8. Java concurrency 2. Synchronization

Июль 26, 2022

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8. Java concurrency 2. Synchronization

Содержание

2. Synchronization Threads communicate primarily by sharing access to fields and the objects reference fields refer to.
3. Thread Interference Example class Counter { private int c = 0; public void increment() { c++;
4. Thread Interference I Interference happens when two operations, running in different threads, but acting on the
5. Thread Interference II Thread A: Retrieve c. Thread B: Retrieve c. Thread A: Increment retrieved value;
6. Thread Interference III Thread A's result is lost, overwritten by Thread B. Because they are unpredictable,
7. Exercise: Thread Inference Modify 511DepoSum project as follows: Create new method add100(int index) in the ListDepo
8. Exercise: Thread Inference See 821Unsync project for the full text. * InfopulseTraining Center
9. Synchronized Methods I public class SynchronizedCounter { private int c = 0; public synchronized void increment()
10. Synchronized Methods II When one thread is executing a synchronized method for an object, all other
11. Exercise: Synchronization Modify 821Unsync project using synchronized add100 method and check result. * InfopulseTraining Center
12. Constructor Synchronization Constructors cannot be synchronized — using the synchronized keyword with a constructor is a
13. Intrinsic Locks and Synchronization When a task wishes to execute a piece of code guarded by
14. Intrinsic Locks If a task is in a call to one of the synchronized methods, all
15. Concurrency Class Fields Especially important to make fields private when working with concurrency Otherwise the synchronized
16. Synchronized Statements Unlike synchronized methods, synchronized statements must specify the object that provides the intrinsic lock:
17. Cooperation Between Tasks How to make tasks cooperate with each other, so that multiple tasks can
18. wait() Method wait( ) allows you to wait for a change in some condition that is
19. Don't do this! public void guardedJoy() { // Simple loop guard. Wastes // processor time. Don't
20. wait() Example (1 of 2) public synchronized guardedJoy() { while(!joy) { try { wait(); } catch
21. notify() / notifyAll() Methods wait( ) suspends the task while waiting for the world to change
22. wait() Example (2 of 2) public synchronized notifyJoy() { joy = true; notifyAll(); } * InfopulseTraining
23. Deadlock Deadlock describes a situation where two or more threads are blocked forever, waiting for each
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Слайд 2

Synchronization
Threads communicate primarily by sharing access to fields and the objects

reference fields refer to.
Such communication is extremely efficient
Errors possible: thread interference
The tool needed to prevent these errors is synchronization.

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Слайд 3

Thread Interference Example
class Counter {
private int c = 0;
public

void increment() {
c++;
}
public void decrement() {
c--;
}
public int value() {
return c;
}
}

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Слайд 4

Thread Interference I
Interference happens when two operations, running in different threads,

but acting on the same data, interleave
Single expression c++ can be decomposed into three steps:
Retrieve the current value of c.
Increment the retrieved value by 1.
Store the incremented value back in c.

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Слайд 5

Thread Interference II
Thread A: Retrieve c.
Thread B: Retrieve c.
Thread A: Increment

retrieved value; result is 1.
Thread B: Decrement retrieved value; result is -1.
Thread A: Store result in c; c is now 1.
Thread B: Store result in c; c is now -1.

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Слайд 6

Thread Interference III
Thread A's result is lost, overwritten by Thread B.
Because

they are unpredictable, thread interference bugs can be difficult to detect and fix.

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Слайд 7

Exercise: Thread Inference
Modify 511DepoSum project as follows:
Create new method add100(int index)

in the ListDepo class that adds 100.0 to the deposit with given index. Sleep the current thread for 1 sec before saving result to the deposit
Create ThreadTest class implements Runnable interface with field ListDepo field. Run method of the class should invoke add100 method
Try to modify the same deposit from two threads using ThreadTest class

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Слайд 8

Exercise: Thread Inference
See 821Unsync project for the full text.
*
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Слайд 9

Synchronized Methods I
public class SynchronizedCounter {
private int c = 0;

public synchronized void increment() {
c++;
}
public synchronized void decrement() {
c--;
}
public synchronized int value() {
return c;
}
}

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Слайд 10

Synchronized Methods II
When one thread is executing a synchronized method for

an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object
Synchronized method exits guarantees that changes to the state of the object are visible to all threads

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Слайд 11

Exercise: Synchronization
Modify 821Unsync project using synchronized add100 method and check result.

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Слайд 12

Constructor Synchronization
Constructors cannot be synchronized — using the synchronized keyword with

a constructor is a syntax error.
Synchronizing constructors doesn't make sense, because only the thread that creates an object should have access to it while it is being constructed

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Слайд 13

Intrinsic Locks and Synchronization
When a task wishes to execute a

piece of code guarded by the synchronized keyword, it
checks to see if the lock is available
then acquires it,
executes the code
and releases it.

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Слайд 14

Intrinsic Locks
If a task is in a call to one of

the synchronized methods, all other tasks are blocked from entering any of the synchronized methods of that object until the first task returns from its call
A static synchronized method invocation the thread acquires the intrinsic lock for the Class object associated with the class

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Слайд 15

Concurrency Class Fields
Especially important to make fields private when working

with concurrency
Otherwise the synchronized keyword cannot prevent another task from accessing a field directly, and thus producing collisions

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Слайд 16

Synchronized Statements
Unlike synchronized methods, synchronized statements must specify the object that

provides the intrinsic lock:
public void addName(String name) {
synchronized(this) {
lastName = name;
nameCount++;
}
nameList.add(name);
}

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Слайд 17

Cooperation Between Tasks
How to make tasks cooperate with each other, so

that multiple tasks can work together to solve a problem?
To accomplish this we use the mutex, which in this case guarantees that only one task can respond to a signal
This eliminates any possible race conditions, which is safely implemented using the Object methods wait( ) and notifyAll( )

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Слайд 18

wait() Method
wait( ) allows you to wait for a change in

some condition that is outside the control of the forces in the current method
Often, this condition will be changed by another task
You don’t want to idly loop while testing the condition inside your task; this is called busy waiting, and it’s usually a bad use of CPU cycles

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Слайд 19

Don't do this!
public void guardedJoy() {
// Simple loop guard.

Wastes
// processor time. Don't do this!
while(!joy) { }
System.out.println("Joy has been achieved!");
}

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Слайд 20

wait() Example (1 of 2)
public synchronized guardedJoy() {
while(!joy) {

try { wait(); }
catch (InterruptedException e) {}
}
System.out.println("Joy and efficiency have been achieved!");
}

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Слайд 21

notify() / notifyAll() Methods
wait( ) suspends the task while waiting for

the world to change
Only when a notify( ) or notifyAll( ) occurs - suggesting that something of interest may have happened - does the task wake up and check for changes
Thus, wait( ) provides a way to synchronize activities between tasks

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Слайд 22

wait() Example (2 of 2)
public synchronized notifyJoy() {
joy = true;

notifyAll();
}

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Слайд 23

Deadlock
Deadlock describes a situation where two or more threads are blocked

forever, waiting for each other

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8. Java concurrency 2. Synchronization

Содержание

SynchronizationThreads communicate primarily by sharing access to fields and the objects

Thread Interference Exampleclass Counter { private int c = 0; public

Thread Interference IInterference happens when two operations, running in different threads,

Thread Interference IIThread A: Retrieve c.Thread B: Retrieve c.Thread A: Increment

Thread Interference IIIThread A's result is lost, overwritten by Thread B.Because

Exercise: Thread InferenceModify 511DepoSum project as follows:Create new method add100(int index)

Exercise: Thread InferenceSee 821Unsync project for the full text.*InfopulseTraining Center

Synchronized Methods Ipublic class SynchronizedCounter { private int c = 0;

Synchronized Methods IIWhen one thread is executing a synchronized method for

Exercise: SynchronizationModify 821Unsync project using synchronized add100 method and check result.

Constructor SynchronizationConstructors cannot be synchronized — using the synchronized keyword with

Intrinsic Locks and Synchronization When a task wishes to execute a

Intrinsic LocksIf a task is in a call to one of

Concurrency Class Fields Especially important to make fields private when working

Synchronized StatementsUnlike synchronized methods, synchronized statements must specify the object that

Cooperation Between TasksHow to make tasks cooperate with each other, so

wait() Methodwait( ) allows you to wait for a change in

Don't do this! public void guardedJoy() { // Simple loop guard.

wait() Example (1 of 2)public synchronized guardedJoy() { while(!joy) {

notify() / notifyAll() Methodswait( ) suspends the task while waiting for

wait() Example (2 of 2)public synchronized notifyJoy() { joy = true;

DeadlockDeadlock describes a situation where two or more threads are blocked

Похожие презентации

Synchronization
Threads communicate primarily by sharing access to fields and the objects

Thread Interference Example
class Counter {
private int c = 0;
public

Thread Interference I
Interference happens when two operations, running in different threads,

Thread Interference II
Thread A: Retrieve c.
Thread B: Retrieve c.
Thread A: Increment

Thread Interference III
Thread A's result is lost, overwritten by Thread B.
Because

Exercise: Thread Inference
Modify 511DepoSum project as follows:
Create new method add100(int index)

Exercise: Thread Inference
See 821Unsync project for the full text.
*
InfopulseTraining Center

Synchronized Methods I
public class SynchronizedCounter {
private int c = 0;

Synchronized Methods II
When one thread is executing a synchronized method for

Exercise: Synchronization
Modify 821Unsync project using synchronized add100 method and check result.

Constructor Synchronization
Constructors cannot be synchronized — using the synchronized keyword with

Intrinsic Locks and Synchronization
When a task wishes to execute a

Intrinsic Locks
If a task is in a call to one of

Concurrency Class Fields
Especially important to make fields private when working

Synchronized Statements
Unlike synchronized methods, synchronized statements must specify the object that

Cooperation Between Tasks
How to make tasks cooperate with each other, so

wait() Method
wait( ) allows you to wait for a change in

Don't do this!
public void guardedJoy() {
// Simple loop guard.

wait() Example (1 of 2)
public synchronized guardedJoy() {
while(!joy) {

notify() / notifyAll() Methods
wait( ) suspends the task while waiting for

wait() Example (2 of 2)
public synchronized notifyJoy() {
joy = true;

Deadlock
Deadlock describes a situation where two or more threads are blocked