EXCEPTIONS

Handling Asynchronous Exceptions

11.3.2

encountered that can handle the exception; execution then continues by executing

the block of that

catch

 clause. The code that caused the exception is never

resumed.

If no

catch

 clause handling an exception can be found, then the current

thread (the thread that encountered the exception) is terminated, but only after all

finally

 clauses have been executed and the method

uncaughtException

( 20.21.31) has been invoked for the

ThreadGroup

 that is the parent of the cur 

rent thread.

In situations where it is desirable to ensure that one block of code is always

executed after another, even if that other block of code completes abruptly, a

try

statement with a

finally

 clause ( 14.18.2) may be used. If a

try

 or

catch

 block

in a

try

finally

 or

try

catch

finally

 statement completes abruptly, then the

finally

 clause is executed during propagation of the exception, even if no

matching

catch

 clause is ultimately found. If a

finally

 clause is executed

because of abrupt completion of a

try

 block and the

finally

 clause itself com 

pletes abruptly, then the reason for the abrupt completion of the

try

 block is dis 

carded and the new reason for abrupt completion is propagated from there.

The exact rules for abrupt completion and for the catching of exceptions are

specified in detail with the specification of each statement in  14 and for expres 

sions in  15 (especially  15.5).

11.3.1   Exceptions are Precise

Exceptions in Java are

precise

: when the transfer of control takes place, all effects

of the statements executed and expressions evaluated before the point from which

the exception is thrown must appear to have taken place. No expressions, state 

ments, or parts thereof that occur after the point from which the exception is

thrown may appear to have been evaluated. If optimized code has speculatively

executed some of the expressions or statements which follow the point at which

the exception occurs, such code must be prepared to hide this speculative execu 

tion from the user visible state of the Java program.

11.3.2   Handling Asynchronous Exceptions

Most exceptions in Java occur synchronously as a result of an action by the thread

in which they occur, and at a point in the Java program that is specified to possibly

result in such an exception. An asynchronous exception is, by contrast, an excep 

tion that can potentially occur at any point in the execution of a Java program.

Proper understanding of the semantics of asynchronous exceptions is neces 

sary if high quality machine code is to be generated.

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