Programming with Threads in Java

Programming with Threads in Java koji lin@twjug 2012/9/15

Multiple Threads with in 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

On a computer with multiprocessors, processes or threads can run on different processors ~MSDN Threads and Processes

其實只想講上面兩段，結束（誤）

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

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

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

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

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

Java on each OS ● 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)

How JVM creates thread? ● 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

Does JVM do something special?

So, Why Thread in 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

Risks ● Safety Hazards – synchronization ● Liveness Hazards – deadlock – starvation ● Performance Hazards – context switch – synchronization

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

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++; }

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... … }

Visibility problem ● There is no guarantee that the reading thread will see a value written by another thread ● Using lock or volatile variable

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

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

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

Happens-before ● Program order ● Monitor lock – explicit Lock object ● Volatile variable – AtomicXXX ● Thread start, termination ● Interruption

Happens-before ● Finalizer ● Some class libraries – Concurrent containers ● Transitivity – A -> B ,B -> C then A -> C

Volatile Map configOptions; volatile boolean initialized = false; // In Thread A configOptions = new HashMap(); ConfigOptions.put(); initialized = true; // In Thread B while (!initialized) sleep(); // use configOptions

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

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?

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 } });

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

Non-interruptible block ● Synchronous Socket IO – Close socket ● Lock – Using explicit Lock and lockInterruptibly

java.util.concurrent.* ● Atomic* ● Lock – ReentrantLock – ReadWrtieLock ● CountDownLatch ● Semaphore ● ConcurrentHashMap ● Fork/Join (Java SE 7)

Atomic* ● Lock-free thread-safe on single variable ● AtomicInteger, AtomicLong, AtomicReference , etc. – getAndAdd, addAndGet, incrementAndGet, decrementAndGet, compareAndSet, etc. ● AtomicStampedReference, AtomicMarkableReference – ABA problem

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

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

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)

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

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)

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

JDK7 Fork/Join ● Work Stealing

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));

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

Is multithreaded programming hard ?

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.

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

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/

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

Programming with Threads in Java

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