Multithreading in java
- 3. Multithreading concepts Need For Concurrency Multi Processing Multi Threading Multithreading makes user-interface more responsive Better utilization of time
- 4. Creating Threads A thread represents code that will be run concurrently with other code in the same process. Creating a Thread • Use The Thread Class • Use The Runnable Interface
- 5. Which approach is better? Extending Thread is Simpler. Implementing Runnable allows sub classing other classes.
- 7. Implementing Runnable Create a Runnable Object Create Pass the Object to Thread Constructor Pass Override run method Override
- 8. What Is Thread A Thread is a single sequential flow of control within a program. Thread does not have its own address space but uses the memory and other resources of the process in which it executes. There may be several threads in one process. The JVM manages this and shedules them.
- 9. What is Multithreading? Multithreading is a conceptual programming where one program or process is divided into multiple sub programs or tasks those are having independent path of execution and can run in parallel. For example one sub program can read a CSV file and another sub program can enter the data into DB. A program that contains multiple flow of control is known as multithreaded program.
- 10. Thread State
- 11. Thread Priorities Java assign each thread a priority, that determines how that Thread should be treated with respect to others. Thread Priorities are integers that specify relative priority of one thread to another. A Thread Priority is used to decide when to switch from one running thread to the next. This is called context switching.
- 12. Thread Priorities • A thread can voluntarily relinquish control. This is done by explicit yielding, sleeping or blocking on pending IO. In this scenario all other threads are examined , and the highest priority thread that is ready to run is given the CPU. • A Thread can be pre-empted by a higher priority thread. This is called pre-emptive multi tasking.
- 13. Multitasking Multitasking allows several activities to occur concurrently on the computer. A distinction is usually made between : Process based Multitasking Thread based Multitasking
- 14. Advantage of thread based multithreading Thread share the same address space Context switching is less expensive. Cost of communication between threads is relatively low
- 15. Overview of Thread A thread is an independent sequential path of execution within a program. Many threads can run concurrently within a program. At run time Threads in a program exists in a common memory space and can therefore share both data and code.
- 16. The Main Thread When a standalone application run, a user thread is automatically created to execute the main method of the application. From main thread child thread are spawned. If no user threads are there ,the program terminates when main execution finished. All other threads are called child threads ,spawned from main thread inheriting its child thread status. The main method can finish , but the program will keep running until all user threads have complted.
- 17. User Threads & Daemon Threads The run time environment distinguishes between user threads and daemon threads. As long as user thread is alive , the JVM does not terminate. A daemon thread is at the mercy of the runtime system. It is stopped if there are no more user threads running, thus terminating the program. Daemon thread exists only to serve user thread.
- 18. Creating Daemon or User Thread Calling the setDaemon(boolean) method of Thread class marks the status of Thread as either Daemon or User. This method must be called on a Thread object before it starts. Any attempt to change the status of the Thread will throw an IllegalThreadStateException.
- 19. Thread Creation A Thread in Java is represented by an object of the Thread class. Implementing thread is achieved in one of the two ways. Implementing the java.lang.Runnable interface Extending the java.lang.Thread class
- 24. Synchronization Threads share the same memory space i.e they can share resources. However there are some critical situation where it is desirable that only one Thread at a time has access to a shared resources. For example crediting and debiting a shared bank account concurrently among several users without proper discipline will harm the integrity of the account data. Java Provides high level concepts of synchronization in order to control access to shared resources.
- 25. Locks A lock also called as a monitor is used to synchronize access to a shared resource. Threads gain access to a shared resource by first acquiring the lock associated with the resource. At any given time at most one thread can hold the lock and there by have access to the shared resource. In Java , all the objects have a lock, including arrays. By associating a shared resource with a java object and its lock , the object can act as a guard, ensuring synchronized access to the resource.
- 26. Object Lock Mechanism • The Object lock mechanism , enforces the following rules of synchronization. • A thread must acquire the object lock associated with a shared resource, before it can enter the shared resource. The runtime system ensures that no other thread can entered a shared resource if another thread already holds the object lock associated with it. • If a thread can not immediately acquire the object lock, it will be blocked. • If a thread exists a shared resource , the runtime ensures that the object lock is also relinquished. If another thread is waiting for this object lock , it will be notified .
- 27. Two ways of synchronization • There are two ways in which execution of code can be synchronized: • Synchronized Method • Synchronized Code block
- 28. Different way to hold a lock By executing a synchronized instance method of the object. By executing a synchronized block that synchronizes on the object. By executing a synchronized static method of a class .
- 29. Thread States New state-: A thread is created but not yet started. Ready To run state-: when we call start() on a Thread object comes to this state waiting for TS to get its turn. Running state-: In this state the thread gets its turn and started executing run (). Dead State-:Once in this state, A thread can not run again. Non-Runnable states-: A running thread can transit to one of the non-runnable states. A thread remains in non-runnable state until a special transition occurs.
- 30. Non- Runnable States Sleeping: the thread sleeps for a specified amount of time. Blocked for I/O: the threads wait for blocking operation to complete. Blocked for join completion: The thread awaits completion of another thread. Waiting for notification: the thread awaits notification from another thread. Blocked for lock acquisition: The thread wait to acquire the lock.
- 31. Thread.State NEW – Created but not yet started RUNNABLE – Executing the JVM BLOCKED – blocked for lock acquisition WAITING- Waiting for notify, Blocked for join completion TIMED_WAITING- Sleeping, Waiting For notify TERMINATED- Completed
- 32. Thread Prorities Threads are assigned priorities that the thread scheduler can use to determine how the threads will be scheduled. The TS can use thread priorities to determine which thread gets to run. The TS favours giving the CPU time to the thread with the highest priority in ready to run state. Heavy reliance on thread priorities for the behavior of a program can make the program unportable across platforms, as thread scheduling is host-platform dependant.
- 33. Thread Priorities Priorities are integer values 1-lowest (Thread.MIN_PRIORITY) 10-highest(Thread.MAX_PRIORITY) 5-normal(Thread.NORM_PRIORITY)
- 34. Thread Scheduler Scheduler in JVM implementations usually employ one of the two strategies. Preemptive scheduling: If a thread with a higher priority than the current running thread moves to the ready to run state, then the current state can be preempted to the higher priority thread execute. Time-sliced or Round-Robin: A running thread is allowed to execute for a fixed length of time , after which it moves to ready to run state.