EXECUTION

Loading: Implications for Code Generation

12.2.2

the Java language is that the type system cannot be subverted by code written in

Java, not even by implementations of such otherwise sensitive system classes as

ClassLoader

 ( 20.14) and

SecurityManager

 ( 20.17).

An entry may be deleted from the internal table only after unloading ( 12.8)

the class or interface represented by the class object in the entry.

12.2.1   The Loading Process

The loading process is implemented by the class

ClassLoader

 ( 20.14) and its

subclasses. Different subclasses of

ClassLoader

 may implement different load 

ing policies. In particular, a class loader may cache binary representations of

classes and interfaces, prefetch them based on expected usage, or load a group of

related classes together. These activities may not be completely transparent to a

running Java application if, for example, a newly compiled version of a class is

not found because an older version is cached by a class loader. It is the responsi 

bility of a class loader, however, to reflect loading errors only at points in the pro 

gram they could have arisen without prefetching or group loading.

If an error occurs during class loading, then an instance of one of the follow 

ing subclasses of class

LinkageError

 will be thrown at any point in the Java pro 

gram that (directly or indirectly) uses the type:

ClassCircularityError

: A class or interface could not be loaded because

it would be its own superclass or superinterface ( 13.4.4).

ClassFormatError

: The binary data that purports to specify a requested

compiled class or interface is malformed.

NoClassDefFoundError

: No definition for a requested class or interface

could be found by the relevant class loader.

Because loading involves the allocation of new data structures, it may fail

with an

OutOfMemoryError

.

12.2.2   Loading: Implications for Code Generation

A cooperating class loader can enable a code generator to generate code for a

group of class and interface types perhaps an entire package by loading the

binary code for these types as a group. A format can be designed that allows all

the internal symbolic references in such a group to be resolved, before the group is

loaded. Such a strategy may also allow the generated code to be optimized before

loading based on the known concrete types in the group. This approach may be

useful in specific cases, but is discouraged as a general technique, since such a

class file format is unlikely to be widely understood.

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