CMSC 132 Object Oriented Programming II Threads in Java Department of Computer Science University of Maryland College Park 1 Problem Multiple tasks for computer Draw display images on screen Check keyboard mouse input Send receive data on network Read write files to disk Perform useful computation editor browser game How does computer do everything at once Multitasking Multiprocessing 2 Multitasking Time Sharing Approach Computer does some work on a task Computer then quickly switch to next task Tasks managed by operating system scheduler Computer seems to work on tasks concurrently Can improve performance by reducing waiting 3 Multitasking Can Aid Performance Single task Two tasks 4 Multiprocessing Multithreading Approach Multiple processing units multiprocessor Computer works on several tasks in parallel Performance can be improved Dual core AMD Athlon X2 32 processor Pentium Xeon 4096 processor Cray X1 5 Perform Multiple Tasks Using 1 Process Definition executable program loaded in memory Has own address space Variables data structures in memory Each process may execute a different program Communicate via operating system files network May contain multiple threads 6 Perform Multiple Tasks Using 2 Thread Definition sequentially executed stream of instructions Shares address space with other threads Has own execution context Program counter call stack local variables Communicate via shared access to data Multiple threads in process execute same program Also known as lightweight process 7 Motivation for Multithreading 1 Captures logical structure of problem May have concurrent interacting components Can handle each component using separate thread Simplifies programming for problem Example Web Server uses threads to handle Multiple simultaneous web browser requests 8 Motivation for Multithreading 2 Better utilize hardware resources When a thread is delayed compute other threads Given extra hardware compute threads in parallel Reduce overall execution time Example Multiple simultaneous web browser requests Handled faster by multiple web servers 9 Multithreading Overview Motivation background Threads Creating Java threads Thread states Scheduling Synchronization Data races Locks Deadlock 10 Programming with Threads Concurrent programming Writing programs divided into independent tasks Tasks may be executed in parallel on multiprocessors Multithreading Executing program with multiple threads in parallel Special form of multiprocessing 11 Creating Threads in Java Need to specify work performed by thread Two approaches 1 Runnable interface public interface Runnable public void run work thread 2 Extending Thread class public class Thread extends Object 12 Thread Class public class Thread extends Object implements Runnable public Thread public Thread String name Thread name public Thread Runnable R Thread R run public Thread Runnable R String name public void run if no R work for thread public void start begin thread execution 13 More Thread Class Methods public class Thread extends Object public static Thread currentThread public String getName public void interrupt public boolean isAlive public void join public void setDaemon public void setName public void setPriority public static void sleep public static void yield 14 Creating Threads in Java Runnable Approach 1 Define class implementing Runnable interface public interface Runnable public void run 2 Put work to be performed in run method 3 Create instance of the worker class 4 Create thread to run it Create Thread object Pass worker object to Thread constructor Or hand the worker instance to an executor Alternative methods for running threads 15 Creating Threads in Java Example public class MyT implements Runnable public void run work for thread Thread t new Thread new MyT create thread t start begin running thread thread executing in parallel 16 Alternative Thread Creation Approach Thread Class Approach Extend Thread class and override run method Not recommended Example public class MyT extends Thread public void run work for thread MyT t new MyT create thread t start begin running thread thread executing in parallel 17 Why Not Recommended Not a big problem for getting started But a bad habit for industrial strength development Methods of worker and Thread class intermixed Hard to migrate to more efficient approaches Thread Pools 18 Creating Threads in Java Note Thread starts executing only if start is called Runnable is interface So it can be implemented by any class Required for multithreading in applets 19 Threads Thread States Java thread can be in one of these states New Runnable Running Blocked Dead thread allocated waiting for start thread can begin execution thread currently executing thread waiting for event I O etc thread finished Transitions between states caused by Invoking methods in class Thread new start yield sleep wait notify Other external events Scheduler I O returning from run 20 Threads Thread States State diagram new start new runnable yield time slice scheduler running notify notifyAll IO complete sleep expired join complete blocked IO sleep wait join terminate dead Running is a logical state indicates runnable thread is actually running 21 Daemon Threads Java threads types User Daemon Provide general services Typically never terminate Call setDaemon before start Program termination 1 All user threads finish 2 Daemon threads are terminated by JVM 3 Main program finishes 22 Threads Scheduling Scheduler Determines which runnable threads to run Can be based on thread priority Part of OS or Java Virtual Machine JVM Scheduling policy Nonpreemptive cooperative scheduling Preemptive scheduling 23 Threads Non preemptive Scheduling Threads continue execution until Thread terminates Executes instruction causing wait e g IO Thread volunteering to stop invoking yield or sleep 24 Threads Preemptive Scheduling Threads continue execution until Same reasons as non preemptive scheduling Preempted by scheduler 25 Java Thread Example public class ThreadExample extends Thread public void run for int i 0 i 3 i try sleep int Math random 5000 5 secs catch InterruptedException e System out println i public static void main String args new ThreadExample start new ThreadExample start System out println Done 26 Java Thread Example Output Possible outputs 0 1 2 0 1 2 Done 0 1 2 Done 0 1 2 Done 0 1 2 0 1 2 0 0 1 1 2 Done 2 thread 1 thread 2 main thread 1 main thread 2 main thread 1 thread 2 main threads interleaved main thread 1 thread
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