java programming: advanced topics 1 multithreading chapter 5

Java Programming: Advanced Topics

1

Multithreading

Chapter 5


2

Objectives

• Explore the concepts of threads and multithreading

• Understand the lifecycle of a thread• Create, start, and stop threads using

java.lang.Thread and java.lang.Runnable• Create daemon threads• Use the JVM and java.util.Timer objects to

schedule execution of java.util.TimerTask objects


3

Objectives (Cont.)

• Make your multithreaded programs robust• Make class variables thread-safe using the

ThreadLocal class• Synchronize threads using the keyword

synchronized• Set up interthread communication using the

methods Object.wait, Object.notify, and Object.notifyAll

• Create and use ThreadGroup objects that contain sets of threads


4

Threads and Multithreading• Java includes the following features for

multithreading:– The class java.lang.Thread– The interface java.lang.Runnable– The classes java.util.TimerTask and java.util.Timer– The class java.lang.ThreadLocal– Internal mechanism for thread synchronization– The methods wait, notify, and notifyAll inherited from

java.lang.Object– The class java.lang.ThreadGroup


5

Single-Threaded and Multithreaded Programs


6

The Lifecycle of a Thread• Threads start running, pause, resume, and

finally stop dynamically during program execution

• The states of all threads except the thread that the JVM is currently executing cannot change

• By default, the JVM tries to distribute control equally to all threads

• Threads with high priority values tend to preempt lower priority threads


7

Creating and Running Threads • A class can run as a separate thread if it extends

the Thread class or implements the Runnable interface

• The Thread class provides the full infrastructure for multithreading, and implements Runnable

• All classes that implement Runnable must implement the method run

• Implementing Runnable is the more flexible and preferred approach


8

Using the Thread Class• Methods:

– Thread currentThread()– int getPriority()– void interrupt()– boolean isDaemon()– boolean isAlive()– void join()– void join( long milliseconds)


9

Using the Thread Class (Cont.)• Methods:

– void run()– void setDaemon(boolean on)– void setPriority(int priority)– void sleep(long milliseconds)– void start()– void yield()


10

State Transition Diagram of a Thread


11

Sample Class That Extends Thread


12

Using the Runnable Interface

• The java.lang.Runnable interface defines the protocol that must be followed by all threads

• The run() is the only method in the Runnable interface

• You must provide an implementation of the run method in every class that can be run as a spawned thread


13

Sample Class That Implements Runnable


14

Stopping a Thread


15

Creating Service Threads• Daemon threads are threads that run in the

background and typically have no associated user interface– setDaemon method: for a thread to specify that it is a

daemon thread– isDaemon method: to determine whether a thread is a

daemon thread


16

Using the JVM to Schedule Tasks • The JVM creates background threads for you if

you use the Timer and TimerTask classes from the java.util package to schedule tasks for future execution

• Class java.util.TimerTask implements Runnable• Class java.util.Timer provides the facility to run a

TimerTask object on a background thread generated by the JVM


17

A Class That Extends java.util.TimerTask


18

A Class That Creates a java.util.Timer Object


19

Wring Robust Multithreaded Programs

• Declare variables with the keyword volatile to suppress compiler optimizations that have the potential to lose or ignore changes made by different threads to the values of the variables

• Qualify the declaration of a method with the keyword synchronized to be run for a particular object or class by only one thread at a time


20

Making Variable Values Thread-Safe

• Thread-safe variables belong to one thread alone, and different threads have separate copies

• Local variables are thread-safe• Synchronization provides the safety for

instance variables• Class variables are not thread-safe


21

Making Variable Values Thread-Safe (Cont.)

• Classes ThreadLocal and InheritableThreadLocal provide variables that are local to each thread and therefore thread-safe

• The ThreadLocal class can hold an object of any reference type


22

Making Variable Values Thread-Safe (Cont.)

• Methods of the ThreadLocal class:– Object get()– Object initialValue()– void set(Object value)


23

A ThreadLocal Class Variable


24

A ThreadLocal Class Variable (Cont.)


25

A Class That Uses a ThreadLocal Variable


26

A Class That Uses a ThreadLocal Variable (Cont.)


27

Synchronizing Threads

• Declare a class method with synchronized if only one thread at a time should be able to execute the method

• The mechanism of thread synchronization places locks on objects

• Locks are the Java concept for flags maintained by the operating system

• Object and class locks are independent


28

A Synchronized Method


29

When to Synchronize Code

• Synchronize code whenever the value of class or instance variables may change or methods may produce output to the same destination in code that can run on more than one thread

• To determine the level of synchronization that your application requires, consider how the threads within it share classes and objects


30

Synchronizing Methods of Inner Classes

• No special relationship exists between the synchronized methods of an inner class and its enclosing class

• If a method of an inner class needs to participate in the synchronization of fields in the enclosing class, it should use a synchronized statement to obtain a lock for the enclosing class instance


31

Synchronizing Methods of Inner Classes (Cont.)


32

Synchronizing Methods of Inner Classes (Cont.)


33

Communicating Between Threads

• The Object class defines three methods for interthread communication:– void wait()– void notify (long timeout)– void notifyAll (long timeout, int nanoseconds)


34

Making Threads Wait

• Call the wait method if a thread reaches an inactive state while waiting for input

• The wait method suspends the thread and gives other threads a chance to run

• Call wait in a loop:

while ( condition ) { wait() }


35

Waking a Single Thread

• Call the notify method to wake up a thread• The notify method wakes up a single thread

that is blocked because it is in a wait state• A synchronized method must still wait for the

lock on the class or object


36

Waking All Threads

• The notifyAll method sends wake-up messages to all threads waiting for the lock

• All the threads go into a runnable state, but only one can actually acquire the lock and start running


37

Grouping Threads

• Collect Thread objects into ThreadGroup objects

• Thread objects in separate groups can be protected from each other

• Perform Thread operations on an entire group at once


38

Grouping Threads (Cont.)

• ThreadGroup objects provide security because a thread can modify another thread only if:– Both threads reside in the same group or– If the modified thread resides in a group that is

nested within the group of the modifying thread

• After you create a thread, you cannot change its group


39

Summary• A class can run as a separate thread if it

extends the Thread class or implements the Runnable interface

• All classes that implement Runnable must implement the method run

• The Thread class provides methods to control your threads, including yield, sleep, and setPriority

• Daemon threads are threads that run in the background


40

Summary (Cont.)

• Use the Timer and TimerTask classes from the java.util package to schedule tasks for future execution

• Declare a class method with synchronized if only one thread at a time should be able to execute the method

• For interthread communication the wait, notify, and notifyAll methods are used

• For greater security and convenience, you can collect Thread objects into ThreadGroup objects

java programming: advanced topics 1 multithreading chapter 5

Documents