Multithreading in Java Nelson Padua Perez Bill Pugh Department of Computer Science University of Maryland College Park 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 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 Multitasking Can Aid Performance Single task Two tasks 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 Perform Multiple Tasks Using 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 Perform Multiple Tasks Using 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 Motivation for Multithreading 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 Motivation for Multithreading 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 Multithreading Overview Motivation background Threads Creating Java threads Thread states Scheduling Synchronization Data races Locks Wait Notify 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 Creating Threads in Java You have to specify the work you want the thread to do Define a class that implements the Runnable interface public interface Runnable public void run Put the work in the run method Create an instance of the worker class and create a thread to run it or hand the worker instance to an executor Thread Class public class Thread public Thread Runnable R Thread R run public Thread Runnable R String name public void start begin thread execution More Thread Class Methods public class Thread public String getName public void interrupt public boolean isAlive public void join public void setDaemon boolean on public void setName String name public void setPriority int level public static Thread currentThread public static void sleep long milliseconds public static void yield Creating Threads in Java Runnable interface Create object implementing Runnable interface Pass it to Thread object via Thread constructor 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 Alternative Not Recommended Directly extend Thread class public class MyT extends Thread public void run work for thread MyT t new MyT t start create thread begin running thread Why not recommended Not a big problem for getting started but a bad habit for industrial strength development The methods of the worker class and the Thread class get all tangled up Makes it hard to migrate to Thread Pools and other more efficient approaches Threads Thread States Java thread can be in one of these states New Runnable Blocked Terminated thread allocated waiting for start thread can execute thread waiting for event I O etc thread finished Transitions between states caused by Invoking methods in class Thread start yield sleep Other external events Scheduler I O returning from run Threads Thread States State diagram start new new runnable terminate IO sleep join request lock terminated IO complete sleep expired join complete acquire lock blocked Threads Scheduling Scheduler Determines which runnable threads to run Can be based on thread priority Part of OS or Java Virtual Machine JVM Many computers can run multiple threads simultaneously or nearly so Java Thread Example public class ThreadExample implements Runnable public void run for int i 0 i 3 i System out println i public static void main String args new Thread new ThreadExample start new Thread new ThreadExample start System out println Done 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 2 println Done thread 1 println 0 println 1 println 2 thread 2 println 0 println 1 println 2 Daemon Threads Why doesn t the program quit as soon as Done is printed Java threads types User Daemon Provide general services Typically never terminate Call setDaemon before start Program termination If all non daemon threads terminate JVM shuts down Might not see different interleavings The threads in that example are too short Each started thread will probably complete before the next thread starts Let s make more threads that run longer Data Races public class DataRace implements Runnable static volatile int x public void run for int i 0 i 10000 i x x public static void main String args throws Exception Thread threads new Thread 100 for int i 0 i threads length i threads i new Thread new DataRace for int i 0 i threads length i threads i start for int i 0 i threads length i threads i join System out println x x not always 0 Why volatile We ll spend more time on volatile later But volatile tells the compiler other threads might see reads writes of this variable don t change reorder eliminate the reads and writes An optimizing compiler should if it sees x x replace it with a no op if x isn t volatile Thread Scheduling Observations Order thread is selected is indeterminate Depends on scheduler
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