12.4.2

Detailed Initialization Procedure

EXECUTION

class or interface may be requested recursively as part of the initialization of that

class or interface; for example, a variable initializer in class

A

 might invoke a

method of an unrelated class

B

, which might in turn invoke a method of class

A

.

The implementation of the Java Virtual Machine is responsible for taking care of

synchronization and recursive initialization by using the following procedure. It

assumes that the

Class

 object has already been verified and prepared, and that the

Class

 object contains state that indicates one of four situations:

This

Class

 object is verified and prepared but not initialized.

This

Class

 object is being initialized by some particular thread

T

.

This

Class

 object is fully initialized and ready for use.

This

Class

 object is in an erroneous state, perhaps because the verification or

preparation step failed, or because initialization was attempted and failed.

The procedure for initializing a class or interface is then as follows:

1. Synchronize ( 14.17) on the

Class

 object that represents the class or interface

to be initialized. This involves waiting until the current thread can obtain the

lock for that object ( 17.13).

2. If initialization is in progress for the class or interface by some other thread,

then

wait

 ( 20.1.6) on this

Class

 object (which temporarily releases the

lock). When the current thread awakens from the

wait

, repeat this step.

3. If initialization is in progress for the class or interface by the current thread,

then this must be a recursive request for initialization. Release the lock on the

Class

 object and complete normally.

4. If the class or interface has already been initialized, then no further action is

required. Release the lock on the

Class

 object and complete normally.

5. If the

Class

 object is in an erroneous state, then initialization is not possible.

Release the lock on the

Class

 object and throw a

NoClassDefFoundError

.

6. Otherwise, record the fact that initialization of the

Class

 object is now in

progress by the current thread and release the lock on the

Class

 object.

7. Next, if the

Class

 object represents a class rather than an interface, and the

superclass of this class has not yet been initialized, then recursively perform

this entire procedure for the superclass. If necessary, verify and prepare the

superclass first. If the initialization of the superclass completes abruptly

because of a thrown exception, then lock this

Class

 object, label it erroneous,

notify all waiting threads ( 20.1.10), release the lock, and complete abruptly,

throwing the same exception that resulted from initializing the superclass.

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