What is a process vs thread
Processes do not share common memory
Threads share common memory
A process is an isolated piece of software, work by inter-process communication called as intermediate API.
A thread shares memory with threads of the same application, it allows for reducing overhead, and faster data exchange.
Create a thread instance
A thread is a thread of execution in a program.
- Threads have priorities
- Code running in some thread creates a new
Thread
object, initial priority is equal to the creator thread - Thread execution continues until
- the
exit
method of classRuntime
has been called, with security manager's permission. - All the non-daemon threads have died
- the
Declare a class to be a subclass of Thread
. This subclass should override the run
method of class Thread
.
class PrimeThread extends Thread {
long minPrime;
PrimeThread(long minPrime) {
this.minPrime = minPrime;
}
public void run() {
// compute logic goes here
}
}
PrimeThread p = new PrimeThread(143);
p.start();
or simply call from Runnable
Thread thread01 = new Thread(() -> System.out.println("Hello"));
thread01.start();
Thread states
https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/Thread.State.html
- NEW - A thread that has not yet started in this state.
- RUNNABLE - A thread executing in the Java virtual machine in this state
- BLOCKED - A thread that is blocked waiting for a monitor lock in this state
- WAITING - A thread waiting indefinitely for another thread to perform a particular action. A variant is TIMED_WAITING.
- TERMINATED
Runnable vs Callable interfaces
Runnable
has a run
method. Runs a unit of computation in a thread
Callable
interface has a call
method and returns a value.
Create a daemon thread
Daemon threads do supportive tasks for the other threads, and can be abandoned at anytime.
Thread daemon = new Thread(() -> System.out.println("This is a daemon thread");
daemon.setDaemon(true);
daemon.start();
https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/Thread.html#isDaemon()
Implementations of ExecutorService
ForkJoinPool
, ScheduledThreadPoolExecutor
, ThreadPoolExecutor
Java memory model (JMM)
Important for the interaction among threads.
Some notions of JMM
- Actions, are inter-thread. They can be executed by one thread and found by another.
- Program Order, the observable total order of actions in a single thread
- Synchronization Order, the total order between all synchronization actions - in consistent with Program Order.
- Execution - a certain set of ordered actions with consistency rules among them
If a program is synchronized then all of its execution have sequentially consistent.
Deadlock, Livelock and Starvation
Two processes each waiting for a resource the other has but waiting in a non-blocking manner. When each learns they cannot continue they release their held resource and sleep for 30 seconds, then they retrieve their original resource followed by trying to the resource the other process held, then left, then reacquired Since both processes are trying to cope (just badly), this is a livelock.
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