Java Reference
In-Depth Information
an Exception Object
When an exception is thrown but
not caught
in a particular scope, the method-call stack is
“unwound,” and an attempt is made to
catch
the exception in the next outer
try
block.
This process is called
stack unwinding
. Unwinding the method-call stack means that the
method in which the exception was not caught
terminates
, all local variables in that meth-
od
go out of scope
and control returns to the statement that originally invoked that method.
If a
try
block encloses that statement, an attempt is made to
catch
the exception. If a
try
block does not enclose that statement or if the exception is not caught, stack unwinding
occurs again. Figure 11.6 demonstrates stack unwinding, and the exception handler in
main
shows how to access the data in an exception object.
Stack Unwinding
In
main
, the
try
block (lines 8-11) calls
method1
(declared at lines 35-38), which in turn
calls
method2
(declared at lines 41-44), which in turn calls
method3
(declared at lines 47-
50). Line 49 of
method3
throws an
Exception
object—this is the
throw point
. Because the
throw
statement at line 49 is
not
enclosed in a
try
block,
stack unwinding
occurs—
method3
terminates at line 49, then returns control to the statement in
method2
that in-
voked
method3
(i.e., line 43). Because
no
try
block encloses line 43,
stack unwinding
oc-
curs again—
method2
terminates at line 43 and returns control to the statement in
method1
that invoked
method2
(i.e., line 37). Because
no
try
block encloses line 37,
stack unwinding
occurs one more time—
method1
terminates at line 37 and returns control to the statement
in
main
that invoked
method1
(i.e., line 10). The
try
block at lines 8-11 encloses this state-
ment. The exception has not been handled, so the
try
block terminates and the first
matching
catch
block (lines 12-31) catches and processes the exception. If there were no
matching
catch
blocks, and the exception is
not declared
in each method that throws it, a
compilation error would occur. Remember that this is not always the case—for
unchecked
exceptions, the application will compile, but it will run with unexpected results.
1
// Fig. 11.6: UsingExceptions.java
2
// Stack unwinding and obtaining data from an exception object.
3
4
public
class
UsingExceptions
5
{
6
public
static
void
main(String[] args)
7
{
8
try
9
{
10
method1();
11
}
12
catch
(Exception exception)
// catch exception thrown in method1
13
{
14
System.err.printf(
"%s%n%n"
,
exception.getMessage()
);
15
exception.printStackTrace();
16
Fig. 11.6
|
Stack unwinding and obtaining data from an exception object. (Part 1 of 2.)
