Java ReentrantLock (Part 1)
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1 Java ReentrantLock (Part 1) Douglas C. Schmidt Institute for Software Integrated Systems Vanderbilt University Nashville, Tennessee, USA
2 Learning Objectives in this Part of the Lesson Understand how the concept of mutual exclusion in concurrent programs Waiting threads Critical Section Running thread See en.wikipedia.org/ wiki/mutual_exclusion 2
3 Overview of Mutual Exclusion Locks 3
4 Overview of Mutual Exclusion Locks A mutual exclusion lock (mutex) defines a critical section Critical Section See en.wikipedia.org/wiki/critical_section 4
5 Overview of Mutual Exclusion Locks A mutual exclusion lock (mutex) defines a critical section Ensures only one thread can run in a block of code at a time Critical Section 5
6 Overview of Mutual Exclusion Locks A mutual exclusion lock (mutex) defines a critical section Ensures only one thread can run in a block of code at a time Acquire lock Critical Section 6
7 Overview of Mutual Exclusion Locks A mutual exclusion lock (mutex) defines a critical section Ensures only one thread can run in a block of code at a time Critical Section Running thread 7
8 Overview of Mutual Exclusion Locks A mutual exclusion lock (mutex) defines a critical section Ensures only one thread can run in a block of code at a time Other threads are kept at bay so they don t corrupt shared resources that can be set/get by multiple concurrent operations Critical Section Waiting threads Running thread 8
9 Overview of Mutual Exclusion Locks A mutual exclusion lock (mutex) defines a critical section Ensures only one thread can run in a block of code at a time Other threads are kept at bay so they don t corrupt shared resources that can be set/get by multiple concurrent operations Race conditions could occur if multiple threads could run within a critical section Critical Section Waiting threads Running threads Race conditions can arise when a program depends on the sequence or timing of threads for it to operate properly See en.wikipedia.org/wiki/race_condition 9
10 Overview of Mutual Exclusion Locks A mutual exclusion lock (mutex) defines a critical section Ensures only one thread can run in a block of code at a time Other threads are kept at bay so they don t corrupt shared resources that can be set/get by multiple concurrent operations After a thread leaves a critical section another thread can enter & start running Critical Section Waiting threads Release lock 10
11 Overview of Mutual Exclusion Locks A mutual exclusion lock (mutex) defines a critical section Ensures only one thread can run in a block of code at a time Other threads are kept at bay so they don t corrupt shared resources that can be set/get by multiple concurrent operations After a thread leaves a critical section another thread can enter & start running Critical Section Waiting threads Acquire lock 11
12 Overview of Mutual Exclusion Locks A mutual exclusion lock (mutex) defines a critical section Ensures only one thread can run in a block of code at a time Other threads are kept at bay so they don t corrupt shared resources that can be set/get by multiple concurrent operations After a thread leaves a critical section another thread can enter & start running Critical Section Waiting threads Running thread 12
13 Overview of Mutual Exclusion Locks A mutex is typically implemented in hardware via atomic operations Atomic operations appear to occur instantaneously & either change the state of the system successful or have no effect See en.wikipedia.org/wiki/linearizability 13
14 Overview of Mutual Exclusion Locks A mutex is typically implemented in hardware via atomic operations Implemented in Java via the compareandswap*() methods in the Unsafe class See earlier discussion of Java Atomic Classes & Operations 14
15 Human Known Use of Mutual Exclusion Locks 15
16 Human Known Use of Mutual Exclusion Locks A human known use of mutual exclusion locks is an airplane restroom protocol 16
17 Human Known Use of Mutual Exclusion Locks A human known use of mutual exclusion locks is an airplane restroom protocol 17
18 Human Known Use of Mutual Exclusion Locks A human known use of mutual exclusion locks is an airplane restroom protocol lock door 18
19 Human Known Use of Mutual Exclusion Locks A human known use of mutual exclusion locks is an airplane restroom protocol lock door 19
20 Human Known Use of Mutual Exclusion Locks A human known use of mutual exclusion locks is an airplane restroom protocol unlock door This protocol is fully-bracketed, i.e., person who locks must be the same as 20 the person who unlocks
21 End of Java ReentrantLock (Part 1) 21
22 Java ReentrantLock (Part 2) Douglas C. Schmidt Institute for Software Integrated Systems Vanderbilt University Nashville, Tennessee, USA
23 Learning Objectives in this Part of the Lesson Understand how the concept of mutual exclusion in concurrent programs Recognize how Java ReentrantLock provides mutual exclusion to concurrent programs 2
24 Overview of ReentrantLock 3
25 Provide mutual exclusion to concurrent Java programs Overview of ReentrantLock public class ReentrantLock implements Lock, java.io.serializable { See docs.oracle.com/javase/8/docs/api/java/ 4 util/concurrent/locks/reentrantlock.html
26 Provide mutual exclusion to concurrent Java programs Implements Lock interface Overview of ReentrantLock public class ReentrantLock implements Lock, java.io.serializable { See docs.oracle.com/javase/8/docs/api/ 5 java/util/concurrent/locks/lock.html
27 Applies the Bridge pattern Overview of ReentrantLock public class ReentrantLock implements Lock, java.io.serializable { Decouples its interface from its implementation so fair & non-fair semantics can be supported uniformly ReentrantLock Sync FairSync NonFairSync See en.wikipedia.org/wiki/bridge_pattern 6
28 Applies the Bridge pattern Locking handled by Sync Implementor hierarchy Overview of ReentrantLock public class ReentrantLock implements Lock, java.io.serializable { /** Performs sync mechanics */ final Sync sync; 7
29 Applies the Bridge pattern Locking handled by Sync Implementor hierarchy Inherits functionality from AbstractQueuedSynchronizer Overview of ReentrantLock public class ReentrantLock implements Lock, java.io.serializable { /** Performs sync mechanics */ final Sync sync; /** Sync implementation for ReentrantLock */ abstract static class Sync extends AbstractQueuedSynchronizer { } See docs.oracle.com/javase/8/docs/api/java/util/ 8 concurrent/locks/abstractqueuedsynchronizer.html
30 Applies the Bridge pattern Locking handled by Sync Implementor hierarchy Inherits functionality from AbstractQueuedSynchronizer Many Java synchronizers that rely on FIFO wait queues use this framework Overview of ReentrantLock public class ReentrantLock implements Lock, java.io.serializable { /** Performs sync mechanics */ final Sync sync; /** Sync implementation for ReentrantLock */ abstract static class Sync extends AbstractQueuedSynchronizer { } See gee.cs.oswego.edu/dl/papers/aqs.pdf 9
31 Applies the Bridge pattern Locking handled by Sync Implementor hierarchy Overview of ReentrantLock Inherits functionality from AbstractQueuedSynchronizer Optionally implements fair or non-fair lock acquisition model public class ReentrantLock implements Lock, java.io.serializable { public ReentrantLock (boolean fair) { sync = fair? new FairSync() : new NonfairSync(); } This param determines whether FairSync or NonfairSync is used The Reentrantlock fair & non-fair models follow 10 the same pattern used by the Java Semaphore
32 Applies the Bridge pattern Locking handled by Sync Implementor hierarchy Overview of ReentrantLock Inherits functionality from AbstractQueuedSynchronizer Optionally implements fair or non-fair lock acquisition model Ensures strict FIFO fairness, at the expense of performance public class ReentrantLock implements Lock, java.io.serializable { public ReentrantLock (boolean fair) { sync = fair? new FairSync() : new NonfairSync(); } 11
33 Applies the Bridge pattern Locking handled by Sync Implementor hierarchy Overview of ReentrantLock Inherits functionality from AbstractQueuedSynchronizer Optionally implements fair or non-fair lock acquisition model public class ReentrantLock implements Lock, java.io.serializable { public ReentrantLock (boolean fair) { sync = fair? new FairSync() : new NonfairSync(); } Enables faster performance at the expense of fairness 12
34 Applies the Bridge pattern Locking handled by Sync Implementor hierarchy Overview of ReentrantLock Inherits functionality from AbstractQueuedSynchronizer Optionally implements fair or non-fair lock acquisition model public class ReentrantLock implements Lock, java.io.serializable { public ReentrantLock (boolean fair) { sync = fair? new FairSync() : new NonfairSync(); } public ReentrantLock() { sync = new NonfairSync(); } The default behavior favors performance over fairness 13
35 Applies the Bridge pattern Locking handled by Sync Implementor hierarchy Overview of ReentrantLock Inherits functionality from AbstractQueuedSynchronizer Optionally implements fair or non-fair lock acquisition model public class ReentrantLock implements Lock, java.io.serializable { public ReentrantLock (boolean fair) { sync = fair? new FairSync() : new NonfairSync(); } public ReentrantLock() { sync = new NonfairSync(); } FairSync is generally much slower than NonfairSync, so use it accordingly 14
36 ReentrantLock is similar to the monitor lock provided by Java s built-in monitor objects Overview of ReentrantLock void void void boolean boolean lock() Acquires the lock unlock() Attempts to release this lock lockinterruptibly() Acquires the lock unless the current thread is interrupted trylock() Acquires the lock only if it is not held by another thread at the time of invocation trylock(long timeout, Timeunit unit) Acquires the lock if it is not held by another thread within the given waiting time and the current thread has not been interrupted See upcoming lessons on Java Built-in Monitor Object 15
37 ReentrantLock is similar to the monitor lock provided by Java s built-in monitor objects But also provides extended capabilities Overview of ReentrantLock void void void boolean boolean lock() Acquires the lock unlock() Attempts to release this lock lockinterruptibly() Acquires the lock unless the current thread is interrupted trylock() Acquires the lock only if it is not held by another thread at the time of invocation trylock(long timeout, Timeunit unit) Acquires the lock if it is not held by another thread within the given waiting time and the current thread has not been interrupted 16
38 ReentrantLock is similar to the monitor lock provided by Java s built-in monitor objects But also provides extended capabilities Overview of ReentrantLock void void void boolean boolean lock() Acquires the lock unlock() Attempts to release this lock lockinterruptibly() Acquires the lock unless the current thread is interrupted trylock() Acquires the lock only if it is not held by another thread at the time of invocation trylock(long timeout, Timeunit unit) Acquires the lock if it is not held by another thread within the given waiting time and the current thread has not been interrupted In contrast, Java s synchronized methods/statements are not interruptible 17
39 ReentrantLock is similar to the monitor lock provided by Java s built-in monitor objects But also provides extended capabilities Overview of ReentrantLock void void void boolean boolean lock() Acquires the lock unlock() Attempts to release this lock lockinterruptibly() Acquires the lock unless the current thread is interrupted trylock() Acquires the lock only if it is not held by another thread at the time of invocation trylock(long timeout, Timeunit unit) Acquires the lock if it is not held by another thread within the given waiting time and the current thread has not been interrupted Nor are Java s synchronized methods/statements non-blocking 18
40 Overview of ReentrantLock A ReentrantLock supports recursive mutex semantics unlocked (holdcount = 0) lock() T 1 locked (holdcount = 1) Critical Section See en.wikipedia.org/wiki/reentrant_mutex 19
41 Overview of ReentrantLock A ReentrantLock supports recursive mutex semantics The thread that hold the mutex can reacquire it without self-deadlock unlocked (holdcount = 0) lock() T 1 locked (holdcount = 1) Critical Section 20
42 Overview of ReentrantLock A ReentrantLock supports recursive mutex semantics The thread that hold the mutex can reacquire it without self-deadlock unlocked (holdcount = 0) lock() T 1 locked (holdcount = 2) Critical Section 21
43 Overview of ReentrantLock A ReentrantLock supports recursive mutex semantics The thread that hold the mutex can reacquire it without self-deadlock unlocked (holdcount = 0) lock() T 1 locked (holdcount = 2) Critical Section Recursive mutex semantics add a bit more overhead relative to nonrecursive semantics due to additional 22 software logic & synchronization
44 Overview of Key ReentrantLock Methods 23
45 Overview of Key ReentrantLock Methods It key methods acquire & release the lock public class ReentrantLock implements Lock, java.io.serializable { public void lock() { sync.lock(); } public void lockinterruptibly() throws InterruptedException { sync.acquireinterruptibly(1); } public boolean trylock() { return sync.nonfairtryacquire(1); } public void unlock() { sync.release(1); } 24
46 Overview of Key ReentrantLock Methods It key methods acquire & release the lock public class ReentrantLock implements Lock, java.io.serializable { public void lock() { sync.lock(); } public void lockinterruptibly() throws InterruptedException { sync.acquireinterruptibly(1); } public boolean trylock() { return sync.nonfairtryacquire(1); } public void unlock() { sync.release(1); } These methods are defined in the Java Lock interface 25
47 Overview of Key ReentrantLock Methods It key methods acquire & release the lock public class ReentrantLock implements Lock, java.io.serializable { public void lock() { sync.lock(); } public void lockinterruptibly() throws InterruptedException { sync.acquireinterruptibly(1); } public boolean trylock() { return sync.nonfairtryacquire(1); } public void unlock() { sync.release(1); } These methods all simply forward to their implementor methods, most 26 of which are inherited from the AbstractQueuedSynchronizer framework
48 Overview of Key ReentrantLock Methods It key methods acquire & release the lock lock() acquires the lock if it s available public class ReentrantLock implements Lock, java.io.serializable { public void lock() { sync.lock(); } 27
49 Overview of Key ReentrantLock Methods It key methods acquire & release the lock lock() acquires the lock if it s available If lock isn t available its implementation depends on the fairness policy public class ReentrantLock implements Lock, java.io.serializable { public void lock() { sync.lock(); } 28
50 Overview of Key ReentrantLock Methods It key methods acquire & release the lock lock() acquires the lock if it s available If lock isn t available its implementation depends on the fairness policy Non-fair implementations are optimized in hardware public class ReentrantLock implements Lock, java.io.serializable { public void lock() { sync.lock(); } See en.wikipedia.org/wiki/spinlock 29
51 Overview of Key ReentrantLock Methods It key methods acquire & release the lock lock() acquires the lock if it s available If lock isn t available its implementation depends on the fairness policy Non-fair implementations are optimized in hardware Fair implementations park themselves on a wait queue public class ReentrantLock implements Lock, java.io.serializable { public void lock() { sync.lock(); } 30
52 Overview of Key ReentrantLock Methods It key methods acquire & release the lock lock() acquires the lock if it s available lockinterruptibly() acquires lock unless interrupted public class ReentrantLock implements Lock, java.io.serializable { public void lockinterruptibly() throws InterruptedException { sync.acquireinterruptibly(1); } See lesson on Managing the Java Thread Lifecycle 31
53 Overview of Key ReentrantLock Methods It key methods acquire & release the lock lock() acquires the lock if it s available lockinterruptibly() acquires lock unless interrupted In contrast, lock() is not interruptible public class ReentrantLock implements Lock, java.io.serializable { public void lockinterruptibly() throws InterruptedException { sync.acquireinterruptibly(1); } 32
54 Overview of Key ReentrantLock Methods It key methods acquire & release the lock lock() acquires the lock if it s available lockinterruptibly() acquires lock unless interrupted trylock() acquires lock only if it s not held by another thread at invocation time public class ReentrantLock implements Lock, java.io.serializable { public boolean trylock() { sync.nonfairtryacquire(1); } Untimed trylock() doesn t honor 33fairness setting & can barge
55 Overview of Key ReentrantLock Methods It key methods acquire & release the lock lock() acquires the lock if it s available lockinterruptibly() acquires lock unless interrupted trylock() acquires lock only if it s not held by another thread at invocation time unlock() attempts to release the lock public class ReentrantLock implements Lock, java.io.serializable { public void unlock() { sync.release(1); } 34
56 Overview of Key ReentrantLock Methods It key methods acquire & release the lock lock() acquires the lock if it s available lockinterruptibly() acquires lock unless interrupted trylock() acquires lock only if it s not held by another thread at invocation time unlock() attempts to release the lock public class ReentrantLock implements Lock, java.io.serializable { public void unlock() { sync.release(1); } 35
57 Overview of Key ReentrantLock Methods It key methods acquire & release the lock lock() acquires the lock if it s available lockinterruptibly() acquires lock unless interrupted trylock() acquires lock only if it s not held by another thread at invocation time unlock() attempts to release the lock IllegalMonitorStateException is thrown if calling thread doesn t hold lock public class ReentrantLock implements Lock, java.io.serializable { public void unlock() { sync.release(1); } i.e., a ReentrantLock is fully bracketed! 36
58 Overview of Other ReentrantLock Methods 37
59 Overview of Other ReentrantLock Methods There are many other ReentrantLock methods boolean trylock(long timeout, TimeUnit unit) Acquires the lock if it is not held by another thread within the given waiting time and the current thread has not been interrupted boolean isfair() Returns true if this lock has fairness set true boolean islocked() Queries if this lock is held by any thread Condition newcondition() Returns a Condition instance for use with this Lock instance These methods go above & beyond what s available 38 from Java s synchronized statements/methods
60 Overview of Other ReentrantLock Methods There are many other ReentrantLock methods boolean trylock(long timeout, TimeUnit unit) Acquires the lock if it is not held by another thread within the given waiting time and the current thread has not been interrupted boolean isfair() Returns true if this lock has fairness set true boolean islocked() Queries if this lock is held by any thread Condition newcondition() Returns a Condition instance for use with this Lock instance Timed trylock() does honor fairness setting & can t barge 39
61 Overview of Other ReentrantLock Methods There are many other ReentrantLock methods boolean trylock(long timeout, TimeUnit unit) Acquires the lock if it is not held by another thread within the given waiting time and the current thread has not been interrupted boolean isfair() Returns true if this lock has fairness set true boolean islocked() Queries if this lock is held by any thread Condition newcondition() Returns a Condition instance for use with this Lock instance 40
62 Overview of Other ReentrantLock Methods There are many other ReentrantLock methods boolean trylock(long timeout, TimeUnit unit) Acquires the lock if it is not held by another thread within the given waiting time and the current thread has not been interrupted boolean isfair() Returns true if this lock has fairness set true boolean islocked() Queries if this lock is held by any thread Condition newcondition() Returns a Condition instance for use with this Lock instance Not very useful due to non-determinism of concurrency.. 41
63 Overview of Other ReentrantLock Methods There are many other ReentrantLock methods boolean trylock(long timeout, TimeUnit unit) Acquires the lock if it is not held by another thread within the given waiting time and the current thread has not been interrupted boolean isfair() Returns true if this lock has fairness set true boolean islocked() Queries if this lock is held by any thread Condition newcondition() Returns a Condition instance for use with this Lock instance See upcoming lesson on Java ConditionObject 42
64 End of Java ReentrantLock (Part 2) 43
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