Thread Synchronization

}When multiple threads share anobject and it is modified by one or more of them, indeterminate results may occurunless access to the shared object is managed properly.

}The problem can be solved by giving only one thread at a time exclusive accessto code that manipulates the shared object.

◦During that time, other threads desiring to manipulate the object are kept waiting.

◦When the thread with exclusive access to the object finishes manipulating it, one ofthe threads that was waiting is allowed to proceed.

}This process, called thread synchronization, coordinates access to shared data by multiple concurrent threads.◦Ensures that each thread accessing a shared object excludes all other threads from doing so simultaneously—this is called mutual exclusion.

}A common way to perform synchronization is to use Java’s built-in monitors.

  ◦Every object has a monitor and a monitor lock (or intrinsiclock).◦Can be held by a maximum of only one thread at any time.

  ◦A thread must acquire the lock before proceeding with the operation.◦Other threads attempting to performan operation that requires the same lock will be blocked.

}To specify that a thread must holda monitor lock to execute a block of code, the code should be placed in a synchronized statement.

  ◦Said to be guarded by the monitor lock

}The synchronized statements are declared using the synchronized keyword:

–synchronized (object)
{
   statements
} //end synchronized statement

}where object is the object whose monitor lock will be acquired◦objectis normally this if it’s the object in which the synchronized statement appears.

}When a synchronized statement finishes executing, the object’s monitor lock is released.

}Java also allows synchronized methods.

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