Synchronization in Java Nelson Padua Perez Bill Pugh Department of Computer Science University of Maryland College Park Synchronization Overview Unsufficient atomicity Data races Locks Deadlock Wait Notify Unsufficient atomicity Very frequently you will want a sequence of actions to be performed atomically or indivisibly not interrupted or disturbed by actions by any other thread x isn t an atomic operation it is a read followed by a write Can be a intermittent error depends on exact interleaving Insuffient Atomicity Example public class InsuffientAtomicity implements Runnable static int x 0 public void run int tmp x x tmp 1 public static void main String args for int i 0 i 3 i new Thread new InsuffientAtomicity start System out println x may not be 3 Data Race Definition Concurrent accesses to same shared variable where at least one access is a write variable isn t volatile Can expose all sorts of really weird stuff the compiler and processor are doing to improve performance Quiz Time x y 0 Thread 1 start threads Thread 2 x 1 y 1 j y i x Can this result in i 0 and j 0 Answer Yes x y 0 Thread 1 start threads Thread 2 x 1 y 1 j y i x How can i 0 and j 0 How Can This Happen Compiler can reorder statements Or keep values in registers Processor can reorder them On multi processor values not synchronized to global memory The memory model is designed to allow aggressive optimization including optimizations no one has implemented yet Good for performance bad for your intuition about insufficiently synchronized code Synchronization Uses Marks when a block of code must not be interleaved with code executed by another thread Marks when information can must flow between threads Notes Incurs a small amount of runtime overhead if only used where you might need to communicate between threads not significant used everywhere can add up Lock Definition Entity can be held by only one thread at a time Properties A type of synchronization Used to enforce mutual exclusion Thread can acquire release locks Thread will wait to acquire lock stop execution If lock held by another thread Synchronized Objects in Java All Java objects provide locks Apply synchronized keyword to object Mutual exclusion for code in synchronization block block Example Object x new Object void foo synchronized x acquire lock on x on entry hold lock on x in block release lock on x on exit Synchronized Methods In Java Java methods also provide locks Apply synchronized keyword to method Mutual exclusion for entire body of method Synchronizes on object invoking method Example block synchronized void foo code shorthand notation for void foo synchronized this code Synchronized Methods In Java Locks in Java Properties No other thread can get lock on x while in block Does not protect fields of x except by convention other threads can access update fields but can t obtain lock on x By convention lock x to obtain exclusive access to x Locked block of code critical section Lock is released when block terminates No matter how the block terminates End of block reached Exit block due to return continue break Exception thrown Using synchronization public class UseSynchronization implements Runnable static int x 0 static Object lock new Object public void run synchronized lock int tmp x x tmp 1 Questions What would happen if the lock field were not static Why don t we just make the run method synchronized Why don t we just synchronize on x Not sharing same lock public class NotSharingLock implements Runnable static int x 0 Object lock new Object public void run synchronized lock int tmp x x tmp 1 Synchronization Issues Use same lock to provide mutual exclusion Ensure atomic transactions Avoiding deadlock Issue 1 Using Same Lock Potential problem Mutual exclusion depends on threads acquiring same lock No synchronization if threads have different locks Example void run Object o new Object different o per thread synchronized o potential data race Locks in Java Single lock for all threads mutual exclusion Separate locks for each thread no synchronization Issue 2 Atomic Transactions Potential problem Sequence of actions must be performed as single atomic transaction to avoid data race Ensure lock is held for duration of transaction Example synchronized lock int tmp x both statements must be executed together x tmp by single thread Using synchronization public class InsuffientAtomicity implements Runnable static int x 0 static Object lock new Object public void run int tmp synchronized lock tmp x synchronized lock x tmp 1 Issue 3 Avoiding Deadlock In general want to be careful about performing any operations that might take a long time while holding a lock What could take a really long time getting another lock Particularly if you get deadlock Deadlock Example 1 Thread1 synchronized a synchronized b Thread2 synchronized b synchronized a Thread1 holds lock for a waits for b Thread2 holds lock for b waits for a Deadlock Example 2 void moveMoney Account a Account b int amount synchronized a synchronized b a debit amount b credit amount Thread1 moveMoney a b 10 holds lock for a waits for b Thread2 moveMoney b a 100 holds lock for b waits for a Waiting for Godot Sometimes you need to wait for another thread else to do something before you can do something Abstract Data Type Buffer Buffer Transfers items from producers to consumers Very useful in multithreaded programs Synchronization needed to prevent multiple consumers removing same item Buffer usage Producer thread calls buffer add o adds o to the buffer Consumer thread calls buffer remove if object in buffer removes and returns it otherwise waits until object is available to remove Buffer Implementation public class Buffer private LinkedList objects new LinkedList public synchronized add Object x objects add x public synchronized Object remove while objects isEmpty waits for more objects to be added return objects removeFirst if empty buffer remove holds lock and waits prevents add from working deadlock Eliminating Deadlock public class Buffer private Object myObjects private int numberObjects 0 public synchronized add Object x objects add x public Object remove while true waits for more objects to be added synchronize this if objects isEmpty return objects removeFirst if empty buffer remove gives up lock for a moment Works barely if at all Might work But waiting thread is going to be running a full tilt twiddling its thumbs doing nothing burning up your battery life keeping the producer from getting
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