21⟩ What is thread? What are the high-level thread states? Or What are the states associated in the thread?
A thread is an independent path of execution in a system. The high-level thread states are ready, running, waiting and dead.
Java Threads
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38 Java Threads Questions And Answers
22⟩ What is the purpose of the wait(), notify(), and notifyAll() methods?
The wait(), notify() and notifyAll() methods are used to provide an efficient way for thread inter-communication.
23⟩ What is the difference between process and thread?
A thread is a separate path of execution in a program. A Process is a program in execution.
24⟩ What is the difference between the methods sleep() and wait()?
The sleep method is used when the thread has to be put aside for a fixed amount of time. Ex: sleep(1000), puts the thread aside for exactly one second. The wait method is used to put the thread aside for up to the specified time. It could wait for much lesser time if it receives a notify() or notifyAll() call. Ex: wait(1000), causes a wait of up to one second. The method wait() is defined in the Object and the method sleep() is defined in the class Thread.
25⟩ Can Java object be locked down for exclusive use by a given thread? Or What happens when a thread cannot acquire a lock on an object?
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. If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available.
26⟩ What is daemon thread and which method is used to create the daemon thread?
Daemon threads are threads with low priority and runs in the back ground doing the garbage collection operation for the java runtime system. The setDaemon() method is used to create a daemon thread. These threads run without the intervention of the user. To determine if a thread is a daemon thread, use the accessor method isDaemon()
When a standalone application is run then as long as any user threads are active the JVM cannot terminate, otherwise the JVM terminates along with any daemon threads which might be active. Thus a daemon thread is at the mercy of the runtime system. Daemon threads exist only to serve user threads.
27⟩ What do you understand by Synchronization? Or What is synchronization and why is it important? Or Describe synchronization in respect to multithreading? Or What is synchronization?
With respect to multithreading, Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access a particular resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value. Synchronization prevents such type of data corruption which may otherwise lead to dirty reads and significant errors.
E.g. synchronizing a function:
public synchronized void Method1 () {
// method code.
}
E.g. synchronizing a block of code inside a function:
public Method2 (){
synchronized (this) {
// synchronized code here.
}
}
28⟩ What is an objects lock and which objects have locks?
An objects lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the objects lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.
29⟩ What state does a thread enter when it terminates its processing?
When a thread terminates its processing, it enters the dead state.
30⟩ Why would you use a synchronized block vs. synchronized method?
Synchronized blocks place locks for shorter periods than synchronized methods.
31⟩ When you will synchronize a piece of your code?
When you expect that your shared code will be accessed by different threads and these threads may change a particular data causing data corruption, then they are placed in a synchronized construct or a synchronized method.
32⟩ Is there a separate stack for each thread in Java?
Yes. Every thread maintains its own separate stack, called Runtime Stack but they share the same memory. Elements of the stack are the method invocations,
called activation records or stack frame. The activation record contains pertinent information about a method like local variables.
33⟩ How would you implement a thread pool?
public class ThreadPool implements ThreadPoolInt
This class is an generic implementation of a thread pool, which takes the following input
a) Size of the pool to be constructed
b) Name of the class which implements Runnable and constructs a thread pool with active threads that are waiting for activation. Once the threads have finished processing they come back and wait once again in the pool.
This thread pool engine can be locked i.e. if some internal operation is performed on the pool then it is preferable that the thread engine be locked. Locking ensures that no new threads are issued by the engine. However, the currently executing threads are allowed to continue till they come back to the passivePool.
34⟩ What is the SwingUtilities.invokeLater(Runnable) method for?
The static utility method invokeLater(Runnable) is intended to execute a new runnable thread from a Swing application without disturbing the normal sequence of event dispatching from the Graphical User Interface (GUI). The method places the runnable object in the queue of Abstract Windowing Toolkit (AWT) events that are due to be processed and returns immediately. The runnable object's run() method is only called when it reaches the front of the queue.
The deferred effect of the invokeLater(Runnable) method ensures that any necessary updates to the user interface can occur immediately, and the runnable work will begin as soon as those high priority events are dealt with. The invoke later method might be used to start work in response to a button click that also requires a significant change to the user interface, perhaps to restrict other activities, while the runnable thread executes.
35⟩ What is a green thread?
A green thread refers to a mode of operation for the Java Virtual Machine (JVM) in which all code is executed in a single operating system thread. If the Java program has any concurrent threads, the JVM manages multi-threading internally rather than using other operating system threads.
There is a significant processing overhead for the JVM to keep track of thread states and swap between them, so green thread mode has been deprecated and removed from more recent Java implementations. Current JVM implementations make more efficient use of native operating system threads.
36⟩ What is the volatile modifier for?
The volatile modifier is used to identify variables whose values should not be optimized by the Java Virtual Machine, by caching the value for example. The volatile modifier is typically used for variables that may be accessed or modified by numerous independent threads and signifies that the value may change without synchronization.
37⟩ Which class is the wait() method defined in?
The wait() method is defined in the Object class, which is the ultimate superclass of all others. So the Thread class and any Runnable implementation inherit this method from Object. The wait() method is normally called on an object in a multi-threaded program to allow other threads to run. The method should should only be called by a thread that has ownership of the object's monitor, which usually means it is in a synchronized method or statement block.
38⟩ What is a working thread?
A working thread, more commonly known as a worker thread is the key part of a design pattern that allocates one thread to execute one task. When the task is complete, the thread may return to a thread pool for later use. In this scheme a thread may execute arbitrary tasks, which are passed in the form of a Runnable method argument, typically execute(Runnable). The runnable tasks are usually stored in a queue until a thread host is available to run them.
The worker thread design pattern is usually used to handle many concurrent tasks where it is not important which finishes first and no single task needs to be coordinated with another. The task queue controls how many threads run concurrently to improve the overall performance of the system. However, a worker thread framework requires relatively complex programming to set up, so should not be used where simpler threading techniques can achieve similar results.
