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Trail: Essential Classes
Lesson: Concurrency
Section: Synchronization
Concurrency
Processes and Threads
Thread Objects
Defining and Starting a Thread
Pausing Execution with Sleep
Interrupts
Joins
The SimpleThreads Example
Synchronization
Thread Interference
Memory Consistency Errors
Synchronized Methods
Intrinsic Locks and Synchronization
Atomic Access
Liveness
Deadlock
Starvation and Livelock
Guarded Blocks
Immutable Objects
A Synchronized Class Example
A Strategy for Defining Immutable Objects
High Level Concurrency Objects
Lock Objects
Executors
Executor Interfaces
Thread Pools
Concurrent Collections
Atomic Variables
For Further Reading
Questions and Exercises
Home Page > Essential Classes > Concurrency
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Synchronized Methods

> The Java programming language provides two basic synchronization idioms: synchronized methods and synchronized statements. The more complex of the two, synchronized statements, are described in the next section. This section is about synchronized methods.

> To make a method synchronized, simply add the `synchronized` keyword to its declaration:

> >     public class SynchronizedCounter {
> >         private int c = 0;

> >         public synchronized void increment() {
> >             c++;
> >         }

> >         public synchronized void decrement() {
> >             c--;
> >         }

> >         public synchronized int value() {
> >             return c;
> >         }
> >     }

> If `count` is an instance of `SynchronizedCounter`, then making these methods synchronized has two effects:
> - First, it is not possible for two invocations of synchronized methods on the same object to interleave. 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.
> - Second, when a synchronized method exits, it automatically establishes a happens-before relationship with any subsequent invocation of a synchronized method for the same object. This guarantees that changes to the state of the object are visible to all threads.

> Note that 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.

> > Warning: When constructing an object that will be shared between threads, be very careful that a reference to the object does not "leak" prematurely. For example, suppose you want to maintain a `List` called `instances` containing every instance of class. You might be tempted to add the line

> > >     instances.add(this);

> > to your constructor. But then other threads can use `instances` to access the object before construction of the object is complete.

> Synchronized methods enable a simple strategy for preventing thread interference and memory consistency errors: if an object is visible to more than one thread, all reads or writes to that object's variables are done through `synchronized` methods. (An important exception: `final` fields, which cannot be modified after the object is constructed, can be safely read through non-synchronized methods, once the object is constructed) This strategy is effective, but can present problems with liveness, as we'll see later in this lesson.

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