Java Reference
In-Depth Information
In particular, when working with multiple threads, you must often take care to pre‐
vent multiple threads from modifying an object simultaneously in a way that might
corrupt the object's state. Java provides the synchronized statement to help the pro‐
grammer prevent corruption. The syntax is:
synchronized ( expression ) {
expression is an expression that must evaluate to an object or an array. statements
constitute the code of the section that could cause damage and must be enclosed in
curly braces.
Before executing the statement block, the Java interpreter first obtains an exclusive
lock on the object or array specified by expression . It holds the lock until it is fin‐
ished running the block, then releases it. While a thread holds the lock on an object,
no other thread can obtain that lock.
The synchronized keyword is also available as a method modifier in Java, and when
applied to a method, the synchronized keyword indicates that the entire method is
locked. For a synchronized class method (a static method), Java obtains an exclu‐
sive lock on the class before executing the method. For a synchronized instance
method, Java obtains an exclusive lock on the class instance. (Class and instance
methods are discussed in Chapter 3 .)
The throw Statement
An exception is a signal that indicates some sort of exceptional condition or error
has occurred. To throw an exception is to signal an exceptional condition. To catch
an exception is to handle it—to take whatever actions are necessary to recover from
it. In Java, the throw statement is used to throw an exception:
throw expression ;
The expression must evaluate to an exception object that describes the exception
or error that has occurred. We'll talk more about types of exceptions shortly; for
now, all you need to know is that an exception is represented by an object, which
has a slightly specialized role. Here is some example code that throws an exception:
public static double factorial ( int x ) {
if ( x < 0 )
throw new IllegalArgumentException ( "x must be >= 0" );
double fact ;
for ( fact = 1.0 ; x > 1 ; fact *= x , x --)
/* empty */ ; // Note use of the empty statement
return fact ;
When the Java interpreter executes a throw statement, it immediately stops normal
program execution and starts looking for an exception handler that can catch, or
handle, the exception. Exception handlers are written with the try/catch/finally
Search WWH ::

Custom Search