programming with threads in java

Programming with Threads in Java

koji lin@twjug 2012/9/15

java.lang.Thread

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）

Thread Basics

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

Thread and Process

Benefits of Thread

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?

No!!

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 bylock

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

● Responding to interruption– throw exception again

– set interruption status

public class Thread{

public void interrupt(){}

public boolean isInterrupted(){}

public static boolean interrupted(){}

}

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

● 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

interface Lock { void lock(); void unlock(); …}

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

● Based on Fork/Join● More elegant with lambda

public interface ParallelIterable<T> ... {

void forEach(Block<? super T> block)...

...

}

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 ?

Yes

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/#documentation/Darwin/Conceptual/KernelProgramming/Mach/Mach.html