Synchronization in Java Fawzi Emad Chau Wen Tseng Department of Computer Science University of Maryland College Park Synchronization Overview Data races Locks Deadlock Wait Notify Data Race Definition Concurrent accesses to same shared variable where at least one access is a write Properties Order of accesses may change result of program May cause intermittent errors very hard to debug Example public class DataRace extends Thread static int x shared variable x causing data race public void run x x 1 access to x Data Race Example public class DataRace extends Thread static int common 0 public void run int local common data race local local 1 common local data race public static void main String args for int i 0 i 3 i new ThreadExample start System out println common may not be 3 Data Race Example Sequential execution output Data Race Example Concurrent execution output possible case Result depends on thread execution order Synchronization Definition Coordination of events with respect to time Properties May be needed in multithreaded programs to eliminate data races Incurs runtime overhead Excessive use can reduce performance 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 non Mutable Java objects provide locks Apply synchronized keyword to object Mutual exclusion for code in synchronization block block Example Object x new Object 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 foo code shorthand notation for foo synchronized this code Synchronized Methods In Java Locks in Java Properties No other thread can get lock on x while in block Other threads can still access modify x Locked block of code critical section Lock is released when block terminates End of block reached Exit block due to return continue break Exception thrown Synchronization Example Lock Example public class DataRace extends Thread static int common 0 static Object o all threads use o s lock public void run synchronized o single thread at once int local common data race eliminated local local 1 common local public static void main String args o new Object Synchronization Issues 1 Use same lock to provide mutual exclusion 2 Ensure atomic transactions 3 Avoiding deadlock 4 Use wait notify to improve efficiency Issue 1 Using Same Lock Potential problem Mutual exclusion depends on threads acquiring same lock No synchronization if threads have different locks Example foo 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 Lock Example Incorrect Version public class DataRace extends Thread static int common 0 public void run Object o new Object different o per thread synchronized o int local common data race local local 1 common local data race public static void main String args 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 o int local common local local 1 common local all 3 statements must be executed together by single thread Lock Example Incorrect Version public class DataRace extends Thread static int common 0 static Object o all threads use o s lock public void run int local synchronized o local common transaction not atomic synchronized o data race may occur local local 1 even using locks common local Issue 3 Avoiding Deadlock Potential problem Threads holding lock may be unable to obtain lock held by other thread and vice versa Thread holding lock may be waiting for action performed by other thread waiting for lock Program is unable to continue execution deadlock Deadlock Example 1 Object a Object b 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 swap Object a Object b Object local synchronized a synchronized b local a a b b local Thread1 swap a b holds lock for a waits for b Thread2 swap b a holds lock for b waits for a 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 Implementation Class BufferUser Buffer b new Buffer ProducerThread Object x new Object b add x produces items ConsumerThread Object y y b remove consumes items Buffer Implementation public class Buffer private Object myObjects private int numberObjects 0 public synchronized add Object x myObjects numberObjects x public synchronized Object remove while numberObjects 1 waits for more objects to be added return myObjects numberObjects 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 add Object x synchronized this myObjects numberObjects x public Object remove while true waits for more objects to be added synchronize this if numberObjects 0 return myObjects numberObjects if empty buffer remove gives up lock Deadlock Avoiding deadlock In general avoid holding lock for a long time Especially avoid trying to hold two locks May wait a long time trying to get 2nd lock Issue 4 Using Wait Notify Potential problem Threads actively waiting for lock consume resources Solution Can wait in queue for lock in a blocked state to be notified when lock is released Use Thread class methods wait notify notifyAll Note wait notify only work when holding a lock Thread Class Wait Notify Methods wait Calling thread is put into blocked state Calling thread is placed on wait queue for lock Execution continues only if thread reacquires lock notify 1 One thread is taken from wait queue for lock 2 Thread is put in runnable state 3 Thread will try to get lock again 4 If thread fails it is put back in blocked state notifyAll Invokes notify on all threads in wait queue Using Wait Notify State transitions Example Using Wait
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