What does the transient and volatile keywords do?

Discussion in 'Java' started by res7cxbi@verizon.net, Jan 21, 2006.

  1. Guest

    What does the transient and volatile keywords do? Why would i use them?

    I heard that the volatile keyword has not been fully implemented in
    Sun's JVM. But yet most of the JDK (especially the I/O classes) use
    this keyword. What's up with that?
    , Jan 21, 2006
    #1
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  2. IchBin Guest

    wrote:
    > What does the transient and volatile keywords do? Why would i use them?
    >
    > I heard that the volatile keyword has not been fully implemented in
    > Sun's JVM. But yet most of the JDK (especially the I/O classes) use
    > this keyword. What's up with that?
    >


    When your not sure consult the 'Bible', 'Java™ Language Specification'
    http://java.sun.com/docs/books/jls/third_edition/html/classes.html#78119


    8.3.1.3 transient Fields
    Variables may be marked transient to indicate that they are not part of
    the persistent state of an object.

    If an instance of the class Point:

    class Point {
    int x, y;
    transient float rho, theta;
    }

    were saved to persistent storage by a system service, then only the
    fields x and y would be saved. This specification does not specify
    details of such services; see the specification of java.io.Serializable
    for an example of such a service.

    8.3.1.4 volatile Fields
    As described in §17, the Java programming language allows threads to
    access shared variables. As a rule, to ensure that shared variables are
    consistently and reliably updated, a thread should ensure that it has
    exclusive use of such variables by obtaining a lock that,
    conventionally, enforces mutual exclusion for those shared variables.

    The Java programming language provides a second mechanism, volatile
    fields, that is more convenient than locking for some purposes.
    A field may be declared volatile, in which case the Java memory model
    (§17) ensures that all threads see a consistent value for the variable.

    If, in the following example, one thread repeatedly calls the method one
    (but no more than Integer.MAX_VALUE times in all), and another thread
    repeatedly calls the method two:

    class Test {
    static int i = 0, j = 0;
    static void one() { i++; j++; }
    static void two() {
    System.out.println("i=" + i + " j=" + j);
    }
    }

    then method two could occasionally print a value for j that is greater
    than the value of i, because the example includes no synchronization
    and, under the rules explained in §17, the shared values of i and j
    might be updated out of order.

    One way to prevent this out-or-order behavior would be to declare
    methods one and two to be synchronized (§8.4.3.6):

    class Test {
    static int i = 0, j = 0;
    static synchronized void one() { i++; j++; }
    static synchronized void two() {
    System.out.println("i=" + i + " j=" + j);
    }
    }

    This prevents method one and method two from being executed
    concurrently, and furthermore guarantees that the shared values of i and
    j are both updated before method one returns. Therefore method two never
    observes a value for j greater than that for i; indeed, it always
    observes the same value for i and j.

    Another approach would be to declare i and j to be volatile:

    class Test {
    static volatile int i = 0, j = 0;
    static void one() { i++; j++; }
    static void two() {
    System.out.println("i=" + i + " j=" + j);
    }
    }

    This allows method one and method two to be executed concurrently, but
    guarantees that accesses to the shared values for i and j occur exactly
    as many times, and in exactly the same order, as they appear to occur
    during execution of the program text by each thread. Therefore, the
    shared value for j is never greater than that for i, because each update
    to i must be reflected in the shared value for i before the update to j
    occurs. It is possible, however, that any given invocation of method two
    might observe a value for j that is much greater than the value observed
    for i, because method one might be executed many times between the
    moment when method two fetches the value of i and the moment when method
    two fetches the value of j.

    See §17 for more discussion and examples.

    A compile-time error occurs if a final variable is also declared volatile.

    --

    Thanks in Advance...
    IchBin, Pocono Lake, Pa, USA
    http://weconsultants.servebeer.com/JHackerAppManager
    __________________________________________________________________________

    'If there is one, Knowledge is the "Fountain of Youth"'
    -William E. Taylor, Regular Guy (1952-)
    IchBin, Jan 21, 2006
    #2
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  3. Guest

    thanks a lot!
    , Jan 21, 2006
    #3
  4. Roedy Green Guest

    Roedy Green, Jan 21, 2006
    #4
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