CMSC 132 Object Oriented Programming II Synchronization in Java Department of Computer Science University of Maryland College Park 1 Multithreading Overview Motivation background Threads Creating Java threads Thread states Scheduling Synchronization Data races Locks Deadlock 2 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 3 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 throws InterruptedException int max 3 DataRace allThreads new DataRace max for int i 0 i allThreads length i allThreads i new DataRace for DataRace t allThreads t start for DataRace t allThreads t join System out println common may not be 3 4 Data Race Example Sequential execution output 5 Data Race Example Concurrent execution output possible case Result depends on thread execution order 6 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 7 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 Only 1 thread can acquire lock at a time Thread will wait to acquire lock stop execution If lock held by another thread Used to implement monitors Only 1 thread can execute locked code at a time 8 Synchronized Objects in Java Java objects provide locks Apply synchronized keyword to object Will acquire release lock associated with 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 9 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 10 Synchronized Methods In Java 11 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 12 Synchronization Example 13 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 14 Synchronization Issues 1 Use same lock to provide mutual exclusion 2 Ensure atomic transactions 3 Avoiding deadlock 15 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 16 Locks in Java Single lock for all threads mutual exclusion Separate locks for each thread no synchronization 17 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 18 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 19 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 20 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 21 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 22 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 23 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 24 Synchronization Summary Needed in multithreaded programs Can prevents data races Java objects support synchronization Many other tricky issues To be discussed in future courses 25
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