Ricci, Alessandro

English

Almae Matris Studiorum Campus

SISCO LS - II Facoltà Ingegneria - Cesena Concurrent Programming in Java: Overview 1Sistemi Concorrenti e di Rete LSII Facoltà di Ingegneria - Cesenaa.a 2007/2008[module lab 1.1] CONCURRENT PROGRAMMING IN  JAVA: OVERVIEWv1.0SISCO LS - II Facoltà Ingegneria - Cesena Concurrent Programming in Java: Overview 2CONCURRENT PROGRAMMING IN JAVA• Java has been the first mainstream programming language to provide a first native support to concurrent programming– “conservative approach”: everything is still an object– + mechanisms for concurrency• Recently extended with the  java.util.concurrent library to provide a more fine-grained control  for synchronization– First developed as a Java Specification Request: JSR-166 • Doug Lea et at.• An extension with a more coarse-grained support is expected in in Java 7– Fork-Join Framework• Doug Lea, http://gee.cs.oswego.edu/dl/papers/fj.pdfSISCO LS - II Facoltà Ingegneria - Cesena Concurrent Programming in Java: OverviewBASIC MECHANISMS: OVERVIEW• Class Thread (along with a few related utility class) used to initiate and control concurrent activities– Runnable interface• Keyword synchronized and volatile, used to control the execution of code in objects that may participate in multiple threads– for mutual exclusion• Me thods wait, notify, and notifyAll as de f i ned i n java.lang.Object used to coordinate activities across threads – for real synchronization3Concurrent Programming in Java: OverviewSISOP LA - II Facoltà Ingegneria - Cesena 4DEFINING THREADS• Java provides a basic API for defining new types of thread,  and for dynamically create and (partially) manage thread execution– threads are mapped onto OS threads, with  strategies that depend on the specific system– typically a one-to-one approach is adopted • A thread is represented by the abstract class Thread, characterised by the abstract method run, which defines the behaviour of the thread– a concrete thread can be defined by extending Thread class, and implementing the run method• To start thread asynchronous execution, the method start is provided– must be invoked on the instance of a thread object– it returns immediately, and a new activity executing what specified in run method is launched• The thread terminates as soon as the execution of the method run body is completedConcurrent Programming in Java: OverviewSISOP LA - II Facoltà Ingegneria - Cesena 5Thread CLASS• Thread class is provided in the package java.langThreadrun():voidMyWorkerrun():voidpublic class MyWorker extends Thread  {    public MyWorker(String name){    super(name);  }  public void run(){    ...    <active behaviour>    ...  }}Concurrent Programming in Java: OverviewSISOP LA - II Facoltà Ingegneria - Cesena 6MAIN THREAD API• Main features provided by the Thread class:– Thread(String name) • to construct a thread with a specified name– String getName();• get the thread name– void sleep(long ms) • to suspend thread execution for ms milliseconds– void  join()  • wait for the termination of the thread – void interrupt() • causes a sleep, wait or join to abort with an InterruptedException, which can be caught and deal with in an application-specific way– static Thread currentThread() • to get the reference to current thread in executionConcurrent Programming in Java: OverviewSISOP LA - II Facoltà Ingegneria - CesenaSPAWNING THREADS• NOTE– the method executed on the thread object is start, not run • what if we execute the method run instead? what is the behaviour of the program Test if we invoke run instead of start for both the workers?– a Java application has always at least one thread in execution• itʼs the main thread on the application, invoking the static method main7public class Test {  public static void main(String[] args) {Thread myWorkerA = new MyWorker("worker-A");     myWorkerA.start();      Thread myWorkerB = new MyWorker("worker-B");     myWorkerB.start();   }}Concurrent Programming in Java: OverviewSISOP LA - II Facoltà Ingegneria - CesenaMONITORING THREADS: JConsole TOOL• JConsole is the Java Monitoring and Management Console, a graphical tool shipped in J2SE JDK 5.0 (and later versions)– it uses the instrumentation of the Java virtual machine to provide information on performance and resource consumption of applications running on the Java platform – based on the  Java Management Extension (JMX) technology– http://java.sun.com/developer/technicalArticles/J2SE/jconsole.html• Useful (also) to monitor the thread spawned by a running Java programs– including VM threads, such as the one used for garbage collecting• Usage– start the Java application that you want to monitor with the option:          • Example – java -Dcom.sun.management.jmxremote -cp bin sisco.test00.Test– from another shell, launch the jconsole tool by simply typing jconsole and connect to the running application• it is part of the JRE (Java Runtime Environment) 8-Dcom.sun.management.jmxremoteConcurrent Programming in Java: OverviewSISOP LA - II Facoltà Ingegneria - CesenaDEPRECATED• All the public methods to asynchronously act on the control flow of the thread have been deprecated– stop– suspend– resume– destroy– ...• The same functionality is achieved through proper patterns– next lab modules9Concurrent Programming in Java: OverviewSISOP LA - II Facoltà Ingegneria - Cesena 10Runnable Interface• An alternative approach is provided to define a thread, based on Runnable interface, useful when the class used to implement the thread belongs to some class hierarchy – ..already extending some  class, which is not Thread• Note the Runnable object parameter in Thread constructorRunnablerun():voidMyWorkerrun():voidThreadtargetpublic class MyWorker implements Runnable {  public void run(){    ...  }}...Thread th = new Thread(new MyWorker());th.start();Concurrent Programming in Java: OverviewSISOP LA - II Facoltà Ingegneria - CesenaIMPLICIT SYNCHRONIZATION:synchronized• By applying the keyword synchronized as a qualifier to any code block within any method, only one thread at a time can obtain access to the object where synchronized is defined – prevents arbitrary interleaving of the actions in the method bodies> prevents unintended interactions among thread accessing the same objects• Suggestion– to be used in passive objects that are shared and concurrently accessed (for updates) by multiple thread11Concurrent Programming in Java: OverviewSISOP LA - II Facoltà Ingegneria - CesenaEXPLICIT SYNCHRONIZATION• Set of mechanisms used for explicit synchronization among threads, through shared objects– wait method• any synchronized method in any object can contain a wait, which suspend the current thread– notifyAll method• all threads waiting on the target object are resumed upon the invocation of the method notifyAll on the target object• also the notifyAll method must be contained in a synchronized method or block– notify method• one (arbitrarily chosen) thread waiting on the target object is resumed upon invocation of method notify• also the notify method must be contained in a synchronized method or block12Concurrent Programming in Java: OverviewSISOP LA - II Facoltà Ingegneria - CesenaVADEMECUM:ATOMIC ASSIGNMENTS• Java guarantees that most primitive operations are atomic and will work always safely in multithreaded context without explicit synchronization. – These operations include individual access and assignment to all builtin scalar types  (including reference type) except long and double. – Without explicit synchronization concurrent assignment to long and double variables are allowed to be interleaved in ways that could result in reading of inconsistent values• The specification allows compilers to perform optimizations that caches values of variables in ways that may cause assignments  in other thread to be ignored >> this can be avoided by marking a variable as volatile13Concurrent Programming in Java: OverviewSISOP LA - II Facoltà Ingegneria - CesenaVADEMECUM:SYNCHRONIZED BLOCKS• In addition to methods, individual code block within any Java method can be synchronized via • Block synchronization is considered to be a more basic construct than method synchronization.  – A synchronized method is equivalent to one that is not marked as synchronized but has al l of i ts code contained within a synchronized(this) block• Class level static methods and blocks within static methods may be declared as synchronized.  – A non-static method can also lock static data via code block enclosed within synchronized(getClass())14synchronized (anyObject) {   anyCode(); }Concurrent Programming in Java: OverviewSISOP LA - II Facoltà Ingegneria - CesenaVADEMECUM: IMPLICIT SYNCH. AND OBJECT LOCKING• (Implicit) synchronization is implemented by exclusively accessing the underlying and otherwise inaccessible internal lock (called also mutex) that is associated with each Java object (including Class objects for statics). – If the count value (of the mutex) is not zero on entry to a synchronized method or block because another thread holds the lock, the current thread is blocked until the count is zero. – On entry, the count value is incremented. – The count is decremented on exit from each synchronized method or block, even if it is terminated via an exception• Any  method or code block marked as synchronized is executed in its entirely (unless explicitly suspended via wait) before the object is allowed to perform any other synchronized method called from any other thread15Concurrent Programming in Java: OverviewSISOP LA - II Facoltà Ingegneria - CesenaVADEMECUM: SELF-CALL SYNCH METHODS• Code in one synchronized method may make a self-call to another method in the same object without blocking. – Similarly for calls on other objects for which current thread has obtained and not yet released the lock. – Only those calls stemming from other threads are blocked. – Synchronization is retained when calling an unsynchronized method from a synchronized one16Concurrent Programming in Java: OverviewSISOP LA - II Facoltà Ingegneria - CesenaVADEMECUM: SYNCHRONIZED & INHERITANCE• If a method is not marked as synchronized, then it may execute immediately whenever invoked, even while another synchronized method is executing . – Thus, declaring a method synchronized is not sufficient to ensure exclusive access: any other unsynchronized methods may run concurrently with it• The synchronized qualifier for methods can be overridden in subclasses– A subclass overriding a superclass method must explicitly declare it as synchronized if so desired– Otherwise, it is treaded as unsynchronized • in other words, the synchronized qualifier is not automatically inherited• Methods declared in Java interfaces cannot be qualified as synchronized17Concurrent Programming in Java: OverviewSISOP LA - II Facoltà Ingegneria - CesenaVADEMECUM: EXPLICIT SYNCHRONIZATION• Because of the way in which wait sets interact with locks, the methods wait, notify, and notifyAll may be invoked only when the synchronization lock is held on their targets – This is normally ensured by using them only within methods or code blocks synchronized  on their targets• Compliance generally cannot be verified at compile time• Failure to comply causes these operations to throw an IllegalMonitorStateException at run time18Concurrent Programming in Java: OverviewSISOP LA - II Facoltà Ingegneria - CesenaVADEMECUM: WAIT INVOCATION•  A wait invocation results in the following actions:– If the current thread has been interrupted, then the method exits immediately, throwing an InterruptedException • Otherwise, the current thread is blocked– The JVM places the thread in the internal and otherwise inaccessible wait set associated with the target object– The synchronization lock for the target object is released, but all other locks held by the thread are retained • A full release is obtained even if the lock is re-entrantly held due to nested synchronized calls on the target object• Upon later resumption, the lock status is fully restored19Concurrent Programming in Java: OverviewSISOP LA - II Facoltà Ingegneria - CesenaVADEMECUM: NOTIFY INVOCATION• A notify invocation results in the following actions:– If one exists, an arbitrarily chosen thread, say T, is removed by the JVM from the internal wait set associated with the target object. • There is no guarantee about which waiting thread will be selected when the wait set contains more than one thread – T must re-obtain the synchronization lock for the target object, which will always cause it to block at least until the thread calling notify releases the lock. • It will continue to block if some other thread obtains the lock first.– T is then resumed from the point of its wait• A notifyAll works in the same way as notify except that the steps occur (in effect, simultaneously) for all threads in the wait set for the object. – However, because they must acquire the lock, threads continue one at a time20Concurrent Programming in Java: OverviewSISOP LA - II Facoltà Ingegneria - CesenaVADEMECUM: WAIT INTERRUPTION• If Thread.interrupt is invoked for a thread suspended in a wait, the same notify mechanics apply, except that after re-acquiring the lock, the method throws an InterruptedException and the thread's interruption status is set to false. • If an interrupt and a notify occur at about the same time, there is no guarantee about which action has precedence, so either result is possible – Future revisions of JLS may introduce deterministic guarantees about these outcomes.21Concurrent Programming in Java: OverviewSISOP LA - II Facoltà Ingegneria - CesenaVADEMECUM: WAIT-TIMED VERSION• The timed versions of the wait method, wait(long msecs) and wait(long msecs, int nanosecs), take arguments specifying the desired maximum time to remain in the wait set. • They operate in the same way as the untimed version except that if a wait has not been notified before its time bound, it is released automatically. • There is no status indication differentiating waits that return via notifications versus time-outs. – Counter-intuitively, wait(0) and wait(0,0) both have the special meaning of being equivalent to an ordinary untimed wait().• A timed wait may resume an arbitrary amount of time after the requested bound due to thread contention, scheduling policies, and timer granularities. – There is no guarantee about granularity. Most JVM implementations have observed response times in the 1-20ms range for arguments less than 1ms.22

module-lab-1.1-Concurrent Programming in Java - Overview

http://campus.unibo.it/7726/1/module-lab-1.1-Concurrent_Programming_in_Java_-_Overview.pdf

module-lab-1.1-Concurrent Programming in Java - Overview

Abstract

Similar works

Full text

Available Versions

Almae Matris Studiorum Campus