Java并发编程实战指南：掌握多线程安全与性能优化

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《Java并发编程实战》是一本由业内专家Brian Göetz、Tim Peierls、Joshua Bloch、Joseph Bowbeer、David Holmes和Doug Lea共同编著的专业书籍，它针对Java并发编程进行了深入探讨。本书属于Addison-Wesley Professional系列，ISBN号分别为10位和13位。书中内容涵盖了并发编程在实践中的重要性和应用。该书首先介绍了并发编程的历史背景，强调了其在多处理器环境下的优势，包括能够充分利用系统资源、简化模型构建、处理异步事件以及提升用户界面的响应性。然而，同时指出并发编程也伴随着风险，如安全、活性（liveness）和性能方面的挑战。这些风险包括线程安全问题、死锁、以及可能的性能瓶颈。在基础部分，第2章“线程安全”详细阐述了什么是线程安全，讨论了原子性（atomicity）的概念，即在并发环境中确保操作的不可分割性。接着，作者探讨了锁定（locking）机制，它是实现线程同步的关键手段，帮助保护共享数据免受竞态条件的影响。此外，章节还强调了如何通过锁来保护状态，以及线程安全与性能之间的平衡。第三章“共享对象”进一步深入到并发编程的核心——如何在多个线程间安全地共享和操作对象。这包括了对可见性、有序性和内存模型的解释，以及如何设计和实现高效且可维护的并发代码。本书的其余部分可能还会涉及更高级的主题，如并发数据结构、线程池、死锁检测、并行计算、非阻塞算法和Java并发工具类的使用（如synchronized、volatile、ExecutorService等）。作者们通过实例和最佳实践，帮助读者理解和掌握Java并发编程的复杂性，以及如何避免常见陷阱，从而在实际开发中实现高效的并发程序。《Java Concurrency in Practice》是一本实用的指南，不仅适合Java开发者深入理解并发编程原理，也对希望提高并发应用程序性能和正确性的人士具有重要参考价值。通过阅读这本书，读者将能够更好地应对现代软件系统中并发性带来的挑战。

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2

JavaConcurrencyInPractice

1.1.A(Very)BriefHistoryofConcurrency

Intheancientpast,computers didn't haveoperating systems; they executeda single programfrombeginningtoend,

andthatpro gramhaddirectaccesstoalltheresourcesofthemachine.Notonlywasitdifficulttowriteprogramsthat

ran on the bare metal, but running only a single program at a time was an inefficient use of ex

pensive and scarce

computerresources.

Operatingsystems evolvedtoallowmorethanone programtorunatonce,runningindividualprogramsinprocesses:

isolated, independently executing programs to which the operating system allocates resources such as memory, file

handles,and securitycredentials.Iftheyneede

dto,processescouldcommunicatewithoneanotherthrough avariety

ofcoarsegrainedcommunicationmechanisms:sockets,signalhandlers,sharedmemory,semaphores,andfiles.

Several motivating factors led to the development of operating systems that allowed multiple programs to execute

simultaneously:

Resource utilization. Programs sometimes have to wait for ex

ternal operations such as input or output, and while

waitingcandonousefulwork.Itismoreefficienttousethatwaittimetoletanotherprogramrun.

Fairness. Multiple users and programs  may have equal claims on the machine's resources. It is preferable to let them

sharethecomputerviafinergraine

dtimeslicingthantoletoneprogramruntocompletionandthenstartanother.

Convenience. It is often easier or more desirable to write sever

al programs that each perform a single task and have

themcoordinatewitheachotherasnecessarythantowriteasingleprogramthatperformsallthetasks.

Inearlytimesharingsystems,each processwasavirtual vonNeumanncomputer;ithad ame

moryspace storingboth

instructions and data, executing instructions sequentially according to the semantics of the machine language, and

interacting with the outside world via the operating system through a set of I/O primitives. For each instruction

executedtherewasaclearlydefined"nextinstruction",andcontrolflowedthroughtheprogramaccordingtother

ules

oftheinstructionset.Nearlyallwidelyusedprogramminglanguagestodayfollowthissequentialprogrammingmodel, 

wherethelanguagespecificationclearlydefines"whatcomesnext"afteragivenactionisexecuted.

Thesequentialprogrammingmodelisintuitiveandnatural,asitmodelsthewayhumanswor

k:doonethingatatime,

in sequence mostly. Get out of bed, put on your bathrobe, go downstairs and start the tea. As in programming

languages, each of these realworld actions is an  abstraction for a sequence of finergrained actionsopen the

cupboard,selectaflavoroftea,measuresometeaintothepot,seeifthere'senoughwaterinthetea

kettle,ifnotput

somemorewaterin,setitonthestove,turnthestoveon,waitforthewatertoboil,andsoon.Thislaststepwaiting

forthewatertoboilalsoinvolvesadegreeofasynchrony.Whilethewaterisheating,youhaveachoiceofwhattodo

just wait, or do other tasks in that time such as starting th

e toast (another asynchronous task) or fetching the

newspaper, while remaining aware that your attention will so on be needed by the t

eakettle. The manufacturers of

teakettlesandtoasterskn owtheirproductsareoftenusedinanasynchronousmanner,sotheyraiseanaudiblesignal

when they complete their task. Finding the right balance of sequentiality and asynchrony is often a characteristic of

efficientpeopleandthesameistrueofprograms.

Thesameco

ncerns(resourceutilization,fairness,andconvenience)thatmotivatedthedevelopmentofprocessesalso

motivated the development of threads. Threads allow multiple streams of program control flow to coexist within a

process. They share processwide reso urces such as memory and file handles, but each thread has its own progra m

counter,stack,andlocalva

riables.Threadsalsoprovideanaturaldecompositionforexploitinghardwareparallelismon 

multiprocessorsystems;multiplethreadswithinthesameprogramcanbescheduledsimultaneouslyonmultipleCPUs.

Threadsaresometimescalledlightweightprocesses,andmostmodernoperatingsystemstreatthreads,notprocesses,

as the basic units of scheduli

ng. In the absence of explicit coordination, threads execute simultaneously and

asynchronouslywithrespecttooneanother.Sincethreadssharethememoryaddressspaceoftheirowningprocess,all

threadswithinaprocesshaveaccesstothesamevariables andallocateobjectsfromthesameheap,whichallowsfiner

grained da

ta sharing than interprocess mechanisms. But without explicit synchronization to coordinate access to

shareddata,athreadmaymodifyvariablesthatanotherthreadisinthemiddleofusing,withunpredictableresults.

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3

3BChapter1 Introduction11B1.2.BenefitsofThreads

1.2.BenefitsofThreads

Whenusedproperly,threadscanreducedevelopmentandmaintenancecostsandimprovetheperformanceofcomplex 

applications.Threadsmakeiteasiertomodelhowhumansworkandinteract,byturningasynchronousworkflowsinto

mostly sequential ones. They can also turn otherwise convoluted code into  straightline code that is easier to write,

read,andma

intain.

ThreadsareusefulinGUIapplicationsforimprovingtheresponsivenessoftheuserinterface,andinserverapplications

for improving resource utilization and throughput. They also simplify the implementation of the JVMthe garbage

collector usually runs in one or more dedicated threads. Most nontrivial Java applications rely to so

me degree on

threadsfortheirorganization.

1.2.1.ExploitingMultipleProcessors

Multiprocessor systems used to be expensive and rare, found only in large data centers and scientific computing

facilities.Todaytheyarecheapand plentiful;evenlowendserverandmidrangedesktop systemsoftenhavemultiple

processors.Thistrendwillonlyaccelerate;asitgetshardertoscaleupclockrates,processormanufactur

erswillinstead

put more processor cores on a single chip. All the major chip manufacturers have begun this transition, and we are

alreadyseeingmachineswithdramaticallyhigherprocessorcounts.

Sincethebasicunitofschedulingisthethread,aprogramwith onlyonethreadcanrunonatmostoneprocessorata

time.Onatw

oprocessorsystem,asinglethreadedprogramisgivingupaccesstohalftheavailableCPUresources;ona

100processor system, it is giving up access to 99%. On the other hand, programs with multiple active threads can

execute simultaneously on multiple processors. When prope

rly designed, multithreaded programs can improve

throughputbyutilizingavailableprocessorresourcesmoreeffectively.

Using multiple threads can also help achieve better throughput on singleprocessor systems. If a prog ram is single

threaded, the processor remains idle  while it waits for a synchronous I/O operation to complete. In a multithr

eaded

program,anotherthreadcanstillrunwhilethefirstthreadiswaitingfortheI/Otocomplete,allowingtheapplicationto

stillmakeprogressduringtheblockingI/O.(Thisis likereadingthenewspaperwhilewaitingforthewatertoboil,rather

thanwaitingforthewatertoboilbefor

estartingtoread.)

1.2.2.SimplicityofModeling

It is often easier to manage your time when you have only one type of task to perform (fix these twelve bugs) than

when you have several (fix the bugs, interview replacement candidates for the system administrator, complete your

team'sperformanceevaluations,andcreatetheslidesforyourpresentationnextweek).Wh

enyouhaveonlyonetype

of task to do, youcanstartat the top of thepile andkeep working until thepile is exhausted (or you are); you don't

haveto spend any mental energy fi guring out what to work on next.On the otherhand, managingmultiple prioriti

es

anddeadlinesandswitchingfromtasktotaskusuallycarriessomeoverhead.

The same is true for software: a program that pro cesses one type of task sequentially is simpler to write, less error

prone,andeasiertotestthanonemanagingmultipledifferenttypesoftasksatonce.Assigningathreadtoeachtypeof

taskortoeachelementinasimulationaffordstheill

usionofsequentialityandinsulatesdomainlogicfromthedetails of

scheduling, interleaved operations, asynchronous I/O, and resourcewaits.A complicated, asynchronous workflow can

be decomposed into anumber of simpler, synchronousworkflowseach running in aseparate thread, i

nteracting only

witheachotheratspecificsynchronizationpoints.

This benefit is often exploited by frameworks such as servlets or RMI (Remote Method Invocation). The framework

handles the details of request management, thread creation, and load balancing, dispatching portions of the request

handling to the appropriate applica

tion component at the appropriate point in the workflow. Servlet writers do not

needtoworryabouthowmanyotherrequestsarebeingprocessedatthesametimeorwhetherthesocketinputand

output streams block; when a servlet's

service method is called in response to a web request, it can process  the

requestsynchronously as if it were a singlethreaded program. Thiscan simplify compone nt developmentandreduce

thelearningcurveforusingsuchframeworks.

1.2.3.SimplifiedHandlingofAsynchronousEvents

Aserverapplicationthatacceptssocketconnectionsfrommultipleremoteclientsmaybeeasiertodevelopwheneach

connectionisallocateditsownthreadandallowedtousesynchronousI/O.



4

JavaConcurrencyInPractice

If an application goes to read from a socket when no data is available,

read blocks until some data is available. In a

singlethreadedapplication,thismeansthatnotonlydoesprocessingthecorrespondingrequeststall,butprocessingof

all requests stalls while the single thread is blocked. To avoid this problem, singlethreaded server applications are

forcedtousenonblockingI/O,whichisfarmorecomplica

tedanderrorpronethansynchronous I/O.However,ifeach

requesthasitsownthread,thenblockingdoesnotaffecttheprocessingofotherrequests.

Historically,operatingsystemsplacedrelativelylowlimitsonthenumberofthreadsthataprocesscouldcreate,asfew

asseveralhundred(or ev

enless).Asaresult,operatingsystemsdevelopedefficientfacilitiesformultiplexedI/O,such

as the Unix

select and poll system calls, and to access these facilities, the Java class libraries acquired a set of

packages (

java.nio) for nonblocking I/O. However, operating system support for larger numbers of threads has 

improved significantly, making the threadperclient model practical eve n for large numbers of clients on some

platforms.

[1]



[1]TheNPTLthreadspackage,nowpartofmostLinuxdistributions,wasdesignedtosupporthundredsofthousandsofthreads.NonblockingI/O

hasitsownbenefits,butbetterOSsupportforthreadsmeansthattherearefewersituationsforwhichitisessential.

1.2.4.MoreResponsiveUserInterfaces

GUIapplicationsusedtobesinglethreaded,whichmeantthatyouhadtoeitherfrequentlypollthroughoutthecodefor

input events (which is messy and intrusive) or execute all application code indirectly through a "main event loop". If

codecall ed fromthemaineventlooptakestoolongtoexecute,the userinte

rfaceappearsto"freeze"until thatcode

finishes,becausesubsequentuserinterfaceeventscannotbeprocesseduntilcontrolisreturnedtothemaineventloop.

Modern GUI frameworks, such as the AWT and Swing toolkits, replace the main event loop with an event dispatch

thread(EDT).Whenauserinterfaceev

entsuchasabutton pressoccurs,applicationdefinedeventhandlersarecalled

in the event thread. Most GUI frameworks are singlethreaded subsystems, so the main event loop is effectively still

present,butitrunsinitsownthreadunderthecontroloftheGUItoolkitratherthanth

eapplication.

Ifonlyshortlivedtasksexecutein theeventthread,theinterface remainsresponsivesincetheeventthreadisalways

able to process user actions reasonably quickly. However, processinga longrunning task in the event thread, such as

spellchecking alarge documentorfetchingaresourceoverthe network,impairsresponsiveness.Iftheuserperfor

ms

anactionwhilethistaskisrunning,thereisalongdelaybeforetheeventthreadcanprocessorevenacknowledgeit.To

addinsulttoinjury,notonlydoestheUIbecomeunresponsive,butitisimposs ibletocanceltheoffendingtaskevenif

theUIprovid

esacancelbuttonbecausetheeventthreadisbusyandcannothandlethecancelbuttonpresseventuntil

the lengthy task complet es! If, however, the longrunning task is instead executed in a separate thread, the event

threadremainsfreetoprocessUIevents,makingth

eUImoreresponsive.

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