12.3

Linking of Classes and Interfaces

EXECUTION

12.3   Linking of Classes and Interfaces

Linking

 is the process of taking a binary form of a class or interface type and com 

bining it into the runtime state of the Java Virtual Machine, so that it can be exe 

cuted. A class or interface type is always loaded before it is linked. Three different

activities are involved in linking: verification, preparation, and resolution of sym 

bolic references.

Java allows an implementation flexibility as to when linking activities (and,

because of recursion, loading) take place, provided that the semantics of the lan 

guage are respected, that a class or interface is completely verified and prepared

before it is initialized, and that errors detected during linkage are thrown at a point

in the program where some action is taken by the program that might require link 

age to the class or interface involved in the error.

For example, an implementation may choose to resolve each symbolic refer 

ence in a class or interface individually, only when it is used (lazy or late resolu 

tion), or to resolve them all at once while the class is being verified (static

resolution). This means that the resolution process may continue, in some imple 

mentations, after a class or interface has been initialized.

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

an

OutOfMemoryError

.

12.3.1   Verification of the Binary Representation

Verification

 ensures that the binary representation of a class or interface is struc 

turally correct. For example, it checks that every instruction has a valid operation

code; that every branch instruction branches to the start of some other instruction,

rather than into the middle of an instruction; that every method is provided with a

structurally correct signature; and that every instruction obeys the type discipline

of the Java language.

For a more detailed description of the verification process, see the separate

volume of this series,

The Java Virtual Machine Specification

.

If an error occurs during verification, then an instance of the following sub 

class of class

LinkageError

 will be thrown at the point in the Java program that

caused the class to be verified:

VerifyError

: The binary definition for a class or interface failed to pass a set

of required checks to verify that it obeys the semantics of the Java language

and that it cannot violate the integrity of the Java Virtual Machine. (See

 13.4.2,  13.4.4,  13.4.8, and  13.4.16 for some examples.)

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