本文详细介绍了多线程的基本概念，包括线程定义、多线程的优势、抢占式调度与时间片分配的区别、join()方法的功能等。同时，还探讨了wait()与sleep()方法的不同之处、线程启动限制、作为普通对象运行的run()方法调用问题、守护线程的作用、用户线程转为守护线程的限制、关闭钩子的概念及用途、线程中断的应用场景、同步的概念及其目的、synchronized块的使用、对象锁定机制、静态同步的特点以及notify()与notifyAll()的区别，并解释了死锁的现象。
1) What is multithreading?
Multithreading is a process of executing multiple threads simultaneously. Its main advantage is:
- Threads share the same address space.
- Thread is lightweight.
- Cost of communication between process is low.
2) What is thread?
A thread is a lightweight subprocess.It is a separate path of execution.It is called separate path of execution because each thread runs in a separate stack frame.
3)What is the difference between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.
4) What does join() method?
The join() method waits for a thread to die. In other words, it causes the currently running threads to stop executing until the thread it joins with completes its task.
5) What is difference between wait() and sleep() method?
| wait() | sleep() |
|---|---|
| 1) The wait() method is defined in Object class. | The sleep() method is defined in Thread class. |
| 2) wait() method releases the lock. | The sleep() method doesn't releases the lock. |
6) Is it possible to start a thread twice?
No, there is no possibility to start a thread twice. If we does, it throws an exception.
7) Can we call the run() method instead of start()?
yes, but it will not work as a thread rather it will work as a normal object so there will not be context-switching between the threads.
8) What about the daemon threads?
The daemon threads are basically the low priority threads that provides the background support to the user threads. It provides services to the user threads.
9)Can we make the user thread as daemon thread if thread is started?
No, if you do so, it will throw IllegalThreadStateException
10)What is shutdown hook?
The shutdown hook is basically a thread i.e. invoked implicitely before JVM shuts down. So we can use it perform clean up resource.
11)When should we interrupt a thread?
We should interrupt a thread if we want to break out the sleep or wait state of a thread.
12) What is synchronization?
Synchronization is the capabilility of control the access of multiple threads to any shared resource.It is used:
- To prevent thread interference.
- To prevent consistency problem.
13) What is the purpose of Synchronized block?
- Synchronized block is used to lock an object for any shared resource.
- Scope of synchronized block is smaller than the method.
14)Can Java object be locked down for exclusive use by a given thread?
Yes. You can lock an object by putting it in a "synchronized" block. The locked object is inaccessible to any thread other than the one that explicitly claimed it.
15) What is static synchronization?
If you make any static method as synchronized, the lock will be on the class not on object. more details...
16)What is the difference between notify() and notifyAll()?
The notify() is used to unblock one waiting thread whereas notifyAll() method is used to unblock all the threads in waiting state.
17)What is deadlock?
Deadlock is a situation when two threads are waiting on each other to release a resource. Each thread waiting for a resource which is held by the other waiting thread.
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