course there’s a price you pay for this flexibility: It
may take more time to allocate and clean up heap storage than stack storage (if you
even could create objects on the stack in Java, as you can in C++).
4. Constant storage. Constant values are often placed directly in the program code,
which is safe since they can never change. Sometimes constants are cordoned off by
themselves so that they can be optionally placed in read-only memory (ROM), in
embedded systems.2
5. Non-RAM storage. If data lives completely outside a program, it can exist while the
program is not running, outside the control of the program. The two primary
examples of this are streamed objects, in which objects are turned into streams of
bytes, generally to be sent to another machine, and persistent objects, in which the
objects are placed on disk so they will hold their state even when the program is
terminated. The trick with these types of storage is turning the objects into something
that can exist on the other medium, and yet can be resurrected into a regular RAMbased
object when necessary. Java provides support for lightweight persistence, and
mechanisms such as JDBC and Hibernate provide more sophisticated support for
storing and retrieving object information in databases.
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