Interview Questions – Thread

Interview Questions – Thread
Java Synchronization
Every object can be used as a limiting resource or monitor object, and to constrain access to code in a critical section.
[Implementation - Each object has header represented by two 32-bit words - Lock word and Class word. In Lock word (which also contains GC state information), it carries synchronization information, such as whether the object is used in a lock right now.]
How synchronization is implemented?
Synchronization is implemented using monitors. Each object's header is associated with a monitor, which a thread can lock or unlock.
Only one thread at a time may hold a lock on a monitor. Any other threads attempting to lock that monitor are blockeduntil they can obtain a lock on that monitor.
The synchronized statement computes a reference to an object; it then attempts to perform a lock action on that object's monitor and does not proceed further until the lock action has successfully completed. After the lock action has been performed, the body of the synchronized statement is executed.
If execution of the body is ever completed, either normally or abruptly, an unlock action is automatically performed on that same monitor.
Class lock vs Object lock
A synchronized method acquires a monitor before it executes. For a class (static) method, the monitor associated with the Class object for the method's class is used. For an instance method, the monitor associated with this (the object for which the method was invoked) is used.
Synchronizing an instance method uses the object's lock, since we assume we're modifying the state of a specific object - and each object is separate from every other of the same class. If a class has two instance synchronized methods, then it is possible to access each of these synchronized methods by two different objects simultaneously. If the synchronized method is a static method, a class lock is used (since the method is assumed to act the same for all instances). 
If 2 different threads hit 2 different synchronized methods in an object at the same time will they both continue? 
No. Only one method can acquire the lock. 2 different threads can not access instance synchronized method on same object simultaneously. 2 different threads can access instance synchronized method on 2 different objects simultaneously. Because of Class lock, 2 different threads can NOT access static synchronized method on 2 different objects simultaneously. If a thread goes to sleep, it holds any locks it has—it doesn't release them. If a class has both synchronized and non-synchronized methods, multiple threads can still access the class same object's synchronized method and non-synchronized method simultaneously.
One point you have to be careful about is that there is no link between synchronized static methods and synchronized non static methods
class A { static synchronized f() {...} synchronized g() {...} } f() and g() are not synchronized with each other and thus can execute totally concurrently.
On an instance method, to implement mutual exclusion between different instances of the object - for example when accessing an external resource):
g() {synchronized(MyClass.class) {}}
Explain ways of creating a thread, what is the difference? Which is better? 
There are two ways to create a new thread: First, Extends the Thread class, and override the run() method in your class. In order to start a new thread, we need initialize an instance of the subclass and invoke the start() method on it. Second, Implements the Runnable interface, The class will have to implement the run() method in the Runnable interface. In order to start a new thread, we need create an instance of this class. Pass the reference of this instance to the Thread constructor a new thread of execution will be created. Implementing the runnable interface is preferred, as it does not require your object to inherit a thread. When you need multiple inheritance, for example, if you are already inheriting a different class, then only interfaces can help you.
How can threads communicate with each other in java? 
The wait(), notify(), and notifyAll() methods are used to provide an efficient way for threads to communicate with each other. 
What is the difference between yield() and sleep()? 
When a task invokes yield(), it changes from running state to runnable state. It allows the current the thread to release its lock from the object and scheduler gives the lock of the object to the other thread with same priority. When a task invokes sleep(), it changes from running state to waiting/sleeping state. It allows the thread to go to sleep state for the specified milliseconds. When a thread goes into sleep state it doesn't release the lock.
What is difference between notify() and notfiyAll()?
When notify is called, one of the threads waiting for the synchronized resource will be arbitrarily selected and woken up by the thread scheduler in the JVM.
When notifyAll is called, all threads waiting for the lock will be woken up. Only one of them will succeed in acquiring the lock and the rest will go to sleep again.
The notifyAll method is safer than notify but carries a greater overhead than notify.
What is daemon thread and which method is used to create the daemon thread? 
Threads are divided into two types: normal threads and daemon threads. When a new thread is created, it inherits the daemon status of the thread that created it, so by default any threads created by the main thread are also normal threads. Normal threads and daemon threads differ only in what happens when they exit. Daemon threads are sometimes called "service" threads. These are threads that normally run at a low priority and provide a basic and helper service to a program. You don't want the existence of this thread to prevent the JVM from shutting down. This is what daemon threads are for. An example of a daemon thread that is continuously running is the garbage collector thread. This thread is provided by the JVM. When a thread exits, the JVM performs an inventory of running threads, and if the only threads that are left are daemon threads, it initiates an orderly shutdown. When the JVM halts, any remaining daemon threads are abandoned, finally blocks are not executed, stacks are not unwound the JVM just exits.
The setDaemon() method is used to create a daemon thread. These threads run without the intervention of the user. 
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. 
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. 
What is synchronization advantage and disadvantage? 
Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object’s value. This often causes dirty data and leads to significant errors. The disadvantage of synchronization is that it can cause deadlocks when two threads are waiting on each other to do something. Also synchronized code has the overhead of acquiring lock, which can adversely the performance.
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