1. Java Programming: Advanced Topics
Multithreading
Chapter 5
Java Programming: Advanced Topics

2. Java Programming: Advanced Topics
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
Java Programming: Advanced Topics

3. Java Programming: Advanced Topics
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
Java Programming: Advanced Topics

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
Java Programming: Advanced Topics

5. Single-Threaded and Multithreaded Programs
Java Programming: Advanced Topics

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
Java Programming: Advanced Topics

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
Java Programming: Advanced Topics

8. Java Programming: Advanced Topics
Using the Thread Class
Methods:
Thread currentThread()
int getPriority()
void interrupt()
boolean isDaemon()
boolean isAlive()
void join()
void join( long milliseconds)
Java Programming: Advanced Topics

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()
Java Programming: Advanced Topics

10. State Transition Diagram of a Thread
Java Programming: Advanced Topics

11. Sample Class That Extends Thread
Java Programming: Advanced Topics

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
Java Programming: Advanced Topics

13. Sample Class That Implements Runnable
Java Programming: Advanced Topics

14. Java Programming: Advanced Topics
Stopping a Thread
Java Programming: Advanced Topics

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
Java Programming: Advanced Topics

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
Java Programming: Advanced Topics

17. A Class That Extends java.util.TimerTask
Java Programming: Advanced Topics

18. A Class That Creates a java.util.Timer Object
Java Programming: Advanced Topics

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
Java Programming: Advanced Topics

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
Java Programming: Advanced Topics

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
Java Programming: Advanced Topics

22. Making Variable Values Thread-Safe (Cont.)
Methods of the ThreadLocal class:
Object get()
Object initialValue()
void set(Object value)
Java Programming: Advanced Topics

23. A ThreadLocal Class Variable
Java Programming: Advanced Topics

24. A ThreadLocal Class Variable (Cont.)
Java Programming: Advanced Topics

25. A Class That Uses a ThreadLocal Variable
Java Programming: Advanced Topics

26. A Class That Uses a ThreadLocal Variable (Cont.)
Java Programming: Advanced Topics

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
Java Programming: Advanced Topics

28. Java Programming: Advanced Topics
A Synchronized Method
Java Programming: Advanced Topics

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
Java Programming: Advanced Topics

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
Java Programming: Advanced Topics

31. Synchronizing Methods of Inner Classes (Cont.)
Java Programming: Advanced Topics

32. Synchronizing Methods of Inner Classes (Cont.)
Java Programming: Advanced Topics

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)
Java Programming: Advanced Topics

34. Java Programming: Advanced Topics
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() }
Java Programming: Advanced Topics

35. Java Programming: Advanced Topics
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
Java Programming: Advanced Topics

36. Java Programming: Advanced Topics
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
Java Programming: Advanced Topics

37. Java Programming: Advanced Topics
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
Java Programming: Advanced Topics

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
Java Programming: Advanced Topics

39. Java Programming: Advanced Topics
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
Java Programming: Advanced Topics

40. Java Programming: Advanced Topics
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