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

Recommended

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to Programming with Threads in Java (20)

More from koji lin (18)

Recently uploaded (20)

Programming with Threads in Java