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Programming with Threads in Java
Threads allow programs to execute multiple tasks simultaneously. In Java, threads are lightweight processes that exist within a process and share its resources. The key benefits of multithreading include taking advantage of multiprocessor systems and simplifying programming models. However, multithreading also introduces risks like race conditions and deadlocks that must be addressed through synchronization and thread safety.
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Programming with Threads in Java
- 1. Programming with Threadsin Java koji lin@twjug 2012/9/15
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- 3. Multiple Threads within the same program can be scheduled simultaneously on multiple CPUs. Most modern operating systems treat threads, not processes, as the basic units of scheduling ~Java concurrency in practice
- 4. On a computerwith multiprocessors, processes or threads can run on different processors ~MSDN Threads and Processes
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- 7. Threads are everywhere ● JVM creates thread for GC ● AWT, Swing and JavaFX use event dispatch thread ● Timer for deferred tasks ● Application server handles multiple client – Servlet must be thread-safe ● RMI
- 8. What is Thread? ● Process – A program in execution – Providing the resources needed to execute a program – One process can't access or modify other process
- 9. What is Thread? ● Thread – A basic unit of CPU utilization – Lightweight process (LWP) – Multiple threads are executed within a process ● Share process's virtual address space and system resource
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- 12. Multithreading Models ● User threads – Efficient, flexible – Above the kernel, without kernel support ● Kernel threads – kernel can assign one thread to each logical core in a system
- 13. Multithreading Models ● User Level Threading – Many-to-One(N:1) – Green Threads, GNU Portable Threads ● Kernel Level Threading – One-to-One(1:1) – FreeBSD, Linux, Windows, Mac, Solaris... ● Hybrid – Many-to-Many(M:N) – Solaris(before 9), Tru64 Unix
- 14. Java on eachOS ● Windows – Native thread(Windows 95/NT) ● Linux – Native thread since JDK 1.3 – LinuxThread, NPTL(Since Red Hat 9) ● FreeBSD – Native thread since JDK 1.3.1 – libpthread(FreeBSD 5.3) – libthr(FreeBSD 7)
- 15. How JVM createsthread? ● Thread.java – Start0 invokes StartThread ● jvm.cc – VM_ENTRY(void, JVM_StartThread(JNIEnv* env, jobject jthread)) invokes JavaThread ● Thread.cpp – JavaThread::JavaThread invokes os::create_thread ● os_windows.cpp, os_linux.cpp, os_bsd.cpp – Win32 Thread,NPTL, libthr
- 16. Does JVM do somethingspecial?
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- 18. So, Why Threadin Java? ● Thread is inescapable feature of Java ● Take advantage of multiprocessor system ● Simplify modeling ● Thread is cheap ● Don't need to worry about memory model in different environment
- 19. Risks ● Safety Hazards – synchronization ● Liveness Hazards – deadlock – starvation ● Performance Hazards – context switch – synchronization
- 20. Thread safety ● Behaves correctly when accessed from multiple threads, and there is no synchronization or coordination on caller – java.text.SimpleDateFormat is not thread safe – Stateless servlet is safe
- 21. Race conditions ● The output is dependent on the sequence or timing of other uncontrollable events 1) if(!map.containsKey(key)){ map.put(key,value); } 2) int n; int calculate(){ return n++; }
- 22. Synchronized ● Only one thread can execute the block of code protected by the same lock at the same time ● Ensures that each thread entering a synchronized block of code sees the effects of all previous modifications that were guarded by the same lock synchronized(object) { //do something... … }
- 23. Visibility problem ● There is no guarantee that the reading thread will see a value written by another thread ● Using lock or volatile variable
- 24. Immutability ● Immutable object is always thread-safe – Its state cannot be modified after construction – All its fields are final – It is properly constructed (object doesn't escape during construction) ● Even when synchronization is not used to publish the object reference
- 25. Safe publication ● Objects that are not immutable must be safely published ● A properly constructed object can be safely published by: – Initializing an object reference form static initializer – Storing into volatile or AtomicReference – Storing into a final field of properly constructed object – Storing into a field properly guarded by lock
- 26. Java Memory Model(JSR-133) ● Defines the semantics of multithreaded programs – Ensure your program run on all processor architecture ● Happens-before – If no, JVM is free to reorder ● New guarantees for Volatile ● Initialization Safety – final
- 27. Happens-before ● Program order ● Monitor lock – explicit Lock object ● Volatile variable – AtomicXXX ● Thread start, termination ● Interruption
- 28. Happens-before ● Finalizer ● Some class libraries – Concurrent containers ● Transitivity – A -> B ,B -> C then A -> C
- 29. Volatile MapconfigOptions; volatile boolean initialized = false; // In Thread A configOptions = new HashMap(); ConfigOptions.put(); initialized = true; // In Thread B while (!initialized) sleep(); // use configOptions
- 30. Initialization Safety ● When object is properly constructed, then all threads will see the values for its final fields that were set in its constructor, regardless of whether or not synchronization is used ● Similar to a happens-before relationship between the write of a final field in a constructor and the initial load of a shared reference to that object in another thread
- 31. Executor framework ● If we have lots of tasks with threads, we need to consider: – How many thread should we create? – How to stop them? – What happened when a task failed?
- 32. Executor framework ● Executor manages running tasks – Submit a Runnable to be run with Executor#execute() final ExecutorService executor = ...; executor.execute(new Runnable(){ @Override public void run(){ // do the task } });
- 33. Task cancellation ● Using interruption public class Thread{ public void interrupt(){} public boolean isInterrupted(){} public static boolean interrupted(){} } ● Responding to interruption – throw exception again – set interruption status
- 34. Non-interruptible block ● Synchronous Socket IO – Close socket ● Lock – Using explicit Lock and lockInterruptibly
- 35. java.util.concurrent.* ● Atomic* ● Lock – ReentrantLock – ReadWrtieLock ● CountDownLatch ● Semaphore ● ConcurrentHashMap ● Fork/Join (Java SE 7)
- 36. Atomic* ● Lock-free thread-safe on single variable ● AtomicInteger, AtomicLong, AtomicReference , etc. – getAndAdd, addAndGet, incrementAndGet, decrementAndGet, compareAndSet, etc. ● AtomicStampedReference, AtomicMarkableReference – ABA problem
- 37. Lock interface Lock { void lock(); void unlock(); … } ● Only one thread can hold a lock at once ● ReentrantLock – Can be reacquired by same thread – Other threads can't acquire lock until has been released same number of times has been acquired
- 38. ReadWriteLock(1) ● Readers-writers problem – 同時有複數個讀與寫的動作想要執行 , 當有寫入動作 時，其他讀寫都不能執行；而沒有寫入動作時，則可 同時執行多個讀取。 ● Writer starvation/Writer preference ● Fair/Unfair mode – 是否依照抵達順序 – 實作上看似仍會去避免無限期延遲的狀況
- 39. ReadWriteLock(2) ● Fair - 當 reader 取得 lock 後有 writer 在等待， 那麼之後的 reader 將會等到該 writer 取得並釋 放後才能取得。 ● Unfair - 當 Queue 中沒有 reader 時，行為同上 ; 但是當新的 reader 到達時，還有 reader 在 deque ，則新 reader 會跳過等待的 writer 先執行。 (bug id:6816565)
- 40. Semaphore ● Counting Semaphore ● 用於管理有限資源存取 – 例如 Pool ● acquire(), tryAcquire() – 當計數不為 0 時，內部計數減１並允許執行 – 如果計數為 0 則等待直到計數不為 0 ● release() – 內部計數加１
- 41. ConcurrentHashMap ● We love HashMap – An easy to use Key-Value store ● Some new methods aware concurrency – putIfAbsent – remove(key, value) – replace(key, value) – replace(key, old value, new value)
- 42. JDK7 Fork/Join ● Fine-Grain Parallelism 1.Divide large task into small tasks 2.Process each task in separate thread 3.Join results ● ForkJoinPool ● ForkJoinTask – RecursiveTask – RecursiveAction
- 43. JDK7 Fork/Join ● Work Stealing
- 44. JDK8 ParallelIterable public interface ParallelIterable<T> ... { void forEach(Block<? super T> block)... ... } ● Based on Fork/Join ● More elegant with lambda users.parallel().forEach( u -> {...}); users.parallel().sorted( (u1, u2) -> u1.name.compareTo(u2.name));
- 45. Useful tools ● ps -eLF – show thread and process information ● jstack, jcmd – command line, useful on server environment ● Jconsole, VisualVM – visual tool integrate commands and tools
- 46. Is multithreaded programming hard ?
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- 48. More... ● To utilize multiprocessor, which one is better? – Thread or Process ● Performance – How many thread is enough? Or only one thread? ● Other language or platform – Python, Ruby, C, Node.js, .Net, etc.
- 49. Reference ● WIKIPEDIA Thread – http://en.wikipedia.org/wiki/Thread_(computing) ● Extending the Haskell Foreign Function Interface with Concurrency – Simon Marlow, Simon Peyton Jones, and Wolfgang Thaller, Haskell workshop 2004. ● JSR-133 ● http://www.cs.umd.edu/~pugh/java/memoryMod el/jsr-133-faq.html#volatile
- 50. Reference ● Java Concurrency in Practice ● Performance of Multi-Process and Multi- ThreadProcessing on Multi-core SMT Processors – https://www.research.ibm.com/trl/people/inouehrs/p df/IISWC2010_inoue_slides.pdf ● Java Technology on the Linux Platform – http://java.sun.com/developer/ technicalArticles/Programming/linux/ ● http://hg.openjdk.java.net/
- 51. Reference ● Java theory and practice: Fixing the Java Memory Model, Part 2 – http://www.ibm.com/developerworks/library/j- jtp03304/ ● Programming with POSIX Threads ● Kernel Programming Guide(OS X) – https://developer.apple.com/library/mac/#document ation/Darwin/Conceptual/KernelProgramming/Mach /Mach.html