🗊Презентация 8. Java concurrency 2. Synchronization

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8. Concurrency 2. Synchronization
Описание слайда:
8. Concurrency 2. Synchronization

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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.
Описание слайда:
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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Thread Interference Example class Counter { private int c = 0;  public void increment() { c++; }  public void decrement() { c--; }  public int value() { return c; }  }
Описание слайда:
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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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.
Описание слайда:
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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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.
Описание слайда:
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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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.
Описание слайда:
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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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
Описание слайда:
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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Exercise: Thread Inference See 821Unsync project for the full text.
Описание слайда:
Exercise: Thread Inference See 821Unsync project for the full text.

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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; } }
Описание слайда:
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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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
Описание слайда:
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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Exercise: Synchronization Modify 821Unsync project using synchronized add100 method and check result.
Описание слайда:
Exercise: Synchronization Modify 821Unsync project using synchronized add100 method and check result.

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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
Описание слайда:
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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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.
Описание слайда:
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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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
Описание слайда:
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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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
Описание слайда:
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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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); }
Описание слайда:
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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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( )
Описание слайда:
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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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
Описание слайда:
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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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!"); }
Описание слайда:
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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wait() Example (1 of 2) public synchronized guardedJoy() { while(!joy) { try { wait(); } catch (InterruptedException e) {} } System.out.println("Joy and efficiency have been achieved!"); }
Описание слайда:
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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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
Описание слайда:
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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wait() Example (2 of 2) public synchronized notifyJoy() { joy = true; notifyAll(); }
Описание слайда:
wait() Example (2 of 2) public synchronized notifyJoy() { joy = true; notifyAll(); }

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Deadlock Deadlock describes a situation where two or more threads are blocked forever, waiting for each other
Описание слайда:
Deadlock Deadlock describes a situation where two or more threads are blocked forever, waiting for each other

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Manuals http://docs.oracle.com/javase/tutorial/essential/concurrency/index.html
Описание слайда:
Manuals http://docs.oracle.com/javase/tutorial/essential/concurrency/index.html

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